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Physical Review A

Continuous-variable quantum swapping gate between light and atoms

T. Takano, M. Fuyama, Ryo Namiki, Yoshiro Takahashi

Published Jul 15, 2008• 17 citations
Publisher

Abstract

In the face of the recent development of the time-domain detection method for short pulses, typically less than 1 microsecond, we consider a feasible method for a continuous variable quantum swapping gate between light polarization and atomic spin ensemble by three successive Faraday rotation interactions using short pulses. This method does not need any particular initial states and its fidelity can ideally run up to one. It also works as a quantum memory of light and a device to generate a nonclassical state of the spin. To evaluate a quantum nature of the proposed swapping gate, we calculated its average fidelity for a set of the coherent states with a Gaussian distribution and confirmed the efficiency as a quantum gate as well as a quantum memory of light.

DOI
10.1103/physreva.78.010307

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In the face of the recent development of the time-domain detection method for short pulses, typically less than 1 microsecond, we consider a feasible method for a continuous variable quantum swapping gate between light polarization and atomic spin ensemble by three successive Faraday rotation interactions using short pulses. This method does not need any particular initial states and its fidelity can ideally run up to one. It also works as a quantum memory of light and a device to generate a nonclassical state of the spin. To evaluate a quantum nature of the proposed swapping gate, we calculated its average fidelity for a set of the coherent states with a Gaussian distribution and confirmed the efficiency as a quantum gate as well as a quantum memory of light.
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Summary form only given. A new approach to the problem of the quantum interface between light and atoms has been recently developed. The method utilizes free space dispersive interaction between pulses of light and spin polarized atomic ensembles. Entanglement between the polarization state of light and the collective spin state of atoms is established by measurement, more precisely by detection of light in a certain polarization basis. In the first demonstration of this approach we have generated a long-lived entangled state of two separate macroscopic atomic samples by a polarization measurement on light transmitted through the samples. We then have shown that this approach also works for mapping of a quantum state of light onto long-lived atomic spin state paving the road towards realization of the quantum memory for light. Recently we have also extended the free space light-atoms interface beyond the continuous variables in the proposal, where a coherent superposition "cat" state of an atomic ensemble is generated by a measurement sequence. Progress with other communication protocols such as atomic state teleportation and multiparty networks will be presented.
A set of protocols for atoms-photons and atoms-atoms quantum state teleportation and swapping utilizing Einstein-Podolsky-Rosen light is proposed. The protocols work for polarization quantum states of multiphoton light pulses and macroscopic samples of atoms, i.e., for continuous quantum variables. A simple free space interaction of polarized light with a spin polarized atomic ensemble is shown to suffice for these protocols. Feasibility of experimental realization using gas samples of atoms is analyzed.
The continuous-variable controlled-ZZZ gate is a canonical two-mode gate for universal continuous-variable quantum computation. It is considered as one of the most fundamental continuous-variable quantum gates. Here we present a scheme for realizing continuous-variable controlled-ZZZ gate between two optical beams using an atomic ensemble. The gate is performed by simply sending the two beams propagating in two orthogonal directions twice through a spin-squeezed atomic medium. Its fidelity can run up to one if the input atomic state is infinitely squeezed. Considering the noise effects due to atomic decoherence and light losses, we show that the observed fidelities of the scheme are still quite high within presently available techniques.
We present an experimentally feasible protocol for the complete storage and retrieval of arbitrary light states in an atomic quantum memory using the Faraday interaction between light and matter. Our protocol relies on multiple passages of a single light pulse through the atomic ensemble without the impractical requirement of kilometer-long delay lines between the passages. A time-dependent interaction strength enables the storage and retrieval of states with arbitrary pulse shapes. The fidelity approaches unity exponentially without squeezed or entangled initial states, as illustrated by calculations for a photonic qubit.
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