Browsing by Author "Pekola, Jukka P., Prof., Aalto University, Department of Applied Physics, Finland"

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  • Control of heat in superconducting microwave circuits
    (2019) Partanen, Matti
     School of Science | Doctoral dissertation (article-based)
    Circuit quantum electrodynamics offers versatile opportunities for various discoveries infundamental physics, as well as for different applications such as sensitive detectors and,ultimately, even a large-scale quantum computer. Since these devices work at cryogenictemperatures and excess heat is a substantial source of errors, it is of great importance to controlthe heat. In this thesis, several techniques for heat control are experimentally investigated. We study heattransfer carried by photons between two heat baths, as well as heat flow between superconductingresonators and their environment. In particular, the focus is on using normal-metal componentsto enhance the operation of superconducting microwave circuits operating at the level of singleenergy quanta. First, we investigate long-distance single-channel heat conduction near the quantum limit, i.e.,the quantum of thermal conductance. As compared to earlier experiments, the distance is increasedby a factor of 10,000. In addition, we demonstrate emission and absorption of photons in amicrowave resonator by photon-assisted tunneling in normal-metal–insulator–superconductorjunctions. Another approach for absorbing photons is to use a normal-metal resistor in asuperconducting resonator with a tunable resonance frequency. Finally, we realize a circuitconsisting of two resonators with a singularity in the parameter space, known as an exceptionalpoint. In our circuit, the exceptional point enables the fastest possible heat transfer between thetwo resonators without back and forth oscillation. In the future, these methods may find applications in different cryogenic devices, including a fastinitialization of qubits. Furthermore, the methods may be utilized in the experimental studies offundamental physics, including further investigations of exceptional points in differentconfigurations.
  • Electron thermometry, refrigeration and heat transport in nanostructures at sub-kelvin temperatures
    (2017) Feshchenko, Anna
     School of Science | Doctoral dissertation (article-based)
  • Heat Transport in Superconducting Quantum Circuits
    (2019) Senior, Jorden
     School of Science | Doctoral dissertation (article-based)
    Superconducting microwave circuits are a ubiquitous and important tool for devices that exploit the phenomena of superconductivity and cryogenic temperatures as an environment for achieving the generation, manipulation, and detection of quantum states - required for the ongoing development of quantum technologies. In particular, superconducting circuits are a promising platform for the universal quantum computer, which will require an unprecedented density of quantum-coherent elements to perform large-scale quantum-enhanced calculations and simulations, using the framework of cavity quantum electrodynamics.Dissipation and heat in these superconducting circuits is a key source of error and inefficiency, however the thermodynamics in this regime is poorly understood despite its increasing relevancy.This thesis describes the integration of superconducting resonators and artificial atoms derived from superconducting quantum circuits with ultra-sensitive bolometry, for looking at heat transport through superconducting circuits. We will describe the physics and operation of each of these elements, before combining and utilising them to perform heat transport measurements through a superconducting artificial atom coupled to two resonators, each terminated by a normal-metal mesoscopic resistor, with the resistor temperature measured and temperature gradients across the circuit induced by superconducting tunnel-probes. We will present tunable heat transport through this system, firstly when the resonators are symmetric, allowing us to observe the role of dissipation-limited coupling of the resonators to the artificial atom on the locality of the heat transport, then on an asymmetric system, demonstrating a directional rectification of the heat transport. Additionally, we will discuss how each element of the system can be individually characterised, in particular the quality factor of superconducting resonators in the highly-dissipative limit, by exploiting the superconducting transition to perform a background reference. It is suggested that this hybrid quantum system, and these initial experiments provide a promising platform in the emergent field of circuit quantum thermodynamics. We believe that the techniques and tools developed during this thesis present key steps towards the understanding of the thermodynamics of quantum circuits, towards the realisation of devices that can explore heat transport in the quantum limit, such as a quantum heat engine.
  • Heat transport, fluctuations, and Maxwell's demon in electronic nanocircuits
    (2016) Koski, Jonne
     School of Science | Doctoral dissertation (article-based)
    The field of thermodynamics describes the properties of a system that is coupled to one or several heat baths. A new direction in the field is to investigate the thermodynamics at microscopic level, considering the stochastic evolution of individual system state trajectories. A thought experiment presented over 150 years ago, which is known as Maxwell's demon and apparently violates the Second law of thermodynamics, is currently investigated to assess the relation between information and energy. This thesis studies stochastic thermodynamics and Maxwell's demon experimentally in electronic nanocircuits. The first part of the thesis covers the basic thermodynamic concepts and recently discovered fluctuation relations. The original thought experiments of Maxwell's demon and Szilard's engine are presented, after which information in thermodynamics is discussed in more detail. Next, this thesis discusses heat transport in metallic nanoelectronic circuits. A superconductor, which ideally is an excellent thermal insulator, leaks heat in a direct contact to a normal metal due to a phenomenon known as inverse proximity effect. This thesis describes the measurement of this heat leak, and demonstrates how inverse proximity effect can be exploited to fabricate fully normal aluminum-based tunnel junctions. The remaining part of the thesis is about devices based on single-electron phenomena, and the experiments on stochastic thermodynamics presented here are based on them. An experimental setup based on a device known as a single-electron box is used to determine distributions of thermodynamic quantities, by which the fluctuation relations are tested experimental-ly in the case of multiple heat baths, and by which the general properties of the distribution are investigated as a function of temperature. Moreover, the setup is operated as a Szilard's engine, where one bit of information is used to extract energy equal to k_BT ln(2), where k_B is the Boltzmann constant and T is the temperature. Finally, heat transfer in single-electron devices is investigated at the level where the effect of heat generation is measurable as a change in temperature. The thesis shows how a device known as a single electron transistor can be operated to achieve local cooling. Furthermore, when such a device is coupled to a similar one, the setup realizes an autonomous Maxwell's demon. The presented experiment shows how the whole transistor cools down, however the coupled device (the demon) heats up corresponding to the information flow between them.
  • Low dissipation thermometry using superconducting tunnel junctions
    (2015) Faivre, Timothé
     School of Science | Doctoral dissertation (article-based)
    Thermometers are a cornerstone of experimental physics, starting in XVII th century with the premises of thermodynamics. Nowadays, the state of the art thermometers are employed as sensors in bolometers and calorimeters, probing the light originating from the Big Bang. The limitation of these devices is intrinsic. Performing at millikelvin temperatures, their temperatures start to fluctuate as predicted by the Fluctuation-Dissipation Theorem. In this thesis we investigate thermometers able to measure the temperature of an electron gas at sub-kelvin temperatures. We based our approach on tunnel junctions (I) between a superconductor (S) and the system under study, either a normal metal (N) or a weaker superconductor (S'). We tried to decoupled the system from its immediate surrounding by constructing SINIS or SIS'IS structure. This way, the central island is protected from noises and heat coming from the measurement contacts. We focused our work on characterizing a non-invasive thermometer, thus we reduced dissipation in the thermometer to its minimum value. In order to achieve that, we mainly investigate how the low-bias impedance of SINIS and SIS'IS devices reacts with temperature. For SINIS structures, we observed a saturation of the temperature response due to the presence of leakage through the junction. Ballistic Andreev reflection are one of the source of these leakage, and set a minimum working temperature for these device. In the special case where diffusive Andreev reflection are dominating the sub-gap conductance, the low bias impedance of a SINIS structure is responsive toward lower temperatures. The achievable temperature range was only limited by a spurious heat load. In SIS'IS structures, we monitor the transition of the weaker of the superconductor by measuring the supercurrent through the whole device. Such a thermometer response is really large, but the temperature range is also really narrow around the transition temperature of S'. In a sense, this kind of devices is similar to Transition-Edge Sensors (TES), but the presence of tunnel junctions increases the responsivity and reduces the heat leak through the measurement contacts.For all these devices, we developed a unified model allowing ones to reproduce quantitatively the zero bias impedance response to temperature. This model allows one to compare and optimize the sensitivity of the thermometers, given as a Noise Equivalent Temperature (NET). NET as low as a few *mu*K/*sqrt* have been observed for a SIS'IS device, and a SINIS device demonstrated a NET within a factor of two of its theoretical limit set by the temperature fluctuations.
  • Magnetometry by a proximity Josephson junction interferometer
    (2018) Najafi Jabdaraghi, Robab
     School of Science | Doctoral dissertation (article-based)
    In quantum technology, several aspects of superconductivity such as proximity effect have studied to develop a wide range of attractive applications at sub-kelvin temperatures. A new type of interferometer based on the proximity effect, taking place around transparent interfaces between normal- and superconducting metals, the Superconducting Quantum Interference Proximity Transistor (SQUIPT), relies on the phase dependence of the density of states in the proximized weak link. The SQUIPT devices offer the possibility to realize sensitive low-dissipation magnetometers compared to conventional DC SQUIDs. In this thesis, we investigate the development of the sensitive SQUIPT magnetometers. The first part of the thesis covers the characterization of non-hysteretic SQUIPTs with enhanced responsivity. In these structures, we demonstrate magnetic flux modulation of the device characteristics displaying no hysteresis at low temperatures by simply increasing the Josephson inductance of the weak link compared to self-inductance of the superconducting loop in the device. As a consequence, improvement in magnetic field responsivity is achievable. We then turn to the implementation of SQUIPT devices based on different fabrication methods and superconductor materials for improving the device performance. In this aspect, we fabricate and characterize niobium-based SNS devices utilizing two separate lithography and deposition steps with strong Ar ion cleaning in between. We further investigate a prototype hybrid SQUIPT device based on an Nb-Cu-Nb SNS junction with a conventional Al probe in tunnel junction. In the third part of the thesis, we present the flux noise characterization of a SQUIPT device using simultaneous measurement of DC transport properties and shot noise. To probe the noise, we use a cryogenic amplifier operating at frequencies in the range of a few MHz. We adapt this technique for flux noise measurements in SQUIPTs, adaptable also to low-temperature shot noise measurements of other nonlinear devices with high impedance. In order to investigate flux noise of SQUIPTs, we develop a model allowing one to optimize the figures of merit of the magnetometers such as the noise-equivalent flux.
  • Quantum state control with a superconducting qubit
    (2015) Sampath Kumar, Karthikeyan
     School of Science | Doctoral dissertation (article-based)
    This thesis explores circuit quantum electrodynamics (circuit QED), an experimental platform for studying fundamental quantum-optics phenomena in the microwave regime. For the realization of a quantum processor, this architecture offers the advantage of scalability and the use of technologies similar to those used in the semiconductor industry. The circuit QED system studied in this thesis is called transmon, and it consists of a superconducting qubit (a tunable artificial atom) coupled to a one-dimensional coplanar waveguide resonator. The qubit can be designed to have a very large electric dipole moment, leading to the strong coupling regime which was hard to reach in optical cavity QED. The main motivation of this thesis is to study the quantum states that can be prepared using this device and how they can be manipulated under various types of modulation.  In the early 1980s, Richard Feynman proposed that quantum computers would be useful in performing powerful computational tasks and could perform simulations of interacting quantum many-body systems. In this light, the thesis starts by exploring novel ways of designing gate sequences. The quest to understand if there are fundamental limitations for the speeding-up of standard quantum algorithms prompted us to discover a new quantum impossibility result: the quantum no-reflection theorem. Next, we present our experimental results obtained with the transmon. We discuss the physics of dispersive coupling between the qubit and resonator and we explore the nonlinearities induced by the presence of the qubit when the resonator is strongly driven. Then, the thesis presents the first steps taken in the direction of simulating many-body quantum systems, where the motional averaging observed in NMR based systems was simulated in a circuit QED architecture. By changing the modulation of the qubit from random to periodic latching, we observe a Stückelberg interference spectrum in a strong nonadiabatic regime where the Landau-Zener formula is not valid. Finally, the last chapter of the thesis is devoted to the presentation of our experimental results on quantum state control, from Rabi oscillations to stimulated Raman adiabatic passage.
  • Quantum transport and phase transitions in superconducting systems
    (2024) Subero Rengel, Diego Armando
     School of Science | Doctoral dissertation (article-based)
    This dissertation focuses on charge and thermal transport phenomena in mesoscopic superconducting circuits, particularly emphasizing the effects of Coulomb blockade on photonic heat transport at cryogenic temperatures. We investigate tunneling in mesoscopic systems with Coulomb blockade effects, the role of the electromagnetic environment on small Josephson junctions (JJs), and the impact of these factors on the bolometry of microwave photons through superconducting circuits. We propose and test a variant of the Maxwell demon experiment, the gambling demon. Unlike the standard Maxwell demon, the gambling demon decides, based on the acquired information, whether to stop the process following a customary gambling condition. Within this context, we derive and verify second-law-like inequalities accounting for the average work done when gambling is involved. For experimental verification, we use a single electron box connected capacitively to an electrometer, where an electrostatic potential governs the dynamics of electron tunneling into a metallic island. Our findings align closely with theoretical predictions, showing remarkable accuracy within 0.5%. We present results on photon-mediated heat transport through a superconducting circuit. We exploit the Johnson-Nyquist archetype to do that, where two thermal reservoirs are connected via a frequency-dependent transmission line. Here, two different frequency-dependent transmission lines are studied: one is a Cooper pair transistor controlled by an electric field, and the other is a superconducting quantum interference device (SQUID) controlled by a magnetic field. The first experiment with the Cooper pair transistor demonstrates a precise control of the thermal conductance close to its quantum limit with the gate voltage. The second experiment examines the environmental back-action effect on photon-mediated heat transport, revealing that while strong fluctuations produced by the environment affect charge transport through the SQUID as expected, they do not impact heat transport. This indicates that, unlike in the DC charge transport experiment, the Josephson effect survived regardless of the strength of the dissipation, which is a complementary experiment to test the anticipated dissipative phase transition by Schmid and Bulgadaev. In this scenario, we have performed a DC charge transport experiment through a JJ connected to a voltage source via an Ohmic resistor with resistance either greater or smaller than the superconducting resistance quantum RQ~6.5 kΩ to revisit the debated dissipative phase transition in a JJ. Our results support the existence of this transition, evidenced by a distinct dip in electrical conductance at zero voltage bias in the JJ connected to an environmental resistance exceeding RQ. Conversely, in devices where the environmental resistance is less than RQ, a conductance peak appears at zero voltage bias, indicative of the Josephson effect.
  • Statistics of rare events in single-electron devices
    (2019) Singh, Shilpi
     School of Science | Doctoral dissertation (article-based)
    Developments in fabrication and control of nanoscale devices have made precise single-electron counting possible. Due to the improved stability of these devices, increasing amounts of data can be collected leading to unprecedented statistics. These features have enabled the experimental verification of various statistical physics concepts, such as fluctuation relations and Maxwell's demon, with high precision. The recent theory results on extreme fluctuations in the entropy produced by a system, and first passage times, have not yet been verified experimentally. The experimental studies of these theoretical concepts using single-electron devices are the focus of this thesis. The thesis starts with a brief introduction to the physics of single-electronic devices used in the experiments along with the experimental setup used to study them. Next, the experimental methods used to fabricate the samples and the basic sample characterization techniques are presented. Later, the theoretical concepts are discussed and compared to the experimental results. This part starts with the probability distribution of the filtered telegraph signal from a bistable system, here a single-electron transistor. The filtering is done in two different ways: low pass filtering and finite time-averaging of the signal. The former allows us to propose a new method to obtain the transition rates between two states of the bistable system using the cumulants of its distribution. The latter allows us to see the rare fluctuations of current and observe theoretically predicted elliptic tail of the logarithm of the averaged current distribution. Next, the stochastic entropy produced by a system is discussed. This part also includes the properties of its distribution and its minimum value. The theory is presented along with the experimental observations. Finally, an introduction to the theory of first-passage-time distributions is provided.