The ASEAN Plus Photonics Symposium 2026

I really enjoyed attending The ASEAN Plus Photonics Symposium 2026 earlier in April. I was invited to give a talk on nonlinear topological photonics. My talk slides are available here.

The conference programme covered a broad range of topics, including metasurfaces, exciton-polaritons in microcavities, structured light, topological photonic crystals, and the integration of two-dimensional materials into photonic devices. The full programme is available here.

Beyond the individual talks, a few high-level themes stood out to me:

A recurring topic was the push to move metasurfaces from proofs of concept toward practical devices. This shift raises its own set of challenges: how can fabrication be made faster, cheaper, and scalable? Similar questions arise for device characterisation, which must evolve from carefully controlled, fragile experiments to something closer to a production line approach. Efficient electrical control of devices is increasingly important, as is long-term stability: can one return to a device six months later and expect the same performance?

Another strong theme was the importance of active exchange between theory and experiment. For this to be effective, theorists need to understand which effects are experimentally accessible and which are not, while experimentalists must know when to trust theoretical predictions, and when those predictions need to be revisited in light of conflicting data.

One particularly striking example came from Prof. Feng Li's talk on optical skyrmions. He candidly talked about how early experimental observations in the 2010s were not well understood and lacked an adequate theoretical explanation. This kind of openness about the inevitable challenges of research elevates a good talk into an inspiring one.

Related to this was a recognition of the value of intermediate results. Such results are often essential for understanding the capabilities and limitations of new fabrication schemes, or for training new students. While modest in scope, these studies can sometimes lead to unexpected and rewarding discoveries.

The conference venue itself was memorable: located by the beach and decorated with artworks depicting various Physics Nobel Laureates, it provided a pleasant and stimulating setting for discussion.

There was also noticeable attention on the rise of Vietnam as a manufacturing hub. As costs increase in China, manufacturing activity is shifting, and Vietnam is seeing growing investment in semiconductor fabrication. This trend is driving demand for scientists and engineers with expertise in nanofabrication and related areas, accompanied by increased funding for research into integrated photonics.

I do like smaller conferences such as this. With no parallel sessions, you can better exposed to topics outside one's own direct area of research, broadening knowledge and getting to know colleagues you wouldn't normally interact with.

Looking forward to the next edition of the conference in 2027!

Artificial Intelligence Photonics 2026 in San Sebastian: call for abstracts

The Artificial Intelligence Photonics 2026 workshop will be held at Palacio Miramar in San Sebastian, on 15-18 June, 2026. This meeting follows the first edition held in 2023 and is aimed at gathering a critical mass of people working at the intersection of AI and photonics. The preliminary list of confirmed invited speakers comprises:

• Andrea Alù, CUNY, USA
• Natalia Berloff, Oxford, UK
• Peter Bientsman, Ghent U., Belgium
• Lu Fang, Tsinghua; China
• Rachel Grange, ETH, Switzerland
• Chaoran Huang, CUHK, Hong Kong
• Aydogan Ozcan, UCLA, USA 
• Valentina Parigi, LKB, France
• Patti Stabile, Eindhoven, Netherlands
• Jelena Vuckovic, Stanford, USA

Abstract submission is open for contributed talks and posters until 1 April, 2026. The number of attendees will be strictly limited to 100. Students are more than welcome to participate.

The 17th Annual Meeting Photonic Devices, Zuse Institute Berlin, Germany

Call for abstracts for the 17th Annual Meeting Photonic Devices (AMPD2025), taking place at Zuse Institute Berlin, Germany.

Topics include nanophotonic devices and related simulation methods. The organisers aim for open discussions between experiment, theory, and numerical methods.

Invited talks will be given by:

Anna Tasolamprou, National and Kapodistrian University of Athens
Bumki Min, Korea Advanced Institute of Science and Technology (KAIST), Daejeon
Costantino De Angelis, University of Brescia
Costanza Toninelli, European Laboratory for Non-Linear Spectroscopy in Florence
Haejun Chung, Hanyang University, Seoul
Humeyra Caglayan, Tampere University
Jesper Mork, Technical University of Denmark, Lyngby
Kurt Busch, Humboldt University of Berlin
Mohsen Rahmadi, Nottingham Trent University
Nahid Talebi, Kiel University
Olivier Martin, EPFL, Lausanne
Thomas Pertsch, Friedrich Schiller University Jena
Tim Schröder, Humboldt University of Berlin
Further invited speakers to be confirmed.

Please consider joining this workshop as either a speaker (poster or talk) or as an audience member. The workshop is free of registration fees.

Important dates:

Abstract submission deadline: January 31, 2025
Decision about acceptance: February 15, 2025
Registration deadline: March 25, 2025
Workshop: April 02-04, 2025 

Further information is available at https://www.zib.de/workshop-photonic-devices/ampd2025.html

Flatbands: then and now

We published a review article on flatband fine-tuning and its photonic applications in Nanophotonics last week! This follows up on our earlier perspective on photonic flatbands published in APL Photonics in 2018.

How has the field changed in 6 years?

In 2018, we identified promising areas for future research where flatbands had not yet been extensively explored yet: coupled resonator lattices, circuit QED, and photonic crystals.

For the case of coupled resonators, the idea of synthetic dimensions (considering coupling in the frequency domain rather than space) has since emerged as a new direction for non-Hermitian and topological photonics, with the ability to fine-tune short- and long-range hoppings to realize flat band lattices using coupled optical fiber loops.

Circuit QED now sees broad interest as a platform for quantum simulation, especially for studying lattices on hyperbolic space. 

Flatband photonic crystals have received a great amount of attention, driven especially by the rise of moire materials which exhibit flat bands at "magic" twist angles. This breakthrough in condensed matter physics inspired the development of theory (see Phys. Rev. Lett. 126, 136101 (2021), Phys. Rev. Lett. 126, 223601 (2021), and Phys. Rev. Research 4, L032031 (2022), for example), with applications to photonic crystal lasers and shaping free electron radiation being actively explored. 

The huge growth of interest in flat bands in photonic crystals and related platforms such as metasurfaces has been quite remarkable. It is driven by the realization that one does not need to carefully control symmetries or suppress long range couplings, guided by simple tight binding models for flat bands, to design them. Rather, a sufficiently complex system supporting parameter fine-tuning is all that you need to realize flat bands! Equipped with this knowledge, our latest review is timely in that it covers novel phenomena that can emerge in fine-tuned flat band systems. 

A busy January

There's been a lot going on here...

Machine Learning & Physics

Unsupervised learning of quantum many-body scars using intrinsic dimension - Now available on arXiv! We applied manifold learning techniques to identify scar states in the PXP model The take-home message: manifold learning techniques are a powerful alternative to more popular deep learning methods, especially in physics problems where you might not have access to enough training data for deep learning to work well.

Identifying topology of leaky photonic lattices with machine learning - Just published in Nanophotonics! We apply various machine learning methods to distinguish different topological phases in a photonic lattice, assuming one only has access to intensity measurements. This can serve as an alternative to full state tomography or phase retrieval methods, but one needs to be careful when training the models on ideal / pristine systems and then applying them to disordered systems. The journal also published a press release on WeChat!

Quantum Computing

Computing electronic correlation energies using linear depth quantum circuits - Finally published in Quantum Science & Technology, after more than a year and a half working through the peer review system. We use perturbation theory to determine electronic correlation energies in small molecular systems (hydrogen, lithium hydride, etc.) using a large set of shallow circuits, giving an alternative to existing methods which require deeper circuits infeasible for current quantum processors. We also tested the algorithm on cloud quantum processors, observing the detrimental impacts of noise. It would be interesting to run this again now to see how much (or how little) the performance from the different cloud providers has improved!

Landscape approximation of low-energy solutions to binary optimization problems - Published in Physical Review A. We present a method to obtain approximate solutions to binary optimization problems using the localization landscape, a function which is able to place bounds on the regions of Anderson localized eigenstates in disordered media without solving the underlying eigenvalue problem. We lay out the conditions required for these bounds to hold, outline how a quadratic unconstrained binary optimization problem can be transformed to fit these conditions, and provide details on how the quantum state representing the landscape function can be produced and sampled using techniques developed for near-term quantum devices.

 

On a related note, I was interested to see this month a new arXiv preprint in which the localization landscape was used to engineer multifractal resonances in SiN membranes!

Photonic Flatband Resonances

Photonic Flatband Resonances in Multiple Light Scattering - Published in Physical Review Letters. We reveal that flatbands can emerge as collective resonances in fine-tuned arrays of Mie-resonant nanoparticles, leading to giant values of the Purcell factor for dipolar emitters. The article was also highlighted with a Synopsis in Physics Magazine!

Updates

Infrequent posting due to other commitments. Here are a few brief items of note from the past month:

• Beng Yee uploaded his second paper from his PhD research to arXiv: A Unified Framework for Trace-induced Quantum Kernels. This project tackled the problem of how to choose the best quantum kernel for a given learning task using tools from classical multiple kernel learning theory. The bottom line: the optimal problem formulation (e.g. as a kernel model, projected kernel model, or quantum neural network) depends on the relative amount of training and test data, whether one wants to impose constraints to the trained model, and whether one has many qubits with low-fidelity gates or a fewer qubits with high fidelity gates. Read to find out more!
• The December issue of Optics & Photonics News highlights some of the most exciting peer-reviewed research in optics and photonics published over the past year. There is also an accompanying perspective on areas to watch in 2024 and beyond by selected summary authors.
• Two papers recently published in PRL caught my eye: Universal Sampling Lower Bounds for Quantum Error Mitigation suggests the quantum error mitigation being pushed by IBM and others as a means of getting useful applications out of current noisy quantum processors may be foiled by an exponentially growing measurement overhead, and Classifying Topology in Photonic Heterostructures with Gapless Environments shows how a recently-developed real space formulation of topological invariants may be a more useful tool for quantifying the robustness of topological states in photonic systems, particularly those exhibiting radiation losses of optical nonlinearities.
• The 7th International Conference on Optical Angular Momentum will be held 24 - 28 June 2024 in South Africa. The abstract submission deadline is 7 January 2024.
• The next edition of the Quantum Techniques in Machine Learning conference will be held in Melbourne, 25-29 November 2024. The abstract submission deadline is 5 July 2024. 
• In the news headlines: Alibaba shuts quantum computing lab. Seems to be part of a wider trend of industry funding shifting from quantum to generative AI - see also Zapata and Normal Computing.
  

From graphene to borophene

In the beginning there was graphene, and graphene had some very remarkable properties which have attracted enormous interest over the years. For the unfamiliar, graphene is a two-dimensional sheet of carbon atoms arranged in a honeycomb lattice structure. The tight binding energy band structure has the peculiar property that its conduction and valence bands touch at the corners of the Brillouin zone. These corners are known as Dirac points because the low energy electronic degrees of freedom are governed by an effective Dirac equation,

$$ i \partial_t \psi = v_F (\boldsymbol{p} \cdot \boldsymbol{\hat{\sigma}} ) \psi,$$

where $\boldsymbol{\hat{\sigma}}$ are the Pauli matices, $\boldsymbol{p}$ is the in-plane momentum, and $v_F$ is the Fermi velocity, which acts as an effective speed of light.

 

The role of spin is in graphene's effective Dirac equation is played by the sublattice degree of freedom – the underlying honeycomb has two sublattices, termed A and B, that are inequivalent. This is known as a pseudospin. Because of this Dirac equation description, electrons in graphene can emulate a variety of interesting phenomena from high energy physics, such as the Klein paradox.

This interesting Dirac physics is not specific to graphene, but emerges in any periodic potential with a honeycomb lattice structure, including photonic systems. In this case, the electronic wavefunction is replaced by the optical field envelope, and the effective potential can be controlled by modulating the local refractive index. For example, in the case of semiconductor microcavities, the potential modulation can be created by selective etching of the cavity to form a honeycomb structure. The resulting photonic band structure can be observed experimentally by measuring the energy-resolved photoluminescence spectrum from the cavity, which reveals a neat Dirac cone structure where the two energy bands cross.



One of the interesting properties of a Dirac cone is that it has an emergent rotational symmetry. Even though the potential is inhomogeneous and breaks the continuous rotational symmetry, the energy eigenvalues in the vicinity of the Dirac cone are invariant under rotations. This in-plane rotational symmetry leads to a conserved total angular momentum J, which is the sum of the usual orbital angular momentum, and a pseudospin angular momentum associated with the sublattice degree of freedom. Rewriting the effective Dirac Hamiltonian in terms of pseudospin raising and lowering operators $\sigma_{\pm} = \hat{\sigma}_x \pm i \hat{\sigma}_y$,

$$ i \partial_t \psi = v_F ( e^{-i \varphi} \hat{\sigma}_+ +e^{i \varphi} \hat{\sigma_-} ) \psi,$$

we see that a flip of the pseudospin must be accompanied by a change in the orbital angular momentum (corresponding to the angular phase winding terms $\exp(\pm i \varphi)$. Therefore, if a Dirac cone is excited with a spin up state, one can measure a phase vortex in the spin down field component. This pseudospin-mediated vortex generation has been observed using photonic waveguide lattices.

Around 2011, multiple groups proposed various generalizations of graphene to higher-order conical intersections (reviewed here). The effective Hamiltonian at a higher order conical intersection can be obtained by replacing the spin ½ Pauli operators in the Dirac equation with spin s matrices. The resulting band structures similarly have intersecting conical bands with energies determined by the spin projection parallel to the momentum. 

 

There is a qualitative difference between intersections with integer and half-integer pseudospin. For the integer s case, the spin projection can vanish, corresponding to a flat band with zero energy for all momenta. The first studies of higher order conical intersections however were limited to tight binding models that seemed quite difficult to implement in practice, requiring for example laser-assisted hopping or fine-tuned multilayer structures.

Around the same time, different groups realized that the s=1 conical intersection could be observed using a relatively simple square lattice structure known as the Lieb lattice, which is obtained by removing one quarter of the sites from an ordinary square lattice. Starting from a tight binding model, one can show that the band structure close to the Brillouin zone corners is described by a spin-1 variant of the Dirac equation. This band structure is interesting because one has conical bands with a vanishing wave effective mass intersecting a flat band with an infinite wave effective mass.



One of the important consequences of the flat band is that waves in this band do not propagate, they remain localized. This nondiffracting property of flat band states was observed in 2015 by two groups (papers here and here). A second interesting observable difference of the Lieb lattice is that different pseudospin states have can have differing dynamics, dependent on the magnitude of the initial pseudospin. Initial states with pseudospin plus or minus one partially excite the flat band, leading to a splitting of a beam between a rapidly-expanding conical diffraction component, and a residual flat band component. In these conical diffraction experiments, it's also possible to observe a pseudospin-mediated generation of charge two phase vortices.

What about higher values of the pseudospin s? It gets more challenging. Usually we design conical intersections within the framework of a weak coupling (tight binding) approximation in which coupling between second and most distant neighbours is assumed to be zero. This requires a lattice that is sufficiently deep lattice or has a large separation between the sites. But minimizing second neighbour coupling in this manner also makes the nearest neighbour coupling weaker, reducing the overall energy bandwidth and making it more difficult to resolve the different bands at the conical intersection. This problem naturally gets worse the more intersecting bands one has. So while there have been many studies of Dirac cones and Lieb lattices, the extension of these ideas to higher pseudospin systems is more challenging. Feasible proposals are scarce and most have required complicated fine-tuned models that are difficult to implement.

One way to overcome these problems is to consider conical intersections that are protected by permutation symmetries. The idea is to associate a conical intersection with a permutation symmetry matrix. By re-interpreting the symmetry as an adjacency matrix of a graph, one can embed the degeneracy into a periodic lattice. Detuning the wavevector away from p=0 breaks the symmetry, lifting the degeneracy, which produces a conical intersection in the dispersion relation. This approach can be used to systematically create conical intersections of a desired order. More importantly, the resulting lattices typically involve symmetric and relatively close-packed structures, giving rise to larger bandwidths!

For the case of a five-fold degeneracy corresponding to pseudospin 2, the permutation symmetry approach generates a lattice known as chiral borophene. It can be obtained by considering as a unit cell a filled hexagon, removing one of the corners, and rotating the remaining sites. This gives rise to a lattice with broken mirror symmetry, which has two inequivalent chiral variants. The tight binding band structure has 5 intersecting bands at p=0, with a 6th band separated by a large gap.



The effective Hamiltonian describing the band structure close to $p=0$ is a little more complicated than the usual Dirac Hamiltonian,

$$ i \partial_t \psi = c_0 \boldsymbol{p} \cdot \boldsymbol{\hat{S}} + c_1 \boldsymbol{p} \cdot \{ \boldsymbol{\hat{S}}, \hat{S}_z^2 \} - c_2 \hat{1}, $$

with a second term proportional to an anticommutator of the spin-2 matrices. The reason for this is basically that the spin-2 matrices allow for more non-trivial terms that respect the rotational symmetry. The effect of this additional term is to control the relative opening angle between the pairs of conical bands. When the lattice is excited by a state with pseudospin 2, the conservation of total angular momentum means that phase vortices with charge up to 4 can be generated by post-selecting on different output pseudospin states, as shown in the simulation results published here.

The middle band in chiral borophene is not very flat, and actually has considerable dispersion over the Brillouin zone, even in the nearest neighbour tight binding model. Flat dispersion only occurs along high symmetry lines, which corresponds to the existence of non-diffracting line states. Similar to the case of the Lieb lattice, these nondiffracting states can be excited by considering an input with a staggered phase profile. The diffraction of such states is strongly suppressed compared to a similar input with a flat phase profile, as you can see in these experimental results.

The pseudospin-2 occurring in latices such as chiral borophene opens up many interesting properties for wave manipulation and nonlinear optics, including the possibility for cascaded wave mixing between the partially flat and conical bands, generation of high charge vortices, and the introduction of strain or other perturbations to open up topological band gaps, and possible analogies with the physics and propagation of gravitons (which also have spin 2). It will also be interesting to see whether this lattice can be realized as a two-dimensional electronic material.

IPS Meeting 2023

A few things I learned attending the first two days of this year's IPS Meeting, held right here at NUS:

Prof. Giovanni Vignale gave a plenary talk on bulk currents and edge accumulation in anomalous Hall systems. In the conventional quantum Hall effects, accumulation of charge at the edges of the sample are driven by the bulk quantized Hall conductivity. Anomalous quantum Hall systems, on the other hand, do not show an accumulation of spin or valley densities at their edges, despite their corresponding bulk spin or valley Hall conductivities being nonzero. In the case of spin Hall systems it's because bulk electrons will flip their spin when reflecting off the edge of the sample. Thus, the edges accumulate a nonzero charge density, but their spin density remains zero. Interestingly, a similar argument does not hold for the case of valley Hall systems because the applied electric field that drives the current also induces coupling between the valleys in the bulk. Further details can be found here.

The second plenary talk by Prof. Silvija Gradecak focused on the use of imperfect or novel materials to develop new components. A striking example given was the use of 2D materials as diffusion barriers in nanoscale metal contacts in integrated circuits, which promises the ability to further miniaturize electronic components.

Dr. Sen Mu talked about Kardar-Parisi-Zhang (KPZ) physics in the Anderson localization of two-dimensional wavepackets. The KPZ equation describes fluctuations that arise in the density fluctuations of expanding waves in the presence of disorder. These fluctuations are universal and arise in a variety of wave systems, including the spreading of coffee poured out onto a napkin, which he demonstrated for us live! arXiv preprint

Weitao Chen discussed critical dynamics in one-dimensional disordered systems with long range coupling. In critical systems the eigenstates exhibit multifractality, meaning that the different moments of the eigenstates scale with different non-integer exponents with the system size. This is a bit abstract and hard to measure directly in an experiment, but remarkably this multifractality can also be observed by exciting a single site of the lattice and measuring the time-dependent return probability! arXiv preprint

Prof. Di Zhu in another plenary surveyed integrated photonics for the generation, manipulation, and detection of quantum states of light. A recurring theme was that many of the improvements required to scale up integrated quantum photonic systems can be found by looking back to scientific literature from the 1960s! One neat example he gave was scaling up superconducting nanowire single photon detectors: Putting many of them one one chip is hard, because each coaxial microwave read-out line also conducts heat in - if you have too many you will no longer be able to keep the chip cool enough for the detectors to work. The solution? Move from detection based on a lumped circuit model to a transmission line detector, which can (with a bit of signal processing) perform spatially-resolved detection of multiple single photons. A demonstration of this idea was published this year in Physical Review Applied after spending quite some time under peer review by the looks of it.

There were many other interesting talks and posters that I didn't take enough notes on to write about, but it was nevertheless great to see the breadth of physics being done at the different universities and research institutes in Singapore.

International Workshop on Polaritons in Emerging Materials

Last week I had the pleasure of attending an IBS PCS International Workshop on Polaritons in Emerging Materials held in Daejeon, Korea. Smaller workshops such a this one (~40 participants) with a more relaxed schedule (40 minutes per speaker and generous coffee/lunch breaks) are great for getting a more in-depth and candid picture of an unfamiliar research field!

One of the hot topics was polaritons in transition metal dichalcogenides - a rapidly-maturing family of two-dimensional graphene-inspired materials. Prof. Myung-Ki Kim from Korea University talked about plasmon resonances in multi-layer TMDs, Prof. Deep Jariwala from the University of Pennsylvania presented experiments with cavity-free polaritonic structures. The high refractive index of 2D materials such as molybdenum disulphide means that they can already exhibit strong light-matter coupling without requiring embedding in a microcavity. Thanks to the different localization of the photonic and electronic degrees of freedom one can form ultra-thin multilayer structures either as thin sheets of the 2D material (with the thickness controlling the electronic band structure), or as lattices formed by multiple non-interacting single sheets. Prof. Su-Hyun Gong (Korea University) presented waveguides based on multilayer tungsten disulphide can achieve tight (nanoscale) light confinement with lower losses compared to conventional plasonic materials such as gold. Expect to see many more works in this area as high-quality and large-area samples of these exotic materials start to become commercially available.

Another active area was optically-driven rotation and localization of exciton-polariton condensates. Dr. Michael Fraser (RIKEN) presented experiments in which a condensate is stirred via incoherent pumping with two slightly-detuned Laguerre-Gaussian layer beams, leading to an asymmetric reservoir density that undergoes a rotation, producing condensates with vortices. Theoretical analyses of vortex generation and turbulence in stirred exciton-polaritons were presented by Dr. Alexey Yulin (ITMO) and Dr. Helgi Sigurðsson (Warsaw), and Dr. Sergei Koniakhin (IBS PCS). Prof. Alberto Amo (Lille) showed that the dynamics of resonantly-driven condensates in lossy lattices can be remarkably counterintuitive - the strongest localization occurs between the pumped sites, not at them!

While not the main theme of the workshop, topological photonics was represented in talks by Profs. Sven Höfling, Sebastian Klembt (both from Würzburg University), Dr. Xingran Xu (NTU), who focused on lasing and non-Hermitian topological phenomena, and Dr. Alexander Cerjan (Sandia National Labs), who showed how real-space topological markers can be used to quantify the robustness of nonlinear topological edge states.

Prof. Fabrice Laussey (Wolverhampton) gave a captivating talk on quantum light and the importance of taking detector bandwidth into account when modelling quantum light sources. Since quantum light is so weak, signals measured using a finite bandwidth filter will inevitably be dominated by the tails of the much stronger pump beam unless homodyne detection is used. Look for quantum correlations in the spectral minima, not the dips! In related talks, Prof. Andrey Moskalenko (KAIST) analyzed entanglement between cavities generated by coherent optical driving, and Prof. Hyang-Tag Lim (KIST) covered experimental generation of multi-mode N00N states.

Most of the talks should become available to watch on the PCS Youtube account at some point.

What I've been reading lately

Continuity Equation for the Flow of Fisher Information in Wave Scattering

We can get an intuitive understanding of a wide variety of wave systems ranging including photonics, acoustics, and electronic condensed matter by visualizing the flow of intensity, energy, or probability density through them. These flows are useful for understanding the behaviour of conserved quantities, since they can be decomposed into sources, sinks, and solenoidal components. This paper shows that the Fisher information, a measure which bounds the precision with which parameters of interest can be measured, similarly obeys a conservation law enabling its visualization in terms of information flow. Remarkably, the Fisher information flow gives distinct insights into wave propagation in complex media and is complementary to more standard analysis methods based on the energy flow. This work raises many interesting questions and opens new possibilities!

Energy and Power requirements for alteration of the refractive index

This is another paper in a series of perspectives on estimating the capabilities and potential limits to the performance of photonic devices using relatively simple classical oscillator models and sharp physical insights. The take home message is that the power required to achieve a given level of optical modulation depends primarily on the interaction time, which depends on the device geometry (e.g. resonator vs travelling wave), without substantial variation among different materials. This suggests that improvements in power efficiency are more likely to come from improvements in fabrication methods and device design, rather than the discovery of some new material with substantially better physical properties.

Quantum Algorithm for Computing Distances Between Subspaces

There's growing evidence that the best place to look for a quantum advantage for classical machine learning will be geometrical or topological problems that have a natural connection to quantum systems. One example is the Betti number problem, which maps to computing the ground state of supersymmetric many-body Hamiltonians. This work shows that computing distances between k-dimensional subspaces of an n-dimensional space can be done exponentially faster using a fault-tolerant quantum computer. The algorithm exploits the ability to efficiently encode subspaces into quantum states combined with quantum signal processing. Subspace distances have to large scale machine learning and computer vision problems, suggesting the asymptotic exponential advantage promised by a fault-tolerant quantum computer could lead to practical speedups.

From structured light to structured waves

Introductory textbooks on quantum mechanics and electromagnetism typically use plane waves, standing waves formed from a superposition of two counter-propagating plane waves, or twin slit interference to illustrate concepts such as the role of boundary conditions, phase velocity, energy transport, interference, and so on. However, these special cases actually too simple to capture the full breadth of wave physics. A nice example of this is shown in Figure 1 of the review article "Singular Optics: Optical Vortices and Polarization Singularities", which was one of my first introductions to the field:

(a) Twin slit interference results in alternating bright and dark fringes, the latter corresponding to lines of vanishing intensity. (b) In three slit interference the intensity only vanishes at points, which correspond to singularities of the field's phase in (c). Panel (d) shows a close up of a pair of oppositely-charged phase singularities.

In the 2000s and early 2010s, studies of the peculiarities of these kinds of structured wave fields were focused on optics, where the availability of devices such as spatial light modulators made phenomena associated with structured waves conveniently accessible. There is now growing interest in structured waves in other wave systems including electron beams, acoustics, water waves, and condensed matter physics. Recent developments in this direction are summarised in a Journal of Optics article, "Roadmap on structured waves," that was just published today. Thanks to Konstantin Bliokh for the invitation to contribute to this article and the mammoth effort of collating and coordinating the work of the 49 coauthors!

The test of time: photonics

As we all (should) know, journal impact factor is a terrible measure of the quality of an individual article. What is more important than where an article is published is whether it has long lasting impact, and the only way to determine this for sure is to wait!

I used Web of Science to look at the most highly cited original research articles in photonics from ten years ago. Here are the top ten:

1. Photonic Floquet topological insulators (2142 citations)

This was the first work to experimentally demonstrate topological edge states in two-dimensional waveguide arrays. It was this (and related works below) which really popularized the now-booming field of topological photonics. While I'm not surprised to see it among the top articles from 2013, I wasn't expecting it to be number one!

2. Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction (1443 citations)

This work falls within two highly active fields: terahertz photonics and metasurfaces. Like other early works on metasurfaces (a few more appear below), the concepts were demonstrated using metallic structures. Ongoing commercialization today was enabled by the development of low loss all-dielectric metasurfaces over the following decade.

3. Photonic topological insulators (1306 citations)

This paper showed theoretically that photonic systems could be used to emulate quantum spin Hall topological phases, by using metamaterials with a judiciously-engineered magneto-electric coupling to emulate a fermionic time-reversal symmetry.

4. Imaging topological edge states in silicon photonics (1113 citations)

Experimental demonstration of a two-dimensional Chern insulator topological phase using ring resonator lattices. This platform is now used extensively for topological laser experiments and exploring other exotic topological tight binding models, including higher order topological phases.

5. Metasurface holograms for visible light (1094 citations)

6. Three-dimensional optical holography using a plasmonic metasurface (976 citations)

These articles published back-to-back in Nature Communications use the metasurface concept to create a hologram using a metallic film with a subwavelength thickness.

7. Wireless sub-THz communication system with high data rate (970 citations)

This work sagely foresaw that viral tiktok memes would drive demand for higher bandwidth mobile data. Higher bandwidth requires higher carrier frequencies, so here the authors demonstrate experimentally wireless data transmission at frequencies approaching the THz band. 

8. Photonic spin Hall effect at metasurfaces (917 citations)

Polarization-controlled beam deflection using a metallic metasurface.

9. Highly efficient gate-tunable photocurrent generation in vertical heterostructures of layered materials (912 citations)

10. Chip-integrated ultrafast graphene photodetector with high responsivity (889 citations)

The top ten is rounded out by two papers demonstrating that 2D materials including graphene can form the basis for highly efficient photodetectors.

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tl;dr: The most highly cited photonics papers in 2013 were on topological photonics, metasurfaces, terahertz and graphene.

Publish or perish

April was an unusually busy month for me for proof-checking, with five papers published:

Moiré Lattice in One-Dimensional Synthetic Frequency Dimension

This work led by collaborators at Shanghai Jiao Tong University analyzes frequency domain synthetic photonic lattices from a transfer matrix perspective, finding that the interplay between an incommensurate time modulation and off-resonant ring modes (not captured by the usual tight binding approximation) can be used to generate flat bands in a simple system of two coupled ring resonators.

Band relaxation triggered by modulational instability in topological photonic lattices

This was a long-delayed follow-up to our work on modulational instability in topological photonic lattices which we initiated during the start of the covid pandemic. Initially we had tried to understand the nonlinear wave dynamics in terms of an effective thermalization process, but it turned out that we could not observe any genuine thermalization within an experimentally-feasible time scale. This paper presents a detailed characterization of the long-lived pre-thermal state that is generated by the modulational instability. While our studies in this area are entirely theoretical / numerical, the dynamics of complex multimode nonlinear optical systems are now starting to be studied in a variety of experimental platforms, reviewed in Nature Physics last year.

Unravelling quantum chaos using persistent homology and Pseudospin-2 in photonic chiral borophene

I posted about these two papers when they were posted to arXiv late last year. The former made it into Physical Review E after a round of revisions. The latter was rejected by Nature Communications before being resubmitted to Photonics Research and accepted for publication after one round of revisions.

Topological data analysis and machine learning

This was a challenging review to prepare, given the need to concisely capture both the surprisingly-long history of applications of topological data analysis to physics (from the early 2000s) and a more recent wave of papers combining TDA with machine learning techniques. While it is far from perfect I hope it can still be a useful anchor for ongoing research in this area.

I'm hoping to have the next set of (now overdue) drafts finished and on arXiv sometime in June. Watch this space!

Conferences in Ukraine

The first two international physics conferences I had the pleasure of attending, way back in September 2011, were held in Ukraine. This was also my first time travelling outside Australia. 

After 30 hours and 4 flights I landed in Kharkiv for the first conference, the International Workshop on Nonlinear Photonics (NLP*2011), held at the Kharkiv National University, located at the picturesque Svobody Square in the city centre.

Svobody Square. The statue of Lenin was torn down in 2014, replaced by a fountain in 2020, and presumably shelled in 2022.

Kharkiv was the home and origin of many great theoretical physicists. Landau and Lifschitz began writing their classic textbook series Course of Theoretical Physics there. Outside the auditorium in which the conference was held, attendees were greeted by an impressive Soviet-era mural.

The entrance to the conference auditorium. The university buildings were destroyed by Russian army shelling in March 2022.

This workshop was my first chance to meet many leading researchers working on nonlinear optics and singular optics, including the late Marat Soskin, who gave a memorable talk on the creation and destruction of topological defects in nematic liquid crystals. One afternoon my then-colleague and future Ignobel Prize Laureate Ivan Maksymov showed me around the city, which was where he had completed his physics studies.

 The following week I attended the Tenth International Conference on Correlation Optics, held at Chernivsti National University on the other side of the country, two flights and a train ride away.

 

The beautiful grounds of the Chernivsti National University, a UNESCO World Heritage Site, constructed between 1864 and 1882.

The Correlation Optics conference series is still going strong, with the next edition planned to be held in hybrid mode in September 2023. As one attendee aptly put it, "No one really knows what correlation optics is precisely, so its themes can continuously adapt to changes in research trends." At the time, one trend was the increasing availability of nanofabrication facilities leading to a transition from micro-optics to nano-optics.

Before dawn on the morning after the end of the conference, what seemed like all of the international attendees converged on the tiny Chernivsti Airport to catch the only international flight running that day. After "checking in" our baggage, we had to wheel it ourselves onto the tarmac to be loaded onto the small jet plane while we considered holding a post-conference session during the flight. 

I travelled onwards to Germany to visit collaborators at the University of Münster. Due to a chance encounter at ICOAM last year, we resumed our collaboration leading to a paper soon to be published in Photonics Research. But that's another story.

arXiv backlog

I've been too busy finishing papers to carefully read the arXiv postings that looked interesting enough to download. Here is what's caught my attention:

• Textbook myths about early atomic models. The plum pudding model is fake news!
• Topological packing statistics distinguish living and non-living matter. This work develops a method to distinguish different types of point clouds using their distributions of local structural features. Seems like a promising new alternative to persistent homology.
• Strong intensity noise condensation using nonlinear dispersive loss in semiconductor lasers. A follow-up to the group's Fock state laser paper, which seems to be still under review.
• Optical isolation by temporal modulation: size, frequency, and power constraints. The third in a series of perspective-style anti-hype articles by Jacob Khurgin (previous ones on nonlinear optics and high refractive index materials), this one targeting the booming field of temporal metamaterials. Take a look at the entertaining introduction!
• A (simple) classical algorithm for estimating Betti numbers. An efficient path integral Monte Carlo algorithm constrains the region where quantum algorithms for Betti number estimation might provide a quantum advantage.
• Matters Arising: Time-reversal-based quantum metrology with
  many-body entangled states. Is entanglement-enhanced metrology a sham?
  

Recently in glossy journals

Engineering topological states in atom-based semiconductor quantum dots

Very nice work implementing the SSH / Hubbard model using quantum dots, forming a platform for studying the interplay between band topology and quantum interactions. The accompanying press release from the spin-off company (Silicon Quantum Computing) is unfortunately pure hype, however. This is not a molecular simulation - it is an implementation of a model. A neat example of analogue quantum simulation, but this is not a general-purpose reprogrammable integrated quantum circuit. 

Quantum advantage in learning from experiments

 

An exponential quantum advantage for quantum machine learning! The fine print is that the advantage is for learning from quantum data.

An on-chip photonic deep neural network for image classification

 

A photonic neural network that uses optoelectronics to implement nonlinear activation functions. "Direct, clock-less processing of optical data eliminates analogue-to-digital conversion and the requirement for a large memory module, allowing faster and more energy efficient neural networks for the next generations of deep learning systems."

ICOAM 2022 - live stream

The 6th International Conference on Optical Angular Momentum is running this week, featuring an impressive lineup of invited speakers including Sir Michael Berry (of geometric/Berry phase fame), who will be giving a public lecture on Friday.

The organisers have kindly provided a zoom livestream of the sessions so those who unable to attend in person can still watch the talks.

I will be giving my talk remotely on Wednesday morning. Here are my slides. My aim is to provide an accessible introduction to topological data analysis and offer some potentially-interesting directions for future research.

Unfortunately the review article we've been working on is not yet finished - I was hoping to have an arXiv link ready in time for my talk.

ArXiv catchup

 Deadlines abound so I haven't been following arXiv postings that closely. Some papers of note from the last few weeks:

Dielectric Mie Voids: Confining Light in Air. The authors demonstrate a metasurface analogue of photonic crystal fibers. A neat idea that could enable ultra-low loss flat optics.

Beyond Barren Plateaus: Quantum Variational Algorithms Are Swamped With Traps. "We prove that a wide class of variational quantum models -- which are shallow, and exhibit no barren plateus -- have only a superpolynomially small fraction of local minima within any constant energy from the global minimum, rendering these models untrainable if no good initial guess of the optimal parameters is known." End-users beware: this work adds to evidence that variational quantum algorithms may not be scalable up to useful problem sizes.

Persistent homology analysis of a generalized Aubry-André-Harper model. Our own recent work on applying topological data analysis to study localization transitions in photonic lattices. What we found particularly exciting is that this is our first example of TDA discovering an unanticipated effect - the emergence of disorder-free eigenstates for certain model parameters.

Experimentally realized in situ backpropagation for deep learning in nanophotonic neural networks. Spoiler: the nonlinear activation functions are implemented digitally. Incorporating useful nonlinear optical response into integrated photonic neural networks while outperforming conventional electronic circuits is a big challenge. One promising potential solution is to employ measurement+based nonlinearities + feedforward.

Fabrication-Robust Silicon Photonic Devices in Standard Sub-Micron Silicon-on-Insulator Processes. With the continuing interest (and hype) in topological photonics it's important to keep in mind conventional non-topological approaches for designing photonic devices. For example, wider waveguides are more robust to fabrication imperfections, at the expense of being multimode. This is a problem, because waveguide bends will then induce coupling between the different guided modes, corrupting signals. Here the authors demonstrate how a clever choice of bending profile can suppress unwanted inter-modal coupling while not increasing the device footprint.

Breakdown of quantization in nonlinear Thouless pumping. Quantized adiabatic pumping of solitons attracted a lot of interest last year. This theoretical analysis shows how quantization can break down for moderate nonlinearity strengths due to the emergence of loops in the adiabatic energy spectrum, leading to dead-ends in the adiabatic path resulting in sudden non-adiabatic transitions of the soliton.

The quest for high refractive index materials

Expanding the Photonic Palette: Exploring High Index Materials

An entertaining and provocative article on why we should care about broadening the palette of high refractive index materials and where we might find them.

 

While graphene is singled out as the "king" of exotic materials with limited practical applications in optics and photonics, it is worth emphasizing that the huge interest in graphene has been a source of funding, techniques, and inspiration for studies of related quasi-2D materials. Among these, transition metal dichalcogenides seem quite promising for photonics applications including high index metasurfaces.

Thanks to nanoscale views for making me aware of this article.

Singapore Quantum Jobs

Qove Laboratory seeks postdoctoal fellows and PhD students: designing and building quantum technologies for quantum networks based on superconducting circuits, rare-earth ions, and integrated photonics. This is a newly-funded NRF Fellowship project with funding for 5 years.

Senior Research Fellow / Research Fellow positions at the School of Electrical & Electronic Engineering, Nanyang Technological University on development finite difference time domain methods for coupled electromagnetic and quantum systems. I guess this is related to the new Quantum Science & Engineering Centre announced at the end of last year.

Research Director in Quantum Computing and Quantum Communication at JPMorgan Chase’s Future Lab for Applied Research and Engineering. They are after someone with at least 12 years' relevant experience to investigate applications to finance, AI, optimization, and quantum key distribution.

PRX Quantum seeks an Associate Editor. The part-time Associate Editor is welcomed to maintain their current position–be it in academia, industry, or others, while contributing to PRX Quantum. They should also hold high standards for peer review, and be committed to building an exceptional reputation for the journal. Researchers from anywhere in the world can apply. This is a great opportunity!