Coherent single photons

Stimulated emission makes lasers excellent sources of large numbers of coherent photons, which is fine for most applications. But quantum information networks are a problem because they work best with coherent photons that come one at a time, and lasers generally are not amenable to generating single photons. Single-photon sources have been developed for quantum computing, but they lack the coherence needed to create quantum entanglement at a distance using quantum entanglement.

Now, a team at the Cavendish Laboratory at Cambridge University (Cambridge, England) led by Mete Atature has found a way to generate single photons with laser-like coherence. Their starting point was optical pumping of quantum dots, which is one way of producing single photons. They first fabricated a Schottky diode containing self-assembled indium-arsenide quantum dots, which could be individually addressed with a pump laser to generate single photons by resonance fluorescence. Resonant fluorescence does not optically excite the host material, reducing interactions in the solid that decrease coherence of emitted photons, but charge fluctuations and other interactions remain to degrade coherence.

In Nature Communications they report avoiding photon decoherence by weak laser excitation, which generates photons primarily by elastic scattering. This avoided charge fluctuations, and allowed them to generate single photons from one quantum dot that remained coherent with the excitation laser for more than three seconds. Taking advantage of this mutual coherence, they report they could "synthesize near-arbitrary coherent photon waveforms by shaping the excitation laser field." That, in turn, let them show that as long as the photons emitted by the quantum dot remained coherent with the pump laser field, the separate photons were "fundamentally indistinguishable," so quantum interference among them can create quantum entanglement at a distance. That makes it possible to combine quantum computing with quantum communications, producing a more powerful tool for tasks such as quantum cryptography.

Ways to encode a qubit.

"The ability to generate quantum entanglement and  perform quantum teleportation between distant quantum-dot spin qubits with very high fidelity is now only a matter of time," says Atature. That's still a long way from science-fiction teleportation. However, the ability to generate single photons that maintain coherence well enough that they can be combined to produce novel waveforms may lead to real-world capabilities almost as attractive as avoiding airport lines.