The development of an on-demand single photon source has been described as desirable for many applications including quantum computing as well as other fields of science and technology. Different technologies have been proposed to generate on-demand single photons including schemes using spontaneous four wave mixing (SFWM) and spontaneous parametric down-conversion (SPDC). Both of these processes generate a pair of correlated photons using a nonlinear medium, such as a crystal pumped by a laser. Sources using SPDC and SFWM are often called heralded single photon sources because the detection of one photon indicates, or ‘heralds’, the presence of its twin.
Several documents discuss the generation of single photons including the following.
Journal document “Tailoring single-photon and multiphoton probabilities of a single-photon on-demand source” by Migdall et al. [Phys. Rev A, 66, 053805 (2002)] describes using an array of down-converters and detectors pumped simultaneously by the same laser. The laser power is chosen so that each down-converter has some small probability of producing a photon pair, while the number of down-converters is chosen so that there is a high likelihood of at least one pair being created somewhere in the array. The detector associated with each down-converter allows the determination of which of the down-converters has fired. This information is used to control an optical switching circuit directing the other photon of the pair onto a single-output channel. The conversion process of each down converter in the array is independent of the conversion process in the other down converters.
Journal article “Experimental generation of single photons via active multiplexing”, Ma et al, [Phys. Rev A 83, 043814 (2011)] describes ‘m’ SPDC sources pumped by a single pulsed laser. The SPDC sources are coupled by fast photon routers and directed to a single output. The conversion process of each SPDC source in the array is independent of the conversion process in the other SPDC sources.
Journal article “Efficient generation of single and entangled photons on a silicon photonic integrated chip” by Mower et al. [Phys. Rev A 84 052326 (2011)] describes a scheme to integrate a source of highly indistinguishable photons on a silicon-on-insulator photonic integrated circuit by using actively multiplexed parametric photon (AMPP) generation. The AMPP source uses photon pairs generated by a single SPDC element pumped at some period T. A pulsed laser at 780 nm is split into a series of delays of lengths of 4T, 2T, and T, to create an eight-pulse train that then pumps a nonlinear crystal cut for type-II SPDC. The idler photons of each subsequently generated pair are sent to the “heralding decision block,” which consists of a single-photon detector, data processor, bit generator and decision switch-on chip. The detector, gated by the pump laser, sends time-tagged idler arrival events to the data processor. The processor outputs to a bit generator, which modulates the decision switch-on chip, selecting which signal photon will enter the “variable delay circuit” block. A single SPDC element is used to generate the photon pairs in this document whereby the generation of one photon pair is not used to influence the generation of a further photon pair from the same SPDC source.
Journal article “Integrated spatial multiplexing of heralded single-photon sources” by Collins et al. [Nature Communications 4, 2582 (2013)] describes an experimental demonstration of integrated, spatially multiplexed, heralded single-photon sources. Pump pulses are coupled to a silicon waveguide and split to a bank of N nominally identical and monolithically integrated photonic crystal waveguides where photon pairs are generated by SFWM which are in turn routed to an N×2 optical switch to produce a sub-Poissonian heralded single photon output. The conversion process of each SFWM source in the array is independent of the conversion process in the other SFWM sources.
These photon sources use a plurality of heralding events in one or more photon pair sources to generate a photon pair used as the source output, wherein the sources allow for each event to occur and then use post event means and techniques to select one photon as the output, for example by using an optical switch. Optical switches can be a major source of loss in an optical circuit.
Journal article “Deterministic generation of single photons via multiplexing repetitive parametric down-conversions” by Glebov, B. L. et al, [Applied Physics Letters 103, 031115 (2013)] describes multiplexing two repetitive SPDC processes, wherein in each process is undertaken by two separate modules. Each process is configured to possibly generate a pair of daughter photons in modes ‘a’ and ‘b’. A cavity is implemented in mode a so that mode a photons circulate inside the cavity whilst the number of photons generated in mode b is detected by a photon-number resolving detector. When one photon is detected in mode b of a process, pumping of that process is stopped. The conversion process of each SPDC process is independent of the conversion process in the other SPDC process.
Journal article “Photon-number state on-demand source by cavity parametric down-conversion” by Hayat et al., [Applied Physics Letters 89, 171108 (2006)] describes an SPDC based photon-number state on-demand source wherein the signal and idler photons are generated inside a monolithic single cavity. The signal photons are automatically coupled to storage. The pump pulse intensity is adjusted to produce on average ‘n’ SPDC pairs. The idler photons are detected wherein if the number of idler photons counted equals ‘n’ then an output mirror of the cavity is abruptly spoiled, reducing the Q factor and enabling the emission of the n signal photons. Only a single monolithic cavity is used, therefore SPDC conversion process of one SPDC source is not used to control the conversion process of a second SPDC source.
Journal article “On-demand single photon emission based on dynamic photon storage on a photonic integrated circuit” by Heuck et al., [Conference on Laser and Electro Optics, Munich, 2015] describes generating signal and idler photons by degenerate Four Wave Mixing in a storage ring. Three other rings are used to couple frequencies ωp, ωs, ωi (corresponding to pump, signal and idler light frequencies respectively). A generated idler photon is coupled to a detector via the idler ring, which in turn triggers a switch to stop the pump laser from entering the pump ring. The signal photon is kept in the storage ring until a clock signal arrives at the signal ring causing it to temporarily tune into resonance with ωs to release the signal photon. This scheme has a single ‘storage ring’ used to generate the signal and idler photons and uses a separate tunable ‘signal’ ring to couple out the signal photon from the storage ring. The conversion process of photon pairs in the ring is not used to control the conversion process in the other photon pair sources.