Towards calorimetric detection of individual itinerant microwave photons

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Joonas Govenius Name of the doctoral dissertation Towards calorimetric detection of individual itinerant microwave photons Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 179/2016 Field of research Applied Physics Manuscript submitted 13 June 2016 Date of the defence 30 September 2016 Permission to publish granted (date) 19 August 2016 Language English Monograph Article dissertation Essay dissertation Abstract This dissertation focuses on the development of a new type of thermal microwave photodetector based on superconductor–normal-metal–superconductor (SNS) junctions. We motivate the development of the detector mainly by microwave quantum optics applications in the context of superconducting qubits coupled to microwave transmission lines and resonators, i.e., in the context of circuit quantum electrodynamics (cQED). In cQED, singlephoton microwave pulses naturally arise as a results of a qubit exchanging its excitation with a transmission line. While immense progress has been achieved in cQED in general, and in linear microwave amplification in particular, the challenge of developing an efficient and practical detector for single itinerant photons remains open, mostly due to the exceedingly small energy of individual microwave photons. This prevents microwave implementations of a certain class of quantum optical protocols, including the parity measurement protocol proposed in this dissertation. The core of this dissertation is dedicated to introducing our detector design, discussing the thermal properties of the detector in detail, and demonstrating the operation of the detector in a time-gated threshold detection mode. In particular, we demonstrate threshold detection of coherent 8.4 GHz microwave pulses containing roughly 200 photons, or 1.1 zJ of energy. Compared to other thermal detectors, this is an order of magnitude improvement in the energy of the detected pulses. In addition, we characterize the linear response of the SNS junctions as separate components. That is, we embed the junctions in a microwave circuit that is specifically designed to allow determining the electrical admittance of the SNS junctions from the response of the circuit as a whole.This dissertation focuses on the development of a new type of thermal microwave photodetector based on superconductor–normal-metal–superconductor (SNS) junctions. We motivate the development of the detector mainly by microwave quantum optics applications in the context of superconducting qubits coupled to microwave transmission lines and resonators, i.e., in the context of circuit quantum electrodynamics (cQED). In cQED, singlephoton microwave pulses naturally arise as a results of a qubit exchanging its excitation with a transmission line. While immense progress has been achieved in cQED in general, and in linear microwave amplification in particular, the challenge of developing an efficient and practical detector for single itinerant photons remains open, mostly due to the exceedingly small energy of individual microwave photons. This prevents microwave implementations of a certain class of quantum optical protocols, including the parity measurement protocol proposed in this dissertation. The core of this dissertation is dedicated to introducing our detector design, discussing the thermal properties of the detector in detail, and demonstrating the operation of the detector in a time-gated threshold detection mode. In particular, we demonstrate threshold detection of coherent 8.4 GHz microwave pulses containing roughly 200 photons, or 1.1 zJ of energy. Compared to other thermal detectors, this is an order of magnitude improvement in the energy of the detected pulses. In addition, we characterize the linear response of the SNS junctions as separate components. That is, we embed the junctions in a microwave circuit that is specifically designed to allow determining the electrical admittance of the SNS junctions from the response of the circuit as a whole.