Single-photon sources have recently been demonstrated using a variety of devices, including molecules, mesoscopic quantum wells, colour centres, 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. Here we test the indistinguishability of photons emitted by a semiconductor quantum dot in a microcavity through a Hong-Ou-Mandel-type two-photon interference experiment. We 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.

Single photons are a fundamental element of most quantum optical technologies. The ideal single-photon source is an on-demand, deterministic, single-photon source delivering light pulses in a well-defined polarization and spatiotemporal mode, and containing exactly one photon. In addition, for many applications, there is a quantum advantage if the single photons are indistinguishable in all their degrees of freedom. Single-photon sources based on parametric down-conversion are currently used, and while excellent in many ways, scaling to large quantum optical systems remains challenging. In 2000, semiconductor quantum dots were shown to emit single photons, opening a path towards integrated single-photon sources. Here, we review the progress achieved in the past few years, and discuss remaining challenges. The latest quantum dot-based single-photon sources are edging closer to the ideal single-photon source, and have opened new possibilities for quantum technologies.

We observe large spontaneous emission rate modification of individual InAs Quantum Dots (QDs) in 2D a photonic crystal with a modified, high-Q single defect cavity. Compared to QDs in bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase by up to a factor of 8. In contrast, off-resonant QDs indicate up to five-fold rate quenching as the local density of optical states (LDOS) is diminished in the photonic crystal. In both cases we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using Finite Difference Time Domain (FDTD) simulations and find good agreement with experiment.One of the core issues of modern optics is the subject of photon interaction with matter. In the Wigner-Weisskopf approximation, the emission rate is directly proportional to the LDOS [1]. Over the past decade, photonic resonators with increased LDOS have been exploited to enhance emission rate for improving numerous quantum optical devices (e.g., [2,3]). Single photon sources in particular promise to see large improvements [4]. While more attention has been given to increasing emission rate, the reverse is also possible in an environment with decreased LDOS.Here we demonstrate that by designing a photonic crystal structure with a modified single-defect cavity, we can significantly increase or decrease the spontaneous emission (SE) rate of embedded QDs. Photonic crystals (PCs), periodic arrays of alternating refractive index, are near-ideal testbeds for such experiments. Their electromagnetic band structure modifies the LDOS relative to free space and hence the SE rate of embedded QD emitters. We demonstrate that SE of cavity-coupled QDs is enhanced up to 8 times compared to QDs in bulk GaAs. This coupling paves the way to single photon sources with higher out-coupling efficiency and visibility. On the other hand, decoupled QDs emit at up to five-fold decreased rate compared to bulk. This lifetime enhancement is significantly higher than

We demonstrate a new method for generating triggered single photons. After a laser pulse generates excitons inside of a single quantum dot, electrostatic interactions between them and the resulting spectral shifts allow a single emitted photon to be isolated. Autocorrelation measurements show a reduction of the two-photon probability to 0.12 times the value for Poisson light. Strong antibunching persists when the emission is saturated. The emitted photons are also polarized.PACS numbers: 42.50. Dv, Photons from classical light sources, which usually consist of a macroscopic number of emitters, follow Poisson statistics or super-Poisson statistics [1]. With a single quantum emitter, however, one can hope to generate a regulated photon stream, containing one and only one photon in a given time interval. Such an "anti-bunched" source would be useful in the new field of quantum cryptography, where security from eavesdropping depends on the ability to produce no more than one photon at a time [2,3].Continuous streams of anti-bunched photons were first observed from single atoms and ions in traps [4,5]. More recently, experiments demonstrating triggered single photons have used single molecules as the emitters, excited optically either by laser pulses [6,7] or through adiabatic following [8].Solid-state sources have potential advantages. Most importantly, they may be conveniently integrated into larger structures, such as distributed-Bragg-reflector (DBR) microcavities [9,10] to make monolithic devices. In addition, most do not suffer from the photo-bleaching effect that severely limits the lifespan of many molecules. The first experimental effort towards a solid-state singlephoton source was based on electrostatic repulsion of single carriers in a semiconductor micropost p-i-n structure [11]. Milli-Kelvin temperatures were required, however, and sufficient collection efficiency to measure the photon autocorrelation function was not obtained. More recently, continuous anti-bunched fluorescence has been seen from color centers in a diamond crystal [12,13] and from CdSe quantum dots [14].Our method to generate triggered single photons involves pulsed optical excitation of a single quantum dot and spectral filtering to remove all but the last emitted photon. Optically active quantum dots confine electrons and holes to small regions so that their energy levels are quantized [15]. If several electrons or holes are placed in the dot at the same time, they will, to a first approximation, occupy single-particle states as allowed by the Pauli exclusion principle. However, electrostatic interactions between the particles cause perturbations in the eigenstates and energies. For example, if two electronhole pairs (excitons) are created (a "biexcitonic" state), the first pair to recombine emits at a slightly lower energy than the second pair, due to a net attractive interaction [16,17]. We exploit this effect to generate single photons not only through regulated absorption, as in the single-molecule experiments, but also through t...

We have demonstrated efficient production of triggered single photons by coupling a single semiconductor quantum dot to a three-dimensionally confined optical mode in a micropost microcavity. The efficiency of emitting single photons into a single-mode traveling wave is approximately 38%, which is nearly 2 orders of magnitude higher than for a quantum dot in bulk semiconductor material. At the same time, the probability of having more than one photon in a given pulse is reduced by a factor of 7 as compared to light with Poissonian photon statistics.

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The spontaneous emission from an isolated semiconductor quantum dot state has been coupled with high efficiency to a single, polarization-degenerate cavity mode. The InAs quantum dot is epitaxially formed and embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%.

Quantum cryptography with a photon turnstileQ uantum cryptography generates unbreakable cryptographic codes by encoding information using single photons, which until now have relied on highly attenuated lasers as sources 1,2 . But these sources can create pulses that contain more than one photon, making them vulnerable to eavesdropping by photon splitting 3,4 . Here we present an experimental demonstration of quantum cryptography that uses a photon turnstile device, which is more reliable for delivering photons one at a time. This device allows completely secure communication in circumstances under which this would be impossible with an attenuated laser.Our quantum-cryptography system (see supplementary information for full technical details) implements a protocol known as BB84 (ref. 5). The photon turnstile is a single quantum dot in a micropost cavity 6,7 , which is optically excited by a pulsed laser. The security improvements attainable with this device can be quantified by two measurements: the probability that the device will inject a photon into the quantum channel, measured as 0.007 by comparing the count rate at detector 0 (see supplementary information) to the repetition rate of the excitation laser (76 MHz); and the secondorder correlation, denoted by g (2) (see supplementary information).This quantity gives the amount of suppression of multiphoton states from our device relative to attenuated laser light -a laser with perfect intensity stability is characterized by g (2) ǃ1, whereas our turnstile device has g (2) ǃ0.14. The probability that our device will emit a multiphoton state is therefore an order of magnitude smaller than a laser that emits photons at the same rate, meaning that security is improved in the presence of channel losses 8 .In our implementation of BB84, the sender of the message, Alice, encodes information by preparing the polarization of each photon in either the horizontal or right circular polarization for binary 0, and vertical or left circular for binary 1. This is done by an electro-optic modulator. The modulator is driven by a data generator that produces the secret key, giving a random four-level signal that corresponds to the four different polarization states in the BB84 protocol. The state of the data generator is recorded by a time-interval analyser and is stored by a computer.After the polarization is prepared, the photon is sent into the quantum channel, a 1-metre free-space propagation, and is detected by the receiving party, Bob. Bob measures the photons by using passive polarization optics and avalanche photodiodes with dark counts of about 80 s ǁ1 (see supplementary information). The detection probability, due to losses in the optics and photodiodes, is 0.24. Detection events are recorded by a second timeinterval analyser and are stored by a second computer for subsequent comparison with Alice.The error rate of the system is measured as 2.5%. These errors are corrected by using an error-correction algorithm 9 . After error correction, privacy amplification is carri...