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1. Beyond one-axis twisting: Simultaneous spin-momentum squeezing

   https://doi.org/10.48550/arXiv.2206.12491  

   Wilson, JD ; Jäger, SB ; Reilly, JR ; Shankar, A ; Chiofalo, ML ; Holland, MF ( July 2022 , ArXivorg)

   The creation and manipulation of quantum entanglement is central to improving precision measurements. A principal method of generating entanglement for use in atom interferometry is the process of spin squeezing whereupon the states become more sensitive to SU(2) rotations. One possibility to generate this entanglement is provided by one-axis twisting (OAT), where a many-particle entangled state of one degree of freedom is generated by a non-linear Hamiltonian. We introduce a novel method which goes beyond OAT to create squeezing and entanglement across two distinct degrees of freedom. We present our work in the specific physical context of a system consisting of collective atomic energy levels and discrete collective momentum states, but also consider other possible realizations. Our system uses a nonlinear Hamiltonian to generate dynamics in SU(4), thereby creating the opportunity for dynamics not possible in typical SU(2) one-axis twisting. This leads to three axes undergoing twisting due to the two degrees of freedom and their entanglement, with the resulting potential for a more rich context of quantum entanglement. The states prepared in this system are potentially more versatile for use in multi-parameter or auxiliary measurement schemes than those prepared by standard spin squeezing. 

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2. Seeded spin-mixing interferometry with long-time evolution in microwave-dressed sodium spinor Bose-Einstein condensates

   https://doi.org/10.1088/1361-6455/acc36f  

   Zhong, Shan ; Ooi, Hio Giap ; Prajapati, Sankalp ; Zhang, Qimin ; Schwettmann, Arne ( March 2023 , Journal of Physics B: Atomic, Molecular and Optical Physics)

   Abstract We experimentally demonstrate a new type of spin-mixing interferometry in sodium Bose–Einstein condensates (BECs) based on seeded initial states. Seeding is useful because it speeds up the generation of entangled pairs, allowing many collisions to take place quickly, creating large populations in the arms of the interferometer. The entangled probe states of our interferometer are generated via spin-exchange collisions in F  = 1 spinor BECs, where pairs of atoms with the magnetic quantum number m F = 0 collide and change into pairs with m F = ± 1 . Our results show that our seeded spin-mixing interferometer beats the standard quantum limit (SQL) with a metrological gain of 3.69 dB with spin-mixing time t  = 10 ms in the case of single-sided seeding, and 3.33 dB with spin-mixing time t  = 8 ms in the case of double sided seeding. The mechanism for beating the SQL is two-mode spin squeezing generated via spin-exchange collisions. Our results on spin-mixing interferometry with seeded states are useful for future quantum technologies such as quantum-enhanced microwave sensors, and quantum parametric amplifiers based on spin-mixing. 

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3. Enhancing the sensitivity of an atom interferometer to the Heisenberg limit using increased quantum noise

   https://doi.org/10.1364/JOSAB.396358  

   Fang, Renpeng ; Sarkar, Resham ; Shahriar, Selim M. ( June 2020 , Journal of the Optical Society of America B)

   In a conventional atomic interferometer employing$N$atoms, the phase sensitivity is at the standard quantum limit:$1/\sqrt{N}$. Under usual spin squeezing, the sensitivity is increased by lowering the quantum noise. It is also possible to increase the sensitivity by leaving the quantum noise unchanged while producing phase amplification. Here we show how to increase the sensitivity, to the Heisenberg limit of$1/N$, while increasing the quantum noise by$\sqrt{N}$and amplifying the phase by a factor of$N$. Because of the enhancement of the quantum noise and the large phase magnification, the effect of excess noise is highly suppressed. The protocol uses a Schrödinger cat state representing a maximally entangled superposition of two collective states of$N$atoms. The phase magnification occurs when we use either atomic state detection or collective state detection; however, the robustness against excess noise occurs only when atomic state detection is employed. We show that for one version of the protocol, the signal amplitude is$N$when$N$is even, and is vanishingly small when$N$is odd, for both types of detection. We also show how the protocol can be modified to reverse the nature of the signal for odd versus even values of$N$. Thus, for a situation where the probability of$N$being even or odd is equal, the net sensitivity is within a factor of$\sqrt{2}$of the Heisenberg limit. Finally, we discuss potential experimental constraints for implementing this scheme via one-axis-twist squeezing employing the cavity feedback scheme, and show that the effects of cavity decay and spontaneous emission are highly suppressed because of the increased quantum noise and the large phase magnification inherent to the protocol. As a result, we find that the maximum improvement in sensitivity can be close to the ideal limit for as many as 10 million atoms.

    

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4. Long-lived Bell states in an array of optical clock qubits

   https://doi.org/10.48550/arXiv.2111.14653  

   Schine, N. ; Young, A.W. ; Eckner, W.J. ; Martin, M. J. ; Kaufman, A. M. ( July 2022 , ArXivorg)

   The generation of long-lived entanglement on an optical clock transition is a key requirement to unlocking the promise of quantum metrology. Arrays of neutral atoms constitute a capable quantum platform for accessing such physics, where Rydberg-based interactions may generate entanglement between individually controlled and resolved atoms. To this end, we leverage the programmable state preparation afforded by optical tweezers along with the efficient strong confinement of a 3d optical lattice to prepare an ensemble of strontium atom pairs in their motional ground state. We engineer global single-qubit gates on the optical clock transition and two-qubit entangling gates via adiabatic Rydberg dressing, enabling the generation of Bell states, |ψ⟩=12√(|gg⟩+i|ee⟩), with a fidelity of F=92.8(2.0)%. For use in quantum metrology, it is furthermore critical that the resulting entanglement be long lived; we find that the coherence of the Bell state has a lifetime of τbc=4.2(6) s via parity correlations and simultaneous comparisons between entangled and unentangled ensembles. Such Bell states can be useful for enhancing metrological stability and bandwidth. Further rearrangement of hundreds of atoms into arbitrary configurations using optical tweezers will enable implementation of many-qubit gates and cluster state generation, as well as explorations of the transverse field Ising model and Hubbard models with entangled or finite-range-interacting tunnellers. 

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5. Generation and dynamics of entangled fermion–photon–phonon states in nanocavities

   https://doi.org/10.1515/nanoph-2020-0353  

   Tokman, Mikhail ; Erukhimova, Maria ; Wang, Yongrui ; Chen, Qianfan ; Belyanin, Alexey ( September 2020 , Nanophotonics)

   null (Ed.)

   Abstract We develop the analytic theory describing the formation and evolution of entangled quantum states for a fermionic quantum emitter coupled simultaneously to a quantized electromagnetic field in a nanocavity and quantized phonon or mechanical vibrational modes. The theory is applicable to a broad range of cavity quantum optomechanics problems and emerging research on plasmonic nanocavities coupled to single molecules and other quantum emitters. The optimal conditions for a tripartite entanglement are realized near the parametric resonances in a coupled system. The model includes dissipation and decoherence effects due to coupling of the fermion, photon, and phonon subsystems to their dissipative reservoirs within the stochastic evolution approach, which is derived from the Heisenberg–Langevin formalism. Our theory provides analytic expressions for the time evolution of the quantum state and observables and the emission spectra. The limit of a classical acoustic pumping and the interplay between parametric and standard one-photon resonances are analyzed. 

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