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Chinese Physics B

Generation of atomic spin squeezing via quantum coherence: Heisenberg–Langevin approach

Xuping Shao

Published Aug 1, 2020• 0 citations
Publisher

Abstract

Taking into account the dephasing process in the realistic atomic ensemble, we theoretically study the generation of atomic spin squeezing via atomic coherence induced by the coupling and probe fields. Using the Heisenberg–Langevin approach, we find that the perfect spin squeezing in the X component can be obtained while the coupling and probe fields produce the maximum coherence between the ground state sublevels 1 and 2. Moreover, the degree of atomic spin squeezing in the X component can be strengthened with the increasing atomic density and/or Rabi frequency of the mixing field. The theoretical results provide a proof-of-principle demonstration of generating the atomic spin squeezing via quantum coherence in the realistic atomic ensemble which may find potential applications in quantum information processing and quantum networks.

DOI
10.1088/1674-1056/abab7c

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Taking into account the dephasing process in the realistic atomic ensemble, we theoretically study the generation of atomic spin squeezing via atomic coherence induced by the coupling and probe fields. Using the Heisenberg–Langevin approach, we find that the perfect spin squeezing in the X component can be obtained while the coupling and probe fields produce the maximum coherence between the ground state sublevels 1 and 2. Moreover, the degree of atomic spin squeezing in the X component can be strengthened with the increasing atomic density and/or Rabi frequency of the mixing field. The theoretical results provide a proof-of-principle demonstration of generating the atomic spin squeezing via quantum coherence in the realistic atomic ensemble which may find potential applications in quantum information processing and quantum networks.
We propose a scheme to generate and control atomic spin squeezing via atomic coherence induced by the strong coupling and probe fields in the \ensuremath{\Lambda}-type electromagnetically-induced-transparency configuration in an atomic ensemble. Manipulation of squeezing of the two components in the plane orthogonal to the mean atomic spin direction and generation of nearly perfect squeezing in either component can be achieved by varying the relative intensities of the coupling and probe fields. This method provides a flexible and convenient way to create and control atomic spin squeezing, which may find potential applications in high-precision atomic-physics measurement, quantum coherent control, and quantum information processing.
We present a special quantum effect named electromagnetically induced squeezing of atomic spin in the conventional Λ-type atomic system driven by a strong pump field and a relatively weak probe field in an atomic ensemble. Nearly perfect atomic spin squeezing in the y component can be achieved with fully on resonance at high pump-field intensity, large optical depth of the atomic ensemble, and a low coherence decay rate between the two lower levels. This method would greatly facilitate the generation of atomic spin squeezing by using only coherent laser fields, and may find potential applications in realistic quantum metrology, quantum computation, and quantum information processing.
We conduct theoretical studies on the effects of various parameters on generation of multipartite continuous-variable entanglement via atomic spin wave induced by the strong coupling and probe fields in the Λ-type electromagnetically induced transparency configuration in a realistic atomic ensemble by using the Heisenberg-Langevin formalism. It is shown that the increase of the atomic density and/or Rabi frequencies of the scattering fields, as well as the decrease of the coherence decay rate of the lower doublet would strengthen the degree of multipartite entanglement. This provides a clear evidence that the creation of multicolor multipartite entangled narrow-band fields to any desired number with a long correlation time can be achieved conveniently by using atomic spin wave in an atomic ensemble with large optical depth, which may find interesting applications in quantum information processing and quantum networks.
The interaction of classical and quantized electromagnetic fields with an ensemble of atoms in an optical cavity is considered. Four fields drive a double-lambda level scheme in the atoms, consisting of a pair of lambda systems sharing the same set of lower levels. Two of the fields produce maximum coherence, rho12 = -1/2, between the ground state sublevels 1 and 2. This pumping scheme involves equal intensity fields that are resonant with both the one and two-photon transitions of the lambda system. There is no steady-state absorption of these fields, implying that the fields induce a type electromagnetically-induced transparency (EIT) in the medium. An additional pair of fields interacting with the second lambda system, combined with the EIT fields, leads to squeezing of the atom spin associated with the ground state sublevels. Our method involves a new mechanism for creating steady-state spin squeezing using an optical cavity. As the cooperativity parameter C is increased, the optimal squeezing varies as C^{-1/3}. For experimentally accessible values of C, squeezing as large as 90% can be achieved.
Squeezed spin states and squeezed light are both key resources for quantum metrology and quantum information science, but have been separately investigated in experiments so far. Simultaneous generation of these two types of quantum states in one experiment setup is intriguing but remains a challenging goal. Here, we propose a novel protocol based on judiciously engineered symmetric atom-light interaction, and report proof-of-principle experimental results of concurrent spin squeezing of 0.61±0.09 dB and light squeezing of 0.65_{-0.10}^{+0.11} dB in a hot atomic ensemble. The squeezing process is deterministic, yielding fixed squeezing directions for both the light field and the collective atomic spin. Furthermore, the squeezed light modes lie in the multiple frequency sidebands of a single spatial mode. This new type of dual squeezed state is applicable for quantum enhanced metrology and quantum networks. Our method can be extended to other quantum platforms such as optomechanics, cold atoms, and trapped ions.
Spin squeezing via atom-field interactions is considered within the context of the Tavis-Cummings model. An ensemble of N two-level atoms interacts with a quantized cavity field. For all the atoms initially in their ground states, it is shown that spin squeezing of both the atoms and the field can be achieved provided the initial state of the cavity field has coherence between number states differing by 2. Most of the discussion is restricted to the case of a cavity field initially in a coherent state, but initial squeezed states for the field are also discussed. Optimal conditions for obtaining squeezing are obtained. An analytic solution is found that is valid in the limit that the number of atoms is much greater than unity and is also much larger than the average number of photons, ${\ensuremath{\alpha}}^{2},$ initially in the coherent state of the cavity field. In this limit, the degree of spin squeezing increases with increasing $\ensuremath{\alpha},$ even though the field more closely resembles a classical field for which no spin squeezing could be achieved.