Quantum nanoelectronics for microwave photons


We develop components for on-chip manipulation of light at microwave frequencies, i.e. for circuit quantum electrodynamics (cQED). The components vary from meter long waveguide resonators to sub-micron nanowires, typically defined lithographically using a combination of superconducting and normal metals. We focus particularly on the interplay of thermal and electric effects in these systems. Our work is supported by the European Research Council (ERC) Starting Grant, see the press release.

Differential heat conductance in nanoelectronics

Identifying and controlling the heat conductance mechanisms between the electrons in a thermal sensor (e.g. calorimeter or bolometer) and its surroundings is very important when developing next-generation single-photon detectors and radiation detectors in general. In particular, a bolometer (power sensor for radiation) should typically be thermally decoupled from its surroundings in order to achieve maximum sensitivity.

We are working with sensors in the regime where the vibrational modes of solids (phonons) are effectively frozen and the dominant heat loss channel is the quantum-limited exchange of energy via photons between the sensor and its electromagnetic environment, i.e., electron-photon heat conduction [Phys. Rev. B 83, 125113 (2011)]. Here, we have developed new ways of measuring differential heat conductance and also engineered electromagnetic environments to isolate mesoscopic thermal sensors. Our measurements show that the thermal conductance can be reduced far below the single-channel quantum-limited value [Nature 444, 187 (2006)].


Fig. The thermal conductance G between a sensor and its surrounding heat bath (along with the sensor heat capacity) determines the characteristic signal decay time in a thermal photon sensor. Our work focuses on quantifying and minimizing G, especially in the regime where the thermal conductance is dominated by electron-photon coupling between the sensor and electromagnetic environment.

Single-photon microwave detector

We are developing a microwave photon detector for measuring the energy of a microwave packet, i.e. for doing microwave calorimetry. Such a detector is fundamentally different from existing microwave voltage amplifiers because voltage and energy correspond to different quantum mechanical operators. This distinction is particularly essential for non-classical wave packets that contain just a few photons. Check out our Youtube video!


Fig. Three-terminal superconductor–normal-metal–superconductor (SNS) structure which is being studied for the microwave photon detector. The gray polygons mask “shadow” metal structures that are artifacts of the double-angle evaporation technique.

Impedance of normal-metal–superconductor junctions

In this project, we analyze the mechanisms (both resistive and reactive) that contribute to the high frequency electrical impedance of a diffusive superconductor–normal-metal–superconductor (SNS) weak link. From a fundamental perspective, these measurements can reveal the internal dynamics of the weak link. Additionally, details about the high-frequency response are needed to understand the efficiency of microwave absorption in ultrasensitive SNS-based radiation detectors.

Our approach is to strongly couple an SNS-wire or SNS SQUID to the end of a low-loss superconducting coplanar waveguide and probe the termination impedance Z(w) formed by the SNS. We have fabricated long (10 cm) microwave resonators (f < 1 GHz) so that the Z(w) can be probed at many multiples of the fundamental mode. The relative shift and broadening of each resonator harmonic at the different frequencies is used to extract the resistive and reactive parts of Z(w).


Fig. We strongly couple an SNS weak link to the end of a CPW resonator (left) and measure the shift and broadening of each peak to extract reactive and resistive parts of the SNS impedance as a function of frequency. The variable termination impedance from the SNS changes the mode profiles in the cavity (right) and is probed in the measurements.

Single-photon heat conduction

We study theoretically [Phys. Rev. B 85, 075413 (2012)] and experimentally photonic heat conduction between two resistors coupled weakly to a single superconducting microwave cavity. At low enough temperature, the dominating part of the heat exchanged between the resistors is transmitted by single-photon excitations of the fundamental mode of the cavity. Other heat transfer mechanisms due to, e.g., phonons and quasiparticles can be negligible [Phys. Rev. B 86, 035313 (2012)]. In this setup, the resistors [panel (a)] introduces voltage fluctuations in the form of Johnson-Nyquist noise. These fluctuations couple directly to the electromagnetic modes of the cavity hence exciting photons [panel (a)]. These photons will eventually be dissipated on the resistors resulting in detectable temperature changes.

This manifestation of single-photon heat conduction should be experimentally observable with the current state of the art in micro- and nanoelectronics device fabrication. Hence, we are currently pursuing the observation of this effect experimentally. The superconducting cavity can be fabricated using standard photolithographic techniques followed by metal-deposition. At the nanoscale level, the resistors together with the thermometer and temperature control leads can be placed at precise locations in the cavity using electron-beam lithography and angled metal deposition techniques.


Fig. Schematic diagram (not to scale) of two resistors, R1 and R2, situated at both ends of a superconducting coplanar waveguide cavity. The two resisors are embedded close to the two ends of the cavity to make the coupling moderately weak while maintaining the quality factor of the cavity. (b) Experimental implementation of one resistor with corresponding thermometer and temperature control using an NIS junction.

The physical implementation of a thermometer and a temperature controller is shown in panel (b). They consist of a pair of normal metal-insulator-superconductor (NIS) junctions of which if biased correctly, can be utilized as sensitive sub-Kelvin thermometer and heater/cooler [Phys. Rev. Lett. 102, 200801 (2009)]. Hence, resistor R1 can be heated/cooled below the bath temperature and corresponding temperature change mediated by the photonic modes of the cavity can be measured in resistor R2.

Photonic heat conduction enables the possibility of performing remote heating or cooling of low-temperature circuit elements at an extremely narrow bandwidth, potentially enabling interference-free remote temperature control of electronic components working at different frequency bands. In addition, studying these engineered electromagnetic environments provides opportunity to rapidly initialize qubit states in the framework of circuit quantum electrodynamics [Sci. Rep. 3, 1987 (2013)].

Heat conductance over macroscopic distances

We are studying experimentally heat conductance between two normal-metal blocks at sub-kelvin temperatures. The normal-metal blocks are separated by a macroscopic distance, and their temperatures are measured with NIS thermometry.

Page content by: communications-phys [at] aalto [dot] fi (Department of Physics) | Last updated: 11.01.2016.