Projective measurements via linear optics and photon detectors

The possibility of implementing a given photonic projective measurement with linear optics and photon detectors is discussed. This problem can be viewed as a single-shot discrimination of orthogonal pure quantum states. It is particularly shown that any two orthogonal states can be perfectly discriminated using only linear optics, photon counting, coherent ancillary states, and feedforward. This means that one can construct any binary projective measurement with these means, but without any nonclassical ancillary state. The statement holds in the asymptotic limit of a large number of these physical resources. To extend this result, we also address the problem of discriminating a simple set of three orthogonal states. The text was submitted by the author in English.     

Complete four-photon cluster-state analyzer based on cross-Kerr nonlinearity

We propose a method to construct an optical cluster-state analyzer based on cross-Kerr nonlinearity combined with linear optics elements. In the scheme, we employ two four-qubit parity gates and the controlled phase gate （CPG） from only the cross-Kerr nonlinearity and show that all the orthogonal four-qubit cluster states can be completely identified. The scheme is significant for the large-scale quantum communication and quantum information processing networks. In addition, the scheme is feasible and deterministic under current experimental conditions.     

Linear optics implementation for quantum game under quantum noise

It has recently been shown that linear optics alone would suffice to implement efficient quantum computation. Quantum computation circuits using coherent states as the logical qubits can be constructed from very simple linear networks, conditional measurements and coherent superposition resource states. We present the quantum game under quantum noise and a proposal for implementing the noisy quantum game using only linear optics.     

Complete <Emphasis Type="Italic">N</Emphasis>-Photon Greenberger-Horne-Zeilinger State Analysis Using Hyperentanglement

We present a scheme for N-photon Greenberger-Horne-Zeilinger (GHZ) state analysis using hyperentanglement in polarization and time-bin degrees of freedom. The scheme only needs linear optics elements and single-photon detectors, which is feasible with current technology. The set of 2 ^N mutual orthogonal states can be unambiguously distinguished and the protocol is expected to find useful applications in quantum information processing.     

Quantum teleportation of optical quantum gates

We show that a universal set of gates for quantum computation with optics can be quantum teleported through the use of EPR entangled states, homodyne detection, and linear optics and squeezing operations conditioned on measurement outcomes. This scheme may be used for fault-tolerant quantum computation in any optical scheme (qubit or continuous-variable). The teleportation of nondeterministic nonlinear gates employed in linear optics quantum computation is discussed.     

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.     

A Simple Method on Generating any Bi-Photon Superposition State with Linear Optics

We present a simple method on the generation of any bi-photon superposition state using only linear optics.In this scheme, the input states, a two-mode squeezed state and a bi-photon state, meet on a beam-splitter and the output states are post-selected with two threshold single-photon detectors. We carry out corresponding numerical simulations by accounting for practical experimental conditions, calculating both the Wigner function and the state fidelity of those generated bi-photon superposition states. Our simulation results demonstrate that not only distinct nonclassical characteristics but also very high state fidelities can be achieved even under imperfect experimental conditions.     

Scheme for generation of four-atom Greenberger-Horne-Zeilinger states in an optical cavity

We propose a scheme for generating maximally GHZ state for four atoms trapped in a two-mode optical cavity via combination of cavity QED and linear optics system. The GHZ state can be not only generated deterministically with a single resonant interaction in cavity QED, but also can be prepared probabilistically based on cavity QED and linear optics elements. The fidelity of the entangled states is not affected by the atomic spontaneous, cavity decay, and imperfection of the photon-detectors. Finally, we briefly analyze and discuss the experimental feasibility of the proposed scheme.     

基于超纠缠交换的量子密钥分发

给出一种基于超纠缠交换的量子密钥分发方案，这个方案可以同时产生确定密钥和随机密钥，并且它的安全性不受任何损害.这个方案只需要一对在空间(路径)模式和极化模式上超纠缠的光子就能获得4 bit的密钥(2 bit随机密钥和2 bit确定密钥).在目前的实验条件下，这个方案可以通过线性光学来实现. 关键词： 量子密钥分发 超纠缠 线性光学     

Generation and classification of robust remote symmetric Dicke states

In this paper, we present an approach to generating arbitrary symmetric Dicke states with distant trapped ions and linear optics. Distant trapped ions can be prepared in the symmetric Dicke states by using two photon-number-resolving detectors and a polarization beam splitter. The atomic symmetric Dicke states are robust against decoherence, for atoms are in a metastable level. We discuss the experimental feasibility of our scheme with current technology. Finally, we discuss the classification of arbitrary n-qubit symmetric Dicke states under statistical local operation and classical communication and prove the existence of [n/2] inequivalent classes of genuine entanglement of n-qubit symmetric Dicke states.     

Discrimination of quantum states with linear optics and continuous photon counting

We investigate a systematic method of implementation of given measurements with linear optics and continuous measurement. We revisit the so-called Dolinar receiver, which is a quasi-physical model attaining the minimum error discrimination of binary coherent states, and show that it can be derived without heuristic assumptions. Our approach also can be applied to various kinds of measurements.     

Polarization-momentum hyper-entangled two photon states

We present a parametric source which allows the engineering of polarization-momentum hyperentangled two photon states based on linear optics and a single type-I nonlinear crystal. The nonlocal character of these states has been verified by various tests, including the “All Versus Nothing” test of local realism [A. Cabello, Phys. Rev. Lett. 87, 010403 (2001)], which represents a generalization of the GHZ to the case of two entangled particles and two observers. We have also created a complete and deterministic Bell-state measurement by a novel experimental scheme which adopts polarization-momentum hyper-entanglement and requires linear optics and single photon detectors. The text was submitted by the authors in English.     

Hybrid long-distance entanglement distribution protocol

We propose a hybrid (continuous-discrete variable) quantum repeater protocol for long-distance entanglement distribution. Starting from states created by single-photon detection, we show how entangled coherent state superpositions can be generated by means of homodyne detection. We show that near-deterministic entanglement swapping with such states is possible using only linear optics and homodyne detectors, and we evaluate the performance of our protocol combining these elements.     

Linear optical scheme for entanglement concentration of two partially entangled three-photon ${\sf W}$ states

We propose a practical scheme for concentrating entanglement in a pair of unknown partially entangled three-photon W states with only linear optics and photon detectors. In the scheme, Alice, Bob, and Charlie at three distant parties can obtain one maximally entangled three-photon W state with a certain success probability from two identical partially entangled three-photon W states by local operations and classical communication. The proposed setup is very simple, which greatly simplifies the experimental realization of the scheme.     

Computable measure of nonclassicality for light

We propose the entanglement potential (EP) as a measure of nonclassicality for quantum states of a single-mode electromagnetic field. It is the amount of two-mode entanglement that can be generated from the field using linear optics, auxiliary classical states, and ideal photodetectors. The EP detects nonclassicality, has a direct physical interpretation, and can be computed efficiently. These three properties together make it stand out from previously proposed nonclassicality measures. We derive closed expressions for the EP of important classes of states and analyze as an example of the degradation of nonclassicality in lossy channels.     

Binary polarization pupil filter: Theoretical analysis and experimental realization with a liquid crystal display

The Jones matrix formalism has been applied to evaluate the response of an optical system when a non-uniform polarizing pupil is introduced. With this formalism we analyze and experimentally demonstrate the properties of a binary polarization pupil filter having two regions with two orthogonal linear polarization orientations. We first study the case when no analyzer is placed behind the pupil filter, and both, the transversal and the axial behavior are described in terms of the intensity and the local state of polarization. Then it is shown how the response of the optical system can be easily changed through the orientation of an analyzer placed behind the pupil. We experimentally verified the theory using a twisted nematic liquid crystal display, which produces two orthogonal linear polarization states for two different addressed voltages.     

纠缠相干态的量子隐形传态

提出利用双模纠缠相干态作为量子信道，实现纠缠相干态量子隐形传态的方案. 分别在非正交的相干态表象和另一个由相干态构成的正交态表象对双模纠缠相干态的隐形传态进行具体分析. 发现在相干态表象，虽然只要线性光学元件就可以完成隐形传态，但成功的概率小于1/2，而在正交态表象，只要能分辨4个由相干态构成的Bell态，成功的概率就是1. 关键词： 隐形传态 纠缠相干态 纠缠相干态量子信道     

Experiments of quantum nonlocality with polarization-momentum entangled photon pairs

We present the results of some experimental tests of quantum nonlocality performed by two-photon states, entangled both in polarization and momentum, namely hyperentangled states and two-photon four-qubit linear cluster states. These states, which double the number of available qubits with respect to the standard two-photon entangled states, are engineered by a simple experimental method, which adopts linear optics and a single type I nonlinear crystal. The tests of local realism performed with these states represent a generalization of the Greenberger, Home, and Zeilinger (GHZ) theorem to the case of two entangled particles.     

Generic entanglement and standard form for N-mode pure Gaussian states

We investigate the correlation structure of pure N-mode Gaussian resources which can be experimentally generated by means of squeezers and beam splitters, whose entanglement properties are generic. We show that those states are specified (up to local unitaries) by N(N-1)/2 parameters, corresponding to the two-point correlations between any pair of modes. Our construction yields a practical scheme to engineer such generic-entangled N-mode pure Gaussian states by linear optics. We discuss our findings in the framework of Gaussian matrix product states of harmonic lattices, raising connections with entanglement frustration and the entropic area law.     

Filtering out photonic Fock states

Unprecedented optical nonlinearities can be generated probabilistically in simple linear-optical networks conditioned on specific measurement outcomes. We describe a highly controllable quantum filter for photon number states, which takes advantage of such a measurement-induced amplitude nonlinearity. The basis for this filter is multiphoton nonclassical interference which we demonstrate for one- and two-photon states over a wide range of beam splitter reflectivities. Specifically, we show that the transmission probability, conditional on a specific measurement outcome, can be larger for a two-photon state than a one-photon state; this is not possible with linear optics alone.