Interactions between light and matter are extremely important in many different technologies, including sensors, lasers, and quantum information processors. To ensure the strongest possible interactions between atoms and individual photons, physicists often use structures named ‘optical cavities’: featuring two opposite-facing, highly reflective mirrors, placed a few centimetres apart.
Photons of just the right frequency can bounce between these mirrors to form a standing light wave: which oscillates in amplitude over time, but which doesn’t propagate through space. In turn, if atoms are placed inside the cavity at temperatures just above absolute zero, their level of interaction with the photon can be greatly enhanced.
Professor Sebastian Slama from the Center for Quantum Science at the University of Tübingen, Germany, is particularly interested in the processes that unfold when groups of atoms are used in this way. ‘Here, the atoms interact with the cavity light not individually, but in a collective way,’ he explains. ‘Therefore, whatever one of the atoms does, it is “seen” by all other atoms. Physicists call this kind of interaction ‘all-to-all’.’
In some situations, these all-to-all interactions can cause atoms to ‘relax’ into a state of minimum possible energy, forming specific organised structures in the process. Building on previous research into this phenomenon, Professor Slama and his colleagues have explored its effects on a particular type of atom that holds a significant potential for use in future technologies.
When an atom’s unbonded outer electron absorbs a photon with just the right frequency, it becomes excited to a higher, discrete energy level, which occupies a space slightly further away from the atom’s nucleus than the lowest-energy ground state. At very high energy levels, the electron approaches its farthest possible distance from the nucleus, without being stripped away from it entirely.
Now named ‘Rydberg atoms,’ after the Swedish physicist who developed the first detailed theory for their behaviour in the 1880s, these atoms have radii of around 1 micrometre. This makes them some 10,000 times larger than conventional atoms, whose outer electrons are in far lower-energy excited states. As a result, Rydberg atoms can react very strongly when placed inside electric fields, including those of neighbouring Rydberg atoms.
In this case, the interaction between both atoms can prevent two or more neighbouring atoms within a certain surrounding volume from being excited to Rydberg states: an effect named the ‘Rydberg blockade.’ ‘Only one of the atoms in this volume can be excited to a Rydberg state, but it is unknown which,’ Slama describes. ‘Thus, all atoms share this excitation and behave like a new particle, named the ‘Rydberg superatom”.’
In turn, photon absorption within the Rydberg superatom is highly restricted. ‘If a first photon within this system has been absorbed by exciting a Rydberg state, a second photon cannot be absorbed, because this would cause another Rydberg excitation,’ Slama continues. This effect has important implications for technologies that rely on light–matter interactions. In his research, Professor Slama explores the exciting possibilities enabled by combining our knowledge of Rydberg atoms with experiments involving optical cavities.
Rydberg polariton quasiparticles
When groups of ultra-cold rubidium atoms are placed inside an optical cavity, previous studies have shown how their enhanced interaction with light can be combined with their unusual photon absorption properties. The result is a new type of ‘quasiparticle’: a term describing groups of particle excitations, which collectively behave like a single, far more exotic particle. Named a ‘Rydberg polariton,’ this quasiparticle arises from the strong coupling between the photon inside the optical cavity, and a highly excited electron in one of the atoms.
As Einstein famously postulated, the speed of light in a vacuum cannot be varied under any circumstances. Yet with this setup, researchers could essentially introduce tight control over the speeds at which photons travel. ‘Here, photons do no longer travel as a pure photon through the cavity, but they are mixed with the Rydberg excitation,’ Slama explains. ‘The polariton does no longer move with the speed of light, but is propagating at a much lower speed. Moreover, these polaritons interact with each other due to the Rydberg interaction.’
As massless, chargeless particles, photons will almost never interact with each other under normal circumstances. Yet through this approach, researchers could not only produce, but also mediate photon–photon interactions. Building on this remarkable discovery, Professor Slama and his colleagues are now aiming to study these interactions in more detail, and assess their influence over the property of magnetisation.
Unlike the conventional interactions taking place between atoms within an optical cavity, Rydberg atoms are not all-to-all. Rather, the strength of interaction between two particular atoms will depend on the distance separating them. Essentially, this introduces a degree of ‘competition’ between atoms – causing some atoms to influence the properties of the overall system more strongly than others.
Exotic phase transition
The effect has intriguing implications for the ‘phase transitions’ taking place within groups of Rydberg atoms. Typically, phase transitions describe the rearrangements of atoms and molecules within materials as they transition between solids, liquids, and gases. In this case, they relate to a quantum property of atoms known as their ‘spin’ – which can point in two possible directions: either up or down.
Spin is particularly important in explaining the origins of magnetism. When atomic spins are collectively aligned in one direction, their combined influence generates a magnetic field within the overall system. To vary the strength of this field artificially, researchers use specialised laser pulses to induce phase transitions from organised arrays atoms in their lowest-energy ground states, with their spins pointing down, to a new, magnetised phase.
In turn, these transitions must alter the distance-dependent interactions between pairs of Rydberg atoms. ‘Here, Rydberg interactions can be understood as an interaction between neighbouring spins,’ Slama illustrates. ‘They can be both positive and negative, corresponding to the case where neighbouring spins are oriented in the same direction, named “ferromagnetism”; or in the opposite direction, named “anti-ferromagnetism”.’
In the case of both all-to-all interactions between atoms, which are mediated by optical cavities, and competitive interactions between pairs of Rydberg atoms, the characteristics of this magnetisation have remained completely unexplored so far. Since the properties can only be described by accounting for all interactions between each individual pair of atoms, the problem has remained incredibly difficult to solve. In their upcoming research, Slama’s team now aspire to carry out the first ever studies of the phase transitions taking place in these systems; ultimately, shedding new light on their highly advanced properties.
Impacts in quantum computing
Through their results, Slama and his colleagues hope to initiate new advances in the field of quantum computing. Whereas a conventional computer bit can only exist in one of two possible states, a quantum bit, or ‘qubit,’ can exist in a ‘superposition’ of two or more states at the same time, and collapses to just one state when its quantum information is read out. As Slama explains, such a process could be readily implemented using systems of Rydberg atoms, contained within optical cavities.
‘As a Rydberg superatom consists of a number of atoms where only one excitation can be shared at maximum, it can be described as a two-level system,’ he says. ‘This represents a qubit for storing quantum information. Several of these superatoms in a cavity would then form a quantum memory.’
Crucially, the coupling between an optical cavity and a superatom can be readily switched on or off with a laser pulse. Therefore, by illuminating two chosen superatoms at the same time, researchers could enable coupling between any possible pair of qubits in a system, regardless of how far apart they are separated. Ultimately, if Slama’s team achieve their goals, they could pave the way for technologies that implement these advanced techniques, transforming our ability to store and process information.
- Jia, N., Schine, N., Georgakopoulos, A., Ryou, A., Clark, L.W., Sommer, A. and Simon, J. (2018). A strongly interacting polaritonic quantum dot. Nature Physics, 14(6), 550–554.
- Ritsch, H., Domokos, P., Brennecke, F. and Esslinger, T. (2013). Cold atoms in cavity-generated dynamical optical potentials. Reviews of Modern Physics, 85(2), 553.
- Saffman, M., Walker, T.G. and Mølmer, K. (2010). Quantum information with Rydberg atoms. Reviews of modern physics, 82(3), 2313.
Rydberg physics and the collective effects of cold atoms in optical cavities.
Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) 422447846 .
The research group is embedded into the fruitful research environment of the Center for Quantum Science of the University of Tübingen. There, experimental, theoretical, and mathematical experts are collaborating on connected research topics.
Sebastian Slama is a professor of Physics at the Center for Quantum Science of the University of Tübingen. His research interests include nanophotonics with cold atoms, collective effects of cold atoms in optical cavities, self-organisation of cold atoms, Rydberg physics, and long-range interacting spin systems. He received the Fulbright-Cottrell Award in 2016.
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