Quantum optics without photons

Atoms can exhibit wave-like behaviour to form matter waves. Such waves have been used to model the basic processes that underpin how light interacts with matter, providing an experimental platform for future research.

The fundamental theory that describes the interaction between light and matter at microscopic scales is known as quantum optics. One of the most striking and tangible consequences of this theory is that excited atoms in the quantum electrodynamic vacuum — a state that contains no photons, often referred to simply as the vacuum — can decay to their ground state by emitting a photon, a process called spontaneous emission. In 1946, physicist Edward Mills Purcell proposed that this process can be tailored by structures that alter the photonic environment1, such as photonic crystals2, engineered dielectrics (insulators) in which light cannot propagate at certain frequency ranges. In the 1990s, it was predicted3 that spontaneous emission in photonic crystals leads to exotic decays, in which photons adopt a ‘superposition’ state in which they are simultaneously emitted into the surrounding environment but also localized around the emitting atoms. In a paper in Nature, Krinner et al.4 report the first observation of the dynamics of these exotic decays, not using photons, but using a system of trapped, ultracold atoms.

The atoms in Krinner and colleagues’ experiments are trapped in optical lattices5, which are formed by the interference of counter-propagating laser fields — the interference generates a periodic pattern of light intensity in which the atoms are confined. Given their quantum-mechanical nature, the atoms can tunnel between neighbouring sites in the lattice at a rate that can be adjusted by altering the lasers’ intensities. Because such systems are highly controllable and exhibit low decoherence (the atoms are well isolated from their environment and thus behave ideally), they are an almost perfect platform for simulating complex quantum problems found in fields such as condensed-matter and high-energy physics (see go.nature.com/2uied19). Krinner et al. now demonstrate that trapped-atom systems can also be used to simulate quantum optical problems.

The authors’ experiments are based on a proposal6 published in 2008. The idea is to use atoms — in this case, rubidium atoms — that have two internal states, which respond to a one-dimensional optical lattice in different ways (Fig. 1). One state (let’s call it the f state) ‘sees’ regions of high light intensity as deep optical potential-energy wells from which it cannot move, whereas the other (the a state) hardly notices the wells, so that atoms in that state can propagate through the optical lattice as a matter wave. An atom in state f thus represents a matter-wave emitter in an excited state, whereas an atom in state a behaves like a photon that can be emitted through spontaneous emission. To complete the analogy with atomic decay phenomena in a structured photonic environment, the two internal states are coupled to each other using external fields, so that an initial excitation can be transformed into a propagating matter wave.

Figure 1 | Matter-wave emitters. Krinner et al.4 trapped rubidium atoms in a one-dimensional optical lattice formed by the interference of laser fields — the interference generates a periodic pattern of light intensity that corresponds to a series of potential-energy wells, in which atoms can be confined. Each atom has two internal states, one of which (red) is confined to the atom and the other (blue) which is unconfined and overlaps with adjacent empty wells. By tuning the parameters of the system, the authors could make occupied lattice sites emit atoms as waves (dotted arrows) that travel along the lattice. The emission mimics that of photons from atoms confined in photonic crystals (materials engineered so that light cannot propagate at certain frequency ranges).

In their experiments, Krinner et al. model the simplest scenario of spontaneous emission in a photonic crystal, but one that potentially offers the most insight into such processes: the spontaneous emission of a single photon into the vacuum. By tuning the experimental parameters of their system, they observed a phenomenon known as fractional decay5, in which the emitter ends up in a quantum superposition of being both excited and having decayed to the ground state. The authors also report direct evidence that the probability of the emitter remaining in the excited state does not decrease exponentially over time. Both the non-exponential behaviour and the fractional decay are some of the most peculiar effects that photonic crystals induce in quantum emitters. The new measurements are analogous to previously reported measurements of the spontaneous emission of photons in the visible-light7,8 and microwave9 regions of the electromagnetic spectrum, but in those studies, it was not possible to measure the dynamics of the decay.

The authors’ experimental platform offers several useful features in addition to its excellent controllability and low decoherence. First, it is flexible enough to emulate photonic crystals that have different geometries, and to model 3D environments in which further unusual features of spontaneous emission emerge10,11. It also allows access to parameter regimes that are out of reach of optical implementations, such as situations in which the coupling between emitters and the environment is very strong. Moreover, it might enable spontaneous emission to be studied in environments that are even more exotic than photonic crystals, such as in materials known as topological insulators — although the experimental set-up would need to be adapted so that the atoms move in the way that simulates the movement of excitations in such materials, and the efficiency with which decays can be detected would need to be improved.

Together with photonic platforms in the optical and microwave regimes, Krinner and colleagues’ system opens the way to studies of the physics that emerges in unconventional quantum optical set-ups. If the experiments are extended to include many emitters, it might be possible to observe collective spontaneous-emission phenomena that cannot be predicted using current computational methods, or even to engineer interactions among the emitters that cannot be produced using other platforms.

Nature 559, 481-482 (2018)

doi: 10.1038/d41586-018-05738-1

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