Toshiba Research Europe Ltd., Cambridge Research Laboratory

Quantum Information Group, Random Key Generation

Quantum dots are sometimes called 'artificial atoms', as they localise electrons and holes (a positively charged absence of an electron) in analogy with real atoms, which localise electrons and protons. For real atoms, the number of protons and electrons dictates unique chemical properties of the element. For quantum dots, the precise number and configuration of charges defines a unique energy, due to the attractive and repulsive electrostatic forces between electrons and holes. An electron and hole can then recombine to emit a single photon, which has a wavelength unique to the charge configuration of the initial and final quantum dot state. This leads to the observation of a handful of discrete lines in the emission spectrum of a single quantum dot. By filtering out light emitted at a specific wavelength, we can isolate emission of a single photon, and create a single photon emitter.

If the quantum dot is excited with two electrons and two holes, recombination leads to the emission of two photons at different wavelengths. Thus, quantum dots can also be used as a source of pairs of photons, as well as single photons. At Toshiba, we have studied the relationship between the polarizations of these two photons and found the surprising result, that they tend to have the same polarization. This property could be used to encode completely random keys in quantum cryptography.

Further Reading

	Type	Title and author(s)	Source
1	Technical	Quantum dots as a source for passive quantum key encoding by R M Stevenson et al	Phys Rev B 66, 081302R (2002)
2	Technical	Single photon emission from exciton complexes in individual quantum dots by R M Thompson et al	Phys Rev B 64, 201302R (2001)
3	Technical	Self assembled quantum dots as a source of single photons and photon pairs by A J Shields et al	Physica Status Solidi (2003)
4	Technical	Generation of single photons and photon pairs using quantum dots by R M Stevenson et al	Proceedings QCMC (2003)
5	Technical	Single photon emission from few particle states in InAs quantum dots by R M Thompson et al	IOP Conference Series, Compound Semiconductors 170, 501 (2002)
6	Technical	Exciton complexes in individual quantum dots as a single photon source by R M Stevenson et al	Physica E 13, 423 (2002)