Heralded Universal Quantum Gate and Entangler Assisted by Imperfect Double‐Sided Quantum‐Dot‐Microcavity Systems

Efficient ways are presented to accomplish photonic controlled‐phase‐flip gate and entangler with the assistance of imperfect double‐sided quantum‐dot‐microcavity systems, but without ancillary qubits. Compact quantum circuits for implementing entanglement swapping between photon pairs and electron pairs are then designed. Unity fidelities of the schemes can be achieved, and physical imperfections in the construction processes are detected by single‐photon detectors. Also, the efficiencies of the schemes can be further improved by repeating the operation processes when the undesired performances are detected. The evaluations show that the schemes are possible with current experiment parameters.     

A new optical scheme for teleportation of entangled coherent state

We propose a nearly perfect optical scheme for the quantum teleportation of entangled coherent states using optical devices such as nonlinear Kerr media, beam splitters, phase shifters, and photon detectors. Different from those previous schemes, our scheme needs only ``yes' or `no' measurements of the photon number of the related modes, i.e. nonzero- and zero-photon measurements, while in previous schemes one has to exactly identify the even or odd parity character of the photon numbers detected by detectors.     

Teleportation of a general two-mode coherent-state superposition via attenuated quantum channels with ideal and/or threshold detectors

Three novel probabilistic yet conclusive schemes are proposed to teleport a general two-mode coherent-state superposition via attenuated quantum channels with ideal and/or threshold detectors. The calculated total success probability is highest (lowest) when only ideal (threshold) detectors are used.     

Generation of Four-Photon Cluster State with Coherent Light via Cross-Kerr Nonlinearity

We propose two schemes for preparing four-photon cluster state through cross-Kerr nonlinearity. Two coherent fields interact when they enter a nonlinear Kerr medium. If the interaction time is chosen appropriately in each Kerr medium, four-photon cluster state can be generated based on the results of two homodyne detectors in the first scheme. These schemes only use Kerr medium and homodyne measurements on coherent light fields, which can be efficiently made in quantum optical laboratories. In addition, weak cross-Kerr nonlinearity is sufficient. All of the properties make these schemes feasible in experiments.     

Detecting itinerant single microwave photons

Single-photon detectors are fundamental tools of investigation in quantum optics and play a central role in measurement theory and quantum informatics. Photodetectors based on different technologies exist at optical frequencies and much effort is currently being spent on pushing their efficiencies to meet the demands coming from the quantum computing and quantum communication proposals. In the microwave regime, however, a single-photon detector has remained elusive, although several theoretical proposals have been put forth. In this article, we review these recent proposals, especially focusing on non-destructive detectors of propagating microwave photons. These detection schemes using superconducting artificial atoms can reach detection efficiencies of 90% with the existing technologies and are ripe for experimental investigations.     

Two-qubit and three-qubit controlled gates with cross-Kerr nonlinearity

Schemes for two-qubit and three-qubit controlled gates based on cross-Kerr nonlinearity are proposed in this paper.The probability of the success of these gates can be increased by quantum nondemolition detectors,which are used to judge which paths the signal photons pass through.These schemes are almost deterministic and require no ancilla photon.The advantages of these gates over the existing ones include less resource consumption and a higher probability of success,which make our schemes more feasible with current technology.     

Design of the 10 m AEI prototype facility for interferometry studies

The AEI 10 m prototype interferometer facility is currently being constructed at the Albert Einstein Institute in Hannover, Germany. It aims to perform experiments for future gravitational wave detectors using advanced techniques. Seismically isolated benches are planned to be interferometrically interconnected and stabilized, forming a low-noise testbed inside a 100 m³ ultra-high vacuum system. A well-stabilized high-power laser will perform differential position readout of 100 g test masses in a 10 m suspended arm-cavity enhanced Michelson interferometer at the crossover of measurement (shot) noise and back-action (quantum radiation pressure) noise, the so-called Standard Quantum Limit (SQL). Such a sensitivity enables experiments in the highly topical field of macroscopic quantum mechanics. In this article we introduce the experimental facility and describe the methods employed; technical details of subsystems will be covered in future papers.     

Robust polarization-based quantum key distribution over a collective-noise channel

We present two polarization-based protocols for quantum key distribution. The protocols encode key bits in noiseless subspaces or subsystems and so can function over a quantum channel subjected to an arbitrary degree of collective noise, as occurs, for instance, due to rotation of polarizations in an optical fiber. These protocols can be implemented using only entangled photon-pair sources, single-photon rotations, and single-photon detectors. Thus, our proposals offer practical and realistic alternatives to existing schemes for quantum key distribution over optical fibers without resorting to interferometry or two-way quantum communication, thereby circumventing, respectively, the need for high precision timing and the threat of Trojan horse attacks.     

Continuous-Variable Quantum Teleportation of Entangled Coherent States

We propose a quantum teleportation scheme for tripartite entangled coherent state （ECS） with continuous variable. Our scheme is feasible and economical in the sense that we need only linear optical devices such as beam splitters, phase shifters and photon detectors and employ three bipartite maximally ECSs as quantum channels. We also generalize the tripartite scheme into multipartite ease and calculate the minimum average fidelity for the schemes in tripartite and multipartite cases.     

Effective schemes for preparation of Greenberger–Horne–Zeilinger and W maximally entangled states with cross-Kerr nonlinearity and parity-check measurement

Schemes to generate Greenberger–Horne–Zeilinger and W maximally entangled states of distant photons with the help of cross-Kerr nonlinearity and parity-check measurement are proposed in this paper. The schemes are based on optical elements, single polarization photons, cross-Kerr nonlinearity, and the conventional photon detectors, which are feasible with existing experimental technology. The schemes are quite different a higher success probability, without the resorting to collective unitary evolution. All these advantages make present schemes more efficient and more convenient than others in the applications in quantum communication.     

Quantum phase gate in decoherence-free subspace with cavity QED system

We demonstrate how to perform quantum phase gate with cavity QED system in decoherence-free subspace by using only linear optics elements and photon detectors. The qubits are encoded in the singlet state of the atoms in cavities among spatially separated nodes, and the quantum interference of polarized photons decayed from the optical cavities is used to realized the desired quantum operation among distant nodes. In comparison with previous schemes, the distinct advantage is that the gate fidelity could not only resist collective noises, but also immune from atomic spontaneous emission, cavity decay, and imperfection of the photodetectors. We also discuss the experimental feasibility of our scheme.     

Effective protocol for preparation of three-atom Greenberger-Horne-Zeilinger state and W state with the help of cross-Kerr nonlinearity

We propose a protocol to generate a Greenberger-Horne-Zeilinger (GHZ) state andWstate by using simple linear elements and quantum nondemolition detectors (QNDs). With the help of cross-Kerr nonlinearity, our protocol can generate the intended states with only one setup, and the probability of getting a W state is greatly increased when compared with previous schemes [Phys. Rev. A 75 (2007) 044301]. Also, our proposed protocol is realizable in experiments.     

Measurement-device-independent quantum key distribution with hyper-encoding

Measurement device-independent quantum key distribution(MDI-QKD) protocols are immune to all possible attacks on the photon detectors during quantum communication, but their key generation rates are low compared with those of other QKD schemes.Increasing each individual photon's channel capacity is an efficient way to increase the key generation rate, and high-dimensional(HD) encoding is a powerful tool for increasing the channel capacity of photons. In this paper, we propose an HD MDI-QKD protocol with qudits hyper-encoded in spatial mode and polarization degrees of freedom(DOFs). In the proposed protocol, keys can be generated using the spatial mode and polarization DOFs simultaneously. The proposed protocol is unconditionally secure,even for weak coherent pulses with decoy states. The proposed MDI-QKD protocol may be useful for future quantum secure communication applications.     

Knill-Laflamme-Milburn quantum computation with bosonic atoms

A Knill-Laflamme-Milburn (KLM) type quantum computation with bosonic neutral atoms or bosonic ions is suggested. Crucially, as opposite to other quantum computation schemes involving atoms (ions), no controlled interactions between atoms (ions) involving their internal levels are required. Versus photonic KLM computation, this scheme has the advantage that single-atom (ion) sources are more natural than single-photon sources, and single-atom (ion) detectors are far more efficient than single-photon ones.     

Approximate Real–Time Visualization of a Quantum Transition by Means of Continuous Fuzzy Measurement

For the detection of gravitational waves the quantum mechanical properties of the detector have to be taken into account. Not all gravitational wave detectors allow a quantum nondemolition (QND) measurement. Continuous weak or fuzzy measurements are an alternative to study the evolution of a quantum mechanical system under the influence of an external field. In the present paper we investigate this alternative by applying it to a simplified system. We numerically simulate continuous fuzzy measurements of the oscillations of a two-level atom subjected to a resonant external light field. We thereby address the question whether it is possible to measure characteristic features of the evolution of a single quantum system in real time without relying on a QND scheme. We compare two schemes of continuous measurement: continuous measurement with constant fuzziness and with fuzziness changing in the course of the measurement. Because the sensitivity of the two-level atom to the influence of the measurement depends on the state of the atom, it is possible to optimize the continuous fuzzy measurement by varying its fuzziness.     

Quantum information processing in optical images

Optical images can be used to transport, store and process information in a parallel way. We discuss different results obtained in the domain of ‘quantum imaging’, aiming at exploiting at the same time the quantum properties of optical images and their intrinsic parallelism. We define the notion of standard quantum limit (SQL) in optical resolution, set by the quantum noise of usual coherent light, and show that it can be much lower than the diffraction limit. We also prove that this limit can be circumvented by especially designed nonclassical and multimode light. We present an experiment showing that OPOs oscillating inside an exactly confocal cavity actually produce such transverse multimode nonclassical light. We finally describe another experiment which has surpassed the SQL in the case of beam positioning, both in the 1D and 2D cases.     

Repeat-until-success linear optics distributed quantum computing

We demonstrate the possibility to perform distributed quantum computing using only single-photon sources (atom-cavity-like systems), linear optics, and photon detectors. The qubits are encoded in stable ground states of the sources. To implement a universal two-qubit gate, two photons should be generated simultaneously and pass through a linear optics network, where a measurement is performed on them. Gate operations can be repeated until a success is heralded without destroying the qubits at any stage of the operation. In contrast with other schemes, this does not require explicit qubit-qubit interactions, a priori entangled ancillas, nor the feeding of photons into photon sources.     

Efficient algorithm for optimizing adaptive quantum metrology processes

Quantum-enhanced metrology infers an unknown quantity with accuracy beyond the standard quantum limit (SQL). Feedback-based metrological techniques are promising for beating the SQL but devising the feedback procedures is difficult and inefficient. Here we introduce an efficient self-learning swarm-intelligence algorithm for devising feedback-based quantum metrological procedures. Our algorithm can be trained with simulated or real-world trials and accommodates experimental imperfections, losses, and decoherence.     

Quantum improvement of time transfer between remote clocks

Exchanging light pulses to perform accurate space-time positioning is a paradigmatic issue of physics. It is ultimately limited by the quantum nature of light, which introduces fluctuations in the optical measurements and leads to the so-called standard quantum limit (SQL). We propose a new scheme combining homodyne detection and mode-locked femtosecond lasers that lead to a new SQL in time transfer, potentially reaching the yoctosecond range (10(-21)-10(-24) s). We demonstrate that this already very low SQL can be overcome using appropriately multimode squeezed light. Benefitting from the large number of photons and from the optimal choice of both the detection strategy and of the quantum resource, the proposed scheme represents a significant potential improvement in space-time positioning.     

基于提高精度的量子雷达方案及其进展

定位测量一直是人们关注的问题,将量子技术应用于定位任务有望展示出经典定位方案无 法达到的优势。目前,已有部分量子雷达方案被提出,展示了科研工作者从量子信息角度对定位 测量的新思考。本文总结了部分已有的量子雷达方案,介绍了相关方案的概念和分类,并着重对 量子定位、量子照明和三维增强雷达三种方案的基本原理进行论述,同时分析了各方案的优势以 及亟待解决的问题。