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Solid State Sources for Single Photons

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The S4P Consortium undertakes the development of Solid-state Single Photon Sources and their use in Quantum Information Processing and Communications, by addressing the following issues:
(1) the light source should involve a single dipole, so that upon emission it produces a single photon. Such a single emitter intrinsically emits radiation almost isotropically. To remedy this;
(2) the emitter should be embedded in an optical microcavity, designed to increase the spontaneous emission in a restricted number of modes, which can be collected with high efficiency for further use, for example, by coupling into an optical fibre. Finally;
(3) in order to assess its performance for QIPC, the source that will reach fully operational status must be intergrable in some operating quantum information system. The S4P Consortium undertakes the development of Solid-state Single Photon Sources and their use in Quantum Information Processing and Communications, by addressing the following issues:
(1) the light source should involve a single dipole, so that upon emission it produces a single photon. Such a single emitter intrinsically emits radiation almost isotropically. To remedy this;
(2) the emitter should be embedded in an optical microcavity, designed to increase the spontaneous emission in a restricted number of modes, which can be collected with high efficiency for further use, for example, by coupling into an optical fibre. Finally;
(3) in order to assess its performance for QIPC, the source that will reach fully operational status must be intergrable in some operating quantum information system.

OBJECTIVES
The primary objective of this project is to design and develop Solid State Light Sources that can deliver truly single photons on demand. Light sources with fully controlled quantum properties are important elements of enabling technology for photon-based QIPC systems. In reaching the primary objective, three partial objectives will be pursued:
(1) Isolation of a single emitting dipole that can be excited either optically or electrically, in: (a) semiconductor Quantum Dots, (b) F-Centres in Diamond, and (c) Rare-earth Ions and Organic Molecules in thin films, so that upon excitation it produces a single photon.
(2) Design of a microcavity environment to collect efficiently the photons produced by the elementary emitters.
(3) Validation of the Solid State Source for Single Photons based on partial objectives (1) and (2) as a reliable component of a QIPC system, by incorporating it in an operating Quantum Cryptography Testbed.

DESCRIPTION OF WORK
The development of Single Photon Sources will be carried out by means of four workpackages (WPs): WP1: Single-photon emitting materials, which will explore solid-state material systems embodying an elementary radiating dipole, emitting photons one-by-one; WP2: Microcavity design, aiming at the redirection and dynamical modification of emission inside a microcavity, to collect efficiently the single photons produced by the material systems explored in WP1; WP3: Quantum cryptography test-bed, which will incorporate the prototypes produced in WP2 in a functioning single photon optical link transmitting a quantum cryptography key. Thus, WP3 will serve to validate the use of microcavity prototypes produced in WP2 as building blocks in photon-based QIPC systems and more particularly, in Secure Quantum Communications. WP4: Management, assessment and QIPC link-up, which will coordinate and streamline the project, insure its compatibility with the scope of the overall QIPC initiative, and establish links with the other projects of FET-QIPC.

The first two technical WPs will be carried out concurrently and will start at the beginning of the project. The physical phenomena behind WP1 and WP2 are still under study on a fundamental level, and thus involve a high element of risk. For this reason, in each WP, several parallel avenues will be explored both by diversifying the types of systems studied and by furthering the fundamental understanding of the underlying physical effects. WP1 and WP2 will be carried out throughout the project, but by the end of the second year they will lead to an assessment of the different systems under study, within the procedures established in WP4, in order to identify the most promising route to producing single photons. The selected devices will then be adopted for implementation in WP3, to be tested on a Quantum Cryptography Testbed.
In meeting its targets, this project and a large number of novel and important results. The announced targets in workpart 1 centered around the confirmation of single photon emission from InAs quantum dots, CdSe nanocrystals, NV color centers in diamond, and from single molecules, through the observation of the antibunching effect.

However, beyond these targets, the partners obtained the following additional results:
1. Observation of a two-photon cascade from InAs single quantum dots, displaying quantum correlations due to the definite order in the photon emission. This two-photon cascade may be exploited for the development of light sources delivering pairs of correlated photons and may even lead to the observation of quantum mechanical entanglement, an important resource for quantum information processing and teleportation.
2. Study of photon correlations in CdSe nanocrystals over 12 orders of magnitude in time delay, giving information on the statistics of the processes "blinking" undergone by electron-hole pairs.
3. Investigation of the photophysics of NV color centers in diamond, and demonstration of the contribution of the refractive index to the spontaneous emission lifetime of the color centers.
4. First direct experimental observation of cooperative emission of a single photon by a pair of two isolated molecules. Co-operative emission is the signature of the quantum mechanical entanglement of the two coupled molecules and may be exploited in the future for quantum information processing.

Workpart 2 aimed at the design of specific microcavities to collect efficiently the photons produced by the elementary emitters of workpart 1. The highlights of its results are as follows:
1. Among the different types of microcavities designed and implemented in this workpart, micro-pillars proved to be the most suitable for the case of semiconductor quantum dots giving rise to a dynamic funnelling of the photons into a single cavity mode through the Purcell effect. In this way, by fabricating cavities with non-degenerate linear polarizations, fully polarized trains of single photons were produced;
2. An interesting line of research that emerged in the course of the work and was not anticipated in the work program, has been the investigation of the use of surface plasmons in metals as photon collectors and mediators to photon emission. Because of their rigorous two-dimensionality, surface plasmons are presently thought to be the most promising route to the realization of two-dimensional photonic circuits;
3. An important contribution of the partners collaborating in workpart 2 has been the publication of a review paper comparing the efficiencies and the advantages of the different photon collection schemes. Workpart 3 was focused on the validation of the solid-state source for single photons built through workparts 1 and 2 and its incorporation in a functioning quantum cryptography testbed. In view of the characteristics of the different single photon sources developed in WP1 and WP2, the partners decided to adopt the single photon source based on NV centers in diamond, emitting at 690nm. This single photon source is a compact, all-solid-state set-up operating at room temperature, that is probably the simplest single-photon source developed so far. Using this compact source delivering trains of single-photon pulses, a complete quantum key distribution scheme was demonstrated, with the rate of pulses containing two photons reduced by a factor of 14 with respect to an attenuated laser pulse train involving the same rate of one-photon pulses. This makes interception by the so-called "two-photon attacks" virtually impossible. At the same time this set-up held the record for the highest quantum key transfer rate achieved that far, at 8kbits/sec.

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