Quantum information processing with nitrogen–vacancy centers in diamond

Nitrogen-vacancy （NV） center in diamond is one of the most promising candidates to implement room temperature quantum computing. In this review, we briefly discuss the working principles and recent experimental progresses of this spin qubit. These results focus on understanding and prolonging center spin coherence, steering and probing spin states with dedicated quantum control techniques, and exploiting the quantum nature of these multi-spin systems, such as superposition and entanglement, to demonstrate the superiority of quantum information processing. Those techniques also stimulate the fast development of NV-based quantum sensing, which is an interdisciplinary field with great potential applications.

Planar ion chip design for scalable quantum information processing

We investigate a planar ion chip design with a two-dimensional array of linear ion traps for scalable quantum information processing. Qubits are formed from the internal electronic states of trapped ^40Ca^＋ ions. The segmented electrodes reside in a single plane on a substrate and a grounded metal plate separately, a combination of appropriate rf and DC potentials is applied to them for stable ion confinement. Every two adjacent electrodes can generate a linear ion trap in and between the electrodes above the chip at a distance dependent on the geometrical scale and other considerations. The potential distributions are calculated by using a static electric field qualitatively. This architecture provides a conceptually simple avenue to achieving the microfabrication and large-scale quantum computation based on the arrays of trapped ions.

Quantum information processing and metrology with color centers in diamonds

The Nitrogen Vacancy （NV） center is becoming a promising qubit for quantum information processing. The defect has a long coherence time at room temperature and it allows spin state initialized and read out by laser and manipulated by microwave pulses. It has been utilized as a ultra sensi- tive probe for magnetic fields and remote spins as well. Here, we review the recent progresses in experimental demonstrations based on NV centers. We first introduce our work on implementation of the Deutsch- Jozsa algorithm with a single electronic spin in diamond. Then the quantum nature of the bath around the center spin is revealed and continuous wave dynamical decoupling has been demonstrated. By applying dynamical decoupling, a multi-pass quantum metrology protocol is realized to enhance phase estimation. In the final, we demonstrated NV center can be regarded as a ultra-sensitive sensor spin to implement nuclear magnetic resonance （NMR） imaging at nanoscale.

Quantum hyperentanglement and its applications in quantum information processing

Hyperentanglement is a promising resource in quantum information processing with its high capacity character, defined as the entanglement in multiple degrees of freedom(DOFs) of a quantum system, such as polarization, spatial-mode, orbit-angular-momentum, time-bin and frequency DOFs of photons.Recently, hyperentanglement attracts much attention as all the multiple DOFs can be used to carry information in quantum information processing fully. In this review, we present an overview of the progress achieved so far in the field of hyperentanglement in photon systems and some of its important applications in quantum information processing, including hyperentanglement generation, complete hyperentangled-Bell-state analysis, hyperentanglement concentration, and hyperentanglement purification for high-capacity long-distance quantum communication. Also, a scheme for hyper-controlled-not gate is introduced for hyperparallel photonic quantum computation, which can perform two controlled-not gate operations on both the polarization and spatial-mode DOFs and depress the resources consumed and the photonic dissipation.

Single-photon source with sub-MHz linewidth for cesium-based quantum information processing

A single-photon source with narrow bandwidth,high purity,and large brightness can efficiently interact with material qubits strongly coupled to an optical microcavity for quantum information processing.Here,we experimentally demonstrate a degenerate doubly resonant single-photon source at 852 nm by the cavity-enhanced spontaneous parametric downconversion process with a 100%duty cycle of generation.The single photon source possesses both high purity with a second-order correlation g^((2))_(h)(0)=0.021 and narrow linewidth with△_(V_(sp))=(800±13)kHz.The single-photon s Single-photon source with sub-MHz linewidth for cesium-based quantum information processingource is compatible with the cesium atom D2 line and can be used for cesium-based quantum information processing Single-photon source with sub-MHz linewidth for cesium-based quantum information processing0.021.

Quantum information procession with fermions based on charge detection

This paper proposes a fermionic linear optical scheme for the teleportation and entanglement concentration via entanglement swapping based on charge detection. It also proves that this method is useful in generating entangled states such as GHZ states, W states, and cluster states by using fermionic polarizing beam splitters and single spin rotations assisted by a parity check on the fermionic qubits. This scheme is nearly deterministic （i.e., with 100% successful probability） and does not need the joint Bell state measurement required in the previous schemes.

Quality of correlation in quantum information

A strong and stable correlation in quantum information is of high quality for quantum information processing.We define two quantities,selective average correlation and ripple coefficient,to evaluate the quality of correlation in quantum information in a time interval.As a new communication channel,Heisenberg spin chains are widely investigated.We select a two-qubit Heisenberg XXZs pin chain with Dzyaloshinskii-Moriya interaction in an inhomogeneous magnetic field as an example,and use the two quantities to evaluate the qualities of the correlation in quantum information with different measures.The result shows that,if the time evolutions are similar,there needs only evaluating one of them to know when the correlation has high quality for quantum information processing.

Fast achievement of quantum state transfer and distributed quantum entanglement by dressed states

We propose schemes to realize quantum state transfer and prepare quantum entanglement in coupled cavity and cavity-fiber-cavity systems,respectively,by using the dressed state method.We first give the expression of pulses shape by using dressed states and then find a group of Gaussian pulses that are easy to realize in experiment to replace the ideal pulses by curve fitting.We also study the influence of some parameters fluctuation,atomic spontaneous emission,and photon leakage on fidelity.The results show that our schemes have good robustness.Because the atoms are trapped in different cavities,it is easy to perform different operations on different atoms.The proposed schemes have the potential applications in dressed states for distributed quantum information processing tasks.

Quantum-Classical Algorithm for an Instantaneous Spectral Analysis of Signals:A Complement to Fourier Theory

A quantum time-dependent spectrum analysis, or simply, quantum spectral analysis (QSA) is presented in this work, and it’s based on Schrödinger’s equation. In the classical world, it is named frequency in time (FIT), which is used here as a complement of the traditional frequency-dependent spectral analysis based on Fourier theory. Besides, FIT is a metric which assesses the impact of the flanks of a signal on its frequency spectrum, not taken into account by Fourier theory and lets alone in real time. Even more, and unlike all derived tools from Fourier Theory (i.e., continuous, discrete, fast, short-time, fractional and quantum Fourier Transform, as well as, Gabor) FIT has the following advantages, among others: 1) compact support with excellent energy output treatment, 2) low computational cost, O(N) for signals and O(N2) for images, 3) it does not have phase uncertainties (i.e., indeterminate phase for a magnitude = 0) as in the case of Discrete and Fast Fourier Transform (DFT, FFT, respectively). Finally, we can apply QSA to a quantum signal, that is, to a qubit stream in order to analyze it spectrally.

Fidelity between Gaussian mixed states with quantum state quadrature variances

In this paper, from the original definition of fidelity in a pure state, we first give a well-defined expansion fidelity between two Gaussian mixed states. It is related to the variances of output and input states in quantum information pro- cessing. It is convenient to quantify the quantum teleportation （quantum clone） experiment since the variances of the input （output） state are measurable. Furthermore, we also give a conclusion that the fidelity of a pure input state is smaller than the fidelity of a mixed input state in the same quantum information processing.

Quantum Generative Model with Variable-Depth Circuit

In recent years,an increasing number of studies about quantum machine learning not only provide powerful tools for quantum chemistry and quantum physics but also improve the classical learning algorithm.The hybrid quantum-classical framework,which is constructed by a variational quantum circuit(VQC)and an optimizer,plays a key role in the latest quantum machine learning studies.Nevertheless,in these hybrid-framework-based quantum machine learning models,the VQC is mainly constructed with a fixed structure and this structure causes inflexibility problems.There are also few studies focused on comparing the performance of quantum generative models with different loss functions.In this study,we address the inflexibility problem by adopting the variable-depth VQC model to automatically change the structure of the quantum circuit according to the qBAS score.The basic idea behind the variable-depth VQC is to consider the depth of the quantum circuit as a parameter during the training.Meanwhile,we compared the performance of the variable-depth VQC model based on four widely used statistical distances set as the loss functions,including Kullback-Leibler divergence(KL-divergence),Jensen-Shannon divergence(JS-divergence),total variation distance,and maximum mean discrepancy.Our numerical experiment shows a promising result that the variable-depth VQC model works better than the original VQC in the generative learning tasks.

Towards quantum-enhanced precision measurements:Promise and challenges

Quantum metrology holds the promise of improving the measurement precision beyond the limit of classical ap- proaches. To achieve such enhancement in performance requires the development of quantum estimation theories as well as novel experimental techniques. In this article, we provide a brief review of some recent results in the field of quantum metrology. We emphasize that the unambiguous demonstration of the quantum-enhanced precision needs a careful analysis of the resources involved. In particular, the implementation of quantum metrology in practice requires us to take into ac- count the experimental imperfections included, for example, particle loss and dephasing noise. For a detailed introduction to the experimental demonstrations of quantum metrology, we refer the reader to another article ＇Quantum metrology＇ in the same issue.

Quantum Multi-User Detection Based on Coherent State Signals

Multi-user detection is one of the important technical problems for moderncommunications. In the field of quantum communication, the multi-access channel onwhich we apply the technology of quantum information processing is still an openquestion. In this work, we investigate the multi-user detection problem based on thebinary coherent-state signals whose communication way is supposed to be seen as aquantum channel. A binary phase shift keying model of this multi-access channel isstudied and a novel method of quantum detection proposed according to the conclusionof the quantum measurement theory. As a result, the average interference betweendeferent users is presented and the average error probability of the quantum detection isderived theoretically. Finally, we show the maximum channel capacity of this effectivedetection for a two-access quantum channel.

Probabilistic direct counterfactual quantum communication

It is striking that the quantum Zeno effect can be used to launch a direct counterfactual communication between two spatially separated parties, Alice and Bob. So far, existing protocols of this type only provide a deterministic counterfactual communication service. However, this counterfactuality should be payed at a price. Firstly, the transmission time is much longer than a classical transmission costs. Secondly, the chained-cycle structure makes them more sensitive to channel noises. Here, we extend the idea of counterfactual communication, and present a probabilistic-counterfactual quantum communication protocol, which is proved to have advantages over the deterministic ones. Moreover, the presented protocol could evolve to a deterministic one solely by adjusting the parameters of the beam splitters.

Quantum storage of single photons with unknown arrival time and pulse shapes

We present a scheme for the quantum storage of single photons using electromagnetically induced transparency(EIT)in a low-finesse optical cavity,assisted by state-selected spontaneous atomic emission.Mediated by the dark mode of cavity EIT,the destructive quantum interference between the cavity input-output channel and state-selected atomic spontaneous emission leads to strong absorption of single photons with unknown arrival time and pulse shapes.We discuss the application of this phenomenon to photon counting using stored light.

General Decoherence Suppression in Three-Level Atom in V-and ■-Configurations

In this paper, we present the decoupling bang-bang （BB） twin-born pulses to suppress the genera/deco- herence, both amplitude and phase decoherence, in a three-level atom in V- and Ξ-configurations. We give the exact sequence of periodic twinborn pulses in such systems.

One-step discrimination scheme on N-particle Greenberger-Horne-Zeilinger bases

We present an experimentally feasible one-step discrimination scheme on Bell bases with trapped ions, and then generalize it to the case of N-ion Greenberger-Horne-Zeilinger （GHZ） bases. In the scheme, all the orthogonal and complete N-ion GHZ internal states can be exactly discriminated only by one step, and thus it takes very short time. Moreover, the scheme is insensitive to thermal motion and dose not require the individual addressing of the ions. The Bell-state and GHZ-state one-step discrimination scheme can be widely used in quantum information processing based on ion-trap set-up.

^(23)Na relaxometry: An overview of theory and applications

^(23)Na is a nuclear magnetic resonance(NMR)-active isotope with a nuclear spin quantum number of 3/2.^(23)Na relaxation phenomenon is at the core of ^(23)Na NMR measurement and analysis.Due to the dominance of quadrupolar interaction,the relaxation behavior of ^(23)Na is physically and mathematically more complex than that of a typical spin-1/2 isotope.In this review,we overview the semi-classical Redfield theory for deriving the formulations of ^(23)Na relaxation.We show that the relaxation behaviors of ^(23)Na can be quantitatively described by constructing the spectral density functions based on the second-order perturbation theory.In addition,we summarize the applications of ^(23)Na relaxometry in different research fields,including biomedicine,sodium ion batteries,and quantum information processing.Because sodium is an essential element in our body,food and industrial materials,the research on sodium by ^(23)Na NMR emerges as important future directions.The theoretical and practical understandings on ^(23)Na relaxation are the step stones for mastering advanced ^(23)Na NMR techniques.

Experimental simulation of violation of the Wright inequality by coherent light

In this paper, we investigate the simulation of violation of the Wright inequality by the classical optical experiment theoretically and experimentally. The feasibility of the simulation is demonstrated by theoretical analysis based on descriptions of the classical electrodynamics and quantum mechanics, respectively. Then, the simulation of violation of the Wright inequality is realized experimentally. The setup is based on a laser source, free-space optical devices and power meters. The experimental result violates the noncontextuality hidden variable bound, agreeing with the quantum bound. This method can be extended to other types of noncontextuality inequalities.

Experimental quantum simulation of Avian Compass in a nuclear magnetic resonance system

Avian magnetoreception is the capacity for avians to sense the direction of the Earth's magnetic field. Discovered more than forty years ago, it has attracted intensive studies over the years. One promising model for describing this capacity in avians is the widely used reference-and-probe model where radical pairs within the eyes of bird combines to form singlet and triplet quantum states.The yield depends on the angle between the Earth's magnetic field and the molecules' axis, hence the relative value of yield of the singlet state or triplet state enables avians to sense the direction. Here we report the experimental demonstration of avian magnetoreception in a nuclear magnetic resonance quantum information processor. It is shown clearly from the experiment that the yield of the singlet state attains maximum when it is normal to the Earth's magnetic field, and the experimental results agree with theory very well.