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Efficient single-photon sources with definite polarization

For absolutely secure quantum communication using the popular BB84 quantum key distribution (QKD) protocol, single photons are required. Figure 1 shows a schematic of a singlephoton QKD that will prevent any potential eavesdropper (Eve) from intercepting a message without the sender (Alice) and receiver (Bob) noticing. If the source emitted more than one photon at a time, it would be possible for Eve to extract one of these photons without disturbing the other photon(s), which would reveal Eve’s presence. In the BB84 protocol, linear and circular polarization bases provide two different quantum-level representations of 0 and 1. The security of a system based on single photons will not be broken even if extremely powerful quantum computers come into existence that break all currently secure systems, such as bank accounts, internet transactions, and military services. A single-photon source (SPS) is a nonclassical light source with all photons separated in time. For such photon ‘antibunching,’ the second-order coherence function g.2/.t/ should have a minimum at interphoton time t D 0 (see Figure 2). If g.2/.0/ < 0:5, it indicates the absence of photon pairs. The critical issue in producing antibunched photons is the low concentration of emitters, such that, within an excitation-laser spot, only one emitter becomes excited and emits only one photon at a time. A microcavity/nanoantenna enhances the singlephoton emission rate due to the Purcell effect (the density of states is enhanced) and also provides line narrowing, directionality, and, in some geometries, polarization selectivity of singleemitter fluorescence. If the photon has unknown polarization, it must be filtered through a polarizer for the desired polarization for quantum key distribution. If we can produce a photon with definite polarization, the efficiency of the quantum cryptography system is doubled. Colleagues and I have been working toward robust efficient room-temperature SPSs with definite polarization Figure 1. Quantum key distribution with single photons.