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arXiv:1209.1991 · Physical Review Letters

Dissipative Preparation of Spin Squeezed Atomic Ensembles in a Steady State

Emanuele G. Dalla Torre, Johannes Otterbach, Eugene Demler, Vladan Vuletić, Mikhail D. Lukin

Published Mar 19, 2013• 178 citations
arXivHTMLPDF

Abstract

We present and analyze a new approach for the generation of atomic spin-squeezed states. Our method involves the collective coupling of an atomic ensemble to a decaying mode of an open optical cavity. We demonstrate the existence of a collective atomic dark state, decoupled from the radiation field. By explicitly constructing this state we find that it can feature spin squeezing bounded only by the Heisenberg limit. We show that such dark states can be deterministically prepared via dissipative means, thus turning dissipation into a resource for entanglement. The scaling of the phase sensitivity taking realistic imperfections into account is discussed.

DOI
10.1103/physrevlett.110.120402

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We present and analyze a new approach for the generation of atomic spin-squeezed states. Our method involves the collective coupling of an atomic ensemble to a decaying mode of an open optical cavity. We demonstrate the existence of a collective atomic dark state, decoupled from the radiation field. By explicitly constructing this state we find that it can feature spin squeezing bounded only by the Heisenberg limit. We show that such dark states can be deterministically prepared via dissipative means, thus turning dissipation into a resource for entanglement. The scaling of the phase sensitivity taking realistic imperfections into account is discussed.
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We propose an efficient scheme for generating spin-squeezed states at steady state in a spin-mechanical hybrid system, where an ensemble of SiV centers is coupled to a strongly damped nanomechanical resonator. We show that there exists a collective steady state in the system, which is exactly formed by the collective spin states plus the zero excitation state of the mechanical mode. The generation of the steady spin-squeezed state is based on a dissipative quantum dynamical process in which the mechanical dissipation plays a positive role but without destroying the target state. We demonstrate that the spin-squeezed steady state can be deterministically prepared via dissipative means, with the optimal spin squeezing up to 4/N4/N4/N in the ideal case, where NNN is the number of spins. This work provides a promising platform for quantum information processing and quantum metrology.
We propose an effective model leveraging cavity superradiance to achieve entanglement between two spatially separated two-level atomic ensembles trapped in a standing-wave optical cavity. This model is based on permutational symmetry breaking, which leads to the emergence of entangled quantum dark states with nonzero excitations but vanishing decay rate. This steady-state entanglement is created by purely dissipative dynamics, unlike the superradiance of permutationally symmetric two-level atoms, whose steady state is the ground state. Furthermore, we show that the entanglement is significantly enhanced near critical points in the superradiance potential because of the extremely high two-mode squeezing factor between two atomic ensembles. We also discuss spin squeezing in each atomic ensemble in the vicinity of specific critical points in the potential. These findings may play an important role in the generation of correlations immune to dissipation in separated atomic modes.
We describe a method to create and store scalable and long-lived entangled spin-squeezed states within a manifold of many-body cavity dark states using collective emission of light from multilevel atoms inside an optical cavity. We show that the system can be tuned to generate squeezing in a dark state where it will be immune to superradiance. We also show more generically that squeezing can be generated using a combination of superradiance and coherent driving in a bright state, and subsequently be transferred via single-particle rotations to a dark state where squeezing can be stored. Our findings, readily testable in current optical cavity experiments with alkaline-earth-like atoms, can open a path for dissipative generation and storage of metrologically useful states in optical transitions.
For dark resonance, one of the most remarkable coherent effects in light-matter interactions, it has commonly been expected that squeezing and entanglement, if existent, are formed via coherent evolutions against dissipation. Contrary to the expectations, here we show that dissipation generates entanglement between two cavity fields and between two dark-state-based spins. The latter correspond also to the atomic ground-state spin squeezing in a limited parameter domain. The dissipation effects, which are hidden deeply behind the coherence-induced nonlinearities, are extracted by probing into the dressed atom-photon interactions, and are widely applicable for the coherently prepared systems in dark resonances or beyond.
The generation of entanglement in atomic systems plays a central topic in the fields of quantum information storage and processing. Moreover, a special category of entangled states of multi-atom ensembles, spin squeezed states, have been proven to lead to considerable improvement in the sensitivity of precision measurements compared to systems involving uncorrelated atoms. A treatment of entanglement in open systems is, however, incomplete without a precise description of the process of decoherence which necessarily accompanies it. The theory of entanglement and decoherence are the two main topics of this thesis. Methods are described for the generation of strong correlations in large atomic ensembles using either cavity quantum electrodynamics or measurement outcome conditioned quantum dynamics. Moreover, the description of loss of entanglement resulting from the coupling to a noise reservoir (electromagnetic vacuum) is explored. A spin squeezing parameter is used throughout this thesis as both a measure of entanglement strength and as an indication of the sensitivity improvement above the so-called standard quantum limit (sensitivity obtained with uncorrelated particles) in metrology. The first scheme considered consists of a single mode cavity field interacting with a collection of atoms for which spin squeezing is produced in both resonant and off-resonant regimes. In the resonant case, transfer of squeezing from a field state to the atoms is analyzed, while in the off-resonant regime squeezing is produced via an effective nonlinear interaction (one-axis twisting Hamiltonian). A second, more experimentally realistic case, is one involving the interaction of free space atoms with laser pulses; a projective measurement of a source field originating from atomic fluctuations provides a means of preparing atomic collective states such as spin squeezed and Schrodinger cat states. A new unravelling is proposed, that employs the detection of photon number in a single output channel and is capable of producing mesoscopic Schrodinger cat states in a single step. As a first step to understanding the role of cooperative decoherence, of importance in the case of dense pencil-shaped atomic ensembles, the collective spin decoherence of a two multilevel atom system is derived. This calculation is also relevant to entanglement loss for two qubits manipulated using with reading/writing pulses. Finally, a scheme is proposed in which arbitrarily strong entanglement can be produced in a four-wave mixing setup, where the preparation of the atoms in a dark state limits the decoherence to negligible values.