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arXiv:1806.08213 · New Journal of Physics

Limitations on the indistinguishability of photons from remote solid state sources

Benjamin Kambs, Christoph Becher

Published Oct 24, 2018• 73 citations
arXivHTMLPDF

Abstract

In the present work, we derive a formalism that can be used to predict and interpret the time structure and achievable visibilities for two-photon interference (TPI) experiments using photons from two separate sources. The treatment particularly addresses photons stemming from solid state quantum emitters, which are often subject to pure dephasing and spectral diffusion. Therefore, it includes the impact of phase- and emission frequency-jitter besides the influence of differing radiative lifetimes and a relative spectral detuning. While the treatment is mainly aimed at interference experiments after Hong–Ou–Mandel, we additionally offer generalized equations that are applicable to arbitrary linear optical gates, which rely on TPI.

DOI
10.1088/1367-2630/aaea99

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In the present work, we derive a formalism that can be used to predict and interpret the time structure and achievable visibilities for two-photon interference (TPI) experiments using photons from two separate sources. The treatment particularly addresses photons stemming from solid state quantum emitters, which are often subject to pure dephasing and spectral diffusion. Therefore, it includes the impact of phase- and emission frequency-jitter besides the influence of differing radiative lifetimes and a relative spectral detuning. While the treatment is mainly aimed at interference experiments after Hong–Ou–Mandel, we additionally offer generalized equations that are applicable to arbitrary linear optical gates, which rely on TPI.
The ability to entangle distant quantum nodes is essential for the construction of quantum networks and for quantum information processing. For solid-state quantum emitters used as qubits, it can be achieved by photon interference. When the emitter is subject to spectral diffusion, this process can become highly inefficient, impeding the achievement of scalable quantum technologies. We study two-photon interference in the context of a Hong-Ou-Mandel (HOM)-type experiment for two separate quantum emitters, with different detunings with respect to a specific target frequency. We evaluate the second order coherences that characterize photon indistinguishability between the two emitters. We find that the two-photon interference operation that is inefficient in the absence of a control protocol, when the two detunings are different and spectral overlap is lessened, can be highly improved by a periodic sequence of $\ensuremath{\pi}$ pulses at a set target frequency. Photon indistinguishability in solid state and other quantum emitters subject to spectral diffusion can thus be enhanced by the proposed pulse sequence and similar external control protocols despite the fluctuations in the environment.
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We probe the indistinguishability of photons emitted by a semiconductor quantum dot (QD) via time- and temperature-dependent two-photon interference (TPI) experiments. An increase in temporal separation between consecutive photon emission events reveals a decrease in TPI visibility on a nanosecond time scale, theoretically described by a non-Markovian noise process in agreement with fluctuating charge traps in the QD's vicinity. Phonon-induced pure dephasing results in a decrease in TPI visibility from (96±4)% at 10 K to a vanishing visibility at 40 K. In contrast to Michelson-type measurements, our experiments provide direct access to the time-dependent coherence of a quantum emitter on a nanosecond time scale.
Two-photon interference (TPI) underpins many photonic quantum technologies and is commonly regarded as a quantum effect requiring indistinguishable photons. Here, we demonstrate TPI between single photons with ultralong coherence time and an independent continuous-wave laser. The measurements reveal a sharp antibunching dip, governed by the single-photon correlation time, together with a broad Hong-Ou-Mandel dip whose half-width is set by the mutual coherence time of the two sources. We achieve a maximum TPI visibility of 72 ± 2 % , well above the 50 % classical limit, thereby confirming the quantum nature of TPI. At the same time, we observe a broad visibility background and a beat visibility below 50 % , signatures of classical interference. Both quantum and classical visibility features remain unchanged for frequency separations up to 10 4 times the linewidth of the single photons, showing no distinction between distinguishable and indistinguishable photons. By applying both general wave superposition theory and a quantum field approach, we derive the same cross-correlation functions that quantitatively reproduce the data. Our results show that quantum and classical features of TPI originate from the same underlying fourth-order interference process arising from second-order interference within the mutual coherence time. Photon indistinguishability is a sufficient condition for TPI to occur, but not a necessary one. This insight offers different perspectives for the development of quantum photonic technologies based on independent photons.
As a fundamental building block for quantum computation and communication protocols, the correct verification of the two-photon interference (TPI) contrast between two independent quantum light sources is of utmost importance. Here, we experimentally demonstrate how frequently present blinking dynamics and changes in emitter brightness critically affect the Hong--Ou--Mandel-type (HOM) correlation histograms of remote TPI experiments measured via the commonly utilized setup configuration. We further exploit this qualitative and quantitative explanation of the observed correlation dynamics to establish an alternative interferometer configuration, which is overcoming the discussed temporal fluctuations, giving rise to an error-free determination of the remote TPI visibility. We prove full knowledge of the obtained correlation by reproducing the measured correlation statistics via Monte Carlo simulations. As an exemplary system, we make use of two pairs of remote semiconductor quantum dots; however, the same conclusions apply for TPI experiments with flying qubits from any kind of remote solid-state quantum emitters.
Generating identical photons from remote emitter-based bright single-photon sources is an important step for scaling up optical quantum technologies. Here, we study the Hong-Ou-Mandel interference of photons emitted from remote sources based on semiconductor quantum dots (QDs). We make use of a deterministic fabrication technique to position the QDs in a spectrally resonant micropillar cavity and fine-tune their operation wavelength electrically. In doing so, we can match four pairs of sources between five distinct sources, study them under various excitation schemes, and measure their degree of indistinguishability. We demonstrate remote indistinguishabiltity between 44 ± 1% and 69 ± 1% depending on the pair of sources and excitation conditions, record values for QDs in cavities. The relative contribution of pure dephasing and spectral diffusion is then analyzed, revealing that the remaining distinguishability is mostly due to low-frequency noise.