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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Rydberg excitation assisted light shift blockade in Rb atoms for realizing a collective state quantum bit and quantum memory

Yanfei Tu, May E. Kim, M. S. Shahriar

Published Oct 3, 2014• 0 citations
Publisher

Abstract

Previously, we had proposed the technique of light shift imbalance induced blockade which leads to a condition where a collection of non-interacting atoms under laser excitation remains combined to a superposition of the ground and the fist excited states, thus realizing a collective state quantum bit which in turn can be used to realize a quantum computer. In this paper, we show first that the light shift imbalance by itself is actually not enough to produce such a blockade, and explain the reason by the limitation of our previous analysis had reached this constraint. We then show that by introducing Rydberg interaction, it is possible to achieve such a blockade for a wide range of parameters. Analytic arguments used to establish these results are confirmed by numerical simulations. The fidelity of coupled quantum gates based on such collective state qubits is highly insensitive to the exact number of atoms in the ensemble. As such, this approach may prove be viable for scalable quantum computing based on neutral atoms.

DOI
10.1117/12.2075303

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Previously, we had proposed the technique of light shift imbalance induced blockade which leads to a condition where a collection of non-interacting atoms under laser excitation remains combined to a superposition of the ground and the fist excited states, thus realizing a collective state quantum bit which in turn can be used to realize a quantum computer. In this paper, we show first that the light shift imbalance by itself is actually not enough to produce such a blockade, and explain the reason by the limitation of our previous analysis had reached this constraint. We then show that by introducing Rydberg interaction, it is possible to achieve such a blockade for a wide range of parameters. Analytic arguments used to establish these results are confirmed by numerical simulations. The fidelity of coupled quantum gates based on such collective state qubits is highly insensitive to the exact number of atoms in the ensemble. As such, this approach may prove be viable for scalable quantum computing based on neutral atoms.
Rydberg interactions of trapped alkali-metal atoms are used widely to facilitate quantum gate operations in quantum processors and repeaters. In most laboratory realizations using this protocol, the Rydberg states are repelled by the trapping laser fields, requiring that the fields be turned off during gate operations. Here we create a quasi-two-level system in a regime of Rydberg excitation blockade for a mesoscopic Rb ensemble of several hundred atoms confined in a magic-wavelength optical lattice. We observe many-body Rabi oscillations between the ground and collective Rydberg state. In addition we use Ramsey interference techniques to obtain the light shifts of both the lower and upper states of the collective qubit. Whereas the coupling producing the Rabi oscillations is enhanced by a factor of sqrt[N], there is no corresponding enhancement for the light shifts. We derive an effective two-level model which is in good agreement with our observations. Trapped Rydberg qubits and an effective two-level description are expected to have broad applicability for studies of quantum simulation and networking using collective encoding in ensembles of neutral atoms.
Recently, we have shown that for conditions under which the so-called light-shift imbalance induced blockade occurs, the collective excitation of an ensemble of a multilevel atom can be treated as a closed two-level system. In this paper, we describe how such a system can be used as a quantum bit (qubit) for quantum communication and quantum computing. Specifically, we show how to realize a controlled-NOT gate using the collective qubit and an easily accessible ring cavity, via an extension of the so-called Pellizzari scheme. We also describe how multiple, small-scale quantum computers realized using these qubits can be linked effectively for implementing a quantum internet. We describe the details of the energy levels and transitions in an 87Rb^{87}\mathrm{Rb}87Rb atom that could be used for implementing these schemes.
Abstract : The goal of this project was to demonstrate the feasibility of realizing a large quantum computer by linking a series of elementary quantum computers (EQCs), using ensembles of atoms as qubits. We built all the tools for making an EQC.However, experimental work revealed that use of light shift imbalance only is not enough to achieve the required blockade. We have now shown why this is the case, and have identified how the blockade can be restored by using Rydberg interaction assisted light-shift imbalance. However, due to lack of necessary lasers, we were unable to demonstrate the EQC using this scheme. Instead, we focused on exploring other aspects of collective state entanglement for precision metrology. Specifically, we have shown how to realize spin-squeezed collective state atomic interferometer (COSAIN) and collective state atomic clock (COSAC), capable of improving the performance by a factor of root-N, thus reaching the Heisenberg limit. Significant experimental work has been carried out to demonstrate these devices, and this work will be carried to completion in the near future.
We present a complete protocol for cavity-free quantum networking based on collective enhancement in Rydberg atom ensembles. The scheme combines Rydberg blockade, collectively enhanced light--matter coupling, and phase-matched directional emission to remove the need for optical cavities while retaining efficiencies comparable to cavity-assisted interfaces. The protocol proceeds in three steps: (i)~local control--ensemble entanglement generated by Rydberg blockade with gate fidelity Fgate≈99.93%F_{\mathrm{gate}}\approx 99.93\%Fgate​≈99.93%; (ii)~atom--photon conversion through Raman emission from an oblate spheroidal ensemble, yielding directional emission efficiency ηdir≈73%η_{\mathrm{dir}}\approx 73\%ηdir​≈73% and single-node efficiency ηnode≈40%η_{\mathrm{node}}\approx 40\%ηnode​≈40%; and (iii)~remote atom--atom entanglement via Hong--Ou--Mandel interference, producing Bell states with fidelity F>97.5%F>97.5\%F>97.5%. Incorporating quantum memories allows up to M≈100M\approx 100M≈100 retry attempts within a coherence time T2>100 μsT_2>100\,μ\mathrm{s}T2​>100μs, enabling entanglement generation rates of approximately 4 kHz4\,\mathrm{kHz}4kHz over a 20~km separation. Collectively enhanced Rydberg ensembles thus provide a practical, cavity-free interface for scalable distributed quantum computing and secure quantum communication.