The big frontier in the future of integrated device engineering is "quantum" integrated circuits, which exploit entirely quantum effects to achieve much more powerful computation, metrology, and sensing than we can do today with "classical" devices. However, quantum effects are hard to witness and to put to good use, because they want to dissipate away and leave us empty handed. On top of that, the platform of integrated circuits poses significant constraints of size, materials, and component layout. Moreover, there are several possible choices for which quantum system should be at the core of this new technology: atoms, ions, photons, something else? One of the most promising candidates are photons, because they are cheap to produce, they don't require a vacuum or cold temperatures, and we understand very well how they behave.
An ideal photon source generates single photons in the same quantum state consistently and on demand. How can we test the quality of a source? One way is to use an interferometer and observe quantum interference: the more interference, the better the source. The Bristol team have developed both sources and interferometer on-chip, and they have observed for the first time quantum interference with 72% visibility. Although the road to 100% is still long, this work sets a new benchmark for future developments.
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