Photon number detectors are most fundamental and essential devices for quantum information processing and quantum communications. In this article, we introduce the activity of developing of photon number detectors.
In quantum optical computing and quantum cryptography a very important problem is that of developing an efficient photon-number resolving detector. Such a device is critical for the development of linear optical quantum computing (LOQC) [1], and can be also used in quantum cryptography [2]. A recent paper claims to have demonstrated the operation of a superconducting, number-resolving photon detector, developed by a group at NIST [3]. However, this device operates with a quantum efficiency of only about 20%, and we show here that it cannot discriminate photon number, even in principal. However, the NIST device has a very low dark-count rate, much less than 1%, which means that it can quite efficiently discriminate against the absence of photons of particular numbers. We call such a device a number-discriminating non-photon detector. Interestingly enough, even though this detector cannot yield photon number—its ability to discriminate against photons of certain numbers allows it to be used, under suitable assumptions, to reconstruct the ensemble photostatistics of the incoming photon field. However, such a low-efficiency device cannot be used, say, in LOQC, where single-shot photon number discrimination is required.
In optical quantum information processing, two of the most basic elements are the sources of quantum mechanical states of light, and the devices that can detect these states. In this chapter, we narrow this down to photon sources and photodetectors. We will describe first how detectors work, starting from abstract ideal detectors, via a complete description of realistic detectors in terms of POVMS, to a brief overview of current photodetectors. Subsequently, we will define what is a single-photon source, and how we can determine experimentally whether a source produces single photons or something else. Having laid down the ground rules, we will survey some of the most popular ways photons are produced in the laboratory. Finally, we take a look at the production of entangled photon sources and quantum non-demolition measurements of photons. A mathematical model of photodetectors Photodetectors are devices that produce a macroscopic signal when triggered by one or more photons. In the ideal situation, every photon that hits the detector contributes to the macroscopic signal, and there are no ‘ghost’ signals, or so-called dark counts. In this situation we can define two types of detector, namely the ‘photon-number detector’, and ‘detectors without number resolution’. First, the photon number detector is a (largely hypothetical) device that tells us how many photons there are in a given optical mode that is properly localized in space and time. This property is called ‘photon-number resolution’.
Single-photon detectors based on superconducting thin films have become a viable class of technologies for widespread usage in quantum optics. In this tutorial paper we introduce the key performance metrics required of them for quantum information processing and related fields. We review the latest records achieved by such devices, study technical details regarding recent improvements in superconducting nanowire detectors and transition-edge sensors, and present representative applications in cutting-edge research areas that benefit from these advances.
Photons have been a flagship system for studying quantum mechanics, advancing quantum information science, and developing quantum technologies. Quantum entanglement, teleportation, quantum key distribution, and early quantum computing demonstrations were pioneered in this technology because photons represent a naturally mobile and low-noise system with quantum-limited detection readily available. The quantum states of individual photons can be manipulated with very high precision using interferometry, an experimental staple that has been under continuous development since the 19th century. The complexity of photonic quantum computing devices and protocol realizations has raced ahead as both underlying technologies and theoretical schemes have continued to develop. Today, photonic quantum computing represents an exciting path to medium- and large-scale processing. It promises to put aside its reputation for requiring excessive resource overheads due to inefficient two-qubit gates. Instead, the ability to generate large numbers of photons—and the development of integrated platforms, improved sources and detectors, novel noise-tolerant theoretical approaches, and more—have solidified it as a leading contender for both quantum information processing and quantum networking. Our concise review provides a flyover of some key aspects of the field, with a focus on experiment. Apart from being a short and accessible introduction, its many references to in-depth articles and longer specialist reviews serve as a launching point for deeper study of the field.