Qubit Readout
A large portion of my graduate and postdoctoral work focused on the readout of superconducting qubits. The standard approach uses the interaction between the qubit and a microwave resonator mode to encode the state of the qubit in the phase of a microwave signal reflected off or transmitted through the resonator. This phase can then be measured using homodyne or heterodyne measurement at room temperature.
Inspired by ideas from quantum metrology, I helped develop techniques that enhance the sensitivity to this phase, and therefore improve the accuracy of qubit readout, using squeezed microwave fields [4]. My experimental collaborators put some of these ideas into practice, and demonstrated some of the highest efficiency superconducting qubit measurement to date [2,3]. Along the way, we also discovered a minimal Purcell filter that protects a qubit from spontaneous decay into the resonator mode’s environment [4].
For simultaneous multi-qubit readout there can be a tangible benefit to using a more complicated discriminator to measure the resonator signal. My collaborators and I demonstrated that a neural network discriminator can improve simultaneous readout performance on a device of five superconducting qubits [1].
An alternative to encoding the qubit state information in the phase of the microwave signal is to encode it in the amplitude. My collaborators and I developed a qubit-readout scheme that associates the two possible qubit states with either zero or many photons, and uses a kind of superconducting microwave photon detector, the Josephson Photomultiplier, to distinguish the signal amplitudes [6]. We also showed how to extend this scheme to measure the parity of up to four qubits in a single shot [5], which has applications in quantum-error-correction.
Selected Papers
B. Lienhard, A. Vepsäläinen, L. C. G. Govia, et al., “Deep Neural Network Discrimination of Multiplexed Superconducting Qubit States”, arXiv:2102.12481 (2021).
A. Eddins, J. M. Kreikebaum, D. M. Toyli, E. M. Levenson-Falk, A. Dove, W. P. Livingston, B. A. Levitan, L. C. G. Govia, A. A. Clerk, I. Siddiqi, “High-efficiency measurement of an artificial atom embedded in a parametric amplifier”, Phys. Rev. X 9 (1), 011004 (2019).
A. Eddins, S. Schreppler, D. M. Toyli, L. S. Martin, S. Hacohen-Gourgy, L. C. G. Govia, H. Ribeiro, A. A. Clerk, and I. Siddiqi, “Stroboscopic qubit measurement with squeezed illumination”, Phys. Rev. Lett. 120 (4), 040505 (2018).
L. C. G. Govia and A. A. Clerk, “Enhanced qubit readout using locally-generated squeezing and inbuilt Purcell-decay suppression”, New J. Phys. 19, 023044 (2017).
L. C. G. Govia, E. J. Pritchett, B. L. T. Plourde, M. G. Vavilov, R. McDermott, and F. K. Wilhelm, “Scalable two-and four-qubit parity measurement with a threshold photon counter”, Phys. Rev. A 92 (2), 022335 (2015).
L. C. G. Govia, E. J. Pritchett, C. Xu, B. L. T. Plourde, M. G. Vavilov, F. K. Wilhelm, and R. McDermott, “High-fidelity qubit measurement with a microwave-photon counter”, Phys. Rev. A 90 (6), 062307 (2014).