Die Photo of Boson 4. Credit: Alice & Bob

We have previously published many articles about the cat qubit technology being developed by Alice & Bob. Their approach to combat qubit errors is different from what other companies are pursuing. In a quantum computer there are two basic types of errors that can occur, bit-flips and phase-flips. So conventional quantum error codes, like the surface code, need to be design to correct both types of errors. In the Alice & Bob architecture, the bit flips are nearly eliminated by the hardware architecture that they use so that their error codes only need to correct for phase flips. A key advantage of this approach is that it could significantly reduce the number of physical qubits needed to produce a logical qubit by as much as 60 times and enable much smaller machines to achieve quantum utility on real world problems. Assuming this chip is successful in meeting its goals, it will represent just one of a series of many more steps that will be needed to achieve a truly useful fault tolerant quantum computer.

Since their cat qubit approach is of high interest to quantum researchers, Alice & Bob is offering access to it with their Boson 4 test chip through the company’s Felis Cloud System and the Google Cloud Marketplace. Although this test chip only contains two qubits, it does show some interesting properties. The chip is showing bit-flip lifetimes of about 430 seconds, a little over 7 minutes. Per the company’s technical documentation for the chip, error rates during a Z-gate operation with 11 photons showing an excellent bit-flip error rate of about 5.8×10-8 but a phase flip error rate of 27%. As the company admitted, that error rate needs to improve by 1 to 2 orders of magnitude in order to get error correction to work correctly, but the company is working on it.

Alice & Bob is providing researchers with the ability to access not only the chip, but also a cat qubit emulator. They are encouraging researchers to study how to use cat qubits to benchmark the Boston 4 device, how to implement error correction, and to understand the affects of noise on their quantum algorithms.

Looking ahead, the company has a robust roadmap planned with a larger multi-qubit chip called Hydrogen that will be used to demonstrate the detection of errors and an even larger chip called Helium that is expected to to exponentially remove phase-flips through error correction.

Diagram Illustrating Alice & Bob’s Chip Roadmap. Credit: Alice & Bob

The team at Alice & Bob has been working at a rapid pace and over the past few years have been developing two new chip designs per month resulting in an improvement in bit flip lifetimes of over 100,000 over the past three years. And they will be continuing to work on improving the hardware technology with a goal of creating a chip that can provide 100 high fidelity logical qubits using 1500 physical cat qubits. The chart below shows the required parameters they need to achieve to get there.

Chart Comparing Current Performance of Boson 4 versus Performance Long Term Goals. Credit: Alice & Bob

For information about the availability for testing of Alice & Bob’s Boson 4 chip, you can access a press announcement posted on their website here. In addition, you can find good technical documentation about both the chip and the company’s technology on the Alice & Bob website here, here, and here.

May 15, 2024