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arXiv:1905.12581

Delay and polarization routing of single photons

Julian Maisch, Hüseyin Avni Vural, Michael Jetter, Peter Michler, Ilja Gerhardt, Simone Luca Portalupi

Published May 29, 2019• 0 citations
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Abstract

The full control of single photons is important in quantum information and quantum networking. A convenient storage device for photons is the key to memory assisted quantum communication and computing. While even a simple optical fiber can act as a convenient and reliable storage device, its storage time is tightly fixed and cannot be adapted. Therefore, the photon storage should ideally be actively controllable by external means, such as magnetic or electric control fields. In order to multiplex several photons, an active routing would also be desirable. Here we show that single photons of a semiconductor quantum dot can be deliberately delayed by an atomic vapor. Also, the output path can be selected, depending on an external magnetic field. By selecting the input polarization of the photons and by aligning the external magnetic field of the hot atomic vapor, the delay-based storage can be fine tuned to a deliberate value. With an overall delay of 25 ns, we are able to fine tune by more than 600 ps. Depending on the input polarization, the photons are routed into different output ports. The experimental data is fully resembled by a theoretical model, which describes the group velocity delay under consideration of spectral diffusion and considers the complex refractive index of the atomic vapor. The present results enable the use of an atomic vapor as a wavelength selective delay and allows for routing the single photons according to their polarization and an external magnetic field.

DOI
10.48550/arxiv.1905.12581

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The full control of single photons is important in quantum information and quantum networking. A convenient storage device for photons is the key to memory assisted quantum communication and computing. While even a simple optical fiber can act as a convenient and reliable storage device, its storage time is tightly fixed and cannot be adapted. Therefore, the photon storage should ideally be actively controllable by external means, such as magnetic or electric control fields. In order to multiplex several photons, an active routing would also be desirable. Here we show that single photons of a semiconductor quantum dot can be deliberately delayed by an atomic vapor. Also, the output path can be selected, depending on an external magnetic field. By selecting the input polarization of the photons and by aligning the external magnetic field of the hot atomic vapor, the delay-based storage can be fine tuned to a deliberate value. With an overall delay of 25 ns, we are able to fine tune by more than 600 ps. Depending on the input polarization, the photons are routed into different output ports. The experimental data is fully resembled by a theoretical model, which describes the group velocity delay under consideration of spectral diffusion and considers the complex refractive index of the atomic vapor. The present results enable the use of an atomic vapor as a wavelength selective delay and allows for routing the single photons according to their polarization and an external magnetic field.
Abstract Full control of single photons is important in quantum information and quantum networking. In particular, controlling the photon–atom interaction can be an appealing means to realize more complex quantum experiments. As a matter of example, the storage of photons into atomic media represents one key approach to memory‐assisted quantum communication and computing. Here it is shown that the propagation of single photons from a semiconductor quantum dot can be deliberately controlled by an atomic vapor under the application of an external magnetic field. The present results enable the use of an atomic vapor as a precise and reliable wavelength selective delay and allows for routing the single photons according to their polarization and the external magnetic field. With an overall delay of 25 ns, it is possible to fine‐tune the arrival time of the photons by more than 600 ps which matches the scale of the quantum dot's lifetime. The experimental data are fully reproduced by a theoretical model.
Quantum emitters coupled to nanophotonic structures are an excellent platform for controllable single-photon scattering. The tunable light-matter interaction enables the construction of a single-photon switch -- a device that can route a single photon from an input port to a selected output port. Such single-photon switching devices can be integrated into reconfigurable photonic circuits to actively control the photon propagation direction in a quantum network. Ideally, a single-photon switch should operate with high speed, efficiency, and fidelity, preserving the state of the input photon in the routing process. This review brings together key input-output methods from quantum optics, theoretical proposals of emitter-based single-photon routing mechanisms, and experimental demonstrations of single-photon switching devices across different physical platforms, including semiconductor quantum dots, neutral atoms, superconducting qubits, and color centers. We highlight the need for reporting the key figures of merit (speed/efficiency/fidelity) in future single-photon switch demonstrations to support further developments in the field.
Future quantum networks will need flying qubits and stationary nodes. As for the generation of single photons which may act as flying qubits, resonantly excited single-semiconductor quantum dots are ideal in terms of their on-demand single-photon emission, indistinguishability, and brightness. Atomic systems can effectively act as mediators for photon–photon interactions, storage media, or building blocks for stationary qubits. Here, we hybridize these two systems and investigate the non-classical interference of spectral Lorentzian-shaped photons, fine-tuned between the cesium (Cs)-D1 hyperfine resonances. The temporal delay in the dispersive hot atomic cesium vapor amounts up to 50 times the photons’ initial width and reveals beats on the single quanta. The photons’ indistinguishability is preserved even after atomic-enabled delay. This proves that the interaction with the Cs vapor conserves the photons’ coherence. The role of spectral diffusion in the solid-state emitter is studied in single- and two-photon experiments in light of the strong frequency dependence of the atomic medium. Our results pave the way for efficient hybrid interfaces between quantum dots and hot atomic vapors as storage media in future quantum networks.
Hybrid quantum systems are based on the capability to interface elements from complementary fields: only the best properties from all components are utilized, overcoming the limitation of each field. Semiconductor quantum dots (QDs) are well established as sources of bright, pure and highly indistinguishable on-demand single photons. A crucial limitation is given by the relatively short coherence time. This can be overtaken by interfacing them to alkali atoms, which display a very long coherence time and will benefit from the superior properties of QD-based non-classical light sources. As one example of an efficient QD-to-atom interface, we recently demonstrated the filtering of two consecutive single photons generated from a resonantly excited QD by using a Cs-based Faraday anomalous dispersion optical filter (FADOF). The Mollow triplet sidebands were tuned on resonance with the Cs-D1transition and simultaneously filtered with more than 10 % transmission. All other spectral components were suppressed below the noise level. In the present study, a Cs vapor was used to realize a variable optical delay. A resonantly excited QD is used as a source of on-demand single photons, frequency tuned in between the aforementioned Cs-D1 transition. An extensive study on the effects of vapor temperature on the achievable delay has been conducted using single photons with several different linewidth. A record delay of 28 ns was achieved for almost 100% of the transmitted photons. Autocorrelation measurements revealed that an almost ideal g(2)(0) ≈ 0 is preserved after the interaction with the atomic vapor.
In forthcoming quantum networks various quantum systems might be involved to accomplish individual tasks, including storage of quantum states, quantum logic operations, error correction, or entanglement distillation. An interface between a single photon emitter and a potential photon storage could provide one fundamental building block of such a hybrid quantum system. Semiconductor quantum dots (QD) and atomic vapors have already been brought together by single photon spectroscopy of cesium and rubidium [1,2], narrow filtering of QD resonance fluorescence in cesium [3], slowing down QD photons by dispersion between the 6.8 GHz hyperfine-split rubidium D2transitions [4] and in the middle of the four cesium D1 transitions [5] (at around 5.0 GHz in Fig. 1 b).In our experiment the neutrally charged excitonic transition of a strain-tunable InGaAs QD is pumped either resonantly or non-resonantly by a pulsed laser. The QD emission is set between two cesium D1 transitions at 894 nm, which are hyperfine-split only by 1.2 GHz, with a very strong dispersion compared to preceding studies [4,5]. This allows for delay times of a few nanoseconds even at low optical densities, resulting in a propagation velocity at the order of 1/20 of the vacuum speed of light. Such a single photon - atom interface might lay the foundations for a low-loss quantum memory in a future hybrid quantum network.
Quantum memories matched to single photon sources will form an important cornerstone of future quantum network technology. We demonstrate such a memory in warm Rb vapor with on-demand storage and retrieval, based on electromagnetically induced transparency. With an acceptance bandwidth of δf=0.66 GHz, the memory is suitable for single photons emitted by semiconductor quantum dots. In this regime, vapor cell memories offer an excellent compromise between storage efficiency, storage time, noise level, and experimental complexity, and atomic collisions have negligible influence on the optical coherences. Operation of the memory is demonstrated using attenuated laser pulses on the single photon level. For a 50 ns storage time, we measure η_{e2e}^{50 ns}=3.4(3)% end-to-end efficiency of the fiber-coupled memory, with a total intrinsic efficiency η_{int}=17(3)%. Straightforward technological improvements can boost the end-to-end-efficiency to η_{e2e}≈35%; beyond that, increasing the optical depth and exploiting the Zeeman substructure of the atoms will allow such a memory to approach near unity efficiency. In the present memory, the unconditional read-out noise level of 9×10^{-3} photons is dominated by atomic fluorescence, and for input pulses containing on average μ_{1}=0.27(4) photons, the signal to noise level would be unity.