Thermodynamics in quantum circuits aims to find improved functionalities of thermal machines, highlight fundamental phenomena peculiar to quantum nature in thermodynamics, and pointout limitations in quantum information processing due to coupling of the system to its environment. An important aspect to achieve some of these goals is the regime of strong coupling that has remained until now a domain of theoretical works only. Our aim is to demonstrate strong coupling features in heat transport using a superconducting flux qubit that has been shown to reach strong to deep-ultra strong coupling regimes. Here we show experimental evidence of strong coupling by observing a hybridized state of the qubit with the cavities coupled to it, leading to a triplet-like thermal transport via this combined system around the minimum energy of the qubit, at power levels of tens of femtowatts, exceeding by an order of magnitude from the earlier ones. We also demonstrate close to 100% on-off switching ratio of heat current by applying small magnetic flux to the qubit. Our experiment opens a way towards testing debated questions in strong coupling thermodynamics such as what heat in this regime is. We also present a theoretical model that aligns with our experimental findings and explains the mechanism behind heat transport in our device. Furthermore, we provide a new tool for quantum thermodynamics aimed at realizing true quantum heat engines and refrigerators with enhanced power and efficiency, leveraging ultra-strong coupling between the system and environment.
Superconducting circuits provide a versatile and controllable platform for studies of fundamental quantum phenomena as well as for quantum technology applications. A conventional techniqueto read out the state of a quantum circuit or to characterize its properties is based on rf measurement schemes involving costly and complex instrumentation. Here we demonstrate a simple dc measurement of a thermal spectrometer to investigate properties of a superconducting circuit, in this proof-of-concept experiment a coplanar waveguide resonator. A fraction of the microwave photons in the resonator is absorbed by an on-chip bolometer, resulting in a measurable temperature rise. By monitoring the dc signal of the thermometer due to this process, we are able to determine the resonance frequency and the lineshape (quality factor) of the resonator. The demonstrated scheme, which is a simple dc measurement, has a wide band up to 200 GHz, well exceeding that of the typical rf spectrometer. Moreover, the thermal measurement yields a highly frequency independent reference level of the Lorentzian absorption signal, unlike the conventional rf measurement. In the low power regime, the measurement is fully calibration-free. Our technique thus offers an alternative spectrometer for quantum circuits, which is in many ways superior with respect to conventional methods.
We report temperature measurements using a transmon qubit by detecting the population of the first three levels of it, after employing a sequence of π-pulses and performing projectivedispersive readout. We measure the effective temperature of the qubit and characterize its relaxation and coherence times τ1,2 for three devices in the temperature range 20-300 mK. Signal-to-noise (SNR) ratio of the temperature measurement depends strongly on τ1, which drops at higher temperatures due to quasiparticle excitations, adversely affecting the measurements and setting an upper bound of the dynamic temperature range of the thermometer. The measurement relies on coherent dynamics of the qubit during the π-pulses. The effective qubit temperature follows closely that of the cryostat in the range 100-250 mK. We present a numerical model of the qubit population distribution and compare it favorably with the experimental results.
The fragile nature of quantum circuits is a major bottleneck to scalable quantum applications. Operating at cryogenic temperatures, quantum circuits are highly vulnerable to amplifierbackaction and external noise. Non-reciprocal microwave devices such as circulators and isolators are used for this purpose. These devices have a considerable footprint in cryostats, limiting the scalability of quantum circuits. We present a compact microwave diode architecture, which exploits the non-linearity of a superconducting flux qubit. At the qubit degeneracy point we experimentally demonstrate a significant difference between the power levels transmitted in opposite directions. The observations align with the proposed theoretical model. At -99 dBm input power, and near the qubit-resonator avoided crossing region, we report the transmission rectification ratio exceeding 90% for a 50 MHz wide frequency range from 6.81 GHz to 6.86 GHz, and over 60% for the 250 MHz range from 6.67 GHz to 6.91 GHz. The presented architecture is compact, and easily scalable towards multiple readout channels, potentially opening up diverse opportunities in quantum information, microwave read-out and optomechanics.
The study of quantum heat transport in superconducting circuits is significant for further understanding the connection between quantum mechanics and thermodynamics, and for possibleapplications for quantum information. The first experimental realisations of devices demonstrating photonic heat transport mediated by a qubit have already been designed and measured. Motivated by the analysis of such experimental results, and for future experimental designs, we numerically evaluate the photonic heat transport of qubit-resonator devices in the linear circuit regime through electromagnetic simulations using Sonnet software, and compare with microwave circuit theory. We show that the method is a powerful tool to calculate heat transport and predict unwanted parasitic resonances and background.
Quantum heat transport devices are currently intensively studied in theory. Experimental realization of quantum heat transport devices is a challenging task. So far, they have beenmostly investigated in experiments with ultra-cold atoms and single atomic traps. Experiments with superconducting qubits have also been carried out and heat transport and heat rectification has been studied in two terminal devices. The structures with three independent terminals offer additional opportunities for realization of heat transistors, heat switches, on-chip masers and even more complicated devices. Here we report an experimental realization of a three-terminal photonic heat transport device based on a superconducting quantum circuit. Its central element is a flux qubit made of a superconducting loop containing three Josephson junctions, which is connected to three resonators terminated by resistors. By heating one of the resistors and monitoring the temperatures of the other two, we determine photonic heat currents in the system and demonstrate their tunability by magnetic field at the level of 1 aW. We determine system parameters by performing microwave transmission measurements on a separate nominally identical sample and, in this way, demonstrate clear correlation between the level splitting of the qubit and the heat currents flowing through it. Our experiment is an important step in the development of on-chip quantum heat transport devices. On the one hand, such devices are of great interest for fundamental science because they allow one to investigate the effect of quantum interference and entanglement on the transport of heat. On the other hand, they also have great practical importance for the rapidly developing field of quantum computing, in which management of heat generated by qubits is a problem.
Here we present an architecture for the implementation of cyclic quantum thermal engines using a superconducting circuit. The quantum engine consists of a gated Cooper-pair box, capacitivelycoupled to two superconducting coplanar waveguide resonators with different frequencies, acting as thermal baths. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at GHz frequencies. By terminating the resonators of the measured structure with normal-metal resistors whose temperature can be controlled and monitored, a quantum heat engine or refrigerator could be realized. Furthermore, we numerically evaluate the performance of our setup acting as a quantum Otto-refrigerator in the presence of realistic environmental decoherence.
We report on a robust method to achieve strong coupling between a superconducting flux qubit and a high-quality quarter-wavelength coplanar waveguide resonator. We demonstrate the progressionfrom the strong to ultrastrong coupling regime by varying the length of a shared inductive coupling element, ultimately achieving a qubit-resonator coupling strength of 655 MHz, 10% of the resonator frequency. We derive an analytical expression for the coupling strength in terms of circuit parameters and also discuss the maximum achievable coupling within this framework. We experimentally characterize flux qubits coupled to superconducting resonators using one and two-tone spectroscopy methods, demonstrating excellent agreement with the proposed theoretical model.
The fundamental excitations in superconductors – Bogoliubov quasiparticles – can be either a resource or a liability in superconducting devices: they are what enables photondetection in microwave kinetic inductance detectors, but they are a source of errors in qubits and electron pumps. To improve operation of the latter devices, ways to mitigate quasiparticle effects have been devised; in particular, combining different materials quasiparticles can be trapped where they do no harm and their generation can be impeded. We review recent developments in these mitigation efforts and discuss open questions.
We describe a qubit linearly coupled to a heat bath, either directly or via a cavity. The bath is formed of oscillators with a distribution of energies and coupling strengths, bothfor qubit-oscillator and oscillator-oscillator interaction. A direct numerical solution of the Schrödinger equation for the full system including up to 106 oscillators in the bath and analytic solutions are given, verifying quantum decay in short time quadratic (Zeno), long time exponential and eventually power law relaxation regimes. The main new results of the paper deal with applications and implications in quantum thermodynamics setups. We start by providing a correspondence of the oscillator bath to a resistor in a circuit. With the presented techniques we can then shed light on two topical questions of open quantum systems. First, splitting a quantum to uncoupled baths is presented as an opportunity for detection of low energy photons. Second, we address quantitatively the question of separation between a quantum system and its classical environment.