By Andre Saraiva, UNSW

For those eagle-eyed readers who have been viewing the Jobs page on the Quantum Computing Report, you would have known that Amazon has been advertising for quantum hardware engineers at the AWS’ Center for Quantum Computing in Pasadena, CA (inside Caltech’s campus) in Pasadena, California for most of this year. We didn’t know exactly what they were working on, but now Amazon has announced their approach in the form of a 118-page-long blueprint for an in-house quantum computer, They are now openly in the race for qubit fabrication and if you think they enter the race late, you should check out the horses they brought including including Caltech IQIM Director John Preskill, seasoned group leaders such as Oskar Painter from Caltech, Liang Jiang from UChicago and led by Amazon’s Head of Quantum Algorithms Fernando Brandao..

The plan is to bring together two technologies – cat states in superconducting cavities and quantum memories based on acoustic nanoresonators. While both technologies are still maturing, the proposal is far from premature. Among the co-authors of the manuscript are people like Oskar Painter, who has been implementing acoustic nanoresonators for years and has mastered the fabrication and operation of similar devices, and Liang Jiang who has been theoretically supporting the experimental group that leads cat state qubits. Not to say their path forward will be a breeze – getting these two technologies to talk to each other will be a major struggle in itself.

The cat states borrow their names from Schrodinger’s all-enduring hypothetical pet. Just like in the imaginary experiment, cat states are quantum superpositions of multiple elementary particles. In practice, implementations of controllable cat states are done with tens or hundreds of photons in a superconducting cavity. This was theoretically predicted and recently shown to lead to some hardware-level error protection (see our other article about a recent qubit demonstration). In short, by encoding information on the collective state of many particles, local errors acting on each separate particle go unnoticed by the qubits. We recently wrote about one version of this qubit idea, which leverages non-linear Kerr media for the photon manipulations. In all recent demonstrations, the improvement on qubit quality only remained up until three or four photons, at which point non-local errors can kick in. This meant a significant reduction in error rates, but high-fidelity qubit manipulation remains to be seen.

Now, on to the quantum memory. The idea of an acoustic cavity is not too difficult to grasp – imagine yourself in an echoeing cave, where after you shout out your name you hear it repeating itself maybe two or three times. This is because of the sound waves reflecting back-and-forth from the cave walls. This is precisely the principle of an acoustic cavity – except instead of repeating the echo three times, these suspended nanobeams are capable of bouncing sound tens of billions times. Also, the sound that they keep in their pipes is extremely high pitched, a million times higher frequency then what human can hear. That is why they are called resonators – the very long lifetimes are specific to the particular frequency that matches the geometrical features of the cavity (similar to the length of the pipes in a trombone, which change the pitch of the sound that gets amplified by its pipes).

These acoustic devices should be compared to the most common gigahertz cavities, based on microwaves in superconducting resonators. The first difference that stands out is the size – since microwaves travel at the speed of light (much faster than sound), in order to hold on to a gigahertz microwave, the superconducting resonators have to be centimetres long. Acoustic nanoresonators are a few micrometers long. The second advantage – probably the most important one – is in the number of times waves bounce back-and-forth, which scientists call quality factor (or Q factor). Superconducting resonators coupled to qubit devices typically manage to get quality factors of up to a few thousands. Acoustic resonators are not yet tested in direct connection to qubits, but while standing alone the early numbers set Q to tens of billions.

Development Challenges that Need to Be Overcome

Firstly, Amazon must work on the figures of merit for cat state qubits. While the principle behind its added robustness is scientifically sound, from principle to practice there is a long way to go and Amazon comes utterly unexperienced into the game. Their economic power might attract some good engineers with experience to hit the ground running, but they have to catch up with companies such as French Alice&Bob and American Quantum Circuits, Inc.

Also, the impressively high Q factors of the nanoresonators demonstrated by the Painter group are to be taken with a grain of salt – as soon as these resonators are integrated and coupled to complex qubit circuitry, their Q factors degrade sharply. Moreover, the complex fabrication recipe for these suspended nanoresonators has to be harmonised with the fabrication of superconducting circuitry without introducing significant junk onto the various materials that are combined in their vision for a quantum processor.

Coming in Part 2

In the second part of this article, to be published next month, we will delve into the error correction scheme that the Amazon team proposes to use for creating a fault-tolerant quantum computer. Part 2 will also cover how the overall architecture compares with other competing quantum computer technology implementations that we are familiar with.

Dr. Saraiva has worked for over a decade providing theoretical solutions to problems in silicon spin quantum computation, as well as other quantum technologies. He works on MOS Silicon Quantum Dot research and commercially-oriented projects at the University of New South Wales (UNSW).

December 21, 2020