ÄûÃʵ¼º½

Title of the thesis: Measurement techniques for quantum microwaves

Thesis defender: Aarne Keränen
Opponent: Associate Professor Michael Hatridge, Yale University, USA
Custos: Professor Mikko Möttönen, Aalto University School of Science

This thesis introduces a novel technique for measuring quantum statistical properties of propagating microwave photons. We show that a microwave radiation sensor based on superconductor–normal-metal–superconductor (SNS) junctions can directly measure the photon number variance of light in addition to the mean photon number. The presented method implements quantum measurements at the millikelvin stage of a dilution refrigerator, completely removing amplification noise from the measured signal and requiring only a data bus from the cryogenic to the room temperature. We show that the presented technique has an immediate application in the characterization of quantum states of itinerant microwave photons.

In addition to the above main result, the thesis shows a novel method for quantum bit (qubit) readout. Practical quantum computing applications require billions of elementary operations such as the measurement of a qubit state. Therefore, minimizing their error and execution time is important. Our presented method is both faster and more precise than the common dispersive readout, demonstrating a 50% smaller readout error for optimal readout duration.

The thesis further displays methods for making quantum circuits more compact. We demonstrate a rigorous process for the design of lumped-element resonators without utilizing any computer simulations. The achieved physical footprint is approximately an order of magnitude smaller than the area required for a corresponding of coplanar waveguide resonators ubiquitous in superconducting circuits. In addition, we demonstrate frequency-multiplexed readout of the SNS sensors, reducing the number of required control and measurement lines.

In summary, this thesis lays the ground for novel ways of measuring propagating microwaves and superconducting qubits along with improvements in their practical implementations. With future improvements in the utilized sensors and in quantum processors, these results are prone to advance the related quantum technological devices towards practical applications.

Keywords: Josephson junction, radiation sensor, quantum computing

Thesis available for public display 7 days prior to the defence at .