At ultralow temperatures and small powers— conditions at which quantum effects become dominant—electrical circuits behave very differently from those in everyday use. In 2023, EQUS researchers from the UQ Superconducting Quantum Devices Laboratory—led by Chief Investigator Arkady Fedorov—worked to harness these effects, specifically superconductivity, to develop devices with new functionalities.
Superconductivity is important for quantum devices because it reduces energy loss, which is harmful for quantum correlations and thus information storage. One of the most common elements of a superconducting quantum device is a superconducting qubit. Such a qubit is built on a chip using superconducting metals and Josephson junctions, and is similar to a conventional LC (inductor–capacitor or resonant) circuit. At ultralow temperatures, the qubit may be prepared in a quantum state, characterised by different oscillations of currents and fluxes in the circuit.
These states may be used to encode information, and be externally controlled by electronics, meaning they could constitute a building block of a future superconducting quantum computer.
In the nearer-term, superconducting qubits may be used to demonstrate new physics regimes. One example is a new state of quantum matter known as an atom–photon bound state and its wavefunction.
Being able to control the spatial form of this wavefunction would be a very useful resource for future experiments.
Creating and manipulating atom–photon bound states
As a new state of quantum matter, atom–photon bound states offer new properties. They have recently attracted attention as a way of implementing new physical models and as a tool for controlling the coupling between qubits. In 2023, the team studied the interactions between superconducting qubits and electromagnetic waves in a waveguide, with the aim of creating, understanding and controlling atom– photon bound states.
When using a waveguide with a stopband (a band of frequencies that do not propagate in the waveguide), the qubit may create an atom–photon bound state. The majority of this state ‘lives’ on the qubit, but the wavefunction also has a photon component, which extends into the waveguide, localised around the qubit.
Using superconducting qubits, the team engineered an atom–photon state in a three-dimensional rectangular waveguide and demonstrated the effect of the boundaries of the waveguide on their physics. Understanding these boundary effects is important because the current theory assumes the unrealistic. They also developed a theoretical proposal for controlling the directionality of the atom–photon bound states.
In contrast to previous proposals, the new scheme achieves perfect chirality of the atom–photon bound state, and enables its directionality to be switched on demand with only one tunable qubit in the device. The scheme is also easy to implement in state-of-the-art superconducting circuits and in quantum dot architectures. The results show technological promise; in principle, being able to control the spatial form of atom–photon bound states would enable long-range coupling of different qubits, with zero cross-talk.
The future of superconducting quantum devices
In the future, the team are also planning to use superconducting devices to route microwave signals. In collaboration with Chief Investigator Tom Stace’s group, they have developed a proof- of-principle superconducting microwave circulator, which removes the need for external driving and may be placed on-chip.
Superconducting quantum devices also have a range of other promising applications, such as in future quantum computers, quantum clocks and axion dark matter detection, and more broadly as an experimental platform for quantum technologies.
In 2023, the team collaborated with Associate Investigators Alexei Gilchrist, Christina Giarmatzi and Fabio Costa to characterise the noise— specifically, non-Markovian noise—of a superconducting qubit. Understanding noise is essential in the context of error mitigation when scaling up superconducting quantum devices.
In collaboration with Chief Investigator Gerard Milburn’s group, they used a superconducting qubit to test a theoretical bound, due to quantum measurement, on the precision of quantum clocks.
This story was first published in the 2023 EQUS annual report, and was written by Kristen Harley.