光量子计算的实验研究进展

自从进入大数据时代,人们对于数据处理速度的需求越来越大,但是经典计算的发展却逐渐达到瓶颈,越来越难以满足这种需求.由于并行性,量子计算在诸多问题的处理上可以达到指数加速效果,有巨大的信息处理潜力,正好能满足人类对于数据处理速度的需求,被公认为是下一代信息技术的核心技术.围绕量子计算,最近实验演示了2种具有指数加速效果的量子算法：求解线性方程组量子算法和机器学习量子算法;其次介绍了最新实现的多自由度量子隐形传态.量子隐形传态是可扩展的线性光学KLM方案的重要组成部分.多自由度量子隐形传态实验打破了国际学术界从1997年以来只能传输粒子单一自由度的局限,为发展可扩展的量子计算奠定了基础.

Realization of -bit semiclassical quantum Fourier transform on IBM's quantum cloud computer

To overcome the difficulty of realizing large-scale quantum Fourier transform(QFT) within existing technology, this paper implements a resource-saving method(named t-bit semiclassical QFT over Z_(2~n)), which could realize large-scale QFT using an arbitrary-scale quantum register. By developing a feasible method to realize the control quantum gate Rk, we experimentally realize the 2-bit semiclassical QFT over Z_(2~3) on IBM's quantum cloud computer, which shows the feasibility of the method. Then, we compare the actual performance of 2-bit semiclassical QFT with standard QFT in the experiments.The squared statistical overlap experimental data shows that the fidelity of 2-bit semiclassical QFT is higher than that of standard QFT, which is mainly due to fewer two-qubit gates in the semiclassical QFT. Furthermore, based on the proposed method, N = 15 is successfully factorized by implementing Shor's algorithm.

Realization of High-Fidelity Controlled-Phase Gates in Extensible Superconducting Qubits Design with a Tunable Coupler

High-fidelity two-qubit gates are essential for the realization of large-scale quantum computation and simulation.Tunable coupler design is used to reduce the problem of parasitic coupling and frequency crowding in manyqubit systems and thus thought to be advantageous. Here we design an extensible 5-qubit system in which center transmon qubit can couple to every four near-neighboring qubits via a capacitive tunable coupler and experimentally demonstrate high-fidelity controlled-phase(CZ) gate by manipulating central qubit and one nearneighboring qubit. Speckle purity benchmarking and cross entropy benchmarking are used to assess the purity fidelity and the fidelity of the CZ gate. The average purity fidelity of the CZ gate is 99.69±0.04% and the average fidelity of the CZ gate is 99.65±0.04%, which means that the control error is about 0.04%. Our work is helpful for resolving many challenges in implementation of large-scale quantum systems.

基于辅助单比特测量的量子态读取算法

在量子计算过程中,需要通过量子测量读取计算结果.然而,受限于物理实现,对量子态的测量往往存在较大误差,直接影响量子计算结果的正确提取,以及限制量子计算的大规模扩展.本文针对一种特定形式的量子态,提出基于辅助单比特测量的量子态间接读取算法,避免多比特测量带来的大量测量误差.理论和模拟结果表明,当所读取的量子态比特数较大时,该算法相比于直接读取具有更高的正确率,可用于大规模量子纠错和量子态的高保真度读取.

Realization of Fast All-Microwave Controlled-Z Gates with a Tunable Coupler

The development of high-fidelity two-qubit quantum gates is essential for digital quantum computing.Here,we propose and realize an all-microwave parametric controlled-Z(CZ)gates by coupling strength modulation in a superconducting Transmon qubit system with tunable couplers.After optimizing the design of the tunable coupler together with the control pulse numerically,we experimentally realized a 100 ns CZ gate with high fidelity of 99.38%±0.34%and the control error being 0.1%.We note that our CZ gates are not affected by pulse distortion and do not need pulse correction,providing a solution for the real-time pulse generation in a dynamic quantum feedback circuit.With the expectation of utilizing our all-microwave control scheme to reduce the number of control lines through frequency multiplexing in the future,our scheme draws a blueprint for the high-integrable quantum hardware design.

Transitionless driving on local adiabatic quantum search algorithm

We apply the transitionless driving on the local adiabatic quantum search algorithm to speed up the adiabatic process. By studying quantum dynamics of the adiabatic search algorithm with the equivalent two-level system, we derive the transi- tionless driving Hamiltonian for the local adiabatic quantum search algorithm. We found that when adding a transitionless quantum driving term Ht~ （t） on the local adiabatic quantum search algorithm, the success rate is 1 exactly with arbitrary evolution time by solving the time-dependent Schr6dinger equation in eigen-picture. Moreover, we show the reason for the drastic decrease of the evolution time is that the driving Hamiltonian increases the lowest eigenvalues to a maximum of

Quantum adiabatic algorithms using unitary interpolation

We present two efficient quantum adiabatic algorithms for Bernstein–Vazirani problem and Simon’s problem.We show that the time complexities of the algorithms for Bernstein–Vazirani problem and Simon’s problem are O(1)and O(n),respectively,which are the same complexities as the corresponding algorithms in quantum circuit model.In these two algorithms,the adiabatic Hamiltonians are realized by unitary interpolation instead of standard linear interpolation.Comparing with the adiabatic algorithms using linear interpolation,the energy gaps of our algorithms keep constant.Therefore,the complexities are much easier to analyze using this method.

在61比特可编程超导量子处理器上对量子多体态进行量子神经元感知

对具有不同性质和物相的多体量子态进行分类是量子多体物理学中最基本的任务之一.然而,由于巨大数量的相互作用的粒子所产生的指数级的复杂性,大规模量子态的分类对于经典的方法来说极具挑战性.本文提出了一种新的方法,称为量子神经元感知.利用一个61比特的超导量子处理器作为演示,作者表明该方案可以有效地对两种不同类型的多体现象,即遍历相和局域相,进行分类.量子神经元感知过程使他们能够通过只测量一个量子比特来区分这些多体物相,并提供比传统方法(如测量不平衡度)更好的分辨率.本研究证明了量子神经元感知在近期量子处理器应用的可行性和扩展性,并为探索更大规模系统中的量子多体现象开辟了新的途径.

通过60比特24深度量子随机线路采样实现量子计算优势性

高精度地扩展量子比特规模是实现实用量子计算必须克服的基础难题.本文将祖冲之2.0升级至祖冲之2.1,该处理器的基本量子操控保真度分别为:单比特门99.84%,两比特1门99.40%,读取97.74%,相比2.0系统在读取上得到了巨大的提升为了表征系统的整体性能,本文通过执行随机量子电路采样任务(-种量子霸权协议)进行基准测试.实验结果表明,可以实现60比特24深度的随机量子线路采样,线路最终保真度为(0.0366土0.00345)%.该线路的经典模拟复杂度比此前的祖冲之2.0量子霸权线路(55比特20深度)难5000倍,因此可以说,本研究在量子随机线路采样问题上进一步增强了量子计算的算力优势.