The Quantum Optics of Entanglement between Single Photons

Chih‐Sung Chuu, Chun‐Yuan Cheng, Chih‐Hsiang Wu +5 more

Abstract Single photons and entangled photons lie at the heart of photonic quantum technologies, whose optimal performances are normally reached when the purity of the single or entangled photons is high. However, the multiphoton emission, dissipation, and decoherence in practical realizations always lead to the degradation of the single‐ and entangled‐ photon quality. The purification of single or entangled photons is thus valuable to restore the quantum states and enhance the performance of quantum technologies. The applications of wavepacket shaping, an emerging quantum optics tool to manipulate the single‐ and entangled‐ photon wavefunctions, to purify single and entangled photons are reviewed. In particular, by modulating the single photons emitted from optically excited room‐temperature quantum dots, it is shown that the fast‐decaying multiphoton emission can be eliminated to obtain a low value of g (2) (0) that is independent of the excitation power. It is also shown that the two‐photon interference and polarization entanglement of the non‐degenerate biphotons from spontaneous parametric down‐conversion can be restored after modulating the biphotons with a periodic function. The works have potential applications in long‐distance quantum communication and linear optical quantum computation.

David S. Simon, Gregg S. Jaeger, Alexander V. Sergienko

Olivier Morin, Virginia D’Auria, Claude Fabre +6 more

Single-photon entangled states, i.e. states describing two optical paths sharing a single-photon, constitute the simplest form of entanglement. Yet they provide a valuable resource in quantum information science. Specifically, they lie at the heart of quantum networks, as they can be used for quantum teleportation, swapped and purified with linear optics. The main drawback of such entanglement is the difficulty in measuring it. Here, we present and experimentally test an entanglement witness [1] allowing one not only to say whether a given state is path-entangled but also that entanglement lies in the subspace where the optical paths are each filled with one photon at most, i.e. refers to single-photon entanglement. It uses local homodyning only and relies on no assumption about the Hilbert space dimension of the measured system, as explained in the following.

P. Tombesi

Olivier Morin, Jean-Daniel Bancal, Melvyn Ho +5 more

Single-photon entangled states, i.e., states describing two optical paths sharing a single photon, constitute the simplest form of entanglement. Yet they provide a valuable resource in quantum information science. Specifically, they lie at the heart of quantum networks, as they can be used for quantum teleportation, swapped, and purified with linear optics. The main drawback of such entanglement is the difficulty in measuring it. Here, we present and experimentally test an entanglement witness allowing one to say whether a given state is path entangled and also that entanglement lies in the subspace, where the optical paths are each filled with one photon at most, i.e., refers to single-photon entanglement. It uses local homodyning only and relies on no assumption about the Hilbert space dimension of the measured system. Our work provides a simple and trustworthy method for verifying the proper functioning of future quantum networks.

Amaal M. Ali, Natasha Bastien, Wendy-Ann Gibbons +2 more