How long does it take an open quantum system to reach its steady state? In a large class of classical systems – those which satisfy “detailed balance” – the answer is well-understood. In collaboration with several former group members now working at Alice & Bob and École Normale Supérieure, we show that this result can be extended to open quantum systems which satisfy the quantum analogue of detailed balance. We use it to provide a precise estimate of the bitflip rate of imperfect dissipative cat qubits.

The Kerr cat qubit is a bosonic qubit that exhibits a large noise bias. A larger bias translates into fewer resources required for error correction. Yet recent circuit‑QED experiments show that this bias cannot be made arbitrarily large, and the origin of this limitation has remained unclear. Our work identifies the main culprit: the multimode nature of the Kerr cat circuit shortens its lifetime. Thankfully, our theory also suggests that a large noise bias can be recovered by carefully engineering the qubit’s electromagnetic environment.

Quantum metrology promises to drastically improve signal sensing capabilities throughout a range of scientific fields ranging from biology to physics and engineering. To deliver on this promise, experimental sensing protocols resilient to noise must be developed. In our theoretical work, we propose a displacement sensor that harnesses uncoventional states of light known as Gottesman-Kitaev-Preskill (GKP) states. Our sensor achieves sensitivities close the limit allowed by quantum mechanics, while remaining resilient to noise thanks to the error-correcting properties of GKP states. The protocol can be implemented on a variety of platforms, such as superconducting circuits or trapped ions, and has potential applications in force sensing, waveform estimation, and quantum channel learning.

Transmon qubits are a leading candidate to encode quantum information. Many recent works have shown that the strong voltages used to measure transmon qubits can excite them to more energetic states in an uncontrolled way. In collaboration with the University of Pittsburgh and Yale University, we showed that this effect also limits the speed at which the qubit can be controlled, even when using highly off-resonant low-frequency voltages. Our theory explains observations and provides an avenue for improving quantum control in the near future.

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This spring, two new articles involving members of the groupe appear on the arXiv. Below is a brief summary along with the link to access them.

First article : Transmon qubits are a leading candidate to encode quantum information. However, it is known that the strong voltages used to measure these qubits can induce measurement errors by exciting the qubit to a more energetic state. In collaboration with the University of Rochester, we studied these excitations and were able to control them. Our theory explains observations and provides tools to improve measurement in the near future.

Second article : One of the advantages of transmon qubits is their resilience against unavoidable electric noise in their environment. However, it was recently suggested that the measurement of these qubits is not protected against such noise. In collaboration with the Karlsruhe Institute of Technology, we studied the effect of electric noise on transmon measurement. Our theory explains observations and suggests strategies to improve measurement in the near future.

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We recently welcomed Rémy Lyscar to the group. Originally from France and currently a first-year Master’s student in Fundamental Physics at the University of Paris-Saclay, he is doing a three-month internship with the group.

Last fall, we welcomed two new members in the group.

Benjamin Levitan completed his graduate studies at McGill University, obtaining his M.Sc. under the direction of Aash Clerk, and his Ph.D. with Tami Pereg-Barnea. After a first postdoc at the Weizmann Institute of Science, he recently joined the group for a second postdoc.

Tianrui completed her M.A. and Ph.D. under the direction of Prof. Joel E. Moore, followed by her first postdoc with Prof. Ana Maria Rey at JILA. She recently joined the group as a postdoc.

Measurement of the transmon in circuit QED does not have the fidelity expected from theory. In a new paper published in PRX, we present a unified picture of this phenomenon which is in agreement with experiments.