Alice&Bob has set itself a rather daring challenge: a young French start-up has entered the quantum computing race, in which large companies and laboratories from all over the world compete. She released a paper that tells a little more about her method… And about the famous “quantum cats” that are the key…

Yes, we’re talking about cats here, those little felines that purr by the fireplace in winter. In a paper recently published on arXiv and awaiting peer review, the Alice & Bob researchers say they are using a new type of qubit, which they have decided to call “cat qubits.” According to them, this makes it possible to drastically reduce computational errors, which are one of the weak points of quantum computing.

However, the expression, obviously, should be taken to the second degree. When talking about “qubit cats”, scientists are actually referring to the well-known theory of “Schrödinger’s cat”. This is a thought experiment invented by Erwin Schrödinger in 1935. He describes a cat locked in a box with a device that has a 50% chance of killing it in 10 minutes. The idea is that, in a certain way, until the test is done, the cat is in a superposition of two equally probable states. This experiment was never carried out: in fact, it served primarily to reflect on the foundations of the field of quantum physics.

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Indeed, quantum physics is interested in the behavior of matter and light at the microscopic or atomic level. It was by studying this behavior that scientists made a very intriguing discovery: at this scale, matter behaves in a way that doesn’t always match what we know about physics at the macroscopic scale. Then several new physical principles appeared, which are still the subject of numerous studies.

Among them, a principle known as “superposition” is critical to the operation of quantum computers. Quantum superposition implies that something like an atom can be “in two states at the same time”, no matter how counterintuitive it may seem. Thanks to this, it became possible to theorize and create “quantum bits”, also known as qubits, and, more generally, to gain access to quantum computing. To understand what a qubit is, we can refer to the “bits” of classical computing. A bit in a computer is a primary code, a unit of basic information that allows you to compose all the codes that underlie the operation of this machine. A bit is necessarily 0 or 1, and it is these 0s and 1s that increasingly complex codes are based on.

## Trapping Light to Create Quantum Bits

In the computer we find something like an equivalent which we call “qubits” for “quantum bits”. The main difference is that these qubits use the famous principle of superposition: so they can somehow be both 0 and 1 in a superposition of states. The interesting thing is that thanks to these qubits, a quantum computer can use processing power never seen before in classical computing and potentially solve hitherto unsolvable problems.

In classical computing, bits are concretely materialized by transistors that can be turned on or off. For quantum bits, things are a bit more complicated. In fact, research into the best way to materialize these famous qubits is in full swing. Often, qubits are actually atoms that we manage to arrange in a certain order thanks to “atomic traps” that make them communicate with each other. However, there are other ways to create qubits.

Bloch sphere representation of a cat qubit. ©

Eli Guzien and others.

Thus, these “cat qubits” presented by Alice and Bob are an example of a different form that researchers have adopted to perform all these calculations. The basic idea is the same: we are talking about qubits that allow the superposition of two states. Instead, they describe two different ways that light, trapped inside a small hole in a superconducting circuit, can come and go.

Jeremy Guyot and his colleagues at Alice&Bob performed a mathematical analysis of a quantum computer consisting of several circuits of this type. If this method is of particular interest to them, it is because it can avoid the large number of computational errors generated by quantum computers. Indeed, these numerous errors constitute a major weakness in quantum computing: “Quantum computers are inherently much more susceptible to interference,” according to a press release from the University of Innsbruck regarding the error correction solution. These known bugs can greatly hinder efficient program execution.

According to scientists, cat qubits are much less prone to error. Calculation errors occur, in particular, in the case of a “bit flip”, when a quantum bit goes from 0 to 1 or vice versa in a certain way. However, with these feline qubits, this change requires more energy, resulting in fewer errors.

To set an example, the researchers estimated the number of qubits that a computer composed in this way would need to break the encryption that secures bitcoin transactions. They attributed some qubits to the calculation itself, and others to correcting the errors of this very calculation. Their result: 126,133 cat qubits and 9 hours of computation would be enough to crack this code. At first glance, this may seem like a lot. However, according to the scientists, this value is about 160 times lower than the previous lowest estimate of the number of qubits needed to perform such an operation.

Mixed feelings about https://t.co/R597b2N7cg. As far as I can tell, they do a good job of detailed estimates, but the assumption that cat qubits won’t experience bit-switching errors among 100,000 qubits for hours is pretty extreme. Must shout that in abstraction.

— Craig Gidney (@CraigGidney) February 15, 2023

However, it should be remembered that this is really a mathematical calculation, and not yet a real functional computer. Whether such promising tests can be carried out in the laboratory remains an open question. “The assumption that cat qubits will not experience bit-flip errors, even when there are many of them and they are used for hours, is quite extreme,” comments quantum physicist Craig Gidney from Google on Twitter. Therefore, it will be about continuing to closely follow the development of this startup, which is developing at an incredible pace in the highly competitive world of quantum computing…