"We are enabling the investigation of light-matter interactions in a new domain in quantum optics," said postdoctoral fellow Pol Forn-Diaz. "The possibilities are exciting because our circuit could act as a quantum simulator to study other interesting quantum systems in nature."
The ultrastrong coupling between photons and qubits may lead to the exploration of new physics related to biological processes, high temperature superconductors, and even relativistic physics.
To conduct their experiment, the researchers made aluminium circuits in the university's Quantum NanoFab, and then cooled them in dilution refrigerators to 0.01K. These aluminium circuits, known as superconducting qubits, obey the laws of quantum mechanics and can behave as artificial atoms.
To control the quantum state of a superconducting circuit, the researchers sent photons into the circuit using microwave pulses and applied a small magnetic field through a coil inside the dilution refrigerator. By measuring the photon transmission, the researchers could define the resonance of the qubit, indicated by the reflection of the photons off the qubit. Usually, the qubit resonance is centred around a narrow range of frequencies.
"We measured a range of frequencies broader than the qubit frequency itself," said Forn-Diaz. "This means there is a strong interaction between the qubit and the photons. It is so strong that the qubit is seeing most of the photons that propagate in the circuit, which is a distinctive signature of ultrastrong coupling in an open system."