Single-photon sources: Approaching the ideal through multiplexing

Abstract

We review the rapid recent progress in single-photon sources based on multiplexing multiple probabilistic photon-creation events. Such multiplexing allows higher single-photon probabilities and lower contamination from higher-order photon states. We study the requirements for multiplexed sources and compare various approaches to multiplexing using different degrees of freedom.

FIG. 1.

Types of single-photon sources: (a) isolated quantum systems (e.g. a single particle in an optical cavity, with ground g and excited e states), (b) heralded single-photon sources from photon pairs, (c) multiplexed source (as one example, spatial multiplexing is shown).

FIG. 2.

Experimental source multiplexing (MUX) performance for state-of-the-art single-photon parametric down-conversion (PDC) sources. For comparison quantum dot performance is also shown. Left: The heralded fidelity to a single photon versus the brightness (i.e. the probability of finding just one photon per pump laser pulse). Single-source PDC brightness is bounded by the black line and limited to the grey region. Optimal sources are toward the upper right, and PDC source multiplexing has outperformed the best quantum dots in brightness. Right: The heralded g⁽²⁾(0) (= 0 for ideal single photons) versus the heralding probability. For standard PDC, g⁽²⁾(0) is bounded by the black line and limited to the grey region. Currently only bulk time multiplexing and quantum dots have achieved g⁽²⁾(0) better than this limit.

FIG. 3.

(upper) Heralding probability and heralded fidelity for a multiplexed single-photon source with K = 20 (solid) and K = 50 (dashed) sources, versus the efficiency of the heralding detector η_h for two different squeezing strengths λ. (lower) The fidelity is independent of the number of sources, while the heralding probability increases with source number.

FIG. 4.

Proposal for spatial source multiplexing, either using many distinct sources (a) or many spatial modes of a single source (b). Reprinted figure with permission from ref. . Copyright (2002) by the American Physical Society.

FIG. 5.

(a) Schematic for temporal multiplexing using a storage loop, where many temporal modes are pumped, and the one with a herald event is synchronized using a selectable number of roundtrips of the storage loop to the output. Reprinted figure with permission from ref. . Copyright (2002) by the American Physical Society. (b) An alternative synchronization method, binary division, where the photon takes the single path with the correct delay length through the fiber network. Reprinted under Creative Commons Attribution license 3.0 from ref. .

FIG. 6.

Spectral source multiplexing. The idler photon, which shares a correlated joint spectrum with the signal photon, is detected with spectral resolution (ν_i+, ν_i0, or ν_i−), then the corresponding signal photon’s frequency is shifted to the output spectral band ν_s0. Reprinted figure with permission from ref. . Copyright (2017) by the American Physical Society.