arXiv subjects

Xiaokai Hou

Publications and source records attributed to Xiaokai Hou.

11 recordsLinked to original sources

arXiv2025

An effcient variational quantum Korkin-Zolotarev algorithm for solving shortest vector problems

Noisy intermediate-scale quantum cryptanalysis focuses on the capability of near-term quantum devices to solve the mathematical problems underlying cryptography, and serves as a cornerstone for the design of post-quantum cryptographic algorithms. For the shortest vector problem (SVP), which is one of the computationally hard problems in lattice-based cryptography, existing near-term quantum cryptanalysis algorithms map the problem onto a fully-connected quantum Ising Hamiltonian, and obtain the solution by optimizing for the first excited state. However, as the quantum system scales with the problem size, determining the first excited state becomes intractable due to the exponentially increased complexity for large-scale SVP instances. In this paper, we propose a variational quantum Korkin-Zolotarev (VQKZ) algorithm, which significantly reduces the qubit requirement for solving the SVP. Specifically, by transforming the original SVP into a series of subproblems on projected sublattices, the proposed VQKZ algorithm enables near-term quantum devices to solve SVP instances with lattice dimensions 61.39% larger than those solvable by previous methods. Furthermore, numerical simulations demonstrate that the proposed VQKZ algorithm can significantly outperform existing methods in terms of the length of solution vectors.

quant-ph↗

arXiv2023

Narrow-linewidth 852-nm DBR-LD with self-injection lock based on high-fineness optical cavity filtering

Narrow-linewidth lasers have high spectral purity, long coherent length and low phase noise, so they have important applications in cold atom physics, quantum communication, quantum information processing and optical precision measurement. We inject transmitted laser from a narrow-linewidth (15 kHz) flat-concave Fabry-Perot (F-P) cavity made of ultra-low expansion (ULE) optical glass into 852-nm distributed-Bragg-reflector type laser diode (DBR-LD), of which the comprehensive linewidth of 1.67 MHz for the free running case. With the increase of self-injection power, the laser linewidth is gradually narrowed, and the inject-locking current range is gradually increased. The narrowest linewidth measured by the delayed frequency-shifted self-heterodyne (DFSSH) method is 263 Hz. Moreover, to characterize the laser phase noise, we use a detuned F-P cavity to measure the conversion signal from laser phase noise to intensity noise for both the free running case and self-injection lock case. Laser phase noise for the self-injection lock case is significantly suppressed in the analysis frequency range of 0.1-10 MHz compared to the free running case. Especially, the phase noise is suppressed by more than 30dB at the analysis frequency of 100 kHz.

physics.optics↗

arXiv2023

A sequentially generated variational quantum circuit with polynomial complexity

Variational quantum algorithms have been a promising candidate to utilize near-term quantum devices to solve real-world problems. The powerfulness of variational quantum algorithms is ultimately determined by the expressiveness of the underlying quantum circuit ansatz for a given problem. In this work, we propose a sequentially generated circuit ansatz, which naturally adapts to 1D, 2D, 3D quantum many-body problems. Specifically, in 1D our ansatz can efficiently generate any matrix product states with a fixed bond dimension, while in 2D our ansatz generates the string-bond states. As applications, we demonstrate that our ansatz can be used to accurately reconstruct unknown pure and mixed quantum states which can be represented as matrix product states, and that our ansatz is more efficient compared to several alternatives in finding the ground states of some prototypical quantum many-body systems as well as quantum chemistry systems, in terms of the number of quantum gate operations.

quant-ph↗

arXiv2023

Ensemble-learning error mitigation for variational quantum shallow-circuit classifiers

Classification is one of the main applications of supervised learning. Recent advancement in developing quantum computers has opened a new possibility for machine learning on such machines. Due to the noisy performance of near-term quantum computers, error mitigation techniques are essential for extracting meaningful data from noisy raw experimental measurements. Here, we propose two ensemble-learning error mitigation methods, namely bootstrap aggregating and adaptive boosting, which can significantly enhance the performance of variational quantum classifiers for both classical and quantum datasets. The idea is to combine several weak classifiers, each implemented on a shallow noisy quantum circuit, to make a strong one with high accuracy. While both of our protocols substantially outperform error-mitigated primitive classifiers, the adaptive boosting shows better performance than the bootstrap aggregating. The protocols have been exemplified for classical handwriting digits as well as quantum phase discrimination of a symmetry-protected topological Hamiltonian, in which we observe a significant improvement in accuracy. Our ensemble-learning methods provide a systematic way of utilising shallow circuits to solve complex classification problems.

quant-ph↗

arXiv2023

Autler-Townes splitting in the trap-loss fluorescence spectroscopy due to single-step direct Rydberg excitation of cesium cold atomic ensemble

We experimentally investigate trap-loss spectra of the cesium 6S1/2(F=4)-71P3/2 Rydberg state by combining the cesium atomic magneto-optical trap with the narrow-linewidth, continuously-tunable 318.6 nm ultraviolet laser. That is, the atoms in the magneto-optical trap are excited to the Rydberg state due to the ultraviolet laser single-step Rydberg excitation, which leads to the reduction of atomic fluorescence. Based on the trap-loss spectroscopy technology, the Autler-Townes splitting due to strong cooling laser is observed, and the parameter dependence of the AT splitting interval of trap-loss spectroscopy is investigated. Furthermore, the effective temperature of cold atoms is measured by means of simplified time-of-flight fluorescence imaging. In addition, closed-loop positive feedback power stabilization of 318.6 nm ultraviolet laser is carried out. This lays the foundation for further experimental research related to Rydberg atoms using ultraviolet lasers, which is of great significance for the development of quantum computing and quantum information fields.

physics.atom-ph↗

arXiv2023

Suppression of laser beam's polarization and intensity fluctuation via a Mach-Zehnder interferometer with proper feedback

Long ground-Rydberg coherence lifetime is interesting for implementing high-fidelity quantum logic gates, many-body physics, and other quantum information protocols. However, the potential well formed by a conventional far-off-resonance red-detuned optical-dipole trap that is attractive for ground-state cold atoms is usually repulsive for Rydberg atoms, which will result in the rapid loss of atoms and low repetition rate of the experimental sequence. Moreover, the coherence time will be sharply shortened due to the residual thermal motion of cold atoms. These issues can be addressed by a one-dimensional magic lattice trap, which can form a deeper potential trap than the traveling wave optical dipole trap when the output power is limited. In addition, these common techniques for atomic confinement generally have certain requirements for the polarization and intensity stability of the laser. Here, we demonstrated a method to suppress both the polarization drift and power fluctuation only based on the phase management of the Mach-Zehnder interferometer for a one-dimensional magic lattice trap. With the combination of three wave plates and the interferometer, we used the instrument to collect data in the time domain, analyzed the fluctuation of laser intensity, and calculated the noise power spectral density. We found that the total intensity fluctuation comprising laser power fluctuation and polarization drift was significantly suppressed, and the noise power spectral density after closed-loop locking with a typical bandwidth of 1-3000 Hz was significantly lower than that under the free running of the laser system. Typically, at 1000 Hz, the noise power spectral density after locking was about 10 dB lower than that under the free running of a master oscillator power amplifier system.The intensity-polarization control technique provides potential applications.

physics.optics↗

arXiv2022

A duplication-free quantum neural network for universal approximation

The universality of a quantum neural network refers to its ability to approximate arbitrary functions and is a theoretical guarantee for its effectiveness. A non-universal neural network could fail in completing the machine learning task. One proposal for universality is to encode the quantum data into identical copies of a tensor product, but this will substantially increase the system size and the circuit complexity. To address this problem, we propose a simple design of a duplication-free quantum neural network whose universality can be rigorously proved. Compared with other established proposals, our model requires significantly fewer qubits and a shallower circuit, substantially lowering the resource overhead for implementation. It is also more robust against noise and easier to implement on a near-term device. Simulations show that our model can solve a broad range of classical and quantum learning problems, demonstrating its broad application potential.

quant-ph↗

arXiv2022

Quantum spectral clustering algorithm for unsupervised learning

Clustering is one of the most crucial problems in unsupervised learning, and the well-known $k$-means clustering algorithm has been shown to be implementable on a quantum computer with a significant speedup. However, many clustering problems cannot be solved by $k$-means, and a powerful method called spectral clustering is introduced to solve these problems. In this work, we propose a circuit design to implement spectral clustering on a quantum processor with a substantial speedup, by initializing the processor into a maximally entangled state and encoding the data information into an efficiently-simulatable Hamiltonian. Compared with the established quantum $k$-means algorithms, our method does not require a quantum random access memory or a quantum adiabatic process. It relies on an appropriate embedding of quantum phase estimation into Grover's search to gain the quantum speedup. Simulations demonstrate that our method is effective in solving clustering problems and will serve as an important supplement to quantum $k$-means for unsupervised learning.

quant-ph↗

arXiv2022

Robust resource-efficient quantum variational ansatz through evolutionary algorithm

Variational quantum algorithms (VQAs) are promising methods to demonstrate quantum advantage on near-term devices as the required resources are divided between a quantum simulator and a classical optimizer. As such, designing a VQA which is resource-efficient and robust against noise is a key factor to achieve potential advantage with the existing noisy quantum simulators. It turns out that a fixed VQA circuit design, such as the widely-used hardware efficient ansatz, is not necessarily robust against imperfections. In this work, we propose a genome-length-adjustable evolutionary algorithm to design a robust VQA circuit that is optimized over variations of both circuit ansatz and gate parameters, without any prior assumptions on circuit structure or depth. Remarkably, our method not only generates a noise-effect-minimized circuit with shallow depth, but also accelerates the classical optimization by substantially reducing the number of parameters. In this regard, the optimized circuit is far more resource-efficient with respect to both quantum and classical resources. As applications, based on two typical error models in VQA, we apply our method to calculate the ground energy of the hydrogen and the water molecules as well as the Heisenberg model. Simulations suggest that compared with conventional hardware efficient ansatz, our circuit-structure-tunable method can generate circuits apparently more robust against both coherent and incoherent noise, and hence is more likely to be implemented on near-term devices.

quant-ph↗

arXiv2021

A universal duplication-free quantum neural network

Universality of neural networks describes the ability to approximate arbitrary function, and is a key ingredient to keep the method effective. The established models for universal quantum neural networks(QNN), however, require the preparation of multiple copies of the same quantum state to generate the nonlinearity, with the copy number increasing significantly for highly oscillating functions, resulting in a huge demand for a large-scale quantum processor. To address this problem, we propose a new QNN model that harbors universality without the need of multiple state-duplications, and is more likely to get implemented on near-term devices. To demonstrate the effectiveness, we compare our proposal with two popular QNN models in solving typical supervised learning problems. We find that our model requires significantly fewer qubits and it outperforms the other two in terms of accuracy and relative error.

quant-ph↗

arXiv2020

Measuring the hyperfine splittings and deriving the hyperfine-interaction constants of Cesium 7D 5/2 excited state

The measurement of Cesium (Cs) 7D5/2 excited state's hyperfine splitting intervals and hyperfine-interaction constants has been experimentally investigated based on ladder-type (852 nm + 698 nm) three-level Cs system (6S1/2 - 6P3/2 - 7D5/2) with room-temperature Cs atomic vapor cell. By scanning the 698-nm coupling laser's frequency, the Doppler-free high-resolution electromagnetically-induced transparency (EIT) assisted double-resonance optical pumping (DROP) spectra have been demonstrated via transmission enhancement of the locked 852-nm probe laser. The EIT-assisted DROP spectra are employed to study the hyperfine splitting intervals for the Cs 7D5/2 excited state with a room-temperature cesium atomic vapor cell, and the radio-frequency modulation sideband of a waveguide-type electro-optic phase modulator(EOPM) is introduced for frequency calibration to improve the accuracy of frequency interval measurement. The existence of EIT makes the DROP spectral linewidth much narrower, and it is very helpful to improve the spectroscopic resolution significantly. Benefiting from the higher signal-to-noise ratio (SNR) and much better resolution of the EIT-assisted DROP spectra, the hyperfine splitting intervals between the hyperfine folds of (F" = 6), (F" = 5), and (F" = 4) of cesium 7D5/2 state (HFS6"-5" = -10.60(0.17) MHz and HFS5"-4" = -8.54(0.15) MHz) have been measured, and therefore the magnetic-dipole hyperfine-interaction constant (A = -1.70(0.03) MHz) and the electric-quadrupole hyperfine-interaction constant (B = -0.77(0.58) MHz) have been derived for the Cs 7D5/2 state. These constants have important reference value for the improvement of precise measurement and determination of basic physical constants.

physics.atom-ph↗