Stronger quantum supremacy

 On arxiv today: Strong quantum computational advantage using a superconducting quantum processor, by the group of Jian-Wei Pan at USTC.

This preprint reports a large 66 superconducting transmon qubit quantum processor, with performance similar to Google's Sycamore processor.  The random circuit sampling protocol is used to demonstrate a quantum computational advantage with 56 qubits (c.f. the previous 53 qubit Google experiment). More measurements are taken per random circuit (19 million samples vs 3 million) to obtain a higher cross entropy benchmarking fidelity (9 sigma rejection of uniform sampling vs 5 sigma). Because the difficulty of classical simulation increases exponentially with the number of qubits, this small increase in the device size incurs a 2-3 orders of magnitude-increased cost of classically simulating the circuit.

This is a significant breakthrough - Google no longer has a monopoly on large scale programmable superconducting quantum circuits!

Edit: Today the USTC team released another preprint reporting an upgraded and programmable Gaussian boson sampling experiment!

CLEO covid conferencing

I attended (virtually) the CLEO/Europe conference last week. It's been said many times before by others, but networking and informal discussions which often lead to new creative lines of research do not occur to any significant extent at online conferences. The one silver lining is that when things return to normal, conferences will hopefully be run in a hybrid mode enabling those unable to attend in person to at least view talks remotely.

Some of the interesting talks I saw:

Fabio Sciarrino from Sapienza University of Rome gave an overview of Boson Sampling and discussed his group's recent work on using thermoelectric tuning of a laser-written waveguide array to create a semi-programmable Boson Sampling device. This aims to overcome the main limitation of the recent Chinese photonic quantum supremacy experiment (that their device is not programmable). The preprint is available here.

Vera Neef from Rostock University discussed the generation of non-Abelian geometric phases using two photon quantum states propagating through an adiabatically-modulated array of four coupled waveguides. The approach can be extended to photonic simulation of quantum chromodynamics.

Christophe Galland from EPFL gave an overview of his group's recent work on molecular optomechanics and nanocavities. The former is an interesting platform for observing quantum coherent effects at room temperature, by using a laser beam to excite a high frequency vibrational resonance of a molecule; Raman scattering can then be used to probe the resulting vibrational quantum states.

Nathan Goldman from Universite libre de Bruxelles presented a scheme to detect fractional quantum Hall states of bosons. The idea is to prepare a few-body ground state of bosons in a topological (Chern) band trapped by a confining potential, turn off the confining potential, and then apply a uniform acceleration. The resulting transverse Hall shift exhibits quantized plateaus which can be used to measure the many-body Chern number. For larger numbers of bosons, density measurements can be used to obtain the Chern number. The study was published in PRA last year. These effects could be probed using superconducting qubits using an approach similar to this paper, but with a larger lattice and more photons.

Christina Jörg from Pennsylvania State University discussed the creation of bound states in the continuum in photonic crystals using tailored environmental coupling. By embedding a photonic crystal in a suitably-designed thin photonic crystal cladding, one can create bound states in the continuum above the diffraction limit (i.e. with a volume larger than the operating wavelength). The preprint is available here.

Dynamical versus spectral localization in dissipative systems

A short summary of this paper which was published in Nature Photonics last week.

Waves in random or disordered media can exhibit Anderson localization. In Anderson localization, interference between different wave scattering paths (and in particular, constructive interference of backscattering) results in a complete suppression of wave propagation; waves remain localized around their sources indefinitely, with an amplitude (or intensity) decaying exponentially with the distance from the source.

Anderson localization is a universal phenomenon. It was originally predicted in a 1958 publication analyzing electrons in disordered crystalline materials, and has since been observed for a wide variety of waves including matter waves (Bose-Einstein condenstates), optics, and acoustics.

Historically, a thorny issue complicating the observation of Anderson localization in classical wave systems such as optics and acoustics has been the question of how to distinguish Anderson localization from absorption-induced localization; both lead to a similar exponential decay of the wave amplitude.

The present work concerns the generalization of Anderson localization to disordered dissipative optical wave systems with random distributions of gain and/or loss, and the subtle distinction between spectral localization and dynamical localization.

Spectral localization refers to localization of the modes of the medium. Each mode has a specific energy (frequency). The mode may be excited by placing a source (e.g. a speaker in the case of acoustic waves) in the medium tuned to that frequency, in which case the amplitude profile of the generated wave will match the profile of the correspond mode.

Dynamical localization refers to the time evolution behaviour of wavepackets comprising a range of frequencies, excited by switching the source on for a short time.

In wave systems that are Hermitian (conservative, i.e. no gain or loss of energy), spectral and dynamical localization coincide because the time evolution of any wavepacket can be obtained by expanding it as a sum of the medium's modes. The paper shows that this is not the case for wave systems with dissipative disorder; dynamical delocalization can occur despite spectral localization. In other words, waves generated by a monochromatic source will have an exponentially localized amplitude profile, whereas waves emitted by a broadband source will spread to distant parts of the system.

To demonstrate this dynamical delocalization, the authors had to carefully distinguish between energy transport and energy loss due to absorption. In particular, for lossy media the wave amplitudes are always decaying exponentially in time. At any moment in time we can consider the shape of the wave's amplitude distribution (e.g. by increasing the sensitivity of the camera or microphone used to detect the waves) and how it decays with separation from the source. 

In dissipative wave media the modal expansion is still valid, but different modes will have different loss rates. Therefore, the relative amplitudes of the different modes in the expansion will change in time, leading to large changes and in particular spreading in the normalized wave amplitude profile - dynamical delocalization. This is the main result of the study, which observes the phenomenon using a cleverly-designed system of coupled optical fibres.

Has topological photonics plateaued?

Not yet.

Data taken from Web of Science. I compare the publication rate of papers on topological photonics with some other hot topics in photonics during my graduate studies. Of course, the number of publications measures interest in a topic and does not necessarily correlate with real progress.

Whether topological photonics was close to reaching a plateau was a topic of discussion during a workshop I attended in 2019. Close to half of all papers on topological photonics have been published since 2019! During this period the main emerging research directions have been nonlinear effects, non-Hermitian topological structures (i.e. with gain and/or loss), higher order topological phases, and bulk topological defects. Ongoing experimental efforts in all of these directions is likely to sustain interest for the next few years, at least. Will the peak be followed by a sustained plateau or an immediate decline?