We report the development of a photon-number resolving detector based on a fiber-optical setup and a pair of standard avalanche photodiodes. The detector is capable of resolving individual photon numbers, and operates on the well-known principle by which a single mode input state is split into a large number (eight) of output modes. We reconstruct the photon statistics of weak coherent input light from experimental data, and show that there is a high probability of inferring the input photon number from a measurement of the number of detection events on a single run.Many quantum information strategies require the preparation of nonclassical states. For example, the method of linear optical quantum computing proposed by Knill, Laflamme, and Milburn [1] demands the preparation of single photon states as well as maximally entangled photon multiplets. A number of schemes have been proposed for the preparation of such states, including single photon emitters [2,3,4] and conditionally prepared photon pairs from parametric downconversion [5,6,7]. Conditional state preparation requires the ability to distinguish states of different photon number, which is not possible using conventional photodetectors. Photon number resolution is also desirable to enhance the security of quantum cryptographic schemes [8,9]. In this case it is important to measure the photon statistics of the source at the sending and receiving stations. For implementations that use weak coherent states this means distinguishing between the detection of, say, one or two photons.According to the quantum theory of photodetection, the signal obtained from an ideal noise-free detector has a discrete form corresponding to the absorption of an integer number of quanta from the incident radiation. In practice, however, the granularity of the output signal is concealed by the noise of the detection mechanism. When very low light levels are detected using devices with single-photon sensitivity such as photomultipliers or Geiger-mode operated avalanche photodiodes (APDs), the electronic signal can be reliably converted into a binary message telling us with high efficiency whether an absorption event has occurred or not. However, the intrinsic noise of the gain mechanism necessary to bring the initial energy of absorbed radiation to the macroscopic level completely masks the information on exactly how many photons have triggered that event.There are several methods for constructing photon number resolving detectors. Among those demonstrated to date are the segmented photomultiplier [10], the superconducting bolometer [11], and the superconducting transimpedance amplifier [12]. These detectors operate at cryogenic temperatures and have single photon quantum efficiencies ranging from about 20% for the superconducting devices to approximately 70% in the case of the segmented photomultiplier. On the other hand, conventional room temperature APDs have intrinsic quantum efficiencies up to 80%, though they respond only to the presence or absence of radiation. The ease of...

Detectors that can resolve photon number are needed in many quantum information technologies. In order to be useful in quantum information processing, such detectors should be simple, easy to use, and be scalable to resolve any number of photons, as the application may require great portability such as in quantum cryptography. Here we describe the construction of a timemultiplexed detector, which uses a pair of standard avalanche photodiodes operated in Geiger mode. The detection technique is analysed theoretically and tested experimentally using a pulsed source of weak coherent light.

We experimentally investigate a method of directly characterizing the photon-number distribution of nonclassical light beams that is tolerant to losses and makes use of only standard binary detectors. This is achieved in a single measurement by calibrating the detector using some small amount of prior information about the source. We demonstrate the technique on a freely propagating heralded two-photon-number state created by conditional detection of a two-mode squeezed state generated by parametric down-conversion.

Geometric phase may enable inherently fault-tolerant quantum computation. However, due to potential decoherence effects, it is important to understand how such phases arise for mixed input states. We report the first experiment to measure mixed-state geometric phases in optics, using a Mach-Zehnder interferometer, and polarization mixed states that are produced in two different ways: decohering pure states with birefringent elements; and producing a nonmaximally entangled state of two photons and tracing over one of them, a form of remote state preparation.

Detectors that can resolve photon number are needed in many quantum information technologies.In order to be useful in quantum information processing, such detectors should be simple, easy to use, and be scalable to resolve any number of photons, as the application may require great portability such as in quantum cryptography. Here we describe the construction of a time-multiplexed detector, which uses a pair of standard avalanche photodiodes operated in Geiger mode. The detection technique is analysed theoretically and tested experimentally using a pulsed source of weak coherent light.