From NISQ to small logical quantum circuits

After six years of huge interest in NISQ (noisy intermediate-scale quantum) circuits there are still no practical applications where a noisy quantum device can outperform the best classical methods. Noise is too detrimental, and classical methods are too powerful. Experts continue to argue that now is not the time for commercial applications: quantum error correction, hundreds of logical qubits, and millions of error-corrected gates are needed.

Then what's next? Circuits of a moderate size with some limited error correction capabilities. LISQ (logical intermediate-scale quantum) or something else, for short.

What can we expect from these up and coming small scale logical circuits?

First, a lot of the tools developed for the NISQ era will become obsolete. For example, variational quantum circuits involving continuously-parameterised quantum gates cannot be easily implemented in a fault-tolerant manner. Instead, post-variational hybrid quantum-classical algorithms for this era will need to offload the continuously-parameterised part of the algorithm to a classical computer, with the quantum circuit used to measure a set of (hopefully classically-intractable) observables that are used as inputs to the classical tunable model.

Second, the hardware, algorithms, and the error correcting code cannot be considered in isolation. Choosing the right error correcting code will be essential to get the most out of the current hardware. Examples of this can be seen in QuEra's logical circuit demonstration from late last year, where the use of a 3D quantum error correction code allowed them to perform random IQP circuit sampling with error detection, and Quantinuum's recent demonstration of repeated error correction. Similar to the NISQ era, different hardware platforms will have different strengths and limitations in what kinds of circuits they will be able to run.

Finally, the most valuable software tools in the NISQ era were for quantum control and state tomography, essential to get the most out of the noisy hardware. These tools will remain important, since fidelities at the physical qubit level directly affect the amount of quantum error correction overhead required. As we move to logical circuits, the new valuable quantum software will be in the form of compilers that will take all the hassle out of hardware and error code selection out of the end-user and translate a given logical circuit into simple, understandable hardware requirements.

Bob Dewar (1944-2024): theoretical plasma physicist

I was sorry to hear last week that Bob Dewar passed away while on a sabbatical at Cambridge. There are some tributes on the ANU MSI website, where he was an Emeritus Professor and still remained active in research.

Bob was my supervisor for a summer research project I undertook in 2009, at the end of my second year of undergraduate studies. At this time I was still unsure whether I would do a theoretical or experimental project if embarking on a PhD, let alone what topic it would be in, and this was my first experience of a research project in theoretical physics.

On my first day, Bob handed me an ancient monograph - his MSc thesis from 1967! - and assigned me the problem of getting some old Fortran code from the appendix working on a modern system. The code was designed to efficiently evaluate the wake potential left by a charged particle moving through a plasma by making use of clever analytical tricks and special function identities. Code efficiency was vital in that era, when programs were written on punched cards and the whole university had to share time on a single computer! Nowadays with easy access to computer algebra systems we are lazier and the art of special functions and asymptotic expansions is less widely appreciated. It was a fun little project for the summer, capped off with the opportunity to present the work at the Gaseous Electronics Meeting, held near Bateman's Bay that year. Later we (well, mostly Bob) wrote a paper out of this project. Whenever I see the wake left by a boat or swimming duck I am reminded of this work.

Bob was an inspiring mentor, representative of a kinder, more humble era of science where there was the freedom to follow a passion and spend decades digging into a single area of expertise throughout one's career and into retirement.

Postdoctoral Positions at Nankai University, Tianjin, China

Applications are solicited for postdoctoral positions in experimental/theoretical optics and photonics in the research group of Prof. Zhigang Chen/Hrvoje Buljan at Nankai University, China, which is one hour away from the capital Beijing and the alma mater of Shiing-Shen Chern. The areas of emphasis are topological photonics, nonlinear optics, optical trapping and manipulation, and machine-learning photonics. A PhD in physics, optics, or related area is required. It is expected that the candidate should have basic numerical skills and/or optical experimental skills and research experience with a good track of record in publications.

The initial appointment will be for three years, with the possibility of extension pending on performance and research funding. If accepted, the salary is competitive (annual gross salary > RMB 500,000, about $70,000), with possible additional merit award depending on academic performance evaluated at the end of each year. Free on-campus apartment is available, and the living cost is very low compared to income. The position will be funded under the China Postdoctoral International Exchange and Introduction Program, with the purpose of attracting outstanding PhD graduates to join the university and conduct postdoctoral research.

Applicant eligibility:
1. In general, applicants shall be under the age of 35. As for key disciplines supported by the university, the limitation on age can be eased to 38 years old.
2. Both Chinese and foreign graduates obtaining their PhD (from overseas top universities or supervised by an internationally recognized scientist) in recent 3~5 years can apply.
3. PhD candidates who meet the above conditions can also apply if they can start the postdoctoral position by June 2025.

Interested candidates are encouraged to contact Professor Chen or Professor Buljan before April 20, 2024 for application this year. However, the starting date is flexible.

Website: https://topo-photonics.nankai.edu.cn/index.htm

Arxiv April Fools'

This year there are quite a few joke papers cross-listed in the popular physics category. My favourite: "Is Winter Coming?"

Particularly memorable entries from previous years include "Novel approach to Room Temperature Superconductivity problem" and "A solvable string theory in four dimensions."

CQT Colloquium on strongly interacting photons in superconducting qubit arrays

 Yesterday at CQT Jonathan Simon from Stanford University gave a wonderful colloquium talk on "Many-body Ramsey Spectroscopy in the Bose Hubbard Model," covering experimental studies of strongly interacting quantum fluids of photons in arrays of superconducting qubits, spanning work from 2019 on the preparation of photonic Mott insulating states to ongoing studies of entangled many-body states of light.

A good colloquium talk should understandable to a broad audience (ideally, including undergraduates) while still going into enough depth to keep specialists in the topic interested. If you cannot frame your research in terms of some simplified model, chances are you do not yet fully understand it.

Simon did this using the neat example of emergence in 2D point clouds: observing non-trivial emergent properties requires three key ingredients: many particles, interactions between the particles, and dissipation (in this case, friction) to allow the system to relax to some ordered state. When all three are included, the cloud self-organizes into a triangular lattice with properties qualitatively different from those of the individual constituent particles, supporting low energy vibrational modes (phonons).

Typically, a colloquium talk will cover research spanning several years. It is important to have some clear common motivation. In this case, the question of how to make quantum states of light exhibit similar emergent properties? Three ingredients are required: give photons an effective mass, achieve strong photon-photon interactions, and introduce a suitable form of dissipation that allows the system to relax to some interesting equilibrium state while preserving non-trivial many-particle effects.

After this framing, the talk went deep into how these ingredients can be realized using arrays of superconducting qubits, and how the relevant dimensionless quantities (interaction strength vs hopping strength vs photon lifetime) compare to other platforms, such as cold atoms (handy, given the mix of expertise in the audience).

The talk finished with a vision for the future - to connect this "photonic quantum simulator" to a small-scale quantum processor to test NISQ-friendly algorithms, such as shadow tomography of many-body quantum states.

A recording will probably be uploaded to the CQT Youtube page later. In the meantime, related talks given at JQI and Munich are already available online!

ChatGPT, write my article introduction! And editors versus referees

This paper with an introduction brazenly written by ChatGPT attracted a lot of attention last week. How is it that the first line of the introduction could remain in the final version without anyone (authors, editors, referees, proofing staff) noticing? 

Some said this was no big deal - aren't paper introductions boilerplate junk that nobody reads anyway? Yes and no. While an expert in the field might not expect to learn anything new from reading a paper introduction, it is nevertheless important as a means for the authors to convince the reader that they sufficiently understand the context of the research and are in a position to make a novel and significant contribution.

Others argued this was an example of the failure of peer review and the current scientific publishing system - junk papers that no one (not even the authors!) read.

Who exactly is at fault here (apart from the authors, obviously) - the journal editors or the referees?

Actually, it is not the referees' job to proofread manuscripts! Many referees will not bother to laboriously point out all the obvious typos in a manuscript and will purely focus on the scientific content in their reports. Sloppiness that the authors fail to notice themselves will detract from the credibility of the science reported and may be more damning than scathing technical criticism by the referees that might not be adequately addressed in the final paper!

The editors should have caught this in their initial screening. One of the roles of an editor is to curate content and ensure that the valuable time of the volunteer referees is not wasted on obviously incorrect, unconvincing, or not even wrong manuscripts. At the same time, we don't want to waste the authors' time by agreeing to send the manuscript out for review and then being unable to secure willing referees!

At Physical Review A we desk reject about half of the manuscripts we receive without sending out for peer review. While this might sound like a lot, these manuscripts tend to be of much lower quality than those that are eventually published. There are several red flags that make us lean towards desk rejection:

Out of journal scope. Does the manuscript report results that are of interest to the readers of the journal? One simple way to gauge this is to check the reference list of the finished manuscript - if you are only referring to works from other disciplines, this is not by itself grounds for rejection, but it is a hint that you need to be particularly careful with explaining the relevance of your work to the journal's specific audience.

Poor presentation. Obvious typos. Ugly figures. No figures (passable in rare cases). Too many figures. Illegible axis markers. Incorrectly formatted equations and symbols. Basic stuff, but many authors sadly cannot be bothered.

Transfer after rejection from a sister journal. This one is surprisingly common, particularly for research topics which fall in the scope of multiple APS journals. Most often we see transfers from PR Applied and PRB, which have higher impact factors, so the authors decide to try their luck with PRA. But the standards of all these journals are the same, regardless of their impact factors that fluctuate from year to year. This means that rejection from PR Applied or PRB generally precludes publication in PRA, except in special cases.

No significant new physics. This is the most controversial. Who is the editor to decide what is significant - isn't that the job of the referees? We do lean towards giving the benefit of the doubt and sending out to referees for this one. The manuscripts that fail this test generally lack the "so, what?" factor - assuming all the claims are correct, have we learned anything new? It is always possible to tweak models, change terms, make them a bit more complicated, and then apply analysis tools that are standard for the field to get something that is technically correct. But the impact of such technically correct works will be limited unless they open up something new - a novel experimental platform, a way to push the limits of existing theory, and so on.

It is never pleasant for one of your articles to be rejected without review, but it is actually the second best response you can receive! The likely alternative would be for you to wait months before receiving a similar rejection on the basis of anonymous referee reports!

Postdoc Opening at Tohoku University: Condensed Matter and AMO Theory

Tomoki Ozawa's group at the Advanced Institute for Materials Research, Tohoku University, has a postdoc opening (the official title would be Specially Appointed Assistant Professor, or Tokunin-Jokyo in Japanese), which can start as soon as the decision is made or from April 1, 2025 at the latest. The position lasts for three years. 

The group works on theoretical condensed matter physics and AMO (atomic, molecular, and optical) physics, in particular on topological phases and/or many-body physics in these systems. A part of the salary of this position will be from the KAKENHI Kiban-B grant, “Geometrical effects in non-Hermitian quantum systems." 

The application deadline is April 30, 2024, and the application should be sent through Academic Jobs Online from the following link:

https://academicjobsonline.org/ajo/jobs/27320