Nathan’s Quantum Tech Newsletter: №23 — Quantum Supremacy
This is what I’ve seen in quantum tech in the last two months:
00 🔮 Quantum Supremacy
01 🗞 Tech News
10 📰 Research Highlights
11 🎲 Bonus Links
00 🔮 Quantum Supremacy
A research paper has been published in Nature detailing Google’s quantum supremacy experiment. Link
The idea of quantum supremacy is to find a testbed where quantum computers outperform classical computers. In the specific case, the calculation was very much related to the theory of quantum computing itself: not only the calculation is performed with a quantum computer, but it calculates quantities that refer to a quantum computation. Yes, it’s a bit “meta”.
The calculation is that of sampling a probability distribution. If you ask some acquaintances how tall they are, you are sampling from the probability distribution of heights in humans. More precisely, there may well be some hidden correlations that actually mean that your sampling is biased: it is not just that of the heights of living humans, but very likely of those living near to you (if you live in Sweden, the emerging distribution may differ considerably from Italy).
Similarly, it is possible to construct quantum experiments such that the outcomes of each experimental measurement belongs to a probability distribution which is hard to compute. If one were to randomly initialize N qubits, the outcomes would span the exponentially large configuration space, 2^N. If the qubits where completely “randomly initialized”, each possible outcome would take the same probability. Instead, what was done in the quantum supremacy experiment, was much more convoluted — in order to obtain a probability distribution that is hard to guess or simulate without a quantum computer. What that may be? Well, the outcome of measuring a many-qubit quantum state after several gates are applied to it in a run, and then feeding the measured state at the end of the run again to the same set of gate operations, which is called a quantum circuit.
Researchers at Google first performed the trials on a small number of qubits, which could be easily simulated on a normal computer. Then they scaled up the number of qubits, and of multiple runs, testing the performance of the quantum computer. They were able to test quantum circuits involving a number of qubits that in general would make the calculation unfeasible to test, but they exploited the inner structure of such quantum circuits to find patterns that could be easily simulated on a non-quantum computer.
Then they cranked up also the “difficulty” knob, and performed some measurements that could not be tested on any other non-quantum computer. Their estimate in this hard cases would require thousands of years to check.
Shortly before the publication of the paper in Nature, and shortly after its leak on a NASA server (NASA Quail collaborates with Google on a long-standing basis and also did for this paper, among with researchers from other institutions), IBM Research scientists uploaded to the arXiv, the physics online archive, a paper that confuted that estimate. It was shown how a mix of modern techniques in tensor network description and high performance computing protocols could be adopted to cut down the simulation time on even the world fastest supercomputer to days (“Summit” is made by IBM and operated at on of US national labs, Oak Ridge).
IBM’s paper claims even more generally that any such algorithm run on the Sycamore chip designed by Google, even for much deeper quantum circuits, the scaling of the resources would be linear in the classical simulation. Link
The reported errors on the single-qubit operations were around 0.1%, which means that one can manipulate a qubit around 1000 times before it looses coherence. You can learn more from John Martinis talk on the experiment. Link
Interestingly, the kind of gates that are used to control the qubits, iSWAP-like, are also very natural choice for quantum simulation, since they implement interactions typical of spins or particles. This may be one of the most interesting realistic applications for quantum devices in the short term, as also pointed out by David DiVincenzo. Link
If you are into coding, here is a sneak-peak tutorial into looking for yourself what the theory behind the experiment can look like. Link
This experiment generated a huge quantity of articles. Alphabet CEO Sundar Pichai wrote a blog post. Link
Ivanka Trump tweeted about the quantum supremacy experiment. While she has been scathed for underscoring the administration involvement in the result, I would still agree on the fact that this was a concerted effort (involving researchers from all over the world), but fueled by sustained US federal funding over many years (including before Martinis lab became Google’s). Link
For the future, Google aims to upgrade its chips to one million qubits, exploiting one of the crucial advancements implemented in Sycamore, the adjustable couplers for each qubit in the chip, and for two-qubit gates, a technology pioneered at MIT’s Lincoln Lab in Will Oliver group, who as one of the referees for Nature also wrote a comprehensive and clear commentary. Link
Nature also interviewed some of the other players in this space to ask what is beyond quantum supremacy. Link
01 🗞 Tech News
These past two months included an acceleration in funding rounds news, with some of the industry largest announcements, and broad spin-off activity in and beyond the USA.
PsiQuantum, a quantum computing startup led by experimentalist Jeremy ‘O Brien raised over £ 179 mln while in stealth mode. Its platform is that of integrated silicon photonics. Link
IonQ, the spin-off of Chris Monroe at the University of Maryland, raised over US$ 55 mln dollars in new funding rounds from venture capital firms such as GV (formerly known as Google Ventures). Link
A new spinoff arrives from MIT and Harvard, QuEra. It involves Misha Lukin, an eminent physicist active in the theory of quantum systems and more recently involved experiments, together with Markus Greiner, a colleague working for long time and with pioneering results on cold atoms (both Bose Einstein condensates and atomic lattices), together with colleagues at MIT, including Vladan Vuletic (atomic physics and lasers) and Dirk Englund, expert in quantum photonics. This is the first hardware quantum computing startup spinning out from Harvard, neutral atoms systems. Link
UK-based spinoff Nu Quantum raises pre-seed investment of £650,000 led by Amadeus Capital Partners. One of its co-founders is Mete Atatüre, an experimental physicist leading a lab which obtained pioneering results in solid state quantum photonics at the University of Cambridge over several years. Link
Qnami, a Swiss spinoff developing microscopy that can fall within the umbrella name of quantum sensing, closes a 2.6M CHF seed round led by Quantonation, the French venture focusing of deep physics startups. Link
German software startup HQS raises € 2.3 mln in seed round from institutional investors. Link
Unitary Fund started as a seed money venue for quantum software projects by Will Zeng, a physicist previously at Rigetti Computing. It recently received back up from the Department of Energy and other startups. Link
Microsoft has been pursuing a very different route to quantum computing so far, focusing, at least experimentally, on topological quantum computing, which requires implementing qubits on still not well-tamed quasiparticles known as Majorana fermions, which can be obtained in a heterostructure. At the same time, it supported various researchers and framework in computational endeavors for quantum computing tools. In somewhat a stark shift in strategy, aligning more with the software efforts, Microsoft has now announced support of a number of software and hardware startups under Azure Quantum. Link1 Link2
NTT Research, the Bell Labs of Japan, announced a collaboration with a string of institutions, including leading US research universities and Canadian startup 1QBit. The consortium establishes NTT PHI Lab in Palo Alto, which will be led by Yoshi Yamamoto, who has for long been pursuing an alternative path to analog computing with the coherent Ising machine, also known as LASolve. Link.
For more about unconventional analog computing, you can read this piece of mine. Link
10 📰 Research Highlights
Experimental realization of an intrinsically error-protected superconducting qubit. Link
Advances in neutral atoms quantum computers. Link
Parameterized quantum circuits as machine learning models. Link
Uncomputability of Phase Diagrams. Link
Machine learning in the quantum era. Link
Theoretical framework over robust encoding of a qubit in a molecule. Link
Large-scale integration of near-indistinguishable artificial atoms in hybrid photonic circuits. Link
11 🎲 Bonus Links
Chancellor Angela Merkel in front of a quantum computer (IBM’s chip) at the Fraunhofer Institute. Link
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