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

  1. B. Lienhard, A. Vepsäläinen, L. C. G. Govia, et al., “Deep Neural Network Discrimination of Multiplexed Superconducting Qubit States”, arXiv:2102.12481 (2021).

  2. 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).

  3. 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).

  4. 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).

  5. 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).

  6. 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).