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arXiv:1109.6254 · Physical Review Letters

Coalescence of Single Photons Emitted by Disparate Single-Photon Sources: The Example of InAs Quantum Dots and Parametric Down-Conversion Sources

Sergey V. Polyakov, Andreas Müller, Edward B. Flagg, Alexander Ling, Natalia Borjemscaia, Edward Van Keuren, Alan L. Migdall, Glenn S. Solomon

Published Oct 5, 2011• 54 citations
arXivHTMLPDF

Abstract

Single photons produced by fundamentally dissimilar physical processes will in general not be indistinguishable. We show how photons produced from a quantum dot and by parametric down-conversion in a nonlinear crystal can be manipulated to be indistinguishable. The measured two-photon coalescence probability is 16%, and is limited by quantum-dot decoherence. Temporal filtering to the quantum-dot coherence time and accounting for detector time response increases this to 61% while retaining 25% of the events. This technique can connect different elements in a scalable quantum network.

DOI
10.1103/physrevlett.107.157402

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Single photons produced by fundamentally dissimilar physical processes will in general not be indistinguishable. We show how photons produced from a quantum dot and by parametric down-conversion in a nonlinear crystal can be manipulated to be indistinguishable. The measured two-photon coalescence probability is 16%, and is limited by quantum-dot decoherence. Temporal filtering to the quantum-dot coherence time and accounting for detector time response increases this to 61% while retaining 25% of the events. This technique can connect different elements in a scalable quantum network.
In quantum mechanics, particles in identical states are indistinguishable, giving rise to effects with no classical analog. For instance, the bosonic nature of light insures that upon interference two indistinguishable photons will coalesce into a single inseparable state. Through this coalescence, we demonstrate that photons produced from two separate quantum dots are indistinguishable. Further, we show that single photons created in a fundamentally different process - parametric down-conversion in a nonlinear crystal - can be manipulated to be indistinguishable from those from quantum dots. The quantum interference in both experiments occurs with a visibility reduced from unity because the quantum dot photons are not lifetime-limited due to the presence of pure dephasing. The measured visibility closely matches the theoretical visibility predicted for photons with the parameters of those measured here.
Quantum information networks will likely require different quantum systems for different functionality within the network. Indistinguishable photons can be used to interconnect these different subsystems. We discuss methods for coherently manipulating the single photons from different quantum systems and experimentally demonstrate spatial, temporal, and frequency matching of single photons using quantum dot and heralded parametric downconversion single photons. The bosonic nature of light insures that when two indistinguishable photons are superimposed on a beam splitter, they will form a single two-photon state, a process we call coalescence. This coalescence property can be used as both a fundamental test of indistinguishability and in quantum networks—connecting and propagating quantum information.
Abstract Hybrid quantum systems play a crucial role in advancing scalable and versatile quantum networks as they combine the strengths of different quantum platforms. An important challenge for the development of hybrid quantum networks lies in interfacing heterogeneous quantum nodes and distributing entanglement among them. Single photons emitted from these dissimilar quantum nodes typically show distinct spectral and temporal properties. Therefore, they necessitate spectral filtering and temporal synchronization, which introduce significant photon losses and require additional resources. In this work, we successfully generate indistinguishable photons from two distinct quantum systems of a warm atomic ensemble and a solid-state quantum dot. Remarkably, quantum interference between dissimilar sources is achieved without additional spectral filtering and time synchronization, which enables autonomous quantum nodes for a hybrid quantum network. 133Cs atomic ensemble can efficiently generate heralded single photons at the wavelength of 917 nm of the 6P_(3/2)-6D_(5/2) transition, while the single photons emitted from an InAs/GaAs quantum dot can be tuned to match the 133Cs transition wavelength. Our dense warm atomic ensemble and cavity-coupled quantum dot can efficiently generate bright and resonant single photons at detection rates approaching MHz, respectively. More importantly, these single photons exhibit inherent spectral similarities not only in the wavelength but also in the spectral linewidth, achieving a high spectral overlap of 0.92. Such intrinsic compatibility between dissimilar quantum sources is essential to leverage the advantages of different quantum platforms, paving the way toward a large-scale and functional hybrid quantum network.
Single-photon sources based on quantum dots have been shown to exhibit almost ideal properties such as high brightness and purity in terms of clear anti-bunching as well as high two-photon interference visibilities of the emitted photons, making them promising candidates for different quantum information applications such as quantum computing, quantum communication and quantum teleportation. However, as most single-photon sources also quantum dots typically emit light at wavelengths of electronic transitions within the visible or the near infrared range. In order to establish quantum networks with remote building blocks, low-loss single photons at telecom wavelengths are preferable, though. Despite recent progress on emitters of telecom-photons, the most efficient single-photon sources still work at shorter wavelengths. On that matter, quantum frequency down-conversion, being a nonlinear optical process, has been used in recent years to alter the wavelength of single photons to the telecom wavelength range while conserving their nonclassical properties. Characteristics such as lifetime, first-order coherence, anti-bunching and entanglement have been shown to be conserved or even improved due to background suppression during the conversion process, while the conservation of indistinguishability was yet to be shown. Here we present our experimental results on quantum frequency down-conversion of single photons emitted by an InAs/GaAs quantum dot at 903.6 nm following a pulsed excitation of a p-shell exciton at 884 nm. The emitted fluorescence photons are mixed with a strong pump-field at 2155 nm inside a periodically poled lithium niobate ridge waveguide and converted to 1557 nm. Common issues of a large background due to Raman-scattered pump-light photons spectrally overlapping with the converted single photons could largely be avoided, as the pump-wavelength was chosen to be fairly longer than the target wavelength. Additional narrowband spectral filtering at the telecom regime as a result of the small conversion bandwidth and using a high-performance fiber-Bragg-grating solely left the detector dark counts as the only noise source in our setup. Therefore, we could achieve conversion efficiencies of more than 20 %. In order to test the indistinguishability, sequentially emitted photons were fed into a Mach-Zehnder interferometer and spatially as well as temporally overlapped at the output beam splitter. Cross-correlation measurements between both output-ports of the beam splitter exhibit two-photon interference contrasts of more than 40 % prior to and after the down-conversion step. Accordingly, we demonstrate that the process of quantum frequency conversion preserves photon indistinguishability and can be used to establish a versatile source of indistinguishable single photons at the telecom C-Band. Furthermore our scheme allows for converting photons in a wavelength band from 900 nm to 910 nm to the same telecom target wavelength. This enables us to test indistinguishability of frequency-converted photons, originally stemming from different sources with dinstinguishable wavelengths.