Single-photon counting

High-sensitivity detectors that are able to generate an electrical signal when struck by a single photon have two main applications. The first is for tasks involving very low light levels, such as fluorescence microscopy for biological imaging, free-space optical communications or laser ranging. The second is for singlephoton counters in quantum optics, which must often capture individual photons for tasks such as quantum information processing and quantum key distribution. For example, in quantum cryptography for secure data communication, unbreakable codes can be produced by entangling the properties of two photons and measuring the quantum mechanical state of one of them. “I think there is a general trend in the marketplace towards looking at dynamic processes,” says Michael Mellon, general manager at Quantar Technology, a California-based developer of specialized test and measurement equipment. For instance, a sensitive enough detector can ‘see’ when a gate in a microprocessor switches, because the act of switching causes it to emit a few infrared photons. This has allowed computer chip designers to study processes such as how clock pulses propagate across a chip. Quantar’s equipment focuses on photon counting as an imaging technique, which is important when the spatial position of a photon provides information. In spectroscopy, for example, the position of a photon gives information about its wavelength. Mark Itzler, chief technology officer at Princeton Lightwave, says that in the past five years he’s seen an expansion in photon counting technologies, from silicon devices that work mainly at visible wavelengths to devices based on iii–v materials systems such as InGaAs, which cover telecommunications wavelengths in the infrared. When quantum cryptography is widely deployed it will probably need to operate in the near-infrared windows that are commonly used for optical communication (1,300 and 1,500 nm). “However, that’s a future market opportunity, not a current one. The challenge for those of us in the business is to see the opportunities in the longer wavelengths grow,” Itzler says. PRODUCT ROUND-UP The single-photon bench-top receiver from Princeton Lightwave (Cranbury, New Jersey, USA) is based on an InGaAs/ InP single-photon avalanche photodiode (APD) integrated with all the necessary bias and control electronics, and has particular application in quantum optics. The APD is designed to achieve a minimum detection efficiency of 20% at ambient operating temperatures for a wavelength of 1,550 nm, with a maximum dark count rate of 5 × 10–5 ns–1. When using the ultrahighsensitivity receiver, the maximum dark count rate is reduced to 5 × 10–6 ns–1. The front panel of the unit displays the number of photons detected in a selected interval, as well as other diagnostic information. All operating parameters and values can also be accessed through the RS-232 interface. Settings for the receiver include the APD’s temperature, blanking value, trigger delay and discriminator level. Measurement results displayed include the pulse count, gate pulse count and ratio measurement. An internal pulse generator provides an internal or external trigger at repetition rates from 1 Hz to 20 MHz. www.princetonlightwave.com