Experimental results demonstrating optical gates, or switches, based on atomic systems that respond at the single photon level, or photon-photon interactions

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A quantum gate between a flying optical photon and a single trapped atom
A. Reiserer, N. Kalb, G. Rempe, S. Ritter
Nature 508, 237-240 (2014);
Nanophotonic quantum phase switch with a single atom
T. G. Tiecke, J. D. Thompson, N. P. de Leon, L. R. Liu, V. Vuletic, M. D. Lukin Nature 508, 241-244 (2014);
Nonlinear π phase shift for single fibre-guided photons interacting with a single resonator-enhanced atom
J. Volz, M. Scheucher, C. Junge, A. Rauschenbeutel
Nature Photonics 8, 965-970 (2014);
Nonlinear Interaction between Single Photons
T. Guerreiro, A. Martin, B. Sanguinetti, J. S. Pelc, C. Langrock, M. M. Fejer, N. Gisin, H. Zbinden, N. Sangouard, R.T. Thew
Phys. Rev. Lett. 113, 173601 (2014)

Nonlinear optics made significant progress in 2014 with several experimental results demonstrating optical gates, or switches, based on atomic systems that respond at the single photon level, or photon-photon interactions.

In the first work, Reiserer and co-workers implement a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The gate mechanism is deterministic and robust, and is expected to be applicable to almost any matter qubit. It is based on reflection of the photonic qubit from a cavity that provides strong light–matter coupling. To demonstrate its versatility, they use the quantum gate to create atom–photon, atom–photon–photon and photon–photon entangled states from separable input states.
In the second work, Tiecke and co-workers, by strongly coupling a photon to a single atom trapped in the near field of a nanoscale photonic crystal cavity, realize a system in which a single atom switches the phase of a photon and a single photon modifies the atom’s phase. They experimentally demonstrate an atom- induced optical phase shift that is nonlinear at the two-photon level, a photon number router that separates individual photons and photon pairs into different output modes, and a single-photon switch in which a single ‘gate’ photon controls the propagation of a subsequent probe field.

In the third work, Volz and co-workers, implement a strong interaction between individual photons. They demonstrate a fibre-based nonlinearity that realizes an additional two-photon phase shift close to the ideal value of π. They employ a whispering-gallery-mode resonator, interfaced by an optical nanofibre, where the presence of a single rubidium atom in the resonator mode results in a strongly nonlinear response. They show that this results in entanglement of initially uncorrelated incident photons, which represents an important step towards photon-based scalable quantum logics.

In the last work, Guerreiro and co-workers report the nonlinear interaction between two single photons. Each photon is first generated in independent parametric down-conversion sources. They are subsequently combined in a nonlinear waveguide where they are converted into a single photon of higher energy by the process of sum-frequency generation. This results in the direct generation of photon triplets. This work highlights the potential for quantum nonlinear optics with integrated devices, with applications in quantum communication such as device-independent quantum key distribution.