Session QQ01: V: Hong Kong Satellite Meeting

Configurable quantum reservoir computing for multi-task learning

Amidst the rapid advancements in experimental technology, noise-intermediate-scale quantum (NISQ) devices have become increasingly programmable, offering versatile opportunities to leverage quantum computational advantage. We explore the intricate dynamics of programmable NISQ devices for quantum reservoir computing. Using a genetic algorithm to configure the quantum reservoir dynamics, we systematically enhance the learning performance. Remarkably, a single configured quantum reservoir can simultaneously learn multiple tasks, including a synthetic oscillatory network of transcriptional regulators, chaotic motifs in gene regulatory networks, and the fractional-order Chua's circuit. Our configured quantum reservoir computing yields highly precise predictions for these learning tasks, outperforming classical reservoir computing. We also test the configured quantum reservoir computing in foreign exchange (FX) market applications and demonstrate its capability to capture the stochastic evolution of the exchange rates with significantly greater accuracy than classical reservoir computing approaches. Through comparison with classical reservoir computing, we highlight the unique role of quantum coherence in the quantum reservoir, which underpins its exceptional learning performance. Our findings suggest the exciting potential of configured quantum reservoir computing for exploiting the quantum computation power of NISQ devices in developing artificial general intelligence.   

Quantum phase synchronization via exciton-vibrational energy dissipation sustains long-lived coherence in photosynthetic antennas

Coherent energy transfer is a highly efficient energy transfer pathway in photosynthesis. Matching of long-lived quantum coherence to the time scale of energy transfer is a prerequisite. In contrast to short-lived electronic coherence, the presence of exciton-vibrational coherence in photosynthetic systems5,6 can account for the observed long-lasting quantum coherence. However, uncovering the mechanism of such coherence within a biological environment is challenging because of the presence of noise typically encountered at room temperature. This paper presents conclusive evidence of the existence of long-lasting electronic vibronic coherence in the allophycocyanin trimer, in which pigment pairs behave as excitonic dimers with weak exciton-vibrational coupling. Employing ultrafast two-dimensional electronic spectroscopy, our study demonstrates an extension of the exciton-vibrational coherence time within the trimer compared with the isolated pigments. The prolonged quantum coherences were identified as arising from the quantum phase synchronization of the resonant vibrational collective modes for the pigment pair. The anti-symmetric resonant collective modes undergo fast energy dissipation when coupled to the delocalized electronic states of fast dephasing, while the decoupled symmetric resonant collective modes survive. The nuclear motion of the symmetric resonant collective modes leads to the correlated energy fluctuation of the excitonic energy levels on the two individual pigment molecules in the dimer, resulting in significantly lowered energy dissipation and supporting long-lasting quantum coherences. The presence of the quantum phase synchronization was confirmed by two experimental indicators consistent with the expectation, i.e., about half reduction in the vibrational intensities of the resonant modes and their almost zero intensites in dynamical Stokes shift spectrum. This paper provides direct evidence revealing how biological systems effectively employ a quantum synchronization strategy to uphold persistent coherences, and our findings pave the way for protecting coherences against the noisy environment in quantum biology.   

Majorana Zero Modes Induced by the Meissner Effect at Small Magnetic Field

One key difficulty in realizing Majorana zero modes (MZMs) is the required high magnetic field, which causes serious issues, e.g., shrinks the superconducting gap, reduces topological region, and weakens their robustness against disorders. In this talk, we propose that the Meissner effect can bring the topological superconducting phase to a superconductor/topologicalinsulator/superconductor (SC/TI/SC) hybrid system. Remarkably, the required magnetic field strength (<10 mT) to support MZMs has been reduced by several orders of magnitude compared to that (>0.5 T) in the previous schemes. Tuning the phase difference between the top and bottom superconductors can control the number and position of the MZMs. In addition, we account for the electrostatic potential in the superconductor/topological-insulator (SC/TI) interface through the self-consistent Schroedinger-Poisson calculation, which shows the experimental accessibility of our proposal. Our proposal only needs a small magnetic field of less than 10 mT and is robust against the chemical potential fluctuation, which makes the SC/TI/SC hybrid an ideal Majorana platform.   

Crystal thermal transport in altermagnets

In this talk, I will demonstrate the emergence of a pronounced thermal transport in the recently discovered class of magnetic materials – altermagnets [1]. From symmetry arguments and first principles calculations performed for the showcase altermagnet, RuO2, we uncover that crystal Nernst and crystal thermal Hall effects in this material are very large and strongly anisotropic with respect to the Neel vector. We find the large crystal thermal transport to originate from three sources of Berry’s curvature in momentum space: the Weyl fermions due to crossings between well-separated bands, the strong spin-flip pseudo-nodal surfaces, and the weak spin-flip ladder transitions, defined by transitions among very weakly spin-split states of similar dispersion crossing the Fermi surface. Moreover, we reveal that the anomalous thermal and electrical transport coefficients in RuO2 are linked by an extended Wiedemann-Franz law in a temperature range much wider than expected for conventional magnets. Our results suggest that altermagnets may assume a leading role in realizing concepts in spin caloritronics not achievable with ferromagnets or antiferromagnets.

[1]    Xiaodong Zhou, Wanxiang Feng∗, Run-Wu Zhang, Libor Smejkal, Jairo Sinova, Yuriy Mokrousov, and Yugui Yao*, arXiv: 2305.01410, Phys. Rev. Lett. (Editors’ Suggestion, in press)   

Splitting between k-points for Periodic Systems Quantum Simulation

Quantum computation of periodic systems suffers the problem of the linear scaling of qubit requirement with respect to the number of sampled k-points. As a result, it may not be practical to include N_k N_orb qubits for a VQE calculation, where N_k is the number of k-points and N_orb is the number of spin-orbitals per k-point. Here, we propose an approximate ansatz for VQE that splits the single N_k N_orb circuit to multiple circuits with 4N_orb qubits each. To allow for such splitting, the ansatz should be chosen such that the parity of the occupied states of each k-points are fixed to either even or odd. This fixes the parity such that only one k-point needs to be included to calculate the expectation value of a one-body term and at most four k-points needs to be included to calculate the expectation value of a two-body term. Next, the Hamiltonian is partitioned into multiple circuits with each circuit containing 4 k-points. Finally, the cost function is calculated as the sum of the expectation value of each circuit. VQE calculations will be carried out using an exact simulator and the accuracy and efficiency of this ansatz will be compared against classical quantum chemical calculations.   

The triadic Hall effect

As an elementary phenomenon, the anomalous Hall effect is indispensable for probing the complexities of fundamental physics, highlighting the spin-orbit coupling and band topology across magnetic materials. The Onsager's reciprocal relation dictates that the anomalous Hall conductivity is an odd function of magnetization, leading to two longstanding impressions that the anomalous Hall conductivity is linear in magnetization and that the magnetization is perpendicular to the Hall-deflection plane. In this talk, we show that the third-order-in-magnetization term can contribute to the anomalous Hall conductivity, which we refer to as the triadic Hall effect. A straightforward and important consequence is that the magnetization can lie within the Hall-deflection plane, leading to the in-plane anomalous Hall effect. Such an effect is purely due to the triadic contribution, as also confirmed by experiments. We further illuminate the role of the spin-orbital coupling in the triadic Hall effect. Our work reveals the intricate relation between the crystal symmetry and band geometry and paves the way of investigating other triadic contributions to various Hall effect.   

Quantum signal filter based on quantum correlation

TBA   

Optimal Control of Flying Qubits

The transmission of flying qubits carried by itinerant photons is ubiquitous in quantum communication networks. In addition to their logical states, the temporal/frequency profiles of flying qubits must also be tailored into proper shapes to match remote receiver qubits. In this talk, we report a general framework for optimal control of flying qubits. The framework is based on quantum stochastic differential equations (QSDEs) that describe the flying qubit input-output relations actuated by a standing quantum system (e.g., a superconducting qubit or quantum dot). Under the continuous time-ordered photon-number basis, the infinite-dimensional QSDE is reduced to a system of low-dimensional deterministic ordinary differential equations for the non-unitary state evolution of the standing quantum system, and the outgoing flying qubit states can be calculated in the form of randomly occurring quantum jumps. This makes it possible to analyze general cases when the number of excitations is not reserved. The proposed framework lays the foundation for the design of flying-qubit control systems from an optimal control point of view, within which advanced optimal control techniques can be incorporated for practical applications. Some examples, such as the generation, catching and steering of flying photons by two- or three-level artificial atoms, are studied.

[1]  Wenlong Li,  Guofeng Zhang,  and Re-bing Wu, On the control of flying qubits, Automatica, 143:110338, 2022. [2]  Wenlong Li, Xue Dong, Guofeng Zhang, and Re-bing Wu, Flying-qubit control via a three-level atom with tunable waveguide,  Physical Review B, 106:134305, 2022. [3]  Xue Dong, Guofeng Zhang, and Re-bing Wu,  Optimal control of flying-qubit generation, 2023.   

Incompatibility measures in multi-parameter quantum estimation under hierarchical quantum measurements

The incompatibility of the optimal measurements for the estimation of different parameters constraints the achievable precisions in multi-parameter quantum estimation. Understanding the tradeoff induced by such incompatibility is thus a central topic in quantum metrology. Here we provide an approach to study the incompatibility in terms of information geometry under general p-local measurement, which are the measurements performed collectively on at most p copies of quantum states. We demonstrate the power of the approach by present a hierarchy of analytical tradeoff relations induced by the incompatibility.   

Multidimensional High Harmonic Spectroscopy

We will discuss a novel method for monitoring electronic coherences using ultrafast spectroscopy. This method is based on the time-domain high-order harmonic spectroscopy where a coherent superposition of the electronic states is first prepared by the strong optical laser pulse using a three-step mechanism. The coherent dynamics can then be probed by the higher order harmonics generated by the delayed probe pulse. A a semi-perturbative model based on the Liouville space superoperator approach is developed for the bookkeeping of the different orders of the nonlinear response for the high-order harmonic generation using multiple pulses. Coherence between bound electronic states is monitored in the harmonic spectra from both the first and the second order responses and investigate nonadiabatic dynamics of conical intersections and avoided crossings. High harmonic echo allows to detect both the population transfer and coherent dephasing dynamics in multidimensional spectra. We are able to selectively produce a delayed harmonic echo originated from partial rephasing of a given pair of bound states which is not possible in traditional echo experiments that rely exclusively on full rephasing of electronic coherence. Furthermore, the nature of the multi-wave mixing in high harmonic regime allow to modify the statistics of light and give rise of quantum squeezing between higher harmonics suitable for higher signal-to-noise ratio measurements of electronic properties in multi-eV range.