Quantum physics

A strong hybrid couple

A single atom in an optical cavity is shown to interact strongly with an incoming photon and to switch the photon's state. This finding opens up a path towards optical quantum computation and quantum networks. See Letters p.237 & p.241

When two torch beams cross, they go through each other without affecting one another. This means that the fundamental particles of light, photons, do not typically interact with each other, or with matter, unless designed to do so. Photons and matter interact when strong light beams propagate through a dense atomic medium, but this interaction is negligible for weak light pulses in a system of only a few atoms. In a future Internet based on the principles of quantum mechanics1, which promises enhanced security and computational power, information will be carried by single photons, and the quantum state of those photons will need to be manipulated through their interaction with atoms. In this issue, Reiserer et al.2 (page 237) and Tiecke et al.3 (page 241) report independent experiments that bring this goal a step closer. The researchers have designed systems in which a single atom switches the state of a single photon contained in a faint light pulse.

The experiments represent the culmination of decades of research into atom–photon coupling in an optical cavity1. The Fabry–Perót version of an optical cavity consists of two highly reflective mirrors between which a photon bounces many times. This arrangement allows an atom trapped inside the cavity to be strongly coupled with the photon. Reiserer et al. used a Fabry–Perót cavity in which one of the mirrors has a significantly higher reflectivity, and thus lower transmissivity, than the other, so that a photon enters and leaves the cavity mainly through the lower-reflectivity mirror. Tiecke et al. designed a special type of cavity known as a photonic crystal cavity, which has the same function as Reiserer and colleagues' Fabry–Perót cavity but has a tiny cavity volume, which helps to further enhance the atom–photon coupling.

To switch the photon's state using a single atom, the two studies exploited a scheme proposed for optical quantum computation4. To understand this scheme, consider a spring fixed at either end. Just as the spring carries vibrations of only certain frequencies, an optical cavity in which the mirrors are separated by a fixed distance allows light of only certain frequencies, called cavity modes, to enter it. When an incoming single-photon pulse has the same frequency as one of the cavity modes (in other words, the pulse is resonant with the cavity), it enters the cavity and leaves it through the same mirror (Fig. 1a). In this process, the photon pulse, as a wave, undergoes a shift in phase of π radians; the phase quantifies a wave's local amplitude as it oscillates between its minimum and maximum values. Now, if there is an atom inside the cavity, it couples with the cavity modes and shifts their frequencies. Because of this shift, the incoming pulse that was resonant with the cavity in the absence of the atom will no longer be resonant with it, and thus will not enter the cavity. As a result, it bounces directly back, with no phase shift (Fig. 1b).

Figure 1: An optical switch.

Reiserer et al.2 and Tiecke et al.3 have designed systems in which a single atom trapped in an optical cavity, here formed by an arrangement of two mirrors of different reflectivity, switches the state of a photon in an incoming light pulse. a, If the atom is in a quantum state that does not couple with the cavity, equivalent to there being no atom in the device, a single-photon pulse resonant with one of the cavity's optical modes of oscillation will enter the cavity through the lower-reflectivity mirror and leave it with a phase shift of π radians, illustrated by a darker red than that of the original pulse. b, If the atom is in a state that couples with the cavity, it will shift the frequency of the cavity's mode and the pulse will now be off-resonant with it. Therefore, the pulse will not enter the cavity and will bounce back with no phase shift.

In the authors' experiments, such a conditional phase shift is achieved by preparing the atom in two quantum states: one that couples with the cavity ('presence state') and another that does not ('absence state'). The researchers then prepared the atom in a quantum-mechanical superposition of presence and absence states. Together with the conditional phase shift, this superposition state allowed them to implement a quantum logic gate — the basic building block of quantum computation — between the atom and the photon. Such a gate is crucial for creating quantum networks in which information is stored in, and retrieved from, atoms and transmitted to distant locations by means of single-photon pulses.

Reiserer and colleagues went on to demonstrate that the gate generates entanglement between the atom and the photon, and that this quantum effect mediates an interaction between different single-photon pulses. By shining a succession of resonant single-photon pulses on the cavity, the authors showed that successively reflected pulses become quantum entangled. This mediated interaction offers a means to realize quantum logic gates between single-photon pulses, providing a scalable platform for optical quantum computation4.

The demonstrated conditional π phase shift also has applications in nonlinear quantum optics. Tiecke et al. observed a nonlinear response from the cavity in which the single-photon and the two-photon components of an incoming light pulse are routed to different paths. With reduced photon loss and improved localization of the atom inside the cavity, the conditional phase shift can also be used to prepare a Schrödinger's cat state — an intriguing quantum superposition of classically distinct states — for the reflected light pulse5. Finally, the physical principles behind these experiments are not limited to atom–photon coupling. They could be applied to coupling between a quantum dot (an artificial atom) and infrared photons, or between a superconducting quantum bit and microwave photons.


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Correspondence to Luming Duan.

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Duan, L. A strong hybrid couple. Nature 508, 195–196 (2014). https://doi.org/10.1038/508195a

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