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A simple source of single photons that is powered only by anelectric current has been developed by an international team ofresearchers. The device, a modification of the traditionalsemiconductor p–n junction called a p–i–njunction, is made from diamond and is one of the firstsingle-photon emitters to work at room temperature that does notneed to be pumped with a laser – another research groupworking independently has demonstrated a similar device. In theshort term, the device could help to make "unbreakable" quantumcryptography a more viable endeavour. In the longer term, theresearchers hope the devices might open up new avenues for researchin quantum computing and other research in quantum-informationtheory. Quantum cryptography allows two individuals, conventionally calledAlice and Bob, to send a coded message while tracking anyinterception by an eavesdropper, usually called Eve. Information isencoded into quantum states – say the polarization of photons– and if Eve tries to secretly measure these states as theypass from Alice to Bob, then the laws of quantum mechanics ensurethat Eve's actions are revealed to the correspondents. While quantum-cryptography schemes have already been usedcommercially, they rely on the production of single photons –which has proved to be a difficult thing to achieve. One way ofdoing it is using an extremely weak pulsed laser (in the femtowattrange). However, achieving a stable power output at this level isdifficult. Some of the pulses will contain no photons at all, whileothers pulses could contain two or more photons. The latter isparticularly unhelpful because if a pulse contains two or moreidentical photons, Eve could in principle measure the state of onephoton while leaving the other untouched – and Alice and Bobwould be blissfully unaware they are being bugged. Reliable single photons For this reason, research groups all over the world are developingvarious schemes for producing reliable streams of single photons.Systems under investigation include those based on individual,self-assembled organic molecules or semiconductor quantum dots.Unfortunately, the most promising systems only work when cooled tocryogenic temperatures, which is not compatible with the commercialdevelopment of quantum-communication systems. An alternative approach that works at room temperature involves acrystal defect in diamond, called a nitrogen vacancy (NV). Thisoccurs when a nitrogen atom replaces a carbon atom in the diamondlattice and a nearest-neighbour carbon atom is missing (thevacancy). NVs can emit single photons if excited by a laser of theright wavelength – a property called photoluminescence.However, this scheme is also seen as impractical because of theneed for a bulky and expensive laser. Sandwiched NV centre In this latest work, Norikazu Mizuochi and colleagues at the JapanScience and Technology Agency in Saitama, together withinternational collaborators, have shown how an NV centre can bemade to emit single photons by the application of an electricvoltage – a process called electroluminescence. They did thisby creating a light-emitting diode (LED) structure using dopeddiamond as the semiconductor material. The device consists of anundoped or "intrinsic" region of super-high-purity diamond that issandwiched between p- and n-doped diamond layers. Thesuper-high-purity diamond contains the NV centre. Mizuochi explains that, when electrons and holes are drawn into theintrinsic region by the applied voltage, they excite the nitrogenvacancy, thereby causing electroluminescence and the emission ofsingle photons. Jean-Francois Roch of the Quantum and Molecular PhotonicsLaboratory in Paris, who was part of an independent team thatdeveloped a similar device using different methods in 2011, told physicsworld.com that this latest work is impressive. "To be honest," he says, "Imust say that [Mizuochi's] group has done a more thorough analysisthan the one we have done." The research is described in Nature Photonics . I am an expert from pir-detector.com, while we provides the quality product, such as China Wired PIR Detector , China LPG Gas Detector, Burglar Alarm Control Panel,and more.
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