Unconventional Photon Blockade in a Weakly Coupled Cavity Pair

Conventional light sources emit light either as a continuous stream or in pulses of several light particles called photons. In quantum technology, however, many applications require only a few (or even individual) photons, and accordingly demand specialized light sources that emit a small number of photons at a time.

A promising method of generating single photons is by using a two-level emitter–cavity system (i.e., an emitter, such as an atom or a quantum dot, inside an optical cavity). This system allows only a single photon to exist within the cavity due to an effect called “photon blockade”, which is akin to a gate that allows photons to only pass one at a time through it. The emitted light is correspondingly “antibunched”, with photons more regularly spaced than in a classical light source.

However, photon blockade with a single cavity requires a strong cavity-emitter interaction (or “nonlinearity”), which can be challenging to achieve. In contrast, as shown by theorists in 2010, a system of two coupled cavities can emit strongly antibunched light even for a weak nonlinearity. However, the mechanism underlying this effect (called “unconventional photon blockade” or UPB), remains unknown.

In our study conducted in 2011, we explored UPB in coupled cavities and proved analytically that it is the result of a destructive quantum interference of different excitation paths, which ensures that two photons cannot be present in the cavity simultaneously. Our study generated significant interest among many theorists, who subsequently presented a variety of extended calculations. However, the wait for the experimental demonstration was longer, and our findings would finally be confirmed in the following two studies in 2018!

Vaneph et al. conducted an experiment in 2018 designed to achieve UPB with microwave photons using two coupled superconducting cavities. They observed that the transmission of two microwave photons was indeed blocked by introducing a nonlinearity in one cavity. Further, the device showed rapid oscillation between antibunched and bunched emission, which has also been predicted theoretically.

In another 2018 experiment by Snijders et al., scientists realized UPB using only a single cavity by making use of its two orthogonally polarized modes such that a quantum dot emitter inside the cavity was weakly coupled to these modes, making the system equivalent to a coupled cavity configuration. Photon antibunching was successfully observed in this weakly coupled emitter-cavity device, whereas antibunching could not be obtained in conventional photon blockade.

These findings help “shed light” on UPB, which can potentially advance single-photon-based quantum technologies, opening doors to highly secure communication and enhanced resolution in quantum imaging.

Title of the paper:

Origin of strong photon antibunching in weakly nonlinear photonic molecules


Motoaki Bamba, Atac Imamoğlu, Iacopo Carusotto, and Cristiano Ciuti



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