Qubits, Qutrits and Ququads Stored in a Single Photon

P.B.R. Nisbet-Jones et al, New J. Phys. 15 (2013) 053007
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Encoding arbitrary information in elementary quantum systems is a major key to any approach to quantum computing. In many cases photons play the role of information carriers, e.g. in linear optics quantum computing (LOQC), and the arbitrary control of the photonic quantum states in space and time is crucial to the success of these schemes. To this end, we looked into the deterministic single-photon emission from a single atom into an optical cavity and found that the properties of these photons are controllable to an unprecedented degree, outperforming most spontaneous photon sources hitherto used.

Experimentally, we verified the perfect singleness and indistinguishability of the deterministically emitted photons, which are produced with a probability of 80% upon each trigger and at a repetition rate of 1 MHz. Over and above that, we successfully encode arbitrary qubits, qutrits and even ququads within the spatio-temporal mode profile of individual photons. The fidelity of this groundbreaking state preparation technique has been verified in time resolved quantum-homodyne measurements to be better than 96%.

Such a close-to-perfect control of photonic wavefunctions paves the way towards novel applications in quantum computing and communication. For instance, when using qutrits or ququads, powerful ternary or quaternary quantum logic concepts could be implemented without the need for any additional resources.