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Photonics for Quantum (PfQ)

Quantum photonics: how far can we go?

Paul G. Kwiat

Published Aug 27, 2021• 0 citations
Publisher

Abstract

Although existing experiments have implemented protocols employing up to 10 or even 12 photons, the inefficiencies of sources has resulted in exceedingly low rates of these high-photon events. Single and entangled-photon emitters both suffer from probabilistic operation, substantial losses, or both, so that extrapolating even to 20 photons is discouraging. However, recent experiments employing multiplexing techniques can trade overall rate (or component count) for increased probability, e.g., factors of 10-30 enhancement have been demonstrated. How far can such experiments be pushed? Using our best guesstimates on what system capabilities may be feasible, we discuss the near-term future for multi-photon quantum processing.

DOI
10.1117/12.2610076

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Although existing experiments have implemented protocols employing up to 10 or even 12 photons, the inefficiencies of sources has resulted in exceedingly low rates of these high-photon events. Single and entangled-photon emitters both suffer from probabilistic operation, substantial losses, or both, so that extrapolating even to 20 photons is discouraging. However, recent experiments employing multiplexing techniques can trade overall rate (or component count) for increased probability, e.g., factors of 10-30 enhancement have been demonstrated. How far can such experiments be pushed? Using our best guesstimates on what system capabilities may be feasible, we discuss the near-term future for multi-photon quantum processing.
Recent advancements in quantum photonics have driven significant progress in photonic quantum computing (PQC), addressing challenges in scalability, efficiency, and fault tolerance. Experimental efforts have focused on integrated photonic platforms utilizing materials such as silicon photonics and lithium niobate to enhance performance. Parameters like photon loss rates, coupling efficiencies, and fidelities have been pivotal, with state-of-the-art systems achieving coupling efficiencies above 90% and photon indistinguishability exceeding 99%. Quantum error correction schemes have reduced logical error rates to below 10−310^{-3}10−3, marking a step toward fault-tolerant PQC. Photon generation has also advanced with deterministic sources, such as quantum dots, achieving brightness levels exceeding 10610^6106 photon pairs/s/mW and time-bin encoding enabling scalable entanglement. Heralded single-photon sources now exhibit purities above 99%, driven by innovations in fabrication techniques. High-efficiency photon detectors, such as superconducting nanowire single-photon detectors (SNSPDs), have demonstrated detection efficiencies exceeding 98%, dark count rates below 1 Hz, and timing jitters as low as 15 ps, ensuring precise photon counting and manipulation. Moreover, demonstrations of boson sampling with over 100 photons underscore the growing computational power of photonic systems, surpassing classical limits. The integration of machine learning has optimized photonic circuit design, while frequency multiplexing and time-bin encoding have increased system scalability. Together, these advances bridge the gap between theoretical potential and practical implementation, positioning PQC as a transformative technology for computing, communication, and quantum sensing.
Boson sampling is considered as a strong candidate to demonstrate the “quantum advantage / supremacy” over classical computers. However, previous proof-of-principle experiments suffered from small photon number and low sampling rates owing to the inefficiencies of the single-photon sources and multi-port optical interferometers. In this talk, I will report two routes towards building Boson Sampling machines with many photons. In the first path, we developed SPDC two-photon source with simultaneously a collection efficiency of ~70% and an indistinguishability of ~91% between independent photons. With this, we demonstrate genuine entanglement of ten photons. Very recently, we managed to observe 12-photon entanglement using a novel SPDC source. Such a platform will provide enabling technologies for teleportation of multiple properties of photons and efficient scattershot boson sampling. In the second path, using a QD-micropillar, we produced single photons with high purity (>99%), near-unity indistinguishability for >1000 photons, and high extraction efficiency—all combined in a single device compatibly and simultaneously. We build 3-, 4-, and 5-bosonsampling machines which runs >24,000 times faster than all the previous experiments, and for the first time reaches a complexity about 100 times faster than the first electronic computer (ENIAC) and transistorized computer (TRADIC) in the human history. We are currently increasing the rate by optimizing the single-photon extraction efficiency to near unity, background-free resonance fluorescence, and using improved schemes such as boson sampling with photon loss, with the hope of achieving 20-photon boson sampling this year.
Current sources of heralded single photons based on nonlinear optics operate in a probabilistic manner. In order to build quantum-enhanced devices based around the use of single photons, compact, turn-key and deterministic sources are required. A possible solution is to multiplex a number of sources to increase the single-photon generation probability and in so doing reducing the waiting time to deliver large numbers of photons simultaneously, from independent sources. Previously it has been shown that, in the ideal case, 17 multiplexed sources allow deterministic generation of heralded single photons [Christ and Silberhorn, Phys. Rev. A 85, 023829 (2012)]. Here we extend this analysis to include undesirable effects of detector inefficiency and photon loss on a number of multiplexed sources using a variety of different detectors for heralding. We compare these systems for fixed signal-to-noise ratio to allow a direct comparison of performance for real- world heralded single photon sources.
Photon sources for multi-photon entanglement experiments are commonly based on the process of spontaneous parametric down conversion. Due to the probabilistic photon production, such experiments suffer from low multiphoton count rates. To increase this count rate, we present a novel SPDC pump source based on a femtosecond UV enhancement cavity that increases the available pump power while maintaining a high repetition rate of 80MHz. We apply the cavity as photon source for realizing symmetric, multi-partite entangled Dicke states, which are observed with a high rate and high fidelity. We characterize the observed Dicke states of up to six photons using efficient tools exploiting the state's symmetries.
Scalable quantum photonic technologies require the simultaneous production of highly indistinguishable multi-photon states with high probability. This need may be met, and the output probability significantly increased by multiplexed sources, which channel numerous low-probability heralded single-photon sources into a specified output mode using time or spatial multiplexing. However, conventional methods merely replicate these multiplexing units when scaled to multiple output modes, which is fundamentally inefficient because it discards many valid multi-photon events. To address this, we propose and experimentally demonstrate a novel multiplexing source architecture based on a multi-input/multi-output switching network, which can utilize combinatorial photon events. A super-exponential enhancement in multi-port single-photon creation is revealed by theoretical analysis, which significantly reduces the number of heralded single-photon sources needed. We demonstrate multiplexing with two output ports, supported by four integrated heralded single-photon sources. Our strategy has a generation probability of 1.60 times that of conventional methods and 2.58 times that of implementations without multiplexing. This multi-port multiplexed method opens a new avenue for establishing a solid foundation for advanced multi-photon quantum interference and large-scale quantum information processing.