Famous physics cat now alive, dead and in two boxes at once | Science News for Students

Famous physics cat now alive, dead and in two boxes at once

Splitting the feline between boxes is a step toward building microwave-based quantum computers
Jun 14, 2016 — 7:00 am EST
Schrödinger’s cat

The infamous (and fictious) Schrödinger’s cat, is always both alive and dead inside a box — until the box is opened. Then the truth comes out.

Yvonne Gao, Yale University

Physicist Erwin Schrödinger’s cat can’t seem to catch a break. The fictitious feline is famous for being alive and dead at the same time, as long as it remains hidden inside a box. Scientists think about Schrödinger’s cat in this way so that they can study quantum mechanics. This is the science of the very small — and the way that matter behaves and interacts with energy. Now, in a new study, scientists have split Schrödinger’s cat between two boxes.

Animal lovers can relax — there are no actual cats involved in the experiments. Instead, physicists used microwaves to mimic the cat’s quantum behavior. The new advance was reported May 26 in Science. It brings scientists one step closer to building quantum computers out of microwaves.

Schrödinger dreamt up his famous cat in 1935. He made it the unfortunate participant in a hypothetical experiment. It’s what scientists call a thought experiment. In it, Schrödinger imagined a cat in a closed box with a deadly poison. The poison would be released if some radioactive atoms decayed. This decay occurs naturally when a physically unstable form of an element (such as uranium) sheds energy and subatomic particles. The math of quantum mechanics can calculate the odds that the material has decayed — and in this case, released the poison. But it cannot identify, for certain, when that will happen.

So from the quantum perspective, the cat can be assumed to be both dead — and still alive — at the same time. Scientists called this dual state a superposition. And the cat remains in limbo until the box is opened. Only then will we learn if it’s a purring kitty or a lifeless corpse.

Scientists have now created a real laboratory version of the experiment. They created a box — two actually — out of superconducting aluminum. A superconducting material is one that offers no resistance to the flow of electricity. Taking the place of the cat are microwaves, a type of electromagnetic radiation.

The electric fields associated with the microwaves can point in two opposite directions at the same time — just as Schrödinger’s cat can be alive and dead at the same time. These states are known as “cat states.” In the new experiment, physicists have created such cat states in two linked boxes, or cavities. In effect, they have split the microwave “cat” into the two “boxes” at once.

The idea of putting one cat into two boxes is “kind of whimsical,” says Chen Wang. A coauthor of the paper, he works at Yale University, in New Haven, Conn. He argues, however, that it’s not that far off from the real-world situation with these microwaves. The cat state is not only in one box or the other, but stretches out to occupy both. (I know, that’s weird. But even physicists acknowledge that quantum physics tends to be weird. Very weird.)

What’s even weirder is that the states of the two boxes are linked, or in quantum terms, entangled. That means if the cat turns out to be alive in one box, it’s also alive in the other. Chen compares it to a cat with two symptoms of life: an open eye in the first box and a heartbeat in the second box. Measurements from the two boxes will always agree on the cat’s status. For microwaves, this means the electric field will always be in sync in both cavities.

Schrödinger’s cat

The scientists measured how close the cat states were to the ideal cat state they wanted to produce. And the measured states came within roughly 20 percent of that ideal state. This is about what they would expect, given how complicated the system is, the researchers say.

The new finding is a step toward using microwaves for quantum computing. A quantum computer makes use of the quantum states of subatomic particles to store information. The two cavities could serve the purpose of two quantum bits, or qubits. Qubits are the basic units of information in a quantum computer.

One stumbling block for quantum computers has been that errors will inevitably slip into calculations. They slip in because of interactions with the outside environment that muck up the qubits’ quantum properties. The cat states are more resistant to errors than other types of qubits, the researchers say. Their system should eventually lead to more fault-tolerant quantum computers, they say.

“I think they’ve made some really great advances,” says Gerhard Kirchmair. He is a physicist at the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences in Innsbruck. “They’ve come up with a very nice architecture to realize quantum computation.”

Sergey Polyakov says this demonstration of entanglement in the two-cavity system is very important. Polyakov is a physicist at the National Institute of Standards and Technology in Gaithersburg, Md. The next step, he says, “would be to demonstrate that this approach is actually scalable.” By this, he means that it would still work if they added more cavities to the mix to build a bigger quantum computer.

Power Words

(for more about Power Words, click here)

atom    The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

electric field   A region around a charged particle or object within which a force would be exerted on other charged particles or objects.

electromagnetic radiation  Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.

element    (in chemistry)Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

decay   (in physics) The process whereby a radioactive isotope — which means a physically unstable form of some element — sheds energy and subatomic particles. In time, this shedding will transform the unstable element into a slightly different but stable element. For instance, uranium-238 (which is a radioactive, or unstable, isotope) decays to radium-222 (also a radioactive isotope), which decays to radon-222 (also radioactive), which decays to polonium-210 (also radioactive), which decays to lead-206 — which is stable. No further decay occurs. The rates of decay from one isotope to another can range from timeframes of less than a second to billions of years.

hypothetical       An adjective used to describe a proposal or idea based upon a hypothesis (which is itself a proposed explanation for something).

microwaves  An electromagnetic wave with a wavelength shorter than that of normal radio waves but longer than those of infrared radiation (heat) and of visible light.

physics    The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of  explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.

quantum   (pl. quanta)  A term that refers to the smallest amount of anything, especially of energy or subatomic mass.

quantum computer       A computer that makes use of the quantum states of subatomic particles to store information.

quantum entanglement     A physical phenomenon that occurs when groups of particles (typically pairs) interact in ways such that all the particles have the same quantum state.

quantum mechanics    A branch of physics dealing with the behavior of matter on the scale of atoms or subatomic particles.

quantum state         Any of the possible states of a system described by quantum theory.

quantum theory     A way to describe the operation of matter and energy at the level of atoms. It is based on an interpretation that at this scale, energy and matter can be thought to behave as both particles and waves. The idea is that on this very tiny scale, matter and energy are made up of what scientists refer to as quanta — miniscule amounts of electromagnetic energy.

quantum physics    A branch of physics that uses quantum theory to explain or predict how a physical system will operate on the scale of atoms or sub-atomic particles.

quantum superposition    The condition in which a quantum system is in a few different states at the same time.

qubit       Short for quantum bit. It is the basic unit of information that would be stored in a quantum computer. Such computers use of the quantum states of subatomic particles to store information.

radioactive    An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.

Schrödinger’s cat      In 1935, Erwin Schrödinger came up with the idea for a cat that is both alive and dead at the same time. It was meant to suggest the duality of conditions that can co-exist in quantum physics. Schrödinger imagined a cat in a closed box with a deadly poison. Eventually, but not immediately, the poison would kill the cat. But no one knows whether that has happened yet —until the box is opened. Until then, the cat is equally likely to be alive and dead, and indeed is treated as if it is both at the same time.

subatomic    Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

superconductor    Materials that have no resistance to the flow of electricity, typically only when they are cooled below a certain temperature. Superconductors also repel all magnetic fields, which allows them to float in the air when they are placed inside a strong magnetic field.

uranium   The largest naturally occurring element known. It’s called element 92, which refers to the number of protons in its nucleus. One form (isotope) is radioactive, which means it decays into smaller particles. The other form is stable.


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C. Wang. A Schrödinger cat living in two boxesScience. Vol 352, May 27, 2016, p. 1087. doi: 10.1126/science.aaf2941.

Further Reading

L. Sanders. Everyday entanglementScience News. Vol. 178, November 20, 2010, p. 22.

T. Siegfried. Clash of the quantum titansScience News. Vol. 178, November 20, 2010, p. 15.

S. Ornes. “World’s tiniest fridge.” Science News for Students. September 20, 2010.