Motivated by the recent development of fast and ultrasensitive thermometry in nanoscale systems, we investigate quantum calorimetric detection of individual heat pulses in the sub-meV energy range. We propose a hybrid superconducting injector-calorimeter setup, with the energy of injected pulses carried by tunneling electrons. It is shown that the superconductor constitutes a versatile injector, with tunable tunnel rates and energies. Treating all heat transfer events microscopically, we analyze the statistics of the calorimeter temperature fluctuations and derive conditions for an accurate measurement of the heat pulse energies. Our results pave the way for fundamental quantum thermodynamics experiments, including calorimetric detection of single microwave photons.
Motivated by the recent development of fast and ultrasensitive thermometry in nanoscale systems, we investigate quantum calorimetric detection of individual heat pulses in the sub-meV energy range. We propose a hybrid superconducting injector-calorimeter setup, with the energy of injected pulses carried by tunneling electrons. It is shown that the superconductor constitutes a versatile injector, with tunable tunnel rates and energies. Treating all heat transfer events microscopically, we analyze the statistics of the calorimeter temperature fluctuations and derive conditions for an accurate measurement of the heat pulse energies. Our results pave the way for fundamental quantum thermodynamics experiments, including calorimetric detection of single microwave photons.
Quantum calorimetry, the thermal measurement of quanta, is a method of choice for ultrasensitive radiation detection ranging from microwaves to gamma rays. The fundamental temperature fluctuations of the calorimeter, dictated by the coupling of it to the heat bath, set the ultimate lower bound of its energy resolution. Here we reach this limit of fundamental equilibrium fluctuations of temperature in a nanoscale electron calorimeter, exchanging energy with the phonon bath at very low temperatures. The approach allows noninvasive measurement of energy transport in superconducting quantum circuits in the microwave regime with high efficiency, opening the way, for instance, to observe quantum jumps, detecting their energy to tackle central questions in quantum thermodynamics.
This thesis addresses two different topics related to the physics of nanoscale systems. The first topic concerns quantum correlations and entanglement between electrons in solid-state systems, with a focus on how to generate electronic orbital entanglement on a sub-decoherence time scale and how to achieve experimentally more feasible entanglement detection schemes. The second topic concerns heat transport and temperature fluctuations in nanoscale systems, with a focus on how to utilize temperature fluctuations for calorimetric detection of single particles. The thesis comprises five papers.In Paper I, we propose a quantum dot system to generate and detect, using cotunneling processes, orbitally entangled pairs of electrons on a sub-decoherence time scale.In Paper II, we investigate, by applying an entanglement witness, the minimal number of zero-frequency current cross-correlation measurements needed to detect bipartite entanglement between two flying qubits.In Paper III, we consider energy and temperature fluctuations, and the influence of charging effects, in a metallic island tunnel coupled to a normal metallic lead, the so-called single electron box.In Paper IV, we investigate nanoscale quantum calorimetry and propose a setup consisting of a metallic island and a superconducting lead to realize a nanoscale calorimeter that may probe the energies of tunneling electrons.In Paper V, we investigate photon transport statistics of a microwave cavity, including the short-time statistics of single photon emissions and the long-time statistics of heat transport through the cavity.
The development of single photon detectors (SPD's) in the microwave regime is a fundamental challenge for obtaining a better understanding of the phenomena and components employed in quantum information technologies. Particularly, the field of quantum thermodynamics has until now been mainly theory oriented due to the lack of suitable detectors. In this thesis I present the first steps in developing a high fidelity SPD suitable for detecting itinerant microwave photons in a transmission line.
The device is of mesoscopic size and directly integrable into superconducting quantum circuits. Out of the two main approaches in microwave SPD's, atom-like systems and thermal detectors, we pursue the latter. In a thermal detector, the energy of the photon is transferred into the absorber of the detector, causing an abrupt increase in its temperature. This leads to the destruction of the photon, which can be avoided with an atom-like detector. However, an advantage of the thermal approach is that a broadband microwave detector, operated in a continuous manner, can be realized. This is essential for several experiments for example in quantum thermodynamics.
The absorber of our micro calorimeter is a normal metal nanowire. The electron gas in the wire is used for the photon absorption, whereas the phonons work as a heat sink. The absorber is connected to a superconducting electrode via a thin tunnel barrier and grounded through a direct normal metal-superconductor contact, which acts as a heat mirror. The temperature of the electron gas is probed via the differential conductance of a tunnel-junction. For realising a sensitive SPD, it is essential to minimize the heat capacity of the absorber. Thermal properties of small metal structures can deviate significantly from the properties of bulk materials due to the large surface-to-volume ratio. We have measured thermal relaxation in Cu and Ag thin-film nanowires at sub-kelvin temperatures and observed an anomalously long relaxation time in the Cu wires. In addition to a large specific heat, these results may also originate from the slow thermalisation in the granular structure observed in the evaporated Cu wires.; Saavuttaaksemme paremman ymmarryksen kvantti-informaatioteknologiassa hyodynnettavista ilmioista ja komponenteista, on yksittaisten fotonien resoluutiolla toimivien mikroaaltofotoni-ilmaisinten kehittaminen olennainen haaste. Sopivien havaintolaitteiden puutteen vuoksi on erityisesti kvanttitermodynamiikka pysynyt vahvasti teoriaan keskittyneena alana.
Tassa vaitoskirjassa esitetaan ensimmaiset askeleet herkan, yksittaisia siirtolinjassa kiertavia mikroaaltofotoneja havaitsevan anturin kehittamisessa. Laite on kooltaan mesoskooppinen, ja se voidaan kytkea suprajohtaviin piireihin. Termiset detektorit - joita kehitamme tassa tyossa - ja atomin kaltaiset systeemit ovat yleisimmin kaytettyja, toisiaan taydentavia ilmaisimia. Termisessa anturissa havaitun fotonin energia siirtyy laitteen absorbaattoriin, aiheuttaen siina nopean, havaittavan suuruisen lampotilan nousun.…
We experimentally demonstrate the high bandwidth readout of a thermometer based on a superconductor–insulator–normal metal (SIN) tunnel junction, embedded in a rf resonant circuit. Our implementation enables basic studies of the thermodynamics of mesoscopic nanostructures. It can also be applied to the development of fast calorimeters, as well as ultrasensitive bolometers for the detection of far-infrared radiation. We discuss the operational details of this device, and estimate the ultimate temperature sensitivity and measurement bandwidth.
We apply quantum trajectory techniques to analyze a realistic setup of a superconducting qubit coupled to a heat bath formed by a resistor, a system that yields explicit expressions of the relevant transition rates to be used in the analysis. We discuss the main characteristics of the jump trajectories and relate them to the expected outcomes ("clicks") of a fluorescence measurement using the resistor as a nanocalorimeter. As the main practical outcome, we present a model that predicts the time-domain response of a realistic calorimeter subject to single microwave photons, incorporating the intrinsic noise due to the fundamental thermal fluctuations of the absorber and finite bandwidth of a thermometer.
A feverish rate of change: Thermometry is key to thermodynamics, but measuring temperature changes at the nanoscale is challenging. In this investigation, a superconducting weak link (Josephson junction) is used to measure rapidly changing electron temperature in a long superconducting wire, with nanosecond resolution. The approach used here extends the range of existing experiments to faster time scales, paving the way for cutting-edge work on the thermodynamics of low-temperature quantum circuits and the development of ultralow-energy calorimetry and bolometry.