Heralded entanglement purification protocol using high-fidelity parity-check gate based on nitrogen-vacancy center in optical cavity

The decoherence of entangled states caused by the noisy channel is a salient problem for reducing the fidelity of quantum communication. Here we present a heralded two-photon entanglement purification protocol(EPP) using heralded high-fidelity parity-check gate(HH-PCG), which can increase the entanglement of nonlocal two-photon polarization mixed state. The HH-PCG is constructed by the input-output process of nitrogen-vacancy(NV) center in diamond embedded in a single-sided optical cavity, where the errors caused by the imperfect interaction between the NV center-cavity system and the photon can be heralded by the photon detector. As the unwanted components can be filtrated due to the heralded function, the fidelity of the EPP scheme can be enhanced considerably, which will increase the fidelity of quantum communication processing.     

基于金刚石氮-空位色心自旋系综与超导量子电路混合系统的量子节点纠缠

量子纠缠是实现量子计算和量子通信的核心基础,本文提出了在金刚石氮-空位色心(NV centers)自旋系综与超导量子电路耦合的混合系统中实现两个分离量子节点之间纠缠的理论方案.在该混合系统中,把金刚石NV centers自旋系综和与之耦合的超导共面谐振器视为一个量子节点,两个量子节点之间通过一个空的超导共面谐振器连接.具有较长相干时间的NV centers自旋系综作为一个量子存储器,用于制备、存储和发送量子信息;易于外部操控的超导量子电路可执行量子逻辑门操作,快速调控量子信息.为了实现两个分离量子节点之间的纠缠,首先对系统的哈密顿量进行正则变换,将其等价为两个NV centers自旋系综与同一个超导共面谐振器之间的JC耦合;然后采用NV centers自旋-光子混合比特编码的方式,通过调节超导共面谐振器的谐振频率,精确控制体系演化时间,高保真度地实现了两个分离量子节点之间的量子纠缠.本方案还可以进一步扩展和集成,用于构建多节点纠缠的分布式量子网络.     

Multi-copy entanglement purification with practical spontaneous parametric down conversion sources

Entanglement purification is to distill the high quality entanglement from the low quality entanglement with local operations and classical communications. It is one of the key technologies in long-distance quantum communication. We discuss an entanglement purification protocol(EPP) with spontaneous parametric down conversion(SPDC) sources, in contrast to previous EPP with multi-copy mixed states, which requires ideal entanglement sources. We show that the SPDC source is not an obstacle for purification, but can benefit the fidelity of the purified mixed state. This EPP works for linear optics and is feasible in current experiment technology.     

General Quantum Entanglement Purification Protocol using a Controlled-Phase-Flip Gate

Entanglement purification is an important method to guarantee the fidelity of long-distance quantum communication. Here, a general entanglement purification protocol (EPP) for mixed state with bit-flip error and phase-flip error is proposed, resorting to unilateral operations and a controlled-phase-flip (CPF) gate. The CPF gate is working with a high fidelity under balance condition of quantum dot embedded in a double-sided optical cavity. This general EPP scheme can purify the mixed state with both bit-flip error and phase-flip error to a high fidelity entangled state relatively fast in some regimes, owing to the unilateral operations and high-fidelity CPF gate, which can largely decrease the resource consumption. This general EPP provides a convenient way for increasing the entanglement of different quantum systems, which has great potential for guaranteeing the fidelity of long-distance quantum communication in the future.     

Polarization-Entanglement Purification for Ideal Sources Using Weak Cross-Kerr Nonlinearity

We propose an entanglement purification protocol (EPP) for ideal entangled photon sources resorting to weak cross-Kerr nonlinearities. The key element of this EPP is the quantum nondemolition detector (QND) which uses weak cross-Kerr nonlinearity effect to generate phase shifts on the coherent states conditionally. By exploiting the double cross-phase modulation (XPM) method in QND, we present an efficient EPP protocol, which reduces the requirement for coupling strength of the cross-Kerr nonlinearity medium. The proposed protocol may open up promising possibilities for the practicability of quantum information processing using weak cross-Kerr nonlinearities.     

Two-photon quantum interference from separate nitrogen vacancy centers in diamond

We report on the observation of quantum interference of the emission from two separate nitrogen vacancy (NV) centers in diamond. Taking advantage of optically induced spin polarization in combination with polarization filtering, we isolate a single transition within the zero-phonon line of the nonresonantly excited NV centers. The time-resolved two-photon interference contrast of this filtered emission reaches 66%. Furthermore, we observe quantum interference from dissimilar NV centers tuned into resonance through the dc Stark effect. These results pave the way towards measurement-based entanglement between remote NV centers and the realization of quantum networks with solid-state spins.     

Deterministic Entanglement Purification of the Greenberger-Horne-Zeilinger States in Quantum-Dot and Micro-cavity Coupled System

An improved purification of the triplet Greenberger-Horne-Zeilinger (GHZ) state is demonstrated for the electron-spin-entangled state in the quantum-dot (QD) and micro-cavity coupled systems. In order to distill the maximally entangled GHZ state efficiently, we designate a deterministic entanglement purification protocol (EPP) by using a pair of the triplet-electron-spin-entangled systems. It is based on the elegant parity-check operations performed in the cavity-spin-coupling system with the assistance of an ancillary single photon. With the current and feasible technology, the maximally entangled GHZ states can be achieved as much as flexible for the long-distance quantum communications since only single-photon detection and single-electron detection are required in practice.     

Second-order magnetic field gradient-induced strong coupling between nitrogen-vacancy centers and a mechanical oscillator

We consider a cantilever mechanical oscillator (MO) made of diamond. A nitrogen-vacancy (NV) center lies at the end of the cantilever. Two magnetic tips near the NV center induce a strong second-order magnetic field gradient. Under coherent driving of the MO, we find that the coupling between the MO and the NV center is greatly enhanced. We studied how to generate entanglement between the MO and the NV center and realize quantum state transfer between them. We also propose a scheme to generate two-mode squeezing between different MO modes by coupling them to the same NV center. The decoherence and dissipation effects for both the MO and the NV center are numerically calculated using the present parameter values of the experimental configuration. We have achieved high fidelity for entanglement generation, quantum state transfer, and large two-mode squeezing.     

Quantum Storage in a Hybrid System with a Photonic Molecule and a Diamond Nitrogen Vacancy Center

We propose an efficient scheme for the realization of high-fidelity quantum storage in a hybrid system with a photonic molecule and a diamond nitrogen vacancy (NV) center. The scheme holds the advantages of both photonic crystal cavities and NV centers. Meanwhile, the influence of quantum-computing process on quantum memory can be effectively eliminated by separating the processor and memory units. Moreover, the scheme is experimentally feasible with currently available technology.     

Efficient entanglement purification in quantum repeaters

We present an efficient entanglement purification protocol(EPP) with controlled-not(CNOT) gates and linear optics.With the CNOT gates,our EPP can reach a higher fidelity than the conventional one.Moreover,it does not require the fidelity of the initial mixed state to satisfy F > 1/2.If the initial state is not entangled,it still can be purified.With the linear optics,this protocol can get pure maximally entangled pairs with some probabilities.Meanwhile,it can be used to purify the entanglement between the atomic ensembles in distant locations.This protocol may be useful in long-distance quantum communication.     

Efficient entanglement purification in quantum repeaters

We present an efficient entanglement purification protocol (EPP) with controlled-not (CNOT) gates and linear optics. With the CNOT gates, our EPP can reach a higher fidelity than the conventional one. Moreover, it does not require the fidelity of the initial mixed state to satisfy F>1/2. If the initial state is not entangled, it still can be purified. With the linear optics, this protocol can get pure maximally entangled pairs with some probabilities. Meanwhile, it can be used to purify the entanglement between the atomic ensembles in distant locations. This protocol may be useful in long-distance quantum communication.     

冷原子系综内单集体激发态的相干操纵

原子系综内部分原子发生相干态转移后所处量子态被称为集体激发态.如果激发数目在单原子量级则被称为单激发态.在量子存储过程中,单光子以单激发态的形式在原子系综内进行存储.因此,研究单激发态的制备、演化、转化、干涉等过程是量子存储及其应用研究的关键.本文总结了近年来作者所在研究团队针对冷原子系综体系在此研究方向取得的若干成果.主要包括采用动量模式调控、三维光晶格等手段抑制单激发态的退相干,采用环形腔增强原子至光子的转化效率,发展基于拉曼光的单激发态相干转移技术,利用单量子态不同模式间干涉制备光与原子纠缠,利用里德伯阻塞机制提升纠缠制备效率等.此外,简要回顾了基于多个单激发态的量子中继及量子网络实验.     

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.     

多个量子节点确定性纠缠的建立

量子纠缠是一种重要的量子资源,在多个空间分离的量子存储器间建立确定性的量子纠缠,然后在用户控制的时刻将所存储的量子纠缠转移到量子信道中进行信息的分发和传送,这对于实现量子信息网络是至关重要的.本文介绍了用光学参量放大器制备与铷原子D1吸收线对应的非经典光场,而且在三个空间分离的原子系综中确定性量子纠缠的产生、存储和转移.利用电磁感应透明光和原子相互作用的原理,将制备的多组分光场纠缠态模式映射到三个远距离的原子系综以建立原子自旋波之间的纠缠.然后,存储在原子系综中的纠缠态通过三个量子通道,纠缠态的量子噪声被转移到三束空间分离的正交纠缠光场.三束释放的光场间纠缠的存在验证了该系统具有保持多组分纠缠的能力.这个方案实现了三个量子节点间的纠缠,并且可以直接扩展到具有更多节点的量子网络,为未来实现大型量子网络通信奠定了基础.     

Entanglement Purification on Separate Atoms in an Error-Detected Pattern

We propose an entanglement purification protocol (EPP) for purifying separate atoms embedded in single-sided optical cavities. The protocol works in an error-detected pattern, the reflection coefficients only appear as a global coefficient, the success or failure cases are heralded by the detection of photons, these merits make the protocol more robust and practical. We discuss the performance of the protocol with current experimental parameters, the results show the success probability of the present EPP is relatively high. The EPP could provide a promising building block for quantum repeaters and quantum networks in the future.

     

Coupling nitrogen-vacancy centers in diamond to superconducting flux qubits

We propose a method to achieve coherent coupling between nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.     

Phonon-induced Quantum Entanglement in Diamond Nanostructures

We consider the quantum entanglement such as spin squeezing and the reciprocal of the mean quantum Fisher information per particle(RMQFIP) resulted from the phonon-induced spin-spin interactions in diamond nanoresonator. The entangled state can be generated by adjusting the near-resonant coupling and the single phonon coupling strength and the better entanglement and a longer entangled time interval can be achieved by increasing the number of charged nitrogen-vacancy (NV) centers.     

Purification in entanglement distribution with deep quantum neural network

Entanglement distribution is important in quantum communication. Since there is no information with value in this process, purification is a good choice to solve channel noise. In this paper, we simulate the purification circuit under true environment on Cirq, which is a noisy intermediate-scale quantum (NISQ) platform. Besides, we apply quantum neural network (QNN) to the state after purification. We find that combining purification and quantum neural network has good robustness towards quantum noise. After general purification, quantum neural network can improve fidelity significantly without consuming extra states. It also helps to obtain the advantage of entangled states with higher dimension under amplitude damping noise. Thus, the combination can bring further benefits to purification in entanglement distribution.     

A quantum computer based on NV centers in diamond: Optically detected nutations of single electron and nuclear spins

In an effort to realize a two-bit processor for a quantum computer on the basis of single nitrogen-vacancy defect centers (NV centers) in diamond, the optically detected nutations of the electron spin of a single NV center in the ground state and of the nuclear spin of a ¹³C atom located at a diamond lattice site nearest to the NV center are studied. The photodynamics of NV and NV + ¹³C centers under different temperatures and optical excitation conditions is discussed. A seven-level model of a center excited by radiation from an Ar⁺ laser at room temperature is proposed. On the basis of this model, the experimental spectra of optically detected electron paramagnetic and electron-nuclear double resonances of single NV and NV + ¹³C centers in diamond nanocrystals, as well as experimental data on the optically detected nutations of the electron and nuclear spins of these centers caused by the actions of pulsed microwave and radiofrequency fields, respectively, are interpreted.     

基于拉曼协议的量子存储

量子存储器是实现按照需要存储/读出诸如单光子、纠缠或者压缩态等非经典量子态的系统,是实现量子通信和量子计算必不可少的核心器件.量子存储协议多种多样,其中拉曼方案由于具有存储宽带大、可用于存储短脉冲信号的优点而引起了人们的广泛关注.然而实现真正单光子和光子纠缠的拉曼存储具有挑战性.本文简要介绍了量子存储器的主要性能和评价指标,在回顾了量子存储器特别是拉曼量子存储器的发展现状后,重点介绍了本研究组最近基于拉曼协议实现各种量子态存储的系列研究,取得的研究成果对于构建高速量子网络具有重要参考价值.