Generation of Photon Number States on Demand
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The many applications discussed in quantum communication and quantum cryptography require sources able to produce a preset number of photons. Single photons are, for example, a necessary requirement for secure quantum communication,1, 2, 3 for quantum cryptography4 and in special cases also for quantum computing.5 However, photon fields with fixed photon numbers are also interesting from the point of view of fundamental physics since they represent the ultimate non-classical limit of radiation. When the photon number state is generated by strong coupling of excited-state atoms, a corresponding number of ground-state atoms is simultaneously populated. Such a system therefore produces a fixed number of atoms in the lower state as well. This type of atom source is a long sought after gedanken device.6 Single photons have been generated by several processes such as single-atom fluorescence,7 single-molecule fluorescence,8 two-photon down-conversion,9 Coulomb blockade of electrons,10 and one- and two-photon Fock states have been created in the micromaser.11,12 As these sources do not produce the photons on demand, they are better described as “heralded” photon sources, because they are stochastic either in the emission direction or in the time of creation. A source of single photons or even more generally Fock states created on demand has not yet been demonstrated. Cavity quantum electrodynamics (QED) provides us with both the possibility of generating a photon at a particular time and localising its emission direction.
KeywordsSingle Photon Photon Number Quantum Cryptography Optical Cavity Rydberg Atom
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