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arXiv:1505.04991 · Physical review. B./Physical review. B

Exciton-polariton quantum gates based on continuous variables

Oleksandr Kyriienko, T. C. H. Liew

Published Jan 4, 2016• 47 citations
arXivHTMLPDF

Abstract

We propose a continuous-variable analog of quantum controlled-not gates based on a system of exciton polaritons in semiconductor microcavities. This can be realized by engineering a parametric interaction between control and target polariton modes, which can be varied in time. As an explicit setup we use a system of dipolaritons, which allows for enhancement of parametric interaction by auxiliary classical fields and scalable multigate system realization. We find that high operational fidelity for the proposed gate is achievable for realistic system parameters.

DOI
10.1103/physrevb.93.035301

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Abstract Exciton-polariton condensates have attractive features for quantum computation, e.g., room temperature operation, high dynamical speed, ease of probe, and existing fabrication techniques. Here, we present a complete theoretical scheme of quantum computing with exciton-polariton condensates formed in semiconductor micropillars. Quantum fluctuations on top of the condensates are shown to realize qubits, which are externally controllable by applied laser pulses. Quantum tunneling and nonlinear interactions between the condensates allow SWAP, square-root-SWAP and controlled-NOT gate operations between the qubits.
Temporally controlled optical logic provides a promising route toward ultrafast information processing. However, implementing conditional multi-bit logic in non-equilibrium systems with ultrashort lifetimes remains highly non-trivial. Here, we demonstrate a classical 2-bit controlled-NOT (CNOT) gate operated in the time domain in an exciton–polariton microcavity at room temperature. The logic functionality is enabled by the interplay between a transient excitonic reservoir and bosonic cascading dynamics, which together provide a short-lived but controllable memory linking two sequential optical inputs. By encoding logical states in the temporal sequence and energy redistribution of polariton condensates, we realize all four truth-table operations of a classical CNOT gate on picosecond timescales. The gate operation exhibits clear tolerance to timing variation and reproducible recovery dynamics. Our results establish a physical mechanism for temporally conditional logic in non-equilibrium polaritonic systems, offering a general framework for multi-bit optical operations mediated by reservoir-assisted nonlinear dynamics.
Summary form only given. The optical coherent control of quantum mechanical systems enables the direct manipulation of quantum states, and has been demonstrated on excitonic transitions in semiconductors. We have demonstrated the coherent control of the nonlinear optical response of quantum-well excitons in a resonant high-Q microcavity. There are two significant advantages to using excitons in microcavities instead of bare quantum wells: first, the multiple reflections which occur in the cavity strongly enhance the optical nonlinearity, enabling large-signal coherent control, and second, 2-color coherent control may be implemented, where a pump pulse tuned to one of the normal modes of the coupled exciton-microcavity system can control the response of a probe tuned to the other normal mode. The nonlinear response of the normal modes is due to the nonlinearity of the underlying exciton transition.
We propose a Bell-measurement node for quantum repeaters based on a planar semiconductor microcavity operating in the strong-coupling regime. Cavity photons hybridize with quantum-well excitons to form polaritons combining properties of photons and matter quasiparticles. A control photon loaded into one polariton mode changes the polarization response seen by a subsequently incident target photon. This conditional rotation is governed by the interplay of self-induced Larmor precession triggered by spin-dependent exciton-exciton interactions and the polarization beats caused by the splitting of transverse-electric and transverse-magnetic cavity modes. We identify conditions of the experiment that enable implementation of a controlled-Z gate and allow to distinguish all four Bell states in the ideal limit. The one-sided scattering scheme provides a lower interaction threshold than a scalar Kerr reference under the same assumptions. At a selected operating point, the bandwidth-induced identification error scales as the fourth power of the pulse bandwidth in the narrow-band limit. An additional fixed input rotation reduces this error even further. We describe the entanglement swapping between remote memories and determine the minimum quality of the elementary links needed to obtain an entangled output.