A Double Take on Unconventional Photon Blockade

Abstract

M any quantum technologies require light sources that emit a precise number of photons. Examples include quantum cryptography, which uses individual photons for secure communication, and quantum imaging, which harnesses wave packets of a fixed number of photons to achieve enhanced resolution. A promising method for quickly generating single photons exploits an emitter-cavity system and the so-called photon-blockade effect. Here, the system acts as a photon filter for a laser, allowing the transit of only one photon at a time. Now two independent teams have overcome a major engineering challenge to realizing photon blockade in practical systems [1, 2]. These demonstrations take the effect one step closer to implementation in quantum technologies. A typical photon-blockade device for optical photons consists of a cavity containing an emitter with two energy levels, such as an atom or a quantum dot. The presence of the emit-ter alters the cavity's properties in such a way that the cavity can only accommodate a single photon. Consequently, if a pulse of light from a laser is introduced into one end of the cavity, then the other end will only emit one photon. This blocking effect is a bit like a single-lane exit on a multilane highway, which only allows cars to leave the highway one-by-one. Photon blockade was demonstrated in the lab [3, 4] more than a decade ago. But realizing the effect with high photon emission rates required a strong interaction between the emitter and cavity, which entailed challenging, high-precision fabrication of cavities. An alternative route to achieving photon blockade arose in 2010, when theorists predicted that the strong interaction restriction could be relaxed if two cavities were used instead of one [5]. These two coupled cavities would require only weak interactions with the emitter to generate single photons [6], and, as such, they would not need to be as precisely constructed. This alternative approach relies on what is known as the unconventional photon blockade effect. To appreci-Figure 1: A scheme for single-photon emission using the unconventional photon blockade effect. The device consists of two coupled cavities, which are weakly coupled to a dipole emitter (not shown). Population of the system by two photons is blocked via destructive interference of the different excitation paths. (APS/Joan Tykco) ate its distinction from "regular" photon blockade requires some details about how photon blocking works. Cavities have a series of equally spaced energy levels, N = 1, 2, 3,. .. , which correspond to 1, 2, 3,. .. , photons occupying the cavity. For regular photon blockade, the presence of an emitter shifts all the energy levels of the cavity such that only the N = 1 energy level can accept photons. Photons tuned to this energy can therefore only go in and out of the cavity one at a time. Unconventional photon blockade is less restrictive: it blocks the entry of two photons only. This blocking is achieved via destructive interference of disparate excita-tion paths to the N = 2 energy level (Fig. 1). But the effect does not influence the N = 3 and higher energy levels in the same way, allowing emission of "bunches" of three or more photons. Attempts to realize unconventional photon blockade have been hindered by a lack of suitable platforms. But a recent theoretical paper [7] proposed several new experimental approaches. Inspired by one idea for optical photons, Henk Snijders of Leiden University, Netherlands, and collabora-physics.aps.org