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Advanced Quantum Technologies

Purification of Single and Entangled Photons by Wavepacket Shaping

Chih‐Sung Chuu, Chun‐Yuan Cheng, Chih‐Hsiang Wu, Cheng‐Yeh Wei, Sheng‐Yao Huang, Yen‐Ju Chen, Shih‐Wen Feng, Chunyao Yang

Published Jan 20, 2021• 4 citations
Publisher

Abstract

Abstract Single photons and entangled photons lie at the heart of photonic quantum technologies, whose optimal performances are normally reached when the purity of the single or entangled photons is high. However, the multiphoton emission, dissipation, and decoherence in practical realizations always lead to the degradation of the single‐ and entangled‐ photon quality. The purification of single or entangled photons is thus valuable to restore the quantum states and enhance the performance of quantum technologies. The applications of wavepacket shaping, an emerging quantum optics tool to manipulate the single‐ and entangled‐ photon wavefunctions, to purify single and entangled photons are reviewed. In particular, by modulating the single photons emitted from optically excited room‐temperature quantum dots, it is shown that the fast‐decaying multiphoton emission can be eliminated to obtain a low value of g (2) (0) that is independent of the excitation power. It is also shown that the two‐photon interference and polarization entanglement of the non‐degenerate biphotons from spontaneous parametric down‐conversion can be restored after modulating the biphotons with a periodic function. The works have potential applications in long‐distance quantum communication and linear optical quantum computation.

DOI
10.1002/qute.202000122

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Abstract Single photons and entangled photons lie at the heart of photonic quantum technologies, whose optimal performances are normally reached when the purity of the single or entangled photons is high. However, the multiphoton emission, dissipation, and decoherence in practical realizations always lead to the degradation of the single‐ and entangled‐ photon quality. The purification of single or entangled photons is thus valuable to restore the quantum states and enhance the performance of quantum technologies. The applications of wavepacket shaping, an emerging quantum optics tool to manipulate the single‐ and entangled‐ photon wavefunctions, to purify single and entangled photons are reviewed. In particular, by modulating the single photons emitted from optically excited room‐temperature quantum dots, it is shown that the fast‐decaying multiphoton emission can be eliminated to obtain a low value of g (2) (0) that is independent of the excitation power. It is also shown that the two‐photon interference and polarization entanglement of the non‐degenerate biphotons from spontaneous parametric down‐conversion can be restored after modulating the biphotons with a periodic function. The works have potential applications in long‐distance quantum communication and linear optical quantum computation.
The possibility to manipulate the wave packets of single photons or biphotons has enriched quantum optics and quantum information science, with examples ranging from faithful quantum-state mapping and high-efficiency quantum memory to the purification of single photons. Here we demonstrate another fascinating use of wave packet manipulation on restoring quantum interference. By modulating the photons' temporal wave packet, we observe the revival of postselected entanglement that would otherwise be degraded or lost due to poor quantum interference. Our study shows that the amount of the restored entanglement is only limited by the forms of modulation and can achieve full recovery if the modulation function is properly designed. Our work has potential applications in long-distance quantum communication and linear optical quantum computation, particularly for quantum repeaters and large cluster states.
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Abstract Single-photon sources based on semiconductor quantum dots find several applications in quantum information processing due to their high single-photon indistinguishability, on-demand generation, and low multiphoton emission. In this context, the generation of entangled photons represents a challenging task with a possible solution relying on the interference in probabilistic gates of identical photons emitted at different pulses from the same source. In this work, we implement this approach via a simple and compact design that generates entangled photon pairs in the polarization degree of freedom. We operate the proposed platform with single photons produced through two different pumping schemes, the resonant excited one and the longitudinal-acoustic phonon-assisted configuration. We then characterize the produced entangled two-photon states by developing a complete model taking into account relevant experimental parameters, such as the second-order correlation function, Hong–Ou–Mandel visibility, multiphoton emission and pump laser filtering. Our source shows long-term stability and high quality of the generated entangled states, thus constituting a reliable building block for optical quantum technologies.
The creation and manipulation of biphotons is important for many applications in quantum optics and quantum information. Topics that benefit from efficient biphoton sources range from the most fundamental quantum science experiments to the highly applied fields of quantum communication and quantum computation. Biphoton sources have long been hailed as one of the leading methods for creating entangled photon pairs for tests of Bell's inequality, creating heralded photon pairs that are used in on-demand single-photon sources and heralded measurement techniques, and for quantum communication protocols to name a few. Specifically for quantum communication, biphoton sources are commonly used for cutting edge quantum key distribution (QKD) protocols. In the first part of the thesis, I focus on realizing an efficient biphoton source that produces high yield photon pairs. More specifically, I develop an optimized biphoton source using the nonlinear optical process of spontaneous parametric down-conversion in a second-order nonlinear crystal. I develop a formalism for predicting the two important metrics of a biphoton source: the heralding efficiency and joint count rate. I show how, from a large parameter space, one can tailor the phase matching of the nonlinear interaction to create a high quality biphoton source that produces both high heralding efficiency and high joint count rate. I achieve heralding efficiencies of 86$\\pm555\\%$ and joint count rates of 2.58$\\pm0.6kHzpermWpumppower.Ishowthatusingacollinearnondegenerategeometryallowsforheraldingefficienciesofupto99.70.6 kHz per mW pump power. I show that using a collinear nondegenerate geometry allows for heralding efficiencies of up to 99.70.6kHzpermWpumppower.Ishowthatusingacollinearnondegenerategeometryallowsforheraldingefficienciesofupto99.7\\%$ assuming no loss in the system. I verify the theoretical model with experimental results and find good agreement. In the second part of the thesis I turn to manipulating the single photons born from the biphoton source for applications in creating single-photon spectrometers and time-frequency QKD systems. The security of QKD is only guaranteed if the two parties have access to a set of states called mutually unbiased states. I create a set of these states in time and their conjugate states in the frequency basis and show that I can manipulate single photon correlations in time and frequency so that an eavesdropper can be detected if she localizes a photon to a 1 ns time interval. Additionally, in these experiments, I stretch a single photon wavepacket of 5-ps-width to a wavepacket of 8.3-ns-width and subsequently recompress it to at least the resolution of the detectors (sim\\simsim 300 ps). This demonstrates a stretch factor of >1600 for a single-photon pulse using a group velocity dispersive material. To my knowledge, this is the largest reported stretch factor for a single-photon wavepacket produced by a biphoton source. The ability to stretch and recompress a single photon by this amount has applications in creating high-resolution, high-efficiency, single-photon spectrometers as well as advancing time-frequency QKD systems and other temporal pulse shaping applications.
Narrow-band time-frequency entangled biphotons are generated from spontaneous four-wave mixing in cold atom clouds. The coherence time of the entangled biphotons can be extended to sub-microseconds by the slow light effect. The temporal wavefunction of the biphotons can be manipulated through modulating the spectral or spatial mode of the controlling laser beams. Concerning a pair of entangled biphoton and the resulting heralded single photon, it was commonly believed that, time-frequency entanglement damages the temporal purity of the single photon. However, the case is totally different for biphotons with long coherence time which is far beyond the time resolution of single-photondetectors. We demonstrate that, the single photon heralded from these time-frequency entangled biphotons is in a pure temporal state. Therefore, single photons are able to be shaped through the time-frequency entanglement with their partner photons, while the single photons could be found to be still in a pure state and thus useful for quantum information processing and communication technology.