thought experiment superposition

Schrödinger's cat: The favorite, misunderstood pet of quantum mechanics

Reference article: A brief, simple explanation of Schrödinger's cat.

The thought experiment known as Schrödinger's cat is one of the most famous, and misunderstood, concepts in quantum mechanics . By thinking deeply about it, researchers have come to spectacular insights about physical reality. 

Who came up with Schrödinger's cat? 

The Austrian physicist Erwin Schrödinger, who helped found the discipline of quantum mechanics, first conceived of his feline conundrum in 1935 as a commentary on problems originally posed by the luminary Albert Einstein, according to an article in Quanta Magazine . 

While developing their new understanding of the subatomic realm, most of Einstein and Schrödinger's colleagues had realized that quantum entities exhibited extremely odd behaviors. The Danish physicist Niels Bohr championed an understanding that particles like electrons did not have well-defined properties until they were measured. Before that, the particles existed in what's known as a superposition of states, with, for example, a 50% chance of being oriented "up" and a 50% chance of being oriented "down."

Einstein, in particular, did not like this indecisive explanation. He wanted to know how, exactly, the universe knows that someone is measuring something. Schrödinger highlighted this absurdity with his notorious conceptual cat.

Suppose one builds a strange contraption, Schrödinger wrote in a 1935 paper called " The Current Situation in Quantum Mechanics ." The apparatus consists of a box with a sealed vial of cyanide, above which is suspended a hammer attached to a Geiger counter aimed at a small lump of mildly radioactive uranium. Inside the box, there's also a kitty (and remember, this is a thought experiment that's never actually been carried out). 

The box is sealed, and the experiment is left to run for some set amount of time, perhaps an hour. In that hour, the uranium, whose particles obey the laws of quantum mechanics, has some chance of emitting radiation that will then be picked up by the Geiger counter, which will, in turn, release the hammer and smash the vial, killing the cat by cyanide poisoning. 

According to folks like Bohr, until the box is opened and the cat's status is "measured," it will remain in a superposition of both living and deceased. People like Einstein and Schrödinger balked at such a possibility, which doesn't accord with everything our ordinary experience tells us — cats are either alive or dead, not both at the same time.

"[Q]uantum physics lacked an important component, a story about how it lined up with things in the world," wrote science journalist Adam Becker in his book " What Is Real? " (Basic Books, 2018). "How does a phenomenal number of atoms, governed by quantum physics, give rise to the world we see around us?" 

Is Schrödinger's cat real? 

Schrödinger's cat cut to the heart of what was bizarre about Bohr's interpretation of reality: the lack of a clear dividing line between the quantum and everyday realms. While most people think it provides an example in support of particles lacking clearly-defined properties until they are measured, Schrödinger's original intention was the exact opposite—to show that such an idea was nonsensical. Yet, for many decades, physicists largely ignored this problem, moving on to other quandaries. 

But starting in the 1970s, researchers were able to show that quantum particles can be created in states that always correspond to one another — so if one shows an "up" orientation, the other will be "down" — a phenomenon that Schrödinger called entanglement. Such work has been used to underpin the emerging field of quantum computing , which promises to produce calculating machines that are far faster than current technologies. 

In 2010, physicists also managed to create a real-world version of Schrödinger's cat , albeit in a way that doesn't involve felicide (aka, kitty murder). University of California, Santa Barbara, scientists built a resonator, basically a tiny tuning fork, the size of the pixel on a computer screen. They put it into a superposition in which it was both oscillating and not oscillating at the same time, showing that relatively large objects can occupy bizarre quantum states.

More-recent experiments have put groups of up to 2,000 atoms in two different places at the same time , further blurring the dividing line between the microscopic and macroscopic. In 2019, researchers at the University of Glasgow even managed to take a photo of entangled photons using a special camera that snapped a picture whenever a photon showed up with its entangled partner. 

While physicists and philosophers have yet to agree on how to think about the quantum world, Schrödinger's insights have produced many fruitful research avenues and are likely to continue doing so for the foreseeable future. 

Additional resources: 

• Read how one physicist reconciles the conundrum of Schrödinger's cat, from The Conversation . 
• Learn more about the basics of quantum mechanics from Stanford University .
• Watch "The Real Meaning of Schrödinger's Cat," from Ask A Spaceman, with Paul Sutter . 

Sign up for the Live Science daily newsletter now

Get the world’s most fascinating discoveries delivered straight to your inbox.

Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike. 

Physicists find superconductor behavior at temperatures once thought 'impossible'

Longstanding physics mystery may soon be solved, thanks to Einstein and quantum computing

'Sensational discovery' of 2,000-year-old Roman military camp found hidden in the Swiss Alps

Most Popular

• 2 New tick-borne virus discovered in China can affect the brain, scientists report
• 3 Watch Live: Boeing Starliner is about to return to Earth without its crew
• 4 Anthrax has killed over 50 animals in Wyoming — what's the risk to people?
• 5 Pollution harms men's fertility, but traffic noise affects women's

Advertisement

Schrödinger’s cat

By Joshua Howgego

Sven Puth/EyeEm/Getty Images

Devised in 1935 by the Austrian physicist Erwin Schrödinger, this thought experiment was designed to shine a spotlight on the difficulty with interpreting quantum theory .

Quantum theory is very strange. It says that an object like a particle or an atom that adheres to quantum rules doesn’t have a reality that can be pinned down until it is measured. Until then, its properties, such as momentum, are encoded in a mathematical object known as a wave function that essentially says: if you make a measurement, here are a range of possible outcomes. The inevitable question that arose as the theory developed was: what, then, is the thing doing before that?

The most prominent answer in the 1930s came from the Copenhagen interpretation , developed in the Danish city by luminaries of quantum theory, Niels Bohr and Werner Heisenberg . This says that there really is no definitive reality before the measurement, and the object is in an undefined state known as a superposition.

Schrödinger’s thought experiment probed how this plays out when a quantum object is coupled to something more familiar. He imagined a box containing a radioactive atom, a vial of poison and a cat. Governed by quantum rules, the radioactive atom can either decay or not at any given moment. There’s no telling when the moment will come, but when it does decay, it breaks the vial, releases the poison and kills the cat.

If the Copenhagen interpretation is correct, then before any measurement has occurred, the atom, and so also the cat, are in a superposition of being decayed/dead and not decayed/alive. The absurdity of speaking of a simultaneously living and dead moggie was supposed to show that the Copenhagen interpretation must be lacking something.

The experiment played an important part in spurring other ways of thinking about quantum theory, including the many worlds interpretation, which says that the different possible realities of a quantum object crystallise into different parallel universes at the point of measurement.

The quantum experiment that could help find evidence of the multiverse

Scars of collisions with other universes could show up in radiation from the big bang. A new experiment aims to mimic these collisions and help us look for them

These days the thought experiment has taken on a kind of cult status. There are Schrödinger’s cat T-shirts, memes and hundreds of articles on the subject. In 2018, scientists published a more complicated variant of the thought experiment that appears to show that all existing interpretations of quantum theory are incomplete .

• Take our expert-led quantum physics course and discover the principles that underpin modern physics

Sign up to our weekly newsletter

Receive a weekly dose of discovery in your inbox! We'll also keep you up to date with New Scientist events and special offers.

More on Schrödinger’s cat

Quantum holograms can send messages that disappear

Ultracold quantum battery could be charged with quantum tunnelling

Subscriber-only

Tweezers made of light could illuminate the quantum twin paradox

Related articles.

Are there multiple universes?

People in Science

Roger penrose.

Emmy Noether

Reset password New user? Sign up

Existing user? Log in

Schrödinger's Cat

Already have an account? Log in here.

Two worlds "splitting off"- one where the cat is alive and one where the cat is dead. Taken from [MWI].

Schrödinger's cat is a thought experiment designed to show how certain interpretations of quantum mechanics lead to counterintuitive results.

In the experiment, a cat is placed inside a box with a vial of poisonous gas. A mallet is set up such that it breaks the vial of gas if a particular radioactive atom decays, killing the cat. Since the radioactive decay is a quantum system, whether the cat lives or dies is determined by quantum mechanical behavior. This leads to the conclusion that before the box is opened, the cat is simultaneously alive and dead.

Erwin Schrödinger originally proposed the idea as an absurd example showing that the Copenhagen Interpretation of quantum mechanics - the most popular philosophical interpretation at the time - could not possibly be true [1]. However, it has lived on as a thought experiment fueling both physical theories and the popular imagination.

Quantum States and Superposition

The many-worlds interpretation.

The two main ideas behind quantum theory are the idea of quantized , or discrete, states, and the idea of superposition.

A physical system is defined by the set of possible states in which it can be observed . For instance, an electron can be observed either in a spin up state or in a spin down state, but never a combination of the two. Similarly, a particle emitted in radioactive decay is observed to be either emitted or not emitted, never partway emitted. However, particles prepared in identical ways will not always be observed to have the same state. Identical uranium atoms will decay or not decay at random times, though the more time has passed, the more likely the atom is to decay. The state of the system will change such that it becomes more likely to decay.

To determine how states change over time, the idea of a superposition of states is required. A superposition is a vector addition of two states. Quantum states can be in any superposition of the observable states in the system. For instance, an electron can be in a state that is 50% spin up and 50% spin down. As time passes, the electron may change states, perhaps smoothly oscillating between 100% spin up and 100% spin down, passing through the 50/50 state at each time. The Schrödinger equation , the analog of Newton's laws for quantum mechanics, describes exactly how the electron will transition between these different superpositions of states.

But this seems to be at odds with the previous statement. How can a system be an a superposition of states if it can only ever be observed in one state? The Copenhagen interpretation of quantum mechanics holds that when an observer observes a system, it "collapses" from a superposition of multiple states down to a single state in a probabilistic way. This is distinct from the smooth changes in superposition that happen due to the Schrödinger equation.

When Schrödinger's cat is observed, it is either alive or dead. But when it isn't, it is in a superposition of alive and dead.

Technical note: the space of valid states is the set of complex vectors with an eigenbasis given by the observable states and magnitude 1. For linear combinations of eigenvectors, the probability of observing each component of the state vector is given by the magnitude of the coefficient of that component. Linear algebra is important for a deep understanding of quantum mechanics: most results come directly from the properties of operators on complex vector spaces.

The collapse interpretation has an issue, though. Instead of just you observing a cat in a box, imagine putting yourself and the cat in a room and having your friend wait outside the room. You run the experiment, open the box, record your observations, and only then have your friend open the room and see what your observations were. From your perspective, the cat is in a superposition of alive and dead until you open the box, at which point a collapse occurs. You are then in the definite state of having seen the cat, a state which persists until your friend opens the door. But from your friend's perspective, up until they open the door, you are still in a superposition of having seen the cat dead and having seen it alive.

Even worse, this setup can be repeated again and again, such that every new observer is placed in a larger room. Observer \(n\) always thinks that the state has collapsed before observer \(n + 1\). The idea that different observers will disagree on the state of reality in this experiment is problematic.

The many-worlds interpretation of quantum mechanics solves this problem by rejecting the idea of collapse entirely. It instead claims that there is always a superposition of two "world-branches," one where the cat is dead and one where the cat is alive. When you open the box, there is now a superposition of two worlds with two versions of you. In one world, the cat is alive and you see the cat is alive. In the other world, the cat is dead and you see the cat is dead.

[1] Schrödinger, Erwin; Translated by Trimmer, John. The Present Situation in Quantum Mechanics. Proceedings of the American Philosophical Society . Retrieved on 7 Mar 2016 from http://www.tuhh.de/rzt/rzt/it/QM/cat.html

[flickr] Flickr user chwalker01. Retrieved on 7 Mar 2016 from https://www.flickr.com/photos/31690139@N02/2965956885

Problem Loading...

Note Loading...

Set Loading...

• Medical Xpress
• Tech Xplore

Schrödinger's cat explained

In 1935, E. Schrödinger proposed his well-known cat thought experiment suggesting, but not explaining, how a measurement transforms the probable states of an atom into the actual state of a cat (alive or dead). Rather than applying quantum mechanics (the previous approach usually taken), I offer an out-of-the-box, logically consistent explanation using metrology (the science of physical measurement).

In formal quantum mechanics, an atom (or other entity) is in a superposition, or probabilistic combination of all possible states. In this widely accepted view, each state is possible and no state is actual until a measurement occurs. How does a measurement change multiple probable states into one actual state? E. Schrödinger's tongue-in-cheek thought experiment explores this by proposing a comparison of an atom's quantum superposition to the physical measurement of a cat's state.

The following is all the information Schrödinger provided on his thought experiment: "One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat): In a Geiger counter, there is a tiny bit of radioactive substance, so small that perhaps in the course of one hour, one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges, and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if, meanwhile, no atom has decayed. The first atomic decay would have poisoned it. The ψ function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts."

Schrödinger's experiment first appears to contrast the two distributions, the probabilistic time distribution of an atom's state and a cat's binary state (alive or dead) by correlating two observations (measurements): an atom's decay and a cat's death. In fact, by virtue of the apparatus, the actual time of each cat's death is a fixed time after the time of an atom's decay (due to the "diabolical device"). Schrödinger proposed the mean time of each of these equal but time-shifted distributions as one hour. With a mean time of one hour the maximum extent of every cat's actual time of death, distribution is estimated to be two hours.

Next, Schrödinger proposed one observation of the cat's state by a human at one hour. This is a one-time observation of a third distribution, the observed state of one cat over time. This may be compared with the cat's actual state (second distribution). This comparison is more interesting, as it appears to describe the measurement of a cat's state.

L. Euler defines all measurements as relative: "It is not possible to determine or measure one quantity other than by assuming that another quantity of the same type is known and determining the ratio between the quantity being measured and that quantity." The accuracy of each human observation of a cat is adjusted by calibration that correlates the measuring apparatus intervals (in this experiment, the time between observations) to a time reference (e.g., one second). This time reference is a non-local intermediate required to maintain Euler's relative quantity ratios.

The third distribution of each cat's observed time of death is correlated both to the actual time of death and to how often the human observes the cat (i.e., the time between human observations). In Schrödinger's experiment this time between observations is given as one hour. Then the accuracy of this one observation is +/- one hour relative to any possible cat's actual time of death (about a two-hour span).

When the times proposed (both one hour) are applied, the third distribution of the time between observations is uncorrelated to the first distribution of the atom's decay time. This occurs because the accuracy +/- one hour of the time between observations is close to as wide as an atom's decay distribution (two hours). This thought experiment, as presented, is two equal but time-shifted distributions of the atom's decay and cat's actual death (one ψ function) and one uncorrelated human observation (a different function). However, this experiment has drawn interest for 85 years because physical reality does not allow for the cat's state to be a superposition. What is missing?

The first human observation of a dead cat after an observation of life is measured in time relative to the beginning of the experiment (alive cat). These observations of the time of death identify that the distribution over multiple experiments is correlated with the ψ function of an atom's probabilities and that the state of each cat is binary. The actual time of death and the observed time of death are different. Because the observed time of death is also correlated to the time between observations and the time reference defined.

Consider the experiment when the observer (or measuring apparatus) counts the number of times the cat is examined. These observations result in a sequence of alive states ended by one dead state during the about two-hour maximum time to complete one experiment. Counting this sequence of alive observation generates a magnitude, but not a measurement correlated to a physical reference as Euler requires. Such a measurement requires defining and controlling the time between observations, which requires the calibration of the observer/measuring apparatus to a time reference.

As an example in Schrödinger's experiment, calibration would be setting and maintaining the time between observations to 10s (s = second, the time reference). Then the maximum variation of the observed time of death correlated to the actual time of death is +/-10s (i.e., accuracy). The count of observations is uncorrelated to any reference, and therefore not a relative measurement as Euler defined. Applying calibration, this count changes into a sum of the counted time between observations, where each time between observations is correlated to a time reference (second).

Calibration is required to observe a cat's time of death (i.e., a relative measurement). Schrödinger was correct—there is one ψ function of both the atom and the actual cat's state. But there is also a relative measurement including calibration to a reference which produces the observed cat's state. A relative measurement, which is physical reality, exists only when the second function occurs. Since quantum mechanics does not treat a reference, it cannot represent a relative measurement. Without a relative measurement, the most we can see are the probabilities of a superposition.

This story is part of Science X Dialog , where researchers can report findings from their published research articles. Visit this page for information about ScienceX Dialog and how to participate.

L. Euler, Elements of Algebra, Chapter I, Article I, #3. Third edition, Longman, Hurst, Rees, Orme and Co., London England, 1822. www.google.com/books/edition/E … &printsec=frontcover

K. Krechmer, Relative Measurement Theory, The unification of experimental and theoretical measurements, Measurement, Volume 116, February 2018, pages 77-82. www.sciencedirect.com/science/ … ii/S0263224117306887

Bio: Ken Krechmer ([email protected]) studies and teaches Isology at the University of Colorado, Boulder, CO, USA. Isology is the interdisciplinary study of references, standards and standardization. He also studies the formal application of references in quantum mechanics. He was Program Chair, or co-Chair, of several Standards and Innovation in Information Technology (SIIT) conferences. In 2012, he received first prize, and in 2006, received a joint second prize in the IEC Centenary Challenge paper competition. In 1995 and 2000, he won first prize at the World Standards Day paper competition. From 1990 to 2002, he was the founding technical editor of Communications Standards Review. He has also been secretary of TIA TR-29 (facsimile standards) and a U.S. delegate to ITU-T Study Group 8 (fax), 14 (previous modem standards), 15 (xDSL) and 16 (modem, video, conferencing) meetings. He is a Senior Member of the IEEE and a Member of The Society for Standards Professionals.

What Is Schrödinger’s Cat?

Schrödinger’s Cat is a famous thought experiment that illustrates a paradox of quantum superposition. Here’s how it works.

Schrödinger’s Cat is a thought experiment devised by the Austrian physicist Erwin Schrödinger, which he designed to illustrate a paradox of quantum superposition wherein a hypothetical cat may be considered both alive and dead simultaneously because its fate is linked to a random event that may (or may not) occur.

What Is Schrödinger’s Cat in Simple Terms?

Schrödinger’s Cat, as a thought experiment, states that if you seal a cat in a box with something that can eventually kill it, you won’t know if the cat is alive or dead until you open the box. So, until you open the box and observe the cat, the cat is simultaneously dead and alive. 

How Does Schrödinger’s Cat Work?

We often use Schrödinger’s thought experiment to explain the concept of superposition . The experiment states that a hypothetical cat is locked in a box with some radioactive substance controlling a vial of poison. When the substance decays, it triggers a Geiger counter that causes the poison to be released, thereby killing the cat. 

Since the box is locked, and we on the outside don’t know whether or not the radioactive substance has decayed and released the poison, we can’t tell if the cat is dead or alive. So, until we open the box to know for sure, the cat is both dead and alive. Mathematically speaking, there’s a 50 percent chance the cat is dead and a 50 percent chance the cat is alive. 

More From Built In’s Tech Dictionary What Is Superposition?

How Is Schrödinger’s Cat Both Alive and Dead?

In quantum mechanics terms, the cat’s ability to be in an ambiguous state of both alive and dead until it’s observed (i.e. when someone opens the box) is referred to as quantum indeterminacy or the observer’s paradox . The paradox states that an event or an experiment’s observer affects its outcome. In this case, whomever is performing this hypothetical experiment can affect whether the cat remains in an unknown state or they can open the box and know if the cat is dead or alive with 100 percent certainty.

The experiment also points out when the resolution of possibilities occurs. The experiment is intended to make people ask themselves if it was logical for the observation to trigger the answer. After all, wouldn’t the cat be either dead or alive even if we never open the box? 

Schrödinger’s Cat and the Role of the Observer

In quantum mechanics, the observer (the person conducting the experiment) has a role in the results of the experiment. In this case, we are unaware of the cat’s state until the observer opens the box. Until the observer opens the box, the cat exists in a superposition state; that is, the cat is both alive and dead. Only by opening the box and looking at what’s inside (i.e., observing it) is the cat’s state confirmed to be one of the two states. This is called The Copenhagen interpretation of quantum mechanics, which basically explains that a quantum system exists in all of its possible states at the same time. Only when we make an observation can we confirm the true state of the system.

More Quantum Reading From Built In Experts Why Do Quantum Objects Keep Getting Weirder?

Why Do We Use Schrödinger’s Cat?

We still use this thought experiment today to explain quantum physics concepts in an easy-to-understand way. Some people also use Schrödinger’s Cat to talk more philosophically about how the thought experiment can be extended to other situations in life. For example, let’s say you meet your friend for a night out and you’re both unsure about what to have for dinner; until you reach an agreement or one of you decides for the group, the possible food option is “every option that can exist where you and your friend are.” Looking at things from this perspective has led many people to think of everything in life as “quantum” because until the future is here, it technically (according to Schrödinger ) exists in a state of superposition of all possible scenarios.

Frequently Asked Questions

What is schrödinger's cat in simple terms.

Schrödinger’s Cat is a famous thought experiment that demonstrates the idea in quantum physics that tiny particles can be in two states at once until they’re observed. It asks you to imagine a cat in a box with a mechanism that might kill it. Until you look inside, the cat is both alive and dead at the same time.

Is Schrödinger’s Cat a metaphor?

Schrödinger’s Cat is a thought experiment and is also considered a metaphor or a paradox.

Is Schrödinger’s Cat literal?

Schrödinger’s Cat is not meant to be taken literally, but as a thought experiment designed to show the counterintuitive idea of quantum superposition.

Recent Big Data Articles

What Is Superposition and Why Is It Important?

Imagine touching the surface of a pond at two different points at the same time. Waves would spread outward from each point, eventually overlapping to form a more complex pattern. This is a superposition of waves. Similarly, in quantum science, objects such as electrons and photons have wavelike properties that can combine and become what is called superposed.

While waves on the surface of a pond are formed by the movement of water, quantum waves are mathematical. They are expressed as equations that describe the probabilities of an object existing in a given state or having a particular property. The equations might provide information on the probability of an electron moving at a specific speed or residing in a certain location. When an electron is in superposition, its different states can be thought of as separate outcomes, each with a particular probability of being observed. An electron might be said to be in a superposition of two different velocities or in two places at once. Understanding superposition may help to advance quantum technology such as quantum computers .

The concept of quantum superposition might be difficult to visualize. Traditional descriptions have used the analogy of a coin that is heads up and tails up at the same time, or the famous Schrödinger's cat thought experiment, in which physicist Erwin Schrödinger imagined placing a cat in a sealed box along with a poisonous substance that has an equal chance of killing the cat—or not—within an hour. Schrödinger proposed that, at the end of the hour, the cat could be said to be both alive and dead, in a superposition of states until the box is opened, and that the act of observation randomly determines whether the cat is alive or dead. Schrödinger intended the example to demonstrate what he saw as the absurdity of quantum science.

In mathematical terms, superposition can be thought of as an equation that has more than one solution. When we solve x 2 = 4, x can either be 2 or –2. Both answers are correct. Superposed wave functions will be more complicated to solve, but they can be approached with the same mindset.

How can scientists observe superposition?

Many experiments have been conducted that definitively prove the existence of superposition. One example recruits the help of light filters: screens that selectively block light, such as those found in polarized sunglasses or camera lenses.

Most of the light we see around us is a combination of many different waves coming from the sun and other sources. The peaks and valleys of these waves are rotated in different directions at once. In other words, the light is in a superposition of these different polarized states.

As light waves interact with their surroundings, their properties change. Light that reflects off of the surface of a lake or snow-covered ground will be more likely to be polarized horizontally. If this light then encounters a filter that permits only vertically polarized light to pass through, the reflection will be blocked. This is how polarized sunglasses filter out the glare from reflective surfaces on a bright day.

Superposition becomes apparent when we arrange more than one filter in different ways to tease out additional properties of light. Light that passes through a horizontal filter will have a 100 percent chance of passing through a second horizontal filter, i.e., all of it will pass through. If this second filter is gradually rotated toward a vertical orientation, the chance of the light passing through both filters steadily decreases. Half of the light will pass through when the filter reaches the diagonal (45 degrees), and no light will pass through when the filter is vertical.

If superposition did not exist, light would be completely blocked as soon as the second filter was rotated by even a fraction of a degree because all of the light that went through the first filter would be strictly horizontally polarized.

Surprisingly, adding a diagonal filter between the horizontal and vertical filters allows some light to go all the way through the system. This is also a result of superposition. The new filter will permit 50 percent of the light coming through the horizontal filter to pass. Then, because the new filter is also diagonal relative to the vertical filter, the vertical filter will permit 50 percent of the light to pass through.

The diagonal filter acts to "reset" the superposition of the light by making it more likely to be vertically polarized.

Dive Deeper

What is Superposition? Schrödinger’s Cat Experiment Explained

The tale of physics’ most famous cat is one that is familiar to many, but what is the inside story of the feline so demanding it requires its own universe, and how does it illustrate the 'weirdness' of the quantum world.

Home → Features → Natural Sciences → Physics → Quantum Mechanics

Of all the counter-intuitive elements of quantum physics introduced to the public since its inception in the early days of the twentieth century, it is quite possible that the idea that a system can be two (or more) contradictory things at once, could be the most challenging.

As well as defying a well-known aspect of logic — the law of non-contradiction — thus irritating logisticians, this idea of the coexistence of states, or superposition, was even a challenge to the fathers of quantum physics. Chief amongst them Erwin Schrödinger, who suggested a diabolical thought experiment that would show what he believed to the ludicrous nature of a system existing in contradictory states. 

The thought experiment would go on to become perhaps the most well-known in the history of physics, weaving its way on to witty t-shirts, hats, bags and badges, infiltrating pop-culture, TV and film. This is the strange tale of Schrödinger’s cat, and what it can teach us about quantum physics and the nature of reality itself. 

Before delving into the experiment that Schrödinger devised, it is worth examining the circumstances that led him to consider the absurd situation of a cat that is both living and dead at the same time. 

Wanted: Dead or Alive! How the cat got put in the box

In many ways, Erwin Schrödinger’s place in the history of quantum mechanics is overshadowed by his feline-based thought experiment. The Austrian physicist was responsible for laying the foundation of a theoretical understanding of quantum physics with the introduction of his eponymous wave equation in 1926. As Joy Manners describes in the book ‘Quantum Physics: An Introduction’ :

“The Schrödinger equation did for quantum mechanics what Newton’s laws of motion had done for classical mechanics 250 years before.” Joy Manners, Quantum Physics: An Introduction

What Schrödinger’s equation shows is that the state of a system — the collection of all of its measurable qualities — can be described as a wavefunction — represented by the Greek letter Psi (Ψ). This wavefunction contains all the information of a system that it is possible to hold. Each wavefunction is a solution to Schrödinger’s equation, but here’s the crazy part; two wavefunctions can be combined to form a third, and this resultant wavefunction can contain completely contradictory information.

When the wavefunctions of a system are combined it is in a ‘superposition’ state. There is also no limit no how many of these wavefunctions cam be combined to form a superposition. 

Yet, infinite though a wavefunction can be, eternal it is not. The act of taking a measurement on the system in question seems to cause the wavefunction to collapse — something there is as yet no physical or mathematical description for. There are, however, interpretations of what happens, which go to the very heart of reality.

Before tackling these interpretations, first, we should get to our cat in the box before he gets too impatient. 

A most diabolical device 

It was in 1935, whilst living in Oxford fleeing the rise of the Nazis, that Schrödinger first published an article that expressed his concern with the idea of measurement, wave function collapse, and contradictory states in quantum mechanics. Little would he know, it would lead to him becoming history’s most infamous theoretical-cat-assassin. 

Below Schrödinger describes the terrible predicament that his unfortunate moggy finds himself in. 

“A cat is placed in a steel chamber with the following hellish contraption… In a Gieger counter a tiny amount of a radioactive substance, so that maybe within an hour one of the atoms decays, but equally probable is that no atom decays…”

So, there is a 1/2 chance that an atom of the substances decays and causes the Gieger to tick over the hour duration of the experiment. 

“If one decays the counter triggers a little hammer which breaks a container of cyanide.” 

So, if the atom decays over the hour, the cat is killed. If it doesn’t, the cat survives. Treating the box and the cat as a quantum system how would we describe its wavefunction (Ψ)?

The wavefunction of the system now exists in a superposition of the individual wavefunction that describes the cat as being alive, and the one that declares it dead. According to the rules of quantum physics, the cat is currently both dead and alive.

Our unfortunate feline isn’t doomed to live out its existence as some bizarre quantum zombie, though. A quick peek inside the box constitutes a measurement of the system. Thus, by opening the box we collapse the wavefunction and determine the fate of Schrödinger’s cat. It really is curiosity that kills the cat, in this case.

Let’s end our analogy on a happy note. We open our box and fortunately the substance has not undergone decay. The cyanide bottle remains intact. Our moggy survives, unscathed if irritated. The wavefunction collapsed leaving the blue sub-wavefunction intact, but what actually just happened here? How was the cat’s fate determined? 

The short answer is, we don’t know, but we have some interpretations. Next, we compare the two most prominent. 

Way more than nine lives. The many-worlds interpretation 

What we have discussed thus far consists of a very rough description of the Copenhagen interpretation of quantum mechanics. The reason it’s common sense to present this first is that it is generally the interpretation that is most widely accepted and taught.

As you’ve seen, the Copenhagen interpretation describes a system with no established values until a measurement occurs or is taken and a value — in our case ‘alive’ — emerges. If this sounds deeply unsatisfactory, well, it is. One of the questions it leaves open is ‘why does the wavefunction collapse?’ In 1957, an American physicist Hugh Everett III, asked a different question: ‘What if the wavefunction doesn’t collapse at all? What if it grows?’ From this emerged Everett’s ‘relative state formulation’, better known to fans of science fiction, comic books and fantasy as the ‘Many Worlds Hypothesis/interpretation’.

Below we see what happens to the wavefunction in the Copenhagen interpretation. The box is opened and the wavefunction collapses. 

So what happens in the ‘many worlds’ interpretation? Rather than collapsing, as the box is opened the wavefunction expands. The cat does not cease to be in a superposition, but that superposition now includes the researchers and the very universe they inhabit. We become part of the system.

In the many-worlds interpretation, the researchers do not open the box to discover if the cat is dead or alive, they open the box to see if they are in the universe where the cat survived or the universe in which it was dispatched. They and their world have become part of the wavefunction. An entirely new universe in superposition with the old. The only difference. 

One less cat.

Schrodinger’s Kittens: Some words of caution

Again, as with the Copenhagen interpretation, there is no real experimental evidence of many worlds concept. In many ways, any interpretation of quantum mechanics is really more a realm of philosophy than science. Also, when considering ‘many worlds’ it’s worth noting that this is a different concept than the idea of a ‘multiverse’ of different universes created at the beginning of time. 

Further to this, there are some real problems with considering the ‘cat in a box’ as a quantum system. Researchers are constantly finding quantum effects in larger and larger systems, the current record seems to be 2,000 atoms placed in a superposition. To put that into perspective; a humble cat treat contains around 10²² atoms!

Many physicists have suggested reasons why larger systems fail to display quantum effects, with Roger Penrose suggesting that any system that has enough mass to affect space-time via Einstein’s theory of general relativity can’t be isolated. Via the influence of gravity, it is constantly having ‘measurements’ taken. This would definitely apply to even the most minuscule moggy. 

It is worth noting here that the general description of the thought experiment and the opening of the box has led some to speculate that it is the addition of a ‘consciousness’ that actually causes the wavefunction collapse. 

This is an idea that has sold a million or so books on ‘quantum woo’ and it arises from the unfortunate nomenclature of quantum physics. The use of the words ‘measure’ and ‘observe’ imply the intervention of a conscious observer. The truth is that any interaction with another system is enough to collapse a quantum wavefunction, as they tend to exist in incredibly delicate, easily disturbed states. 

Sources and further reading

Schrödinger. E,

Griffiths. D. J, ‘Introduction to Quantum Mechanics,’ [2017], Cambridge University Press.

Broadhurst. D, Capper. D, Dubin. D, et al, ‘Quantum Physics: An Introduction,’ [2008], Open University Press.

Nomura. Y, Poirer. B, Terning. J, ‘Quantum Physics, Mini Black Holes, and the Multiverse,’

Orzel. C, ‘How to Teach Quantum Physics to your Dog,’ [2009], Simon & Schuster. 

Was this helpful?

Related posts.

• Physicists measure quantum entanglement in chemical reactions
• Marie Curie: The Price of Knowledge
• Side stepping Heisenberg’s Uncertainty Principle isn’t easy
• A Journey through Multiverses, Hidden Dimensions, and Many Worlds

Robert is a member of the Association of British Science Writers and the Institute of Physics, qualified in Physics, Mathematics and Contemporary science.

• Editorial Policy
• Privacy Policy and Terms of Use
• How we review products

© 2007-2023 ZME Science - Not exactly rocket science. All Rights Reserved.

• Science News
• Environment
• Natural Sciences
• Matter and Energy
• Quantum Mechanics
• Thermodynamics
• Periodic Table
• Applied Chemistry
• Physical Chemistry
• Biochemistry
• Microbiology
• Plants and Fungi
• Planet Earth
• Earth Dynamics
• Rocks and Minerals
• Invertebrates
• Conservation
• Animal facts
• Climate change
• Weather and atmosphere
• Diseases and Conditions
• Mind and Brain
• Food and Nutrition
• Anthropology
• Archaeology
• The Solar System
• Asteroids, meteors & comets
• Astrophysics
• Exoplanets & Alien Life
• Spaceflight and Exploration
• Computer Science & IT
• Engineering
• Sustainability
• Renewable Energy
• Green Living
• Editorial policy
• Privacy Policy

MIT Technology Review

• Newsletters

A quantum experiment suggests there’s no such thing as objective reality

• Emerging Technology from the arXiv archive page

Back in 1961, the Nobel Prize–winning physicist Eugene Wigner outlined a thought experiment that demonstrated one of the lesser-known paradoxes of quantum mechanics. The experiment shows how the strange nature of the universe allows two observers—say, Wigner and Wigner’s friend—to experience different realities.

Since then, physicists have used the “Wigner’s Friend” thought experiment to explore the nature of measurement and to argue over whether objective facts can exist. That’s important because scientists carry out experiments to establish objective facts. But if they experience different realities, the argument goes, how can they agree on what these facts might be?

That’s provided some entertaining fodder for after-dinner conversation, but Wigner’s thought experiment has never been more than that—just a thought experiment.  

Last year, however, physicists noticed that recent advances in quantum technologies have made it possible to reproduce the Wigner’s Friend test in a real experiment. In other words, it ought to be possible to create different realities and compare them in the lab to find out whether they can be reconciled.

And today, Massimiliano Proietti at Heriot-Watt University in Edinburgh and a few colleagues say they have performed this experiment for the first time: they have created different realities and compared them. Their conclusion is that Wigner was correct—these realities can be made irreconcilable so that it is impossible to agree on objective facts about an experiment.

Wigner’s original thought experiment is straightforward in principle. It begins with a single polarized photon that, when measured, can have either a horizontal polarization or a vertical polarization. But before the measurement, according to the laws of quantum mechanics, the photon exists in both polarization states at the same time—a so-called superposition.

Wigner imagined a friend in a different lab measuring the state of this photon and storing the result, while Wigner observed from afar. Wigner has no information about his friend’s measurement and so is forced to assume that the photon and the measurement of it are in a superposition of all possible outcomes of the experiment.

Wigner can even perform an experiment to determine whether this superposition exists or not. This is a kind of interference experiment showing that the photon and the measurement are indeed in a superposition.

From Wigner’s point of view, this is a “fact”—the superposition exists. And this fact suggests that a measurement cannot have taken place. 

But this is in stark contrast to the point of view of the friend, who has indeed measured the photon’s polarization and recorded it. The friend can even call Wigner and say the measurement has been done (provided the outcome is not revealed).

So the two realities are at odds with each other. “This calls into question the objective status of the facts established by the two observers,” say Proietti and co.

That’s the theory, but last year Caslav Brukner, at the University of Vienna in Austria, came up with a way to re-create the Wigner’s Friend experiment in the lab by means of techniques involving the entanglement of many particles at the same time.

The breakthrough that Proietti and co have made is to carry this out. “In a state-of-the-art 6-photon experiment, we realize this extended Wigner’s friend scenario,” they say.

They use these six entangled photons to create two alternate realities—one representing Wigner and one representing Wigner’s friend. Wigner’s friend measures the polarization of a photon and stores the result. Wigner then performs an interference measurement to determine if the measurement and the photon are in a superposition.

The experiment produces an unambiguous result. It turns out that both realities can coexist even though they produce irreconcilable outcomes, just as Wigner predicted.  

That raises some fascinating questions that are forcing physicists to reconsider the nature of reality.

The idea that observers can ultimately reconcile their measurements of some kind of fundamental reality is based on several assumptions. The first is that universal facts actually exist and that observers can agree on them.

But there are other assumptions too. One is that observers have the freedom to make whatever observations they want. And another is that the choices one observer makes do not influence the choices other observers make—an assumption that physicists call locality.

If there is an objective reality that everyone can agree on, then these assumptions all hold.

But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.

Of course, there is another way out for those hanging on to the conventional view of reality. This is that there is some other loophole that the experimenters have overlooked. Indeed, physicists have tried to close loopholes in similar experiments for years, although they concede that it may never be possible to close them all.

Nevertheless, the work has important implications for the work of scientists. “The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them,” say Proietti and co. And yet in the same paper, they undermine this idea, perhaps fatally.

The next step is to go further: to construct experiments creating increasingly bizarre alternate realities that cannot be reconciled. Where this will take us is anybody’s guess. But Wigner, and his friend, would surely not be surprised.

Ref: arxiv.org/abs/1902.05080 : Experimental Rejection of Observer-Independence in the Quantum World

Keep Reading

Most popular.

What is AI?

Everyone thinks they know but no one can agree. And that’s a problem.

• Will Douglas Heaven archive page

How to fix a Windows PC affected by the global outage

There is a known workaround for the blue screen CrowdStrike error that many Windows computers are currently experiencing. Here’s how to do it.

• Rhiannon Williams archive page

A controversial Chinese CRISPR scientist is still hopeful about embryo gene editing. Here’s why.

He Jiankui, who went to prison for three years for making the world’s first gene-edited babies, talked to MIT Technology Review about his new research plans.

• Zeyi Yang archive page

Google DeepMind’s new AI systems can now solve complex math problems

AlphaProof and AlphaGeometry 2 are steps toward building systems that can reason, which could unlock exciting new capabilities.

Stay connected

Get the latest updates from mit technology review.

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at [email protected] with a list of newsletters you’d like to receive.

Quantum Physics Lady

Home » Schrodinger’s Cat Experiment

Schrodinger’s Cat Experiment

[This article is under construction.]

In fact, the mathematics was saying even more. The wave function has a mathematical property called “linearity.” This property means that when the photon superposition interacts with the photographic plate, the superposition should “infect” the photographic plate. The photographic plate becomes correlated with the photon. It should, itself, go into a superposition like the photon.

Instead the wave function collapses down to a particle. Rather than  going into a superposition, the photographic plate stays solidly there.

The Schrodinger Cat thought experiment highlights the problem of the cause of the collapse of the wave function. Here’s the experiment: A radioactive atom is in a superposition of two states: 1) decay in which it emits an electron and 2) stability in which it doesn’t. The atom is in a box with a hapless cat and a Geiger counter. If the atom decays and emits the electron, it triggers the Geiger counter, which releases a hammer, which breaks a vial, which releases a poison gas, which kills the cat.

The property of linearity of the wave function tells physicists that the superposition of the atom, both decayed and undecayed, would put the Geiger counter into a superposition. The superposition of the Geiger counter would, in turn, infect the hammer, which would go into a superposition. And so on, until the cat, itself, is in a superposition of being both dead and alive. But this does not describe the reality that we experience. Who has seen a cat that is both dead and alive? What actually happens in the physical universe that saves us from zombie cats?

Some interpretations of quantum mechanics do not hold that the wave function actually collapses. The Many Worlds Interpretation is the most well-known of these.  In the heyday of the Copenhagen Interpretation , some physicists proposed that consciousness collapses the wave function and prevents the atom’s superposition from infecting the entire Rube Goldberg cat-killing machine. In later decades, the theory of decoherence was developed to explain wave function collapse.

It’s a wonderful world — and universe — out there.

Come explore with us!  

Science News Explores

Scientists say: thought experiment.

These imaginary scenarios can reveal real-world misconceptions and gaps in knowledge

Here, the Greek philosopher Aristotle educates Alexander the Great. Aristotle often used thought experiments to challenge assumptions and propose scientific theories.

mikroman6/Moment/Getty Images

Share this:

• Google Classroom

By Katie Grace Carpenter

July 29, 2024 at 6:30 am

Thought experiment (noun, “THAWT Ex-PAIR-eh-mint”)

A thought experiment is a hypothetical scenario addressing a big question.

The scenario can sometimes seem absurd. But considering such scenarios can highlight the consequences of a particular idea or theory.

One famous thought experiment is known as Schrödinger’s cat . Physicist Erwin Schrödinger devised it in 1935. He did this to illustrate a point about quantum theory.

Quantum theory attempts to describe physics at the tiniest scale. One aspect of quantum theory is a concept known as superposition. Superposition says that particles can exist in different states at the same time. Quantum theory also says that when these particles interact, they adopt just one state.

But many scientists took this a step further. They said that just observing such a particle caused it to pick a state. Schrödinger found that idea ridiculous. So, he came up with a thought experiment.

His scenario describes a box with a cat inside. This box connects to a single atom . This atom represents the superposition idea. The atom has a chance of decaying . That means the atom has two potential states: decayed and undecayed.

The cat’s fate is bound to the atom in this thought experiment. If the atom randomly changes state from undecayed to decayed, then poison “kills” the imaginary cat — Schrödinger’s cat, as it came to be known.

And here’s the important part. In this scenario, the cat, box and atom are all unobserved. With no observer, that atom would hypothetically be in a state of superposition. It would be both decayed and undecayed at the same time. But the cat’s life is linked to this atom’s state. So, if the atom exists in two states, so does the cat. The cat is both alive and dead. And its state — dead or alive — would only be determined if someone observed the cat or the atom.

That, Schrödinger pointed out, is absurd. But it doesn’t mean quantum theory is flawed. In fact, this thought experiment has become a helpful device for understanding the absurd nature of subatomic particles .

Thought experiments are not limited to science. Other fields, such as philosophy, history and economics use them, too. Thought experiments help us reconsider information we already know. But they can also help propose new ways of thinking through and discussing complex questions.

In a sentence

Thought experiments help to illustrate the quantum mechanics principle of superposition .

Check out the full list of  Scientists Say .

Educators and Parents, Sign Up for The Cheat Sheet

Weekly updates to help you use Science News Explores in the learning environment

Thank you for signing up!

There was a problem signing you up.

More Stories from Science News Explores on Science & Society

Want to spot a deepfake? Focus on the eyes  

A childhood dog inspired this veterinarian to help others 

How much more can Olympic speed records fall?

Space tourists could face out-of-this-world health risks

Laser-based tech can identify illegal elephant ivory

Where are the flying cars? 

This paleontologist studies ancient mammal movement — virtually

This spice could be the basis of a smart, infection-fighting bandage

August 9, 2024

Experiments Prepare to Test Whether Consciousness Arises from Quantum Weirdness

Researchers wish to probe whether consciousness has a basis in quantum mechanical phenomena

By Hartmut Neven & Christof Koch

nopparit/Getty Images

The brain is a mere piece of furniture in the vastness of the cosmos, subject to the same physical laws as asteroids, electrons or photons. On the surface, its three pounds of neural tissue seem to have little to do with quantum mechanics , the textbook theory that underlies all physical systems, since quantum effects are most pronounced on microscopic scales. Newly proposed experiments, however, promise to bridge this gap between microscopic and macroscopic systems, like the brain, and offer answers to the mystery of consciousness.

Quantum mechanics explains a range of phenomena that cannot be understood using the intuitions formed by everyday experience. Recall the Schrödinger’s cat thought experiment , in which a cat exists in a superposition of states, both dead and alive. In our daily lives there seems to be no such uncertainty—a cat is either dead or alive. But the equations of quantum mechanics tell us that at any moment the world is composed of many such coexisting states, a tension that has long troubled physicists.

Taking the bull by its horns, the cosmologist Roger Penrose in 1989 made the radical suggestion that a conscious moment occurs whenever a superimposed quantum state collapses. The idea that two fundamental scientific mysteries—the origin of consciousness and the collapse of what is called the wave function in quantum mechanics—are related, triggered enormous excitement.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing . By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

Penrose’s theory can be grounded in the intricacies of quantum computation . Consider a quantum bit, a qubit, the unit of information in quantum information theory that exists in a superposition of a logical 0 with a logical 1. According to Penrose, when this system collapses into either 0 or 1, a flicker of conscious experience is created, described by a single classical bit.

Penrose, together with anesthesiologist Stuart Hameroff, suggested that such collapse takes place in microtubules , tubelike, elongated structural proteins that form part of the cytoskeleton of cells, such as those making up the central nervous system.

These ideas have never been taken up by the scientific community as brains are wet and warm, inimical to the formation of superpositions, at least compared to existing quantum computers that operate at temperatures 10,000 times colder than room temperature to avoid destroying superposition states.

Penrose’s proposal suffers from a flaw when applied to two or more entangled qubits. Measuring one of these entangled qubits instantaneously reveals the state of the other one, no matter how far away. Their states are correlated, but correlation is not causation, and, according to standard quantum mechanics, entanglement cannot be employed to achieve faster-than-light communication. However, per Penrose’s proposal, qubits participating in an entangled state share a conscious experience. When one of them assumes a definite state, we could use this to establish a communication channel capable of transmitting information faster than the speed of light, a violation of special relativity.

In our view, the entanglement of hundreds of qubits, if not thousands or more, is essential to adequately describe the phenomenal richness of any one subjective experience: the colors, motions, textures, smells, sounds, bodily sensations, emotions, thoughts, shards of memories and so on that constitute the feeling of life itself.

In an article published in the open-access journal Entropy , we and our colleagues turned the Penrose hypothesis on its head, suggesting that an experience is created whenever a system goes into a quantum superposition rather than when it collapses. According to our proposal, any system entering a state with one or more entangled superimposed qubits will experience a moment of consciousness.

You, the astute reader, must by now be saying to yourself: But wait a minute here—I don’t ever consciously experience a superposition of states. Any one experience has a definitive quality; it is one thing and not the other. I see a particular shade of red, feel a toothache. I don’t simultaneously experience red and not-red, pain and not-pain.

The definitiveness of any conscious experience naturally arises within the many-worlds interpretation of quantum mechanics . A metaphysical position first put forward by physicist Hugh Everett in 1957, the many-worlds view, posits time’s evolution as an enormously branched tree, with every possible outcome of a quantum event splitting off its own universe. A single qubit entering a superposition gives birth to two universes, in one of which the qubit’s state is 0 while in a twin universe everything is identical except that the qubit’s state is 1.

Entanglement potentially offers something else for brain scientists by providing a natural solution to what is called the binding problem, the subjective unity of every experience that has long posed a key challenge to the study of consciousness. Consider seeing the Statue of Liberty: her face, the crown on her head, the torch in her raised right hand, and so on. All these distinctions and relationships are bound together into a single perception whose substrate might be numerous qubits, all entangled with each other.

To make these esoteric ideas concrete, we propose three experiments that would increasingly shape our thinking on these matters. The first experiment, progressing right now thanks to funding from the Santa Monica–based Tiny Blue Dot Foundation, seeks to provide evidence of the relevance of quantum mechanics to neuroscience in two very accessible test beds: tiny fruit flies and cerebral organoids, the latter lentil-sized assemblies of thousands of neurons grown from human-induced pluripotent stem cells. It is known that the inert noble gas xenon can act as anesthetic in animals and people. Remarkably, an earlier experiment claimed that its anesthetic potency, measured as the concentration of the gas that induces immobility, depends on the specific isotopes of xenon. Two isotopes of an element contain the same number of positively charged protons but different numbers of noncharged neutrons in their nuclei. The chemical properties of isotopes—that is, what they interact with—are similar, by and large, even though their masses and magnetic properties differ slightly.

If fruit flies and organoids can be used to detect different xenon isotopes, the hunt will be on for the exact mechanisms by which a gas that is inert and that remains aloof from binding to proteins or other molecules achieves this. Is it the tiny difference in the mass of these isotopes (131 versus 132 nucleons) that makes the difference? Or is it their nuclear spin, a quantum mechanical property of the nucleus? These xenon isotopes differ substantially in their nuclear spin; some have zero spin and others 1 / 2 or 3 / 2 .

These xenon experiments will inform a second follow-on experiment in which we will attempt to couple qubits to brain organoids in a way that allows entanglement to spread between biological and technical qubits. The final experiment, which at this stage is still a purely conceptual one, aims to enhance consciousness by coupling engineered quantum states to a human brain in an entangled manner. The person may then experience an expanded state of consciousness like those accessed under the influence of ayahuasca or psilocybin.

Both quantum engineering and the design of brain-machine interfaces are progressing rapidly. It may not be beyond human ingenuity to directly probe and expand our conscious mind by making use of quantum science and technology.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.

This week: the arXiv Accessibility Forum

Help | Advanced Search

Quantum Physics

Title: superposition in measuring apparatus: a thought experiment.

Abstract: The measurement problem in quantum mechanics arises from the apparent collapse of a superposition state to a definite outcome when a measurement is made. Although treating the measuring apparatus as a classical system has been a successful approach in explaining quantum phenomena, it raises fundamental questions about the nature of measurement and the validity of wave function collapse. In this paper, we discuss a thought experiment that explores superposition in the measuring apparatus when it is treated as a quantum system. The experiment uses the Hong-Ou-Mandel effect in a two-photon interference setup, and its outcome is indicated by the coincidence count. Specifically, a zero count implies the existence of superposition, while a non-zero count indicates a wave function collapse. The discussions provide insight into the measurement problem, particularly regarding wave function collapse and nested measurement, and highlight the importance of indistinguishability to it. It provides a framework that probes the exact conditions necessary for a wave function collapse to happen.

Submission history

Access paper:.

• Other Formats

References & Citations

• INSPIRE HEP
• Google Scholar
• Semantic Scholar

BibTeX formatted citation

Bibliographic and Citation Tools

Code, data and media associated with this article, recommenders and search tools.

• Institution

arXivLabs: experimental projects with community collaborators

arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website.

Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them.

Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs .