Fiber-Optical Switch Controlled by a Single Atom

Abstract

We demonstrate highly efficient switching of optical signals between two optical fibers controlled by a single atom. The key element of our experiment is a whispering-gallery mode bottlemicroresonator, which is coupled to a single atom and interfaced by two tapered fiber couplers. This system reaches the strong coupling regime of cavity quantum electrodynamics (CQED), leading to a vacuum Rabi splitting in the excitation spectrum. We systematically investigate the switching efficiency of our system, i.e., the probability that the CQED fiber-optical switch redirects the light into the desired output. We obtain a large redirection efficiency reaching a raw fidelity of more than 60% without post-selection. Moreover, by measuring the second order correlation functions of the output fields, we show that our switch exhibits a photon number-dependent routing capability.PACS numbers: 42.50. Pq,42.50.Ct,42.60.Da, Fiber-optical switches are devices that enable optical signals to be rerouted to different fiber output ports and play a vital role in optical communication networks. Scaling such a device into the quantum domain, where a single quantum system controls the flow of light, would enable the implementation of quantum communication and information protocols with atoms and photons as well as the preparation of non-classical light, useful for interferometric schemes in quantum metrology [1,2].The physical realization of such a quantum switch requires an enhanced light-matter interaction that can, e.g., be reached by coupling an atom to an optical microresonator. However, this requires to reach the singleatom strong coupling regime, where the so-called critical atom number N 0 = 2κγ/g 2 [3], has to be much smaller than one. Here, κ and γ are the decay constants of the cavity field and the atomic dipole and g is the single photon-single atom coupling strength. This regime has been investigated in the optical domain in numerous groundbreaking experiments using high finesse Fabry-Pérot microresonators [4][5][6][7][8][9][10]. While these experiments clearly demonstrate the high potential of CQED systems for future applications, light absorption and scattering in the mirrors limit the efficiency of coupling light into and out of the resonator to typically a few tens of percent.In this context, whispering-gallery-mode (WGM) microresonators combine very high atom-light coupling strengths [11][12][13] and low coupling losses in the same system. WGM resonators are monolithic dielectric structures, such as microspheres [14] and microtori [15], in which the light is guided near the surface by continuous total internal reflection [16]. The light can be coupled in and out by frustrated total internal reflection with near 100% efficiency using tapered fiber couplers [17], thereby largely outperforming all other types of optical resonators.Strong coupling of single atoms and solid state quan- tum emitters to WGM microresonators has recently been demonstrated in a number of experiments [18][19][20][21][22][23][24]. Moreover, using toroi...

Quantum nonlinear optics — photon by photon

“…For example, the strong dependence of light propagation on photon number allows the sorting or counting of photons non-destructively 48,56,76 , which, in combination with feedback, could be used to implement various sources of non-classical light fields. Quantum nonlinearities also enable classical nonlinear optical devices, such as routers 77,78 or all-optical switches, to be operated at their fundamental limit. One example is the case of a singlephoton transistor 79 , in which even a single 'gate' photon can switch hundreds of signal photons 57 .…”

Nanophotonic quantum phase switch with a single atom

“…We experimentally demonstrate an atom-induced optical phase shift [8] that is nonlinear at the two-photon level [9], a photon number router that separates individual photons and photon pairs into different output modes [10], and a single-photon switch where a single "gate" photon controls the propagation of a subsequent probe field [11, 12]. These techniques pave the way towards integrated quantum nanophotonic networks involving multiple atomic nodes connected by guided light.A quantum optical switch [11,[13][14][15][16]] is challenging to implement because the interaction between individual photons and atoms is generally very weak. Cavity quantum electrodynamics (cavity QED), where a photon is confined to a small spatial region and made to interact strongly with an atom, is a promising approach to overcome this challenge [4].…”

“…1a,b). Compared to transient coupling of unconfined atoms [13, 22], trapping an atom allows for experiments exploiting long atomic coherence times, and enables scaling to quantum circuits with multiple atoms. We use a one-sided optical cavity with a single port for both input and output [8].…”

“…A quantum optical switch [11,[13][14][15][16]] is challenging to implement because the interaction between individual photons and atoms is generally very weak. Cavity quantum electrodynamics (cavity QED), where a photon is confined to a small spatial region and made to interact strongly with an atom, is a promising approach to overcome this challenge [4].…”

“…Cavity quantum electrodynamics (cavity QED), where a photon is confined to a small spatial region and made to interact strongly with an atom, is a promising approach to overcome this challenge [4]. Over the last two decades, cavity QED has enabled advances in the control of microwave [17][18][19] and optical fields [13,[20][21][22][23]. While integrated circuits with strong coupling of microwave photons to superconducting qubits are currently being developed [24], a scalable path to integrated quantum circuits involving coherent qubits coupled via optical photons has yet to emerge.…”