If you are a quantum technology watcher you have probably seen the flurry of reports around recent work by Chinese researchers to demonstrate quantum supremacy using an optical-based quantum computing technique. Their work (Quantum computational advantage using photons) was published in Science earlier this month.
There’s good recap of the work by Tom Simonite in Wired, putting it into context with other efforts, such as Google’s, to achieve quantum supremacy.
“The Chinese team also used a statistical test to stake its claim of quantum superiority, but its quantum data carriers take the form of photons traveling through optical circuits laid out on a lab bench, guided by mirrors. Each photon read out at the end of the process can be thought of as is roughly equivalent to reading out a qubit on a processor like Google’s, revealing the result of a calculation.”
“The researchers reported measuring as many as 76 photons from the Jiuzhang machine but averaged a more modest 43. Members wrote code to simulate the work of the quantum system on Sunway TaihuLight, China’s most powerful supercomputer and the world’s third fastest, but it couldn’t come close. The researchers calculate the supercomputer would have required more than 2 billion years to do what Jiuzhang did in a little over 3 minutes.”
Achieving quantum computing supremacy – performing a calculation that is essentially undoable on classical systems – is often presented as an important proof-point on the way to building quantum computers that can do practical work. China has been vigorously ramping up quantum information sciences research and its focus on optical methods varies from the current, most advanced U.S. efforts which rely on superconducting and ion trap technologies. That said, a wide variety of qubit technologies is being explored worldwide. Intel, for example, is betting big on quantum dots.
(For a quick review of the Google work see HPCwire’s coverage, Google Goes Public with Quantum Supremacy Achievement; IBM Disagrees.)
The latest work is significant both in that it uses an optical approach and also that it seemingly avoids a “loophole” in Google’s work on quantum supremacy. IBM, argued, and most agree, that the sample size was such that a large system with sufficient memory could have performed the Google calculation in a reasonable time. The approach taken by the Chinese researchers builds on an approach suggested by Scott Aaronson (University of Texas) and Alex Arkhipov (MIT).
As described by in Science paper, “Boson sampling, proposed by Aaronson and Arkhipov, was the first feasible protocol for quantum computational advantage. In boson sampling and its variants, non-classical light is injected into a linear optical network, and in the output highly random, photon-number- and path-entangled state is measured by single-photon detectors. The dimension of the entangled state grows exponentially with both the number of photons and the modes, which fast renders the storage of the quantum probability amplitudes impossible. The state-of-the-art classical simulation algorithm is to calculate one probability amplitude (Permanent of the submatrix) at a time. The Permanent is classically hard and at least one Permanent should be evaluated for each sample, thus the sample size loophole can be avoided. In addition, boson samplers use photons which can be operated at room temperature and are robust to decoherence.”
The figures below taken from the paper show the experimental set up.
As always, it’s best to read these kinds of works directly. Here’s a link to the Science article which is freely available.
Work on quantum computing, encouraged by various governments, has mushroomed in recent years. The U.S. Quantum Initiative is just one example. ACM recently published the inaugural issue of a new journal dedicated to quantum computing. Currently, so-called noisy intermediate-scale quantum (NISQ) computers dominate the QC landscape (such and it is). The long-term goal of producing fault-tolerant universal quantum computers remains distant, but efforts to build sufficiently robust NISQ systems and narrowly-tailored applications (e.g. logistics) able to take advantage of them are in full swing.
Link to Wired article: https://www.wired.com/story/china-stakes-claim-quantum-supremacy/