Quantum computing using isolated qubits

We describe a method for scalable quantum computing using isolated matter qubits and photonic messenger qubits. We employ graph state quantum computation in which the entangling operation is probabilistic. The entangling operation is extremely robust under photon loss and interferometric instability. In addition, this scheme allows for arbitrary complex qubit-based quantum communication.     

How good must single photon sources and detectors be for efficient linear optical quantum computation?

We present a scheme for linear optical quantum computation that is highly robust to imperfect single photon sources and inefficient detectors. In particular we show that if the product of the detector efficiency with the source efficiency is greater than 2/3, then efficient linear optical quantum computation is possible. This high threshold is achieved within the cluster state paradigm for quantum computation.     

Deterministic linear optics quantum computation with single photon qubits

We suggest an efficient scheme for quantum computation with linear optical elements, where the qubits are encoded in single photon states. The scheme reduces the resources required per logical gate by several orders of magnitude, compared to an earlier proposal of Knill, Laflamme, and Milburn, while the resource overhead per gate is independent of the length of the computation. A central feature of the scheme, enabling these improvements, is the prior construction of a "linked" photon state designed according to the particular quantum circuit one wishes to process. Once this state has been successfully prepared, the computation is pursued deterministically by a sequence of teleportation steps.     

Distributed quantum information processing via quantum dot spins

We propose a scheme to engineer a non-local two-qubit phase gate between two remote quantum-dot spins. Along with one-qubit local operations, one can in principal perform various types of distributed quantum information processing. The scheme employs a photon with linearly polarisation interacting one after the other with two remote quantum-dot spins in cavities. Due to the optical spin selection rule, the photon obtains a Faraday rotation after the interaction process. By measuring the polarisation of the final output photon, a non-local two-qubit phase gate between the two remote quantum-dot spins is constituted. Our scheme may has very important applications in the distributed quantum information processing.     

利用光子回声技术产生光的三体纠缠态的研究

量子纠缠是实现量子通信和量子计算的重要资源,其中多体纠缠更是构建量子网络实现全局量子计算的基础。本文主要研究如何利用经典的光子回声技术实现光量子态的三体纠缠。在自由空间中向掺杂稀土离子的固态离子系综中射入经典光场,当离子体系达到相位重构条件时即可获得三束存在纠缠的量子光场。基于本方案纠缠产生的技术特点,发现其在提高量子中继器的工作效率上有着潜在的应用价值。     

Teleportation of a cat-state via attenuated quantum channel using only threshold detectors

An experimental scheme is proposed for faithful teleportation of a unknown optical cat-state via attenuated quantum channel due to energy loss or photon absorption during the process of entanglement sharing. The scheme is probabilistic, yet conclusive, and the effective classical communication costs just Log₂3 bits, instead of five bits which are necessary for full record of the measurement outcome. The scheme uses only threshold (i.e., yes/no) detectors so that exact photon counting is not needed. However, it requires application of a nonlinear element called cross-phase modulator. Feasibility of the scheme is also discussed with respect to EIT-based modern techniques.     

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.     

Generation of pair coherent state using weak cross-Kerr media

We firstly give a nonlocal method for generating pair coherent state with two traveling wave fields in distinct districts. The experimental scheme proposed is based on a two-mode photon number matching process, which employs weak cross-Kerr media and on/off detection. Then we discuss the details for implementing this scheme, showing that it is robust against the low quantum efficiency of photon detectors and offers nearly perfect pair coherent states. Finally, we show how a two-mode Schrödinger cat state and a generalized two-mode correlated photon number state can be prepared via this matching process.     

大气损耗对量子干涉雷达的影响机理

以马赫-曾德尔干涉仪作为基本模型对量子干涉雷达的探测原理进行分析,讨论了目标探测过程中光场量子态的具体演化情况,并采用宇称算符作为相位检测算符分析了量子干涉雷达的回波信号,将其与基于振幅检测的经典雷达回波信号进行比较,证明量子干涉雷达具有超越衍射极限的超分辨率特性.此外,针对大气损耗的进一步研究显示:量子干涉雷达分辨率受大气损耗影响较小,且可通过增大脉冲光子数N克服其影响;而量子干涉雷达的灵敏度则受到较大影响,尤其当两路光的损耗情况不同时,灵敏度随N的增加呈现先升高后降低的趋势;当两路光损耗情况相同时,系统灵敏度随N的增加而升高且正比于1/N~(1/2).综上,可根据探测光的大气损耗情况适当调节参考光的衰减来克服大气损耗带来的不良影响.     

Optical quantum computation using cluster States

We propose an approach to optical quantum computation in which a deterministic entangling quantum gate may be performed using, on average, a few hundred coherently interacting optical elements (beam splitters, phase shifters, single photon sources, and photodetectors with feedforward). This scheme combines ideas from the optical quantum computing proposal of Knill, Laflamme, and Milburn [Nature (London) 409, 46 (2001)]], and the abstract cluster-state model of quantum computation proposed by Raussendorf and Briegel [Phys. Rev. Lett. 86, 5188 (2001)]].     

Photon-emission properties of quantum-dot-based single-photon sources under different excitations

The photon-emission efficiencies and photon indistinguishabilities of a single-photon source, which employs a cavity coupled with a quantum dot, are studied under above-band and resonant excitations. The results are obtained by solving master equations and by applying the quantum regression theorem. According to the study, the photon indistinguishability increases with the Purcell factor under resonant excitation, which is consistent with the increase in emission efficiency; however, these two figures of merit are inconsistent for the above-band excitation scheme. Moreover, the efficiencies, defined as the average photon number emitted in one excitation cycle, are almost the same for the two different excitation schemes, whereas the excitation power needed to reach that efficiency is much lower under resonant excitation than that for above-band excitation. These results will be helpful in improving the performances of the applications concerning indistinguishability and efficiency.     

Quantum multiparameter estimation with multi-mode photon catalysis entangled squeezed state

We propose a method to generate the multi-mode entangled catalysis squeezed vacuum states (MECSVS) by embedding the cross-Kerr nonlinear medium into the Mach−Zehnder interferometer. This method realizes the exchange of quantum states between different modes based on Fredkin gate. In addition, we study the MECSVS as the probe state of multi-arm optical interferometer to realize multi-phase simultaneous estimation. The results show that the quantum Cramer−Rao bound (QCRB) of phase estimation can be improved by increasing the number of catalytic photons or decreasing the transmissivity of the optical beam splitter using for photon catalysis. In addition, we also show that even if there is photon loss, the QCRB of our photon catalysis scheme is lower than that of the ideal entangled squeezed vacuum states (ESVS), which shows that by performing the photon catalytic operation is more robust against photon loss than that without the catalytic operation. The results here can find applications in quantum metrology for multiparatmeter estimation.     

Strong Analog Classical Simulation of Coherent Quantum Dynamics

A strong analog classical simulation of general quantum evolution is proposed, which serves as a novel scheme in quantum computation and simulation. The scheme employs the approach of geometric quantum mechanics and quantum informational technique of quantum tomography, which applies broadly to cases of mixed states, nonunitary evolution, and infinite dimensional systems. The simulation provides an intriguing classical picture to probe quantum phenomena, namely, a coherent quantum dynamics can be viewed as a globally constrained classical Hamiltonian dynamics of a collection of coupled particles or strings. Efficiency analysis reveals a fundamental difference between the locality in real space and locality in Hilbert space, the latter enables efficient strong analog classical simulations. Examples are also studied to highlight the differences and gaps among various simulation methods.     

用腔场中的二能级势阱离子实现量子逻辑门

利用光腔中的势阱粒子同时与外激光场和腔场发生相互作用的特性，我们提出了一种量子逻辑门的实现方案。在该方案中，我们采用文献[10-12]中的模型。文献[11-12]中实现的逻辑门是以离子内态和运动态作为量子比特，腔态充当辅助比特在计算过程中保持在基态。而[10]要求离子内态保持为基态，利用离子运动态和腔态构成量子比特。与文献[10-12]不同的是，我们实现的量子逻辑门是以粒子内态和腔态作为比特，而势阱离子的运动态作为辅助比特始终保持在基态。而且，我们对该方案的实验要求进行了讨论。     

Hyper CNOT and Hyper Bell-State Analysis Assisted by Quantum Dots in Double-Side Optical Microcavities

There are many important works about the construction of universal quantum logic gates which are key elements in quantum computation. However, most of them focus on quantum transformations on the same degree of freedom (DOF) of quantum systems. We propose a CNOT gate performed on the polarization DOF and spatial mode DOF of one photon system assisted by a quantum dot in double-side optical microcavities. This hyper CNOT gate is implemented by using spin selective photon reflection from the cavity, without auxiliary spatial modes or polarization modes. This interface can also be used to construct a hyper photonic Bell-state analyzer. The high fidelities of the hyper CNOT gates may be achieved with low side leakage and cavity loss.     

Quantum teleportation with a quantum dot single photon source

We report the experimental demonstration of a quantum teleportation protocol with a semiconductor single photon source. Two qubits, a target and an ancilla, each defined by a single photon occupying two optical modes (dual-rail qubit), were generated independently by the single photon source. Upon measurement of two modes from different qubits and postselection, the state of the two remaining modes was found to reproduce the state of the target qubit. In particular, the coherence between the target qubit modes was transferred to the output modes to a large extent. The observed fidelity is 80%, in agreement with the residual distinguishability between consecutive photons from the source. An improved version of this teleportation scheme using more ancillas is the building block of the recent Knill, Laflamme, and Milburn proposal for efficient linear optics quantum computation.     

EIT-based quantum memory for single photons from cavity-QED

We investigate the feasibility of implementing an elementary building block for quantum information processing. The combination of a deterministic single photon source based on vacuum stimulated Raman adiabatic passage (V-STIRAP), and a quantum memory based on electromagnetically induced transparency (EIT) in atomic vapour is outlined. Both systems are able to produce and process temporally shaped wavepackets which provide a way to maintain the indistinguishability of the photons. We also propose an efficient and robust ‘repeat-until-success’ quantum computation scheme based on this hybrid architecture.     

Fault tolerant quantum computation with very high threshold for loss errors

Many proposals for fault tolerant quantum computation (FTQC) suffer detectable loss processes. Here we show that topological FTQC schemes, which are known to have high error thresholds, are also extremely robust against losses. We demonstrate that these schemes tolerate loss rates up to 24.9%, determined by bond percolation on a cubic lattice. Our numerical results show that these schemes retain good performance when loss and computational errors are simultaneously present.     

Time-Bin Phase-Encoding Measurement-Device-Independent Quantum Key Distribution with Four Single-Photon Detectors

Measurement-device-independent quantum key distribution(MDI-QKD) eliminates all loopholes on detection.Previous experiments of time-bin phase-encoding MDI-QKD allow a factor of 3/4 loss in the final key for the incapability of identifying two successive detection events by a single photon detector.Here we propose a new scheme to realize the time-bin phase-encoding MDI-QKD.The polarization states are used to generate the time bins and the phase-encoding states.The factor of loss in the final key is eliminated by using four single photon detectors at the measurement site.We show the feasibility of our scheme with a proof-of-principle experimental demonstration.The phase reference frame is rotated extremely slowly with only passive stabilization measures.The quantum bit error rate can reach 0.8% in the Z-basis and 26.2% in the X-basis.