Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)
Quantum dots interfaced with alkali atoms: Filtering, delaying and quantum interfering single photons
Hüseyin Avni Vural, Simone Luca Portalupi, Markus A. Müller, Jonas H. Weber, Julian Maisch, Simon Kern, Matthias Widmann, R. Löw, Jörg Wrachtrup, Michael Jetter, Ilja Gerhardt, Peter Michler
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.
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.
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.
Abstract Hybrid quantum systems integrating semiconductor quantum dots (QDs) and atomic vapours become important building blocks for scalable quantum networks due to the complementary strengths of individual parts. QDs provide on-demand single-photon emission with near-unity indistinguishability comprising unprecedented brightness—while atomic vapour systems provide ultra-precise frequency standards and promise long coherence times for the storage of qubits. Spectral filtering is one of the key components for the successful link between QD photons and atoms. Here we present a tailored Faraday anomalous dispersion optical filter based on the caesium-D1 transition for interfacing it with a resonantly pumped QD. The presented Faraday filter enables a narrow-bandwidth (Δω=2π × 1 GHz) simultaneous filtering of both Mollow triplet sidebands. This result opens the way to use QDs as sources of single as well as cascaded photons in photonic quantum networks aligned to the primary frequency standard of the caesium clock transition.
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.
Semiconductor quantum dots (QDs) are commonly considered as artificial atoms due to the 3D confinement of the electrons. These nanostructures are very promising for the realization of integrated devices such as single photon sources for quantum cryptography applications [1,2]. However, contrarily to genuine atoms, QDs are condensed matter systems and they consequently suffer from the coupling to their environment [3] which degrades the quality of the emitted photons in view of quantum cryptography applications. One way to circumvent this coupling is to excite strictly resonantly the QD at low temperature [4-10]. This difficult task has been achieved by designing an original excitation scheme to spatially decouple the excitation and detection paths. We first report that experimental studies of the resonant emission (RE) of single QDs can be strongly limited, even impossible, since most of the QDs show a very strong quenching of the RE of the fundamental transition. This issue is a broadly met experimental situation which is highly problematic for cavity quantum electrodynamics experiments in single QDs. We propose an efficient way to overcome this issue based on the use of an ultra-weak non-resonant laser that optically gates the QD RE [8] which controls the residual doping inherent to the QDs fabrication. It induces a complete recovery of the properties of an artificial atom, and the single QDs behave as quasi-ideal two-level systems. Up to now [7,9], the measurements on the QDs RE reported in the literature mainly focus on the incoherent fluorescence signal for achieving efficient single photon emission. In this regime, the spectrum linewidth is always limited to the radiative limit. We present experimental studies on the coherent laser light scattering by a single QD [7]. Besides the antibunching effect showing the non-classical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. By reducing the resonant excitation power, the so-called Mollow triplet shrinks to a single narrow Lorentzian line, while the relative intensity of this narrow peak increases and tends to overwhelm the RE spectrum. This implies that single QDs can produce single photons with a coherence time that is not limited anymore by the QD electronic properties, resulting in what we name an " ultra-coherent " character [7]. In conclusion, the resulting ultra-coherent single photon source promises high degrees of indistinguishability of the emitted photons which is a crucial requirement for quantum information applications. References [1] P. Michler et al., Science 290 (2000). [6] S. Ates et al., PRL 103 (2009). [2] C. Santori et al., Nature 419 (2002). [7] H. S. Nguyen et al., APL 99 (2011). [3] A. Berthelot et al., Nat. Phys. 2 (2006). [8] H. S. Nguyen et al., PRL 108 (2012). [4] A. Muller et al., PRL 99 (2007). [9] C. Matthiesen et al., PRL 1082 (2012). [5] R. Melet et al., PRB 78 (2008). [10] A. Reinhard et al., Nat. Phot. 6 (2012).
Quantum memories can substantially increase the efficiency of long-distance communications by synchronizing entanglement swapping operations in quantum repeater nodes. To build a quantum memory, electromagnetically induced transparency (EIT) in atomic vapors can be exploited to coherently store light pulses even at room temperatures. As a quantum source of light, semiconductor quantum dots (QD) offer bright on-demand single photons with high purity.4 Interfacing QDs with atomic vapors has been shown by “slow light” but a quantum memory for QDs is yet to be demonstrated. In this work, we develop an EIT quantum memory hosted in warm cesium vapor. Storage of faint coherent light pulses on the single photon level shows high storage efficiency. A measured bandwidth in the order of 200 MHz makes the memory compatible with the Fourier-limited emission of QDs embedded in micropillar cavities. We show the first attempts to interface the emission from a QD-micropillar with our quantum memory by finetuning the emission wavelength of the emitters to one of the hyperfine transitions in Cs, where the EIT memory takes place. This work sets the base for a hybrid quantum memory based on atomic ensembles for an on-demand semiconductor single-photon source.
Faraday Filtering on the Cs-D1-Line for Quantum Hybrid Systems
Narrow-band filtering of light is widely used in optical spectroscopy. Atomic filters, which rely on the Faraday effect, allow for GHz-wide transmission spectra, which are intrinsically matched to an atomic transition. We present an experimental realization and a theoretical study of a Faraday anomalous dispersion optical filter (FADOF) based on cesium and its D1-line-transition (62S1/2→62P1/2) around 894nm. We also present the prospects and visions for combining this filter with the single photon emission of a single quantum dot (QD), which matches with the atomic transition.