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arXiv:1706.09720 · Quantum Science and Technology

Interfacing a quantum dot with a spontaneous parametric down-conversion source

Tobias Huber, Maximilian Prilmüller, Michael Sehner, Glenn S. Solomon, Ana Predojević, Gregor Weihs

Published Aug 3, 2017• 11 citations
arXivHTMLPDF

Abstract

Quantum networks require interfacing stationary and flying qubits. These flying qubits are usually nonclassical states of light. Here we consider two of the leading source technologies for nonclassical light, spontaneous parametric down-conversion and single semiconductor quantum dots. Down-conversion delivers high-grade entangled photon pairs, whereas quantum dots excel at producing single photons. We report on an experiment that joins these two technologies and investigates the conditions under which optimal interference between these dissimilar light sources may be achieved.

DOI
10.1088/2058-9565/aa7b65

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Quantum networks require interfacing stationary and flying qubits. These flying qubits are usually nonclassical states of light. Here we consider two of the leading source technologies for nonclassical light, spontaneous parametric down-conversion and single semiconductor quantum dots. Down-conversion delivers high-grade entangled photon pairs, whereas quantum dots excel at producing single photons. We report on an experiment that joins these two technologies and investigates the conditions under which optimal interference between these dissimilar light sources may be achieved.
Semiconductor quantum dots (QDs) are promising artificial atoms for quantum information processing: they can generate single photons flying quantum bits; they show single photon sensitivity promising to develop quantum gates and the spin of a carrier in a QD can be a quantum memory. The scalability of a quantum network requires efficient interfaces between stationary and flying quantum bits. In the last few years, our group has made important progresses in this direction using cavity quantum electrodynamics. With a deterministic positioning of a single QD in a microcavity, we control the QD spontaneous emission on demand [1]. With such technique highly efficient single photon sources with brightness as large as 80% are demonstrated [2]. By minimizing the charge noise around the QD in a gated structure [3], we demonstrate the generation of fully indistinguishable photon. The source brightness is shown to exceed by one or two orders of magnitude the one of a parametric down-conversion source of same quality [4]. Symmetrically, these devices perform as excellent interfaces between a flying quantum bit and a stationary one, where coherent control of a quantum bit can be done when only few photons [5]. References [1] A. Dousse, et al. , Phys. Rev. Lett. 101, 267404 (2008) [2] O. Gazzano, et al. , Nature Communications 4, 1425 (2013) [3] A. Nowak. et al., Nature Communications 5, 3240 (2014) [4] N. Somaschi, et al. Nature Photonics 10.1038/nphoton.2016.23 (2016). [5] V. Giesz, et al., Nature Communications doi:10.1038/ncomms11986 (2016)
The generation of entangled photon pairs is a fundamental cornerstone of quantum optics, and an essential technology in the high profile field of quantum communication and computing. Currently the most widely used techniques for generating entangled photon pairs are nonlinear optical processes, such as parametric down conversion, which produce a probabilistic number of pairs per excitation cycle. Such a source is of limited use in quantum information/processing applications where a regular stream of single entangled photon pairs is usually required. We have produced such a triggered source from a semiconductor device for the first time, using a two-photon cascade in a single quantum dots, demonstrating efficiencies up to 70%, close to the level required for useful applications. Single quantum dots could prove to be the first robust and compact triggered source of entangled photons.
Photons as flying qubits are ideal carriers of quantum information. However, efficient quantum interfaces which transfer flying into stationary qubits and vice versa are needed to realize complex quantum networks. Stationary qubit systems (atoms, ions, or solid state implementations) couple to photons via characteristic optical transitions. In order to establish key quantum operations, such as coherent photon storage, entanglement swapping, etc., high-quality photons with a tailored spectral, spatial, and temporal mode are needed. One way to generate these photons is to use the stationary qubit systems themselves. Another approach which does not impose limitations in terms of the photon’s wavelength or bandwidth relies on all-optical single photon sources. An example is spontaneous parametric down conversion (SPDC) inside an optical cavity with well-defined linewidth. In SPDC, a pump photon incident on a nonlinear medium is split into two photons of lower energy that are often referred to as signal and idler. Resonating the fluorescent fields, the setup becomes an optical parametric oscillator (OPO) far below the threshold. The usually THz broad emission spectrum of SPDC then follows the Airy function of the resonator. The spectral bandwidth of the generated photons can easily reach values of several MHz, which is comparable to atom resonance linewidths. By introducing additional losses or simply changing the cavity mirror reflectivities the bandwidth is tunable up to several 100 MHz. This is comparable to the bandwidth of single photons generated by e.g. quantum dots.
Entangled photon pairs are a ubiquitous resource in quantum technologies, used in quantum key distribution and quantum networking as well as fundamental tests of non-locality. For scalable quantum networks, pairs that are indistinguishable in all unentangled degrees of freedom are essential, as they enable high-fidelity entanglement swapping across network nodes. To date the most-studied sources of "swappable" entangled photon pairs have been based on spontaneous parametric down-conversion (SPDC) in non-linear crystals. However, the probabilistic nature and unavoidable trade-off between brightness and unwanted multi-photon emission limits their performance in lossy channels. Here, we demonstrate a high-fidelity source of "swappable" entangled photon pairs using a semiconductor quantum dot (QD) coupled to a tunable microcavity. By actively modulating the QD emission between orthogonal polarisation states, delaying one path in a low-loss Herriott cell, and recombining the two on a balanced beam splitter, we generate entangled photon pairs with a fidelity of 96.1±0.596.1\pm0.596.1±0.5 %. We identify and mitigate fidelity-limiting factors, achieving a maximum fidelity of 98.1±0.598.1\pm0.598.1±0.5 % through time-resolved post-selection. The scheme suppresses residual multi-photon events concentrated near the excitation pulse and has only a modest impact on the rate. Furthermore, the photons are mutually indistinguishable, enabling efficient entanglement swapping. Our results establish semiconductor QDs as a viable platform for quantum network-compatible swappable entangled photon pair generation, with feasible entanglement generation rates exceeding 0.5 Gpairs/s.
Abstract The development of scalable sources of non-classical light is fundamental to unlocking the technological potential of quantum photonics. Semiconductor quantum dots are emerging as near-optimal sources of indistinguishable single photons. However, their performance as sources of entangled-photon pairs are still modest compared to parametric down converters. Photons emitted from conventional Stranski–Krastanov InGaAs quantum dots have shown non-optimal levels of entanglement and indistinguishability. For quantum networks, both criteria must be met simultaneously. Here, we show that this is possible with a system that has received limited attention so far: GaAs quantum dots. They can emit triggered polarization-entangled photons with high purity (g (2) (0) = 0.002±0.002), high indistinguishability (0.93±0.07 for 2 ns pulse separation) and high entanglement fidelity (0.94±0.01). Our results show that GaAs might be the material of choice for quantum-dot entanglement sources in future quantum technologies.