arXiv subjects

Jiancun Gao

Publications and source records attributed to Jiancun Gao.

4 recordsLinked to original sources

arXiv2022

Dynamics simulation and numerical analysis of arbitrary time-dependent $\mathcal{PT}$-symmetric system based on density operators

$\mathcal{PT}$-symmetric system has attracted extensive attention in recent years because of its unique properties and applications. How to simulate $\mathcal{PT}$-symmetric system in traditional quantum mechanical system has not only fundamental theoretical significance but also practical value. We propose a dynamics simulation scheme of arbitrary time-dependent $\mathcal{PT}$-symmetric system based on density operators, and the results are compatible with previous methods based on pure-state vectors. Based on the above, we are able to study the influence of quantum noises on the simulation results with the technique of vectorization of density operators and matrixization of superoperators (VDMS), and we show the depolarizing (Dep) noise is the most fatal and should be avoided as much as possible. Meanwhile, we also give a numerical analysis. We find that the problem of chronological product usually has to be solved not only in the numerical calculation, but also even in the experiment, because the dilated higher-dimensional Hamiltonian is usually time-dependent. Through theoretical analysis and numerical calculation, we find that on the premise of meeting the goal of calculation accuracy and saving computing resources, the time step of calculation and the cut-off term of Magnus series have to be carefully balanced.

quant-ph↗

arXiv2021

Loophole-free plug-and-play quantum key distribution

Robust, simple, and flexible quantum key distribution (QKD) is vital for realising practical applications of this technique. Contrary to typical phase-coded QKD schemes, the plug-and-play QKD design requires only one arm-length-insensitive interferometer without active feedback, in which the noise is automatically compensated by the two-way structure. However, there are certain possible loopholes in the typical plug-and-play designs, which require consideration and strict monitoring. This study proposes a loophole-free plug-and-play QKD scheme with two-way protocol and presents an experimental demonstration of said scheme. The security is analysed under a collective attack scenario assisted by the decoy state method. The scheme was implemented in a 50.4 km commercial fibre without active feedback. The system showed highly robust performance with an ultra-low error rate and maintained an ultra-high visibility of 0.9947+/-0.0002 through significant environmental changes over 24 hours.

quant-ph↗

arXiv2020

Quantum Gradient Algorithm for General Polynomials

Gradient-based algorithms, popular strategies to optimization problems, are essential for many modern machine-learning techniques. Theoretically, extreme points of certain cost functions can be found iteratively along the directions of the gradient. The time required to calculating the gradient of $d$-dimensional problems is at a level of $\mathcal{O}(poly(d))$, which could be boosted by quantum techniques, benefiting the high-dimensional data processing, especially the modern machine-learning engineering with the number of optimized parameters being in billions. Here, we propose a quantum gradient algorithm for optimizing general polynomials with the dressed amplitude encoding, aiming at solving fast-convergence polynomials problems within both time and memory consumption in $\mathcal{O}(poly (\log{d}))$. Furthermore, numerical simulations are carried out to inspect the performance of this protocol by considering the noises or perturbations from initialization, operation and truncation. For the potential values in high-dimension optimizations, this quantum gradient algorithm is supposed to facilitate the polynomial-optimizations, being a subroutine for future practical quantum computer.

quant-ph↗

arXiv2018

Implementation and Security Analysis of Practical Quantum Secure Direct Communication

Fast development of supercomputer and perspective quantum computer is posing increasing serious threats to communication security. Based on the laws of quantum mechanics, quantum communication offers provable security of communication, and is a promising solution to counter such threats. Quantum secure direct communication (QSDC) is one of the important branches of quantum communication. Different from other branches of quantum communication, it transmits secret information directly. Recently, remarkable progress has been made in the proof-of-principle experimental demonstrations of QSDC. However, it remains a technical feast to march QSDC into practical application. Here, we report an implementation of practical quantum secure communication system. The security is analyzed in the Wyner wiretap channel theory. The system uses a coding scheme based on concatenation of low density parity check (LDPC) codes, which works in a regime with realistic environment of high noise and high loss. The present system operates with a repetition rate of 1 MHz, and at a distance of 1.5 kilometers. The secure communication rate is 50 bps, which can effectively send text message and files such as image and sounds with a reasonable size.

quant-ph↗