Breaking through the noise – Dynamics of a dispersively coupled transmon qubit

Antti Vaaranta, Trainee in the Quantum Applications team at Bluefors and a recent graduate from the University of Helsinki, studied in his MSc thesis the environmental effects on a dispersive transmon qubit. Based on the thesis work, he, Marco Cattaneo (PhD student at the University of Helsinki and IFISC (CSIC-UIB), and Russell Lake (Director, Quantum Applications at Bluefors) wrote an article to Physical Review A (currently under review). You will find an arxiv link to the preprint at the end.

Understanding the source and effects of noise on gate-based computing devices enables physicists and engineers to develop higher performance quantum devices. In their research Antti, Marco, and Russell study the noise emanating from the qubit control line. They derive an exact theoretical model describing the dynamics of a transmon qubit experiencing this source of noise and solve it to gain insight into the qubit time evolution in different experimentally obtainable parameter regimes.

In a quantum system, noise causes dissipation of energy as well as loss of superposition, both of which are the two main factors limiting qubit lifetimes. One important source of noise is the qubit drive line attenuator, which acts as a bosonic bath, feeding thermal photons into the superconducting chip. Essentially, these photons are the noise that causes the quantum state to decay.

In the derived theoretical model, the drive line attenuator is taken to be a resistor, and inductively coupled to the rest of the superconducting circuit, which includes the qubit with a dispersive coupling to its readout resonator. The derivation starts from the Caldeira-Leggett model for the resistor, which enables it to be treated as a chain of parallel LC-oscillators. In this way, it is possible to derive a quantum circuit Hamiltonian using the tools of circuit quantum electrodynamics that also encompasses the inherently non-unitary nature of the resistor.

The circuit Hamiltonian is needed for the derivation of a Lindblad master equation, which describes the dynamics of the quantum state of the qubit. This equation is essentially an equation of motion for an open quantum system interacting with its environment, such as the case analyzed here; a superconducting circuit of the qubit interacting with the drive line attenuator.

The article details these findings to both summarize and share the team’s gained knowledge. The authors are proud to show a direct way of computing qubit dynamics, where the values of the circuit components are directly connected to the time evolution of the quantum state.

They have used these conclusions to make an educated prediction, that the resistor’s contribution to decoherence can be decreased in the dispersive weak regime. They have also quantified the importance of proper thermalization and cooling of the qubit drive line attenuators in order to reach longer qubit lifetimes.

While noise producing attenuators are necessary for operational qubit control and wiring, we can all carry on driving research and development and work hard to break through the noise. For a more in-depth exploration of the research and findings, the authors invite you to read the full article at  https://arxiv.org/abs/2206.08636.

Poster session: Saturday, August 27, 2022, at 2:00 pm at the SQA event.