What is Single Photon Purity?
This is one of several metrics used to quantify the quality of a single photon source. Single Photon Purity measures the probability that a source generates exactly one photon when triggered. This is related to but is distinct from other metrics such as photon indistinguishability which measures how similar two photons emitted from the same source are to each other.
A generally accepted mathematical definition of single photon purity is given by $$ [P = 1 – g^{(2)}] $$ Where \( g^{(2)} \) represents the intensity autocorrelation function integrated about zero delay time, which for an ideal single photon source is zero. This implies an ideal single photon purity value of one.
Importance of Single Photon Purity in Photonic Quantum Computing
Quantum Interference: Many of the proposed algorithms for quantum computers rely on precisely managed interference between quantum states. This interference is disrupted by multi-photon contamination arising from impure single photon sources. Therefore, High single photon purity is a prerequisite for high quality photonic quantum computing.
Scalability: Achieving utility scale applications of quantum computing requires pushing existing hardware to larger qubit counts. Doing so efficiently requires low error rate qubits which can be achieved by systems that generate high purity photons
Frequently Asked Questions About Single Photon Purity
- How is Single Photon Purity Measured? This metric is measured through a Hanbury Brown and Twiss (HBT) setup. This is a simple experimental configuration where the output of a single photon sources is impinged on a beam splitter and both output ports are measured. If the source only emits one photon at a time, then it would be impossible for both detectors to click simultaneously. Thus, by monitoring this event probability we measure the purity of the single photon source.
- What factors impact Single Photon Purity? There are several sources of error that can lead to impure photon emission. Some examples include Imperfect excitation, where the single photon source is excited multiple times by a single excitation pulse and Background light, stray environmental photons can couple into the same mode that our generated photons occupy thus reducing their purity.