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AIP conference proceedings

Protecting an EPR state by quantum engineering of decoherence

Russell Bloomer, Matthew Pysher, Olivier Pfister, Timothy C. Ralph, Ping Koy Lam

Published Jan 1, 2011• 0 citations
Publisher

Abstract

Quantum‐state decoherence is the major obstacle to the implementation of quantum computing and quantum information. Here we present an experimental proof of principle for the quantum engineering of decohering interactions. An optically generated Einstein‐Podolski‐Rosen (EPR) entangled state was subjected to severe loss by placing a 50% beam splitter in the path of each entangled light beam. The resulting severe degradation of the signature quantum correlations between the two beams was subsequently alleviated when another, independent EPR state impinged upon the other input ports of the beam splitter, effectively entangling the losses inflicted to the initial EPR state. The additional EPR beam pair was classically coherent with the primary one but had no quantum correlations with it. This result may also find application as a “quantum tap” for entanglement.

DOI
10.1063/1.3630186

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Quantum‐state decoherence is the major obstacle to the implementation of quantum computing and quantum information. Here we present an experimental proof of principle for the quantum engineering of decohering interactions. An optically generated Einstein‐Podolski‐Rosen (EPR) entangled state was subjected to severe loss by placing a 50% beam splitter in the path of each entangled light beam. The resulting severe degradation of the signature quantum correlations between the two beams was subsequently alleviated when another, independent EPR state impinged upon the other input ports of the beam splitter, effectively entangling the losses inflicted to the initial EPR state. The additional EPR beam pair was classically coherent with the primary one but had no quantum correlations with it. This result may also find application as a “quantum tap” for entanglement.
We present an experimental analysis of quadrature entanglement produced from a pair of amplitude squeezed beams. The correlation matrix of the state is characterized within a set of reasonable assumptions, and the strength of the entanglement is gauged using measures of the degree of inseparability and the degree of Einstein-Podolsky-Rosen (EPR) paradox. We introduce controlled decoherence in the form of optical loss to the entangled state, and demonstrate qualitative differences in the response of the degrees of inseparability and EPR paradox to this loss. The entanglement is represented on a photon number diagram that provides an intuitive and physically relevant description of the state. We calculate efficacy contours for several quantum information protocols on this diagram, and use them to predict the effectiveness of our entanglement in those protocols.
It is theoretically proved that the two output light beams can be used as a pair of EPR(Einstin-Podolsky-Rosen) entangled beams when the incident light upon a beam splitter is a single mode quadrature-squeezed coherent state or a squeezed quantum soliton. It is the simplest scheme for preparing entangled states to our knowledge. The formulas explicitly show the correlation between the two nonclassical optical fields: squeezed states and entangled states. The affection of squeezing degree on entanglement is also investigated.
Quantum entanglement shared by more than two partites is the essential bases for developing quantum communication networks and quantum computation. The multipartite entanglement of continuous variables can be generated by distributing or combining squeezed states of light using linear optics. We experimentally obtained a tripartite entangled state of bright optical field by means of a nondegenerate optical parametric amplifier (NOPA) and optical beam-splitters. The NOPA is operated in the state of parametric deamplification by locking the relative phase between the pump laser and the injected seed wave to (2n+1)π, so the generated Einstein-Podolsky-Rosen (EPR) beam for continuous variables is an entangled state of light with the quantum correlation of phase quadratures and the quantum anticorrelation of amplitude quadratures [1]. The output EPR beam consists of two frequency-degenerate optical modes with horizontal and vertical polarizations which are distributed to three parties with two linear beam-splitters. We have theoretically and experimentally demonstrated the three parties are in a tripartite entangled state with three-mode “position” (amplitude quadrature) correlation and relative “momentum” (phase quadrature) correlation [2]-[4]. The obtained GHZ-like state is a fully inseparable tripartite continuous-variables state violating the three inequalities used for the criterion of the inseparability [5][6]. The controlled dense coding quantum communication is experimentally accomplished by exploiting the tripartite entanglement. The three entangled modes are sent to a sender (Alice), a receiver (Bob) and a controller (Claire), respectively. The information transmission capacity of the quantum channel between Alice and Bob is controlled by Claire. The channel capacity completed under Claire’s help is always larger than that without her help.
Entanglement of light and matter is an essential resource for effective quantum engineering. In particular, collective states of atomic ensembles are robust against decoherence while preserving the possibility of strong interaction with quantum states of light. While previous approaches to continous-variable quantum interfaces relied on quadratures of light, here we present an approach based on spatial structure of light-atom entanglement. We create and characterize a 12-dimensional entangled state exhibiting quantum correlations between a photon and an atomic ensemble in position and momentum bases. This state allows us to demonstrate the original Einstein-Podolsky-Rosen (EPR) paradox with two different entities, with an unprecedented delay time of 6 μμμs between generation of entanglement and detection of the atomic state.
Einstein-Podolsky-Rosen (EPR) entanglement is a criterion that is more demanding than just certifying entanglement. We theoretically and experimentally analyze the low-resource generation of bipartite continuous-variable entanglement, as realized by mixing a squeezed mode with a vacuum mode at a balanced beam splitter, i.e., the generation of so-called vacuum-class entanglement. We find that in order to observe EPR entanglement the total optical loss must be smaller than 33.333.333.3 %. However, arbitrarily strong EPR entanglement is generally possible with this scheme. We realize continuous-wave squeezed light at 155015501550 nm with up to 9.99.99.9 dB of nonclassical noise reduction, which is the highest value at a telecom wavelength so far. Using two phase-controlled balanced homodyne detectors we observe an EPR covariance product of $0.502\ifmmode\pm\else\textpm\fi{}0.006<1$, where 111 is the critical value. We discuss the feasibility of strong Gaussian entanglement and its application for quantum key distribution in a short-distance fiber network.
We experimentally demonstrate the creation and characterization of Einstein-Podolsky-Rosen (EPR) correlation between optical beams in a time-gated fashion. The correlated beams are created with two independent continuous-wave optical parametric oscillators and a half beam splitter. We define the temporal modes using a square temporal filter with duration TTT and make time-resolved measurements on the generated state. We observe correlations between the relevant conjugate variables in the temporal mode which correspond to EPR correlation. Our scheme is extendable to continuous-variable quantum teleportation of a non-Gaussian state defined in the time domain such as a superposition of coherent states.