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arXiv:1505.02667 · Physical Review A

Coherence in a cold-atom photon switch

Weibin Li, Igor Lesanovsky

Published Oct 20, 2015• 40 citations
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Abstract

We consider an all-optical photon switch in an atomic gas which is gated via a so-called Rydberg spin wave, i.e., a single Rydberg excitation that is coherently shared by the whole ensemble. A current challenge is to preserve the coherence of the Rydberg spin wave during the operation of the switch, which would enable, for example, its coherent optical readout. By employing a simple model description we investigate how the coherence of the Rydberg spin wave is affected during the operation of the switch. The coherence becomes increasingly protected with growing interatomic interaction strength. For the strongly interacting limit we derive analytical expressions for the spin-wave fidelity as a function of the optical depth and bandwidth of the incoming photon.

DOI
10.1103/physreva.92.043828

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We consider an all-optical photon switch in an atomic gas which is gated via a so-called Rydberg spin wave, i.e., a single Rydberg excitation that is coherently shared by the whole ensemble. A current challenge is to preserve the coherence of the Rydberg spin wave during the operation of the switch, which would enable, for example, its coherent optical readout. By employing a simple model description we investigate how the coherence of the Rydberg spin wave is affected during the operation of the switch. The coherence becomes increasingly protected with growing interatomic interaction strength. For the strongly interacting limit we derive analytical expressions for the spin-wave fidelity as a function of the optical depth and bandwidth of the incoming photon.
We develop a theoretical framework to characterize the decoherence dynamics due to multi-photon scattering in an all-optical switch based on Rydberg atom induced nonlinearities. By incorporating the knowledge of this decoherence process into optimal photon storage and retrieval strategies, we establish optimized switching protocols for experimentally relevant conditions, and evaluate the corresponding limits in the achievable fidelities. Based on these results we work out a simplified description that reproduces recent experiments ( Nat. Commun. 7 12480 ) and provides a new interpretation in terms of many-body decoherence involving multiple incident photons and multiple gate excitations forming the switch. Aside from offering insights into the operational capacity of realistic photon switching capabilities, our work provides a complete description of spin wave decoherence in a Rydberg quantum optics setting, and has immediate relevance to a number of further applications employing photon storage in Rydberg media.
This thesis reports the experimental realisation of a single-photon switch and transistor.To this end, Rydberg blockade in electromagnetically induced transparency (EIT) is used to create an interaction between two light pulses at the single photon level.In the experiment, a gate pulse containing one incoming photon on average is stored as a Rydberg excitation in an ultracold atomic gas using EIT.Subsequently a target light pulse is sent through the atomic ensemble.Without the gate pulse, the medium would be transparent due to EIT and the transmission would be high.If a gate excitation has been stored, however, Rydberg blockade suppresses the transmission of the target light pulse.The stored gate photon can be retrieved after the transmission of the target light.A retrieved photon heralds successful storage.In the corresponding postselected subensemble the transmission of the target light is reduced by an extinction factor of 0.05.This reduction is robust if the number of incoming gate photons or target photons is varied.In a conceptually improved version of the experiment, a single-photon transistor is realised that shows a gain above unity.The gain quantifies the change in the transmitted target photon number per incoming gate photon.For one incoming gate photon on average, a gain of 20 is observed.The improved experiment uses a Förster resonance, the benefit of which becomes directly evident in an improved gain and extinction.If a Rydberg excitation is created during the gate pulse, then the transmission of the target light pulse drops substantially.By setting a discrimination threshold for the number of detected target photons, it is possible to detect in a single shot with a fidelity of 86% whether a Rydberg excitation was created during the gate pulse.Since the gate excitation is not retrieved by the target light, this detection method is nondestructive.i iii 6.2.Experimental Sequence . . . . . . .
The long-range interaction between Rydberg-excited atoms endows a medium with large optical nonlinearity. Here, we demonstrate an optical switch to operate on a single photon from an entangled photon pair under a Rydberg electromagnetically induced transparency configuration. With the presence of the Rydberg blockade effect, we switch on a gate field to make the atomic medium nontransparent thereby absorbing the single photon emitted from another atomic ensemble via the spontaneous four-wave mixing process. In contrast to the case without a gate field, more than 50% of the photons sent to the switch are blocked, and finally achieve an effective single-photon switch. There are on average 1-2 gate photons per effective blockade sphere in one gate pulse. This switching effect on a single entangled photon depends on the principal quantum number and the photon number of the gate field. Our experimental progress is significant in the quantum information process especially in controlling the interaction between Rydberg atoms and entangled photon pairs.
The effect of strong interatomic interactions on an optical field stored in Rydberg states of a cold atomic gas is investigated. Due to their large dipole moments Rydberg atoms interact very strongly with each other. The strength of these interactions can be significant over length scales of several microns. An effect known as dipole blockade leads to a suppression in the number of Rydberg excitations supported in a medium of finite size. The experiments described in this thesis aim to exploit this effect to create a medium with a non-linear response which occurs at the level of single-photons. A cold atomic cloud is tightly confined such that only a few Rydberg excitations are supported. It is shown that the dipole blockade phenomenon leads to strong photon-photon interactions, resulting in the generation of quantum states of light. A microwave field is used to control the strength and range over which the interactions between the stored optical photons occur. In addition, it is shown that the propagation of the optical field through the medium is non-trivial. Preliminary evidence is presented suggesting that the slow-light group delay in the medium is dependent on the number of propagating photons.
We report on the realization of an all-optical transistor by mapping gate and source photons into strongly interacting Rydberg excitations with different principal quantum numbers in an ultracold atomic ensemble. We obtain a record switch contrast of 40% for a coherent gate input with mean photon number one and demonstrate attenuation of source transmission by over ten photons with a single gate photon. We use our optical transistor to demonstrate the nondestructive detection of a single Rydberg atom with a fidelity of 0.72(4).
All-optical switching is a technique in which a gate light pulse changes the transmission of a target light pulse without the detour via electronic signal processing. We take this to the quantum regime, where the incoming gate light pulse contains only one photon on average. The gate pulse is stored as a Rydberg excitation in an ultracold atomic gas using electromagnetically induced transparency. Rydberg blockade suppresses the transmission of the subsequent target pulse. Finally, the stored gate photon can be retrieved. A retrieved photon heralds successful storage. The corresponding postselected subensemble shows an extinction of 0.05. The single-photon switch offers many interesting perspectives ranging from quantum communication to quantum information processing.