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APPLIED PHYSICS: DETECTING SINGLE PHOTONS

The following points are made by Daniel E. Prober (Nature 2003 425:777):

1) Vision is the richest of the human senses, and detectors of light have long featured in science and technology. In fields as diverse as telecommunications, medicine and astronomy, there is demand for exquisitely sensitive detectors of the quantum of light, the photon. But as yet there is no single commercial device that can simultaneously detect individual visible photons and record the color (or energy) and arrival time of each photon. If such a detector existed, it should also produce images with many picture elements (pixels) -- and be affordable.

2) The search for the ultimate photon detector has been driven by astronomers, who are often faced with a limited number of photons to measure, emitted from some object in our Galaxy or beyond, and limited time in which to measure them. At present, astronomy is well served by the charge-coupled detector (CCD), familiar to many people as the CCD sensor in their digital camera or camcorder. Megapixel CCDs are now common. But this detector cannot resolve individual photons, because the noise occurring randomly in its readout electronics is too large to allow it. Moreover, the physics of the detector precludes the measurement of photon color, unless color filters are used. These reduce efficiency, but without such filters a CCD would see only shades of grey.

3) To record single photons cleanly and to discern their energy and arrival time requires a detector that operates at low temperatures. This gets rid of the thermal agitation in the device that would disrupt a single-photon signal, and also means that materials and techniques can be used that are fundamentally different from those employed in the CCD. The first advance in cryogenic detector technology was the silicon "microbolometer"(2), in which a small piece of silicon is held at 0.05 K but heats up when a photon is absorbed. The subsequent rise and fall of the temperature is recorded, to determine the photon energy. These detectors are employed by astronomers for rocket-based observations of photons at X-ray wavelengths, with an energy 100 to 1000 times that of visible photons. But their sensitivity is not sufficient to record visible photons, with an energy of 1.5 to 3 electronvolts.

4) Over the past decade, new devices have been developed that are more sensitive and that can record visible photons(3-5). These detectors fall into two classes. The first is the bolometer, which uses the same heating effect as mentioned above to determine photon energy. The thermometer inside such a device, recording the temperature change, is a strip of partly superconducting metal(3); the readout amplifier is also superconducting. As superconductivity -- the flow of current without resistance -- is a low-temperature property, the entire structure of the device is a cryogenic environment (which is easier to engineer than a device with only some superconducting components).

5) Day et al(1) now propose a device, based on the principle of kinetic inductance, that could function as an individual pixel in a photon detector. Of importance is that they demonstrate that the device has the necessary properties to allow the incorporation of many such pixels into the "ultimate" photon detector, one that could also be read out with practical, available electronics.

References (abridged):

1. Day, P. K., LeDuc, H. G., Mazin, B. A., Vayonakis, A. & Zmuidzinas, J. Nature 425, 817-821 (2003)

2. Moseley, S. H., Mather, J. C. & McCammon, D. J. Appl. Phys. 56, 1257-1262 (1984)

3. Stahle, C. K., McCammon, D. & Irwin, K. D. Phys. Today 52, 32-37 (1999)

4. Mather, J. C. Nature 401, 654-655 (1999)

5. Twerenbold, D. Rep. Prog. Phys. 59, 349-426 (1996)

Nature http://www.nature.com/nature

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INDISTINGUISHABLE PHOTONS FROM A SINGLE-PHOTON DEVICE

The following points are made by C. Santori et al (Nature 2002 419:594):

1) Single-photon sources have recently been demonstrated using a variety of devices, including molecules(1-3), mesoscopic quantum wells(4), color centers(5), trapped ions and semiconductor quantum dots. Compared with a Poisson-distributed source of the same intensity, these sources rarely emit two or more photons in the same pulse. Numerous applications for single-photon sources have been proposed in the field of quantum information, but most -- including linear-optical quantum computation -- also require consecutive photons to have identical wave packets. For a source based on a single quantum emitter, the emitter must therefore be excited in a rapid or deterministic way, and interact little with its surrounding environment.

2) When identical single photons enter a 50 50 beam splitter from opposite sides, quantum mechanics predicts that both photons must leave in the same direction, if their wave packets overlap perfectly. This two-photon interference effect originates from the Bose Einstein statistics of photons. This bunching effect was first observed using pairs of highly correlated photons produced by parametric downcoversion, but it should also occur with single, independently generated photons. Most proposed applications for single-photon sources in the field of quantum information (with the notable exception of quantum cryptography) involve two-photon interference. Such applications include quantum teleportation, post-selective production of polarization-entangled photons, and linear-optics quantum computation. It is therefore important to demonstrate that consecutive photons emitted by a single-photon source are identical and exhibit mutual two-photon interference effects.

3) The authors report a test of the indistinguishability of photons emitted by a semiconductor quantum dot in a microcavity through a Hong Ou Mandel-type two-photon interference experiment. The authors find that consecutive photons are largely indistinguishable, with a mean wave-packet overlap as large as 0.81, making this source useful in a variety of experiments in quantum optics and quantum information.

References (abridged):

1. De Martini, F., Di Giuseppe, G. & Marrocco, M. Single-mode generation of quantum photon states by excited single molecules in a microcavity trap. Phys. Rev. Lett. 76, 900-903 (1996)

2. Brunel, C., Lounis, B., Tamarat, P. & Orrit, M. Triggered source of single photons based on controlled single molecule fluorescence. Phys. Rev. Lett. 83, 2722-2725 (1999)

3. Lounis, B. & Moerner, W. E. Single photons on demand from a single molecule at room temperature. Nature 407, 491-493 (2000)

4. Kim, J., Benson, O., Kan, H. & Yamamoto, Y. A single-photon turnstile device. Nature 397, 500-503 (1999)

5. Beveratos, A. et al. Room temperature stable single-photon source. Eur. Phys. J. D 18, 191-196 (2002)

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