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Physical Review A

Efficient two-mode squeezing and quantum-state teleportation of macroscopic atomic ensembles

Mingfeng Wang, Yi Zhang, Nian-Quan Jiang, Yi-Zhuang Zheng

Published Jan 29, 2009• 9 citations
Publisher

Abstract

We present a protocol for deterministically generating an optimal two-mode-squeezed state of distant free-space atomic ensembles. The protocol relies on two passes of a single coherent pulse through two separate atomic ensembles placed in homogeneous magnetic fields. In contrast to previous single-pass schemes, our approach significantly enhances the degree of entanglement due to the exponential behavior of the Einstein-Podolsky-Rosen variance. We also provide an efficient scheme to perform Bell measurement of atomic states, enabling the implementation of high-fidelity atomic quantum teleportation.

DOI
10.1103/physreva.79.012327

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We present a protocol for the teleportation of the quantum state of a pulse of light onto the collective spin state of an atomic ensemble. The entangled state of light and atoms employed as a resource in this protocol is created by probing the collective atomic spin, Larmor precessing in an external magnetic field, off resonantly with a coherent pulse of light. We take here full account of the effects of Larmor precession and show that it gives rise to a qualitatively different type of multimode entangled state of light and atoms. The protocol is shown to be robust against the dominating sources of noise and can be implemented with an atomic ensemble at room temperature interacting with free-space light. We also provide a scheme to perform the readout of the Larmor precessing spin state enabling the verification of successful teleportation as well as the creation of spin squeezing.
We present two protocols, one for the storage of light in an atomic ensemble and the subsequent retrieval, and another one for the generation of entanglement between light and atoms. They rely on two passes of a single pulse through the ensemble, Larmor precessing in an external field. Both protocols work deterministically and the relevant figures of merit---such as the fidelity or the EPR variance---scale exponentially in the coupling strength. We solve the corresponding Maxwell-Bloch equations describing the scattering process and determine the resulting input-output relations which only involve one relevant light mode that, in turn, can be easily accessed experimentally.
We present an experimental feasible scheme to generate two-mode entangled states of two spatially separated atomic spin ensembles, using superconducting quantum circuits consisting of two coplanar waveguide resonators and a charge qubit. We show that, with the charge qubit as a tunable coupler and the resonator modes as a quantum bus, the collective atomic spins can be steered into a two-mode squeezed state exhibiting Einstein-Podolsky-Rosen entanglement through adiabatic following of the ground state of the system.
We propose an efficient scheme for the generation of two-mode squeezed states for two separated atomic ensembles trapped in distant cavities. The scheme is based on selective couplings between the collective atomic modes and two linearly transformed common field modes mediated by an optical fiber or a third cavity. The quanta of the transformed atomic modes are exhausted due to the linear coupling with the transformed field modes, bringing the original atomic modes into the two-mode squeezed states. The experimental implementation of the scheme would be an important step toward quantum communication and networking with continuous variables.
We develop methods for performing quantum teleportation of the total spin variables of an unknown state, using quantum nondemolition measurements, spin projection measurements, and classical communication. While, theoretically, teleportation of high-dimensional states can be attained with the assumption of generalized Bell measurements, this is typically experimentally nontrivial to implement. We introduce two protocols and show that on average, the teleportation succeeds in teleporting the spin variables of a spin coherent state with average zero angular error in the ideal case, beating classical strategies based on quantum state estimation. In a single run of the teleportation, there is an angular error at the level of $\ensuremath{\sim}0.1$ radians for large ensembles. A potential physical implementation for the scheme is with atomic ensembles and quantum nondemolition measurements performed with light. We analyze the decoherence of the protocols and find that the protocol is robust even in the limit of large ensemble sizes.