Research Highlights

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Blending atoms and light

Published online 20 October 2016

Strong quantum coupling in a superconducting circuit leads to new exotic hybrid states of light and matter.

Tim Reid

Light and matter are not always separate and distinct things, according to quantum physics. Researchers at the Qatar Environment and Energy Research Institute (QEERI) in Qatar, the National Institute of Information and Communications Technology (NICT) of Japan and the Nippon Telegraph and Telephone Corporation (NTT) in Japan are exploring what happens when light and atoms become strongly coupled, producing exotic states of matter that don’t always comply with physics as we know it. 

These new states further our understanding of the interaction between light and matter at a fundamental level, which could lead to improved optoelectronic devices such as solar cells. 

The novel light–matter systems could also prove to be invaluable tools for quantum engineering, and the development of ultra-powerful computers.

A good system to use for investigating light–matter interaction comprises an atom trapped in a small cavity. With the correct electromagnetic resonance conditions, there will be quantum coupling between the atom and the cavity, resulting in spontaneous light emission, such as in a laser. However, when the coupling becomes very strong, the emission stops and effectively turns inwards; the atom and cavity continually exchange quantum parcels — or quanta — of energy, forming a hybrid state that can be thought of as a molecule of light and matter.

Sahel Ashhab at QEERI, part of Hamad Bin Khalifa University, and co-workers examined a previously unexplored regime known as deep strong coupling, using highly magnetic artificial atoms called superconducting flux qubits1

They achieved hybrid light–matter states in a tiny cavity within a superconducting circuit.

The researchers achieved a coupling strength in the qubit that meant that all the energy states — including the ground state — of the light and matter in the cavity were highly entangled. They used spectroscopy to confirm that this hybrid system exhibited many unconventional state transitions.

“For over forty years, scientists have debated whether it is possible to achieve the extreme conditions needed to demonstrate these unconventional states,” says Ashhab. “It was very exciting when we saw that our measurement results matched the theoretical predictions.”


  1. Yoshihara, F. et al. Superconducting qubit–oscillator circuit beyond the ultrastrong-coupling regime. Nat. Phys. (2016).