The predictions of EPFL physicists for a new kind of single photon source have come to life in a recent collaboration with the universities of Leiden and Santa Barbara. The so-called Unconventional Photon Blockade was experimentally realized with a semiconductor quantum dot embedded in a micropillar optical cavity.



Thanks to massive investment from the high-tech giants, emerging quantum technologies have recently made the leap onto industry. While the first quantum computers essentially rely on superconducting circuits, photonics stand as the most promising platform for integrable quantum chips, and already lie at the heart of quantum encryption devices available on the market.

Processing quantum information with light relies on extracting the individual photons that encode the information from the random stream of a laser source. The most conventional approach involves trapping the laser light inside an optical cavity characterized by a strongly nonlinear response.

This trapping is referred to as "strong light-matter coupling". During this, the cavity cannot absorb more than one photon at a time and therefore re-emits them, behaving like a turnstile for the laser: the so-called Photon Blockade. However, experimentally realizing a strongly nonlinear optical cavity remains a very challenging and expensive engineering task.

The Unconventional Photon Blockade (UPB) is another paradigm, first proposed in 2010. It relaxes the need for strong nonlinearity and results from a quantum interference effect that minimizes the chances of having two photons trapped at the same time in the cavity thanks to an auxiliary element, such as a second cavity, a quantum dot etc.

In a new study published in Physical Review Letters, the UPB was experimentally implemented by the groups of Wolfgang Loffler (University of Leiden) and Dirk Bouwmeester (University of Santa Barbara) with theoretical support from Hugo Flayac and Vincenzo Savona at EPFL. The effect was measured after a laser light was sent to a micropillar cavity hosting a semiconductor quantum dot far away from the strong coupling regime.

The work opens the way to considerably cheaper and flexible single-photon sources. Flayac and Savona are now attempting to adapt the mechanism to photonic crystal cavities, which are simple and cheap pieces of silicon with suitably arranged arrays of holes operating at room temperature.

The study completes a collaboration spanning another three related papers published or to be published in Physical Review Letters:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.017401

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.233601

https://journals.aps.org/prl/accepted/e907eY1dDac1c66277288187824510fdbe518b39f

https://journals.aps.org/prl/accepted/6e078Y8dX9d1a35ce6e361293517a3dfcc9cff36f