V. Aggarwal, V. Bansal, D. Peddireddy, and Z. Jacob, “Tensor Ring Parametrized Variational Quantum Circuits for Large Scale Quantum Machine Learning”, arXiv, 2022.

Quantum Machine Learning (QML) is an emerging research area advocating the use of quantum computing for advancement in machine learning. Since the discovery of the capability of Parametrized Variational Quantum Circuits (VQC) to replace Artificial Neural Networks, they have been widely adopted to different tasks in Quantum Machine Learning. However, despite their potential to outperform neural networks, VQCs are limited to small scale applications given the challenges in scalability of quantum circuits. To address this shortcoming, we propose an algorithm that compresses the quantum state within the circuit using a tensor ring representation. Using the input qubit state in the tensor ring representation, single qubit gates maintain the tensor ring representation. However, the same is not true for two qubit gates in general, where an approximation is used to have the output as a tensor ring representation. Using this approximation, the storage and computational time increases linearly in the number of qubits and number of layers, as compared to the exponential increase with exact simulation algorithms. This approximation is used to implement the tensor ring VQC. The training of the parameters of tensor ring VQC is performed using a gradient descent based algorithm, where efficient approaches for backpropagation are used. The proposed approach is evaluated on two datasets: Iris and MNIST for the classification task to show the improved accuracy using more number of qubits. We achieve a test accuracy of 83.33% on Iris dataset and a maximum of 99.30% and 76.31% on binary and ternary classification of MNIST dataset using various circuit architectures. The results from the IRIS dataset outperform the results on VQC implemented on Qiskit, and being scalable, demonstrates the potential for VQCs to be used for large scale Quantum Machine Learning applications.


Z. Xu, X. Gao, J. Bang, Z. Jacob, and T. Li, “Non-reciprocal energy transfer through the Casimir effect”, Nature Nanotechnology, 2021, doi: 10.1038/s41565-021-01026-8.

One of the fundamental predictions of quantum mechanics is the occurrence of random fluctuations in a vacuum caused by zero-point energy. Remarkably, quantum electromagnetic fluctuations can induce a measurable force between neutral objects, known as the Casimir effect1, and it has been studied both theoretically2,3 and experimentally4-9. The Casimir effect can dominate the interaction between microstructures at small separations and is essential for micro-and nanotechnologies10,11. It has been utilized to realize nonlinear oscillation12, quantum trapping13, phonon transfer14,15, and dissipation dilution16. However, a non-reciprocal device based on quantum vacuum fluctuations remains an unexplored frontier. Here we report quantum-vacuum-mediated non-reciprocal energy transfer between two micromechanical oscillators. We parametrically modulate the Casimir interaction to realize a strong coupling between the two oscillators with different resonant frequencies. We engineer the system's spectrum such that it possesses an exceptional point17-20 in the parameter space and explore the asymmetric topological structure in its vicinity. By dynamically changing the parameters near the exceptional point and utilizing the non-adiabaticity of the process, we achieve non-reciprocal energy transfer between the two oscillators with high contrast. Our work demonstrates a scheme that employs quantum vacuum fluctuations to regulate energy transfer at the nanoscale and may enable functional Casimir devices in the future.

M. A. Javidian, V. Aggarwal, and Z. Jacob, “Identification of Latent Graphs: A Quantum Entropic Approach”, NeurIPS 2021, 2021.

Quantum causality is an emerging field of study which has the potential to greatly advance our understanding of quantum systems. In this paper, we put forth a new theoretical framework for merging quantum information science and causal inference by exploiting entropic principles. For this purpose, we leverage the tradeoff between the entropy of hidden cause and conditional mutual information of observed variables to develop a scalable algorithmic approach for inferring causality in the presence of latent confounders (common causes) in quantum systems. As an application, we consider a system of three entangled qubits and transmit the second and third qubits over separate noisy quantum channels. In this model, we validate that the first qubit is a latent confounder and the common cause of the second and third qubits. In contrast, when two entangled qubits are prepared and one of them is sent over a noisy channel, there is no common confounder. We also demonstrate that the proposed approach outperforms the results of classical causal inference for Tubingen database when the variables are classical by exploiting quantum dependence between variables through density matrices rather than joint probability distributions. Thus, the proposed approach unifies classical and quantum causal inference in a principled way.

Y. Wang, C. Khandekar, X. Gao, T. Li, D. Jiao, and Z. Jacob, “Broadband circularly polarized thermal radiation from magnetic Weyl semimetals”, Optical Materials Express, vol. 11, no. 11. pp. 3880–3895, 2021.

We numerically demonstrate that a planar slab made of magnetic Weyl semimetal (a class of topological materials) can emit high-purity circularly polarized (CP) thermal radiation over a broad mid- and long-wave infrared wavelength range for a significant portion of its emission solid angle. This effect fundamentally arises from the strong infrared gyrotropy or nonreciprocity of these materials, which primarily depends on the momentum separation between Weyl nodes in the band structure. We clarify the dependence of this effect on the underlying physical parameters and highlight that the spectral bandwidth of CP thermal emission increases with increasing momentum separation between the Weyl nodes. We also demonstrate, using the recently developed thermal discrete dipole approximation (TDDA) computational method, that finite-size bodies of magnetic Weyl semimetals can emit spectrally broadband CP thermal light, albeit over smaller portion of the emission solid angle compared to the planar slabs. Our work identifies unique fundamental and technological prospects of magnetic Weyl semimetals for engineering thermal radiation and designing efficient CP light sources.

A. K. Boddeti et al., “Long-Range Dipole-Dipole Interactions in a Plasmonic Lattice”, Nano letters, 1 vol., no. 22, pp. 22–28, 2021.

Spontaneous emission of quantum emitters can be enhanced by increasing the local density of optical states, whereas engineering dipole–dipole interactions requires modifying the two-point spectral density function. Here, we experimentally demonstrate long-range dipole–dipole interactions (DDIs) mediated by surface lattice resonances in a plasmonic nanoparticle lattice. Using angle-resolved spectral measurements and fluorescence lifetime studies, we show that unique nanophotonic modes mediate long-range DDI between donor and acceptor molecules. We observe significant and persistent DDI strengths for a range of densities that map to ∼800 nm mean nearest-neighbor separation distance between donor and acceptor dipoles, a factor of ∼100 larger than free space. Our results pave the way to engineer and control long-range DDIs between an ensemble of emitters at room temperature.

F. Kalhor, L.-P. Yang, L. Bauer, and Z. Jacob, “Quantum sensing of photonic spin density using a single spin qubit”, Physical Review Research, vol. 3. p. 043007, 2021.

Nitrogen-vacancy (NV) centers in diamond have emerged as promising room-temperature quantum sensors for probing condensed matter phenomena ranging from spin liquids, two-dimensional (2D) magnetic materials, and magnons to hydrodynamic flow of current. Here we propose and demonstrate that the nitrogen-vacancy center in diamond can be used as a quantum sensor for detecting the photonic spin density, the spatial distribution of light’s spin angular momentum. We exploit a single spin qubit on an atomic force microscope tip to probe the spinning field of an incident Gaussian light beam. The spinning field of light induces an effective static magnetic field in the single spin qubit probe. We perform room-temperature sensing using Bloch sphere operations driven by a microwave field (XY8 protocol). This nanoscale quantum magnetometer can measure the local polarization of light in ultra-sub-wavelength volumes. We also put forth a rigorous theory of the experimentally measured phase change using the NV center Hamiltonian and perturbation theory involving only virtual photon transitions. The direct detection of the photonic spin density at the nanoscale using NV centers in diamond opens interesting quantum metrological avenues for studying exotic phases of photons, nanoscale properties of structured light as well as future on-chip applications in spin quantum electrodynamics.

L.-P. Yang and Z. Jacob, “Non-classical photonic spin texture of quantum structured light”, Communications Physics, vol. 4, 221, 2021.

Classical structured light with controlled polarization and orbital angular momentum (OAM)of electromagnetic waves has varied applications in optical trapping, bio-sensing, optical communications, and quantum simulations. However, quantum noise and photon statistics of three-dimensional photonic angular momentum are relatively less explored. Here, we develop a quantum framework and put forth the concept of quantum structured light for space-time wavepackets at the single-photon level. Our work deals with three-dimensional angular momentum observables for twisted quantum pulses beyond scalar-field theory as well as the paraxial approximation. We show that the spin density generates modulated helical texture and exhibits distinct photon statistics for Fock-state vs. coherent-state twisted pulses. We introduce the quantum correlator of photon spin density to characterize nonlocal spin noise providing a rigorous parallel with electronic spin noise. Our work can lead to quantum spin-OAM physics in twisted single-photon pulses and opens explorations for phases of light with long-range spin order.

F. Khosravi, M. M. Sonner, and Z. Jacob, “Ultrafast electron cycloids driven by the transverse spin of a surface acoustic wave”, Science Advances, vol. 7, no. 31, 2021.

Spin-momentum locking is a universal wave phenomenon promising for applications in electronics and photonics. In acoustics, Lord Rayleigh showed that surface acoustic waves exhibit a characteristic elliptical particle motion strikingly similar to spin-momentum locking. Although these waves have become one of the few phononic technologies of industrial relevance, the observation of their transverse spin remained an open challenge. Here, we observe the full spin dynamics by detecting ultrafast electron cycloids driven by the gyrating electric field produced by a surface acoustic wave propagating on a slab of lithium niobate. A tubular quantum well wrapped around a nanowire serves as an ultrafast sensor tracking the full cyclic motion of electrons. Our acousto-optoelectrical approach opens previously unknown directions in the merged fields of nanoacoustics, nanophotonics, and nanoelectronics for future exploration.

F. Bao, H. Choi, Z. Jacob, and V. Aggarwal, “Quantum-accelerated imaging of N stars”, Optics Letters, vol. 46, no. 13, pp. 3045–3048, 2021.

Imaging point sources with low angular separation near or below the Rayleigh criterion are important in astronomy, e.g., in the search for habitable exoplanets near stars. However, the measurement time required to resolve stars in the sub-Rayleigh region via traditional direct imaging is usually prohibitive. Here we propose quantum-accelerated imaging (QAI) to significantly reduce the measurement time using an information-theoretic approach. QAI achieves quantum acceleration by adaptively learning optimal measurements from data to maximize Fisher information per detected photon. Our approach can be implemented experimentally by linear-projection instruments followed by single-photon detectors. We estimate the position, brightness, and the number of unknown stars 10∼100 times faster than direct imaging with the same aperture. QAI is scalable to a large number of incoherent point sources and can find widespread applicability beyond astronomy to high-speed imaging, fluorescence microscopy, and efficient optical read-out of qubits.

M. van Niekerk et al., “Two-dimensional extreme skin depth engineering for CMOS photonics”, J. Opt. Soc. Am. B, vol. 38, no. 4, pp. 1307–1316, 2021.

Extreme skin depth engineering (e-skid) can be applied to integratedphotonics to manipulate the evanescent field of a waveguide. Here wedemonstrate that e-skid can be implemented in two directions in order todeterministically engineer the evanescent wave allowing for denseintegration with enhanced functionalities. In particular, by increasingthe skin depth, we enable the creation of two-dimensional (2D) e-skiddirectional couplers with large gaps and operational bandwidth. Here weexperimentally validate 2D e-skid for integrated photonics in acomplementary metal--oxide semiconductor (CMOS) photonics foundry anddemonstrate strong coupling with a gap of 1.44 {\textmu}m.