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arXiv:2306.17428 · Physical Review Applied

Pure-State Photon-Pair Source with a Long Coherence Time for Large-Scale Quantum Information Processing

Bo Li, Yu-Huai Li, Yuan Wen Cao, Juan Yin, Cheng-Zhi Peng

Published Jun 29, 2023• 6 citations
arXivHTMLPDF

Abstract

This study presents an experimental technique to tackle technical obstacles in generating photon pairs in pure states with extended coherence times. Such photon sources are vital for large-scale quantum information processing. The authors successfully eliminate the frequency correlation of parametric photons using optimal filtering, which enables high-quality Hong-Ou-Mandel interference between two photon sources. This breakthrough has promising practical applications in quantum science and can facilitate engineering solutions for long-distance quantum interference.

DOI
10.1103/physrevapplied.19.064083

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This study presents an experimental technique to tackle technical obstacles in generating photon pairs in pure states with extended coherence times. Such photon sources are vital for large-scale quantum information processing. The authors successfully eliminate the frequency correlation of parametric photons using optimal filtering, which enables high-quality Hong-Ou-Mandel interference between two photon sources. This breakthrough has promising practical applications in quantum science and can facilitate engineering solutions for long-distance quantum interference.
We present an experimental demonstration of heralded single photons prepared in pure quantum states from a parametric down-conversion source. It is shown that, through controlling the modal structure of the photon pair emission, one can generate pairs in factorable states and thence eliminate the need for spectral filters in multiple-source interference schemes. Indistinguishable heralded photons were generated in two independent spectrally engineered sources and Hong-Ou-Mandel interference observed between them without spectral filters. The measured visibility of 94.4% sets a minimum bound on the mean photon purity.
Quantum interference plays an essential role in understanding the concepts of quantum physics. Moreover, the interference of photons is indispensable for large-scale quantum information processing. With the development of quantum networks, interference of photons transmitted through long-distance fiber channels has been widely implemented. However, quantum interference of photons using free-space channels is still scarce, mainly due to atmospheric turbulence. Here, we report an experimental demonstration of Hong-Ou-Mandel interference with photons transmitted by free-space channels. Two typical photon sources, i.e., correlated photon pairs generated in spontaneous parametric down conversion (SPDC) process and weak coherent states, are employed. A visibility of 0.744 ± 0.013 is observed by interfering with two photons generated in the SPDC process, exceeding the classical limit of 0.5. Our results demonstrate that the quantum property of photons remains even after transmission through unstable free-space channels, indicating the feasibility and potential application of free-space-based quantum interference in quantum information processing.
Hong-Ou-Mandel interference is essential for various quantum communication protocols, including quantum teleportation, entanglement swapping, and measurement-device-independent (MDI) quantum key distribution (QKD). A significant challenge is producing single photons that remain indistinguishable over long distances. Time jitter introduced in synchronization between remote nodes reduces temporal overlap, making photons no longer indistinguishable. To mitigate this effect, previous solutions are forced to increase the temporal overlap, typically by using narrowband filtering strategies to extend photon coherence time. However, this dramatically decreases photon rates, making them impractical for real-life applications. Here we address this dilemma by employing common laser pulses to generate intrinsically synchronized single photons, eliminating the need for extended coherence time. Without compromising photon indistinguishability, we achieved a four-fold coincidence count rate more than two orders of magnitude higher than previous methods over 50 kilometers of fiber transmission. This breakthrough enabled the implementation of MDI-QKD with passive decoy states, demonstrating the viability of our approach.
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Single photons - discrete wavepackets of light - are one of the most fundamental entities in physics. In recent years, the ability to consistently create and manipulate both single photons and pairs of photons has facilitated everything from tests of quantum theory to the implementation of quantum-enhanced precision measurements. These activities all fall within the scope of the rapidly-growing field of quantum information - the exploitation of the properties of quantum states (and specifically their capability to exist in superpositions) to accomplish tasks that would not be possible with classical objects. One stated goal of research in quantum information is to build a device consisting of a network of quantum logic gates that can evaluate quantum algorithms. The photonic implementation of individual logic gates has already been demonstrated. However, partly due to standard methods of preparing single photons, current schemes have severe limitations in terms of scaling up from a single logic gate to multiple concatenated operations. Until now it has not been proven that single photons can be generated in pure and indistinguishable quantum states, something upon which the successful operation of optical quantum logic gates relies. This thesis presents an experimental demonstration of simultaneous generation of almost identical single photons in highly pure states from two independent sources based on parametric downconversion. This is a process of photon pair generation during the passage of a light beam through a nonlinear crystal; one photon from the resulting pair is detected to herald the other. The work herein describes, refines, and implements a technique that minimises the strong quantum correlations usually present within each pair by spectral engineering of the source. This allows the heralded single photons to be in pure states, a property that is confirmed by observing a high-visibility two-photon interference effect without spectral filtering.