The building blocks of quantum computers are often thought to imitate the famous thought experiment known as Schrödinger’s cat, in which quantum physics essentially suspends a cat in a box in a nebulous state between life and death: The cat only definitely becomes alive or dead when someone looks in the box. Now, by mimicking Schrödinger’s cats as closely as possible, a French startup reveals it could help make extraordinarily powerful quantum computers a reality sooner than previously thought—a strategy Amazon is also pursuing.

Classical computers generally switch transistors either on or off to symbolize data as ones or zeroes. In contrast, quantum computers use quantum bits—qubits—that, because of the surreal nature of quantum physics, can exist in a state of superposition where they are both 1 and 0 at the same time. This essentially lets each qubit carry out two calculations simultaneously. The more qubits are quantum-mechanically linked, or entangled, the more calculations they can perform at once, to an exponential degree.

The new strategy depends on so-called “cat states,” pairs of very different quantum states as diametrically opposed to one another as the “alive” and “dead” feline once famously postulated by Erwin Schrödinger.

The quantum-computing application that first drew a lot of popular attention is code breaking. For example, much of modern cryptography relies on the extraordinary difficulty that classical computers face when it comes to mathematical problems such as factoring huge numbers. However, using Shor’s algorithm for factoring integers, quantum computers can in principle rapidly crack even highly secure RSA-2048 encryption.

However, a critical drawback of current quantum computers is how their inner workings are prone to errors. Scientists can compensate for these high error rates using operations that make use of redundant qubits to overcome errors. Still, these quantum error correction strategies often demand high overheads in terms of hardware. For example, they may require more than 1,000 redundant “physical” qubits for every useful “logical” qubit.

Another strategy essentially focused on quality over quantity by designing qubits that are inherently stable against error. Recently, French startup Alice & Bob, based in Paris, reveals its strategy could slash the number of qubits required for Shor’s algorithm by a factor of 60.

Scientists can theoretically encode a qubit using virtually any pair of states of a quantum system—for example, two of a molecule’s potentially many different energy levels. In contrast, Alice & Bob’s strategy depends on so-called “cat states,” which consist of pairs of states as diametrically opposed to one another as the states of life and death experienced by the original Schrödinger’s cat.

The researchers found that qubits based on cat states are highly resistant to a common source of error known as bit flip, when a qubit’s state flips from 1 to 0 or the reverse. This approach does render cat qubits more vulnerable to a different common source of error known as phase flip, when they switch between one of two opposite phases. However, the company notes that having only one common source of error to correct instead of two drastically simplifies quantum-computer design.

Cracking RSA-2048 encryption is expected to require around 22 million qubits. Using cat-state qubits, however, researchers calculate they can bring that number down to 350,000.

The qubits that Alice & Bob is working on are superconducting devices, much like those Google, IBM, and others are using in their quantum computers. Specifically, the startup relies on superconducting microwave resonators that can fluctuate between two oscillatory states.

The company, in collaboration with the French Alternative Energies and Atomic Energy Commission, estimated what hardware its strategy might need to outperform a classical computer when it came to breaking codes dependent on huge numbers 600 decimal digits long, about what RSA-2048 involves. This problem would be impossible in any realistic timeframe on a conventional supercomputer but could fall prey to a large-enough quantum computer in less than a day.

Previous research suggested that using present-day, error-ridden quantum computers, Shor’s algorithm would need about 22 million qubits to break RSA-2048. In contrast, Alice & Bob found it would need only about 350,000 of its cat qubits for the same task.

“And there is still room for improvement not just in the hardware, but with the algorithms and architecture as well,” says Théau Peronnin, CEO and cofounder of Alice & Bob. “There’s still a huge opportunity to further reduce the overhead needed to run these codes.”

The startup aims to have a 14-qubit system by the end of this year, although Peronnin says it will also need at least another six months to calibrate the system. In the next few years, he says, the company hopes to develop a 40-qubit device capable of implementing one logical qubit, a system capable of displaying an exponential reduction in errors.

The company’s cat-state qubits currently degrade in 10 seconds. The company aims to improve that to 8 hours—and then to one month.

“There is at the moment no proper indisputable logical qubit,” Peronnin says. “Google’s latest experiments are right on the threshold of this milestone—they can add as many qubits as they want, and while they will not show improvements, it won’t degrade, either. For our group to develop a very long-lived logical qubit, we need 40 cat qubits.”

Currently, Alice & Bob seeks to improve its cat qubits further. For instance, the company’s devices show bit-flip times of roughly 10 seconds, about four orders of magnitude better than previous cat qubits, but the startup wants to improve this to at least 8 hours, and more like a month, Peronnin says. One way to face this challenge is to protect quantum computers against cosmic rays from outer space, which can lead superconducting-qubit-based quantum computers to suffer catastrophic errors roughly once every 10 seconds.

Alice & Bob also wants to reduce its cat qubit phase-flip error rate as well. “There are many ways we can improve this through fabrication quality, which is not yet state of the art,” Peronnin says.

A major challenge that Alice & Bob faces is how Amazon is using research from Alice & Bob’s team to help guide the development of its own cat qubits.

“When we first learned that Amazon was working on cat qubits, obviously our first reaction as a young startup was to sweat heavily,” Peronnin says. “But then we slept on it and realized that it might be the best thing to have happened to us. We’re not competing about who’s going to win; it’s cat qubits versus the rest of the possible technologies, so for us, it’s great external validation.”

All in all, “Amazon is a formidable competitor, and together we’re pushing this technology’s frontier,” Peronnin says. “As far as we know, Amazon has not yet reproduced our results, and even if they do, there is some IP we can discuss later. It’s great to have such competition. We feel like David versus Goliath, and this really gets our team pumped up.”

- The Race to Build a Fault-Tolerant Superconducting Quantum Computer ›
- Amazon's New Quantum Computer Design Relies On Tiny Schrödinger's Cats ›

Charles Q. Choi is a science reporter who contributes regularly to *IEEE Spectrum*. He has written for *Scientific American*, *The New York Times*, *Wired*, and *Science*, among others.