Browsing by Author "Kaivola, Matti, Prof."

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  • Approaching the zero-temperature limit in superfluid dynamics and dissipation
    (2012) Hosio, Jaakko
     School of Science | Doctoral dissertation (article-based)
    Most collective physical systems freeze and become immobile at zero temperature. Thus, there exist few systems where hydrodynamics can be experimentally studied in the zero-temperature limit. Most notable among these are the helium superfluids which remain in liquid state down to zero temperature and may support dissipationless superflow at sufficiently low flow velocities. The measurements of this thesis present the first information on the interplay of laminar and turbulent flow at higher velocities in the zero-temperature regime and the associated dissipation in these flow states. In contrast to earlier beliefs, the results show that there exist residual dissipation mechanisms in both cases which cause damping even in the zero-temperature limit. A remarkable feature of superfluids is the quantization of flow through the creation of quantized vortex lines. These are formed at higher flow velocities, usually at some critical velocity. At higher temperatures the motion of vortices is damped by their interaction with the normal excitations, but this source of dissipation vanishes rapidly towards zero temperature. Thus, the motion of vortices should become dissipationless in the zero-temperature regime. However, as in viscous fluids, the smaller the dissipation the easier the flow is perturbed and becomes turbulent. Accordingly, vortex flow was expected to be turbulent in most experimentally achievable situations in the zero-temperature limit. In this thesis superfluid dynamics is explored in a rotating ultra-low-temperature refrigerator with nuclear magnetic resonance and with measurements of Andreev scattering of ballistic quasiparticle excitations from quantized vortex lines in a cylindrical sample of superfluid helium-3. In an axially symmetric smooth-walled container, vortex flow turned out to be laminar, but perturbations, such as breaking the axial symmetry with obstacles or by changing the surface friction, was found to lead to turbulence. To stabilize laminar flow, the minimization of surface interactions is found to be of major importance. In spite of the sub-millikelvin temperatures, which are needed for the present studies, the advantage of superfluid helium-3 over the experimentally more accessible helium-4 superfluid is the more than two orders of magnitude larger vortex core diameter which reduces decisively disturbances in the flow of the vortex ends along solid walls.
  • Decoherence in superconducting quantum circuits
    (2012) Li, Jian
     School of Science | Doctoral dissertation (article-based)
    Superconducting quantum devices have drawn the attention of physicists greatly in recent years, not only because they are one of the most favorable candidates for developing a solid state quantum computer, but also because they can be employed as test systems for understanding quantum mechanics at nearly macro scale. Superconducting quantum devices can be divided into two groups: the superconducting resonators (linear devices), which have equally spaced discrete energy levels, and the artificial atoms (non-linear devices) consisting of Josephson junctions which have unequally spaced discrete energy levels. By combining these two kinds of devices together, a solid state counterpart of optical cavity quantum electrodynamics (QED) known as circuit QED emerges. Experiments of cavity QED and quantum optics can be reproduced in either circuit QED systems or in bare superconducting artificial atoms. As an example, we have observed the Autler-Townes effect in a three-level artificial atom called phase qutrit. More than one decade has passed since the first experimental demonstration of superconducting two-level artificial atom (qubit). However, to build a practical quantum computer with thousands of superconducting qubits there is still a long way to go (we are at the stage of three qubits now). One of the main obstacles preventing us from scaling up a superconducting quantum computer, as well as some other kinds of quantum computers, has been decoherence, which is believed to be dominated by the low frequency noise due to the two-level system (TLS) fluctuators located inside the Josephson junction barrier. We have experimentally simulated the dynamics of a qubit longitudinally coupled to a randomly fluctuating TLS. A phenomenon known as motional averaging has been observed. In this thesis, theoretical models for decoherence in both resonator and artificial atoms have been established and used for explaining phenomena observed in the experiments performed in our laboratory. The phenomenon of decoherence in coupled bipartite systems has been also studied, and besides the already known entanglement sudden death phenomenon, stable entanglement (robust under decoherence) generated from an arbitrary initial state has been found in these systems.
  • Dynamics of quantized vortices in applied flow in superfluid 3He-B
    (2011) Graaf, Robert Jan de
     Perustieteiden korkeakoulu | Doctoral dissertation (article-based)
    This thesis is mostly focussed on studies of dynamics of superfluid ³He-B at temperatures below 0.4Tc where the flow of quantized vortex lines was expected to be generally turbulent. The damping in vortex motion changes many orders of magnitude in a small temperature interval making vortices in superfluids an ideal tool to study turbulence. The quantum nature of vortices in superfluids allows for exotic hydrodynamics that does not exist in classical fluids. Earlier research had showed that vortices become unstable and lead to turbulence when the superfluid Reynolds number exceeds unity. The question remained open whether vortical flow is inherently unstable at lower temperatures. This thesis addresses issues surrounding the instability of quantized vortices in applied flow in the zero-temperature limit. Using the non-invasive nuclear magnetic resonance measurement technique, we have studied the dynamics of vortices in transient states during spin-up experiments where the rotation velocity of the system changes in a step-like manner. We found transition temperatures where the vortices connected to the cylindrical container become unstable and, ultimately, start a turbulent burst of vortex formation. This is in contrast to the laminar motion at higher temperatures, where the vortex ends smoothly slide in helical motion on the cylindrical surface. The exact conditions for this onset temperature to turbulence are established in terms of the applied flow, and the perturbation of the superfluid state by so-called seed vortices. The spin-up and spin-down experiments in the zero-temperature limit show different vortex dynamics. The vortex motion in applied flow is laminar for a cylindrical container, while in a cubical geometry the motion is expected to be partly turbulent. Our experiments on turbulent front propagation after injection of seed vortices from the AB-phase boundary (via the Kelvin-Helmholtz instability) into the rotating Landau state show a change over from quasi-classical turbulence at high temperatures, to quantum turbulence in the low temperature regime where the energy cascade of Kelvin wave excitations starts to contribute to the dissipative process. The effect of a bottleneck in this energy cascade is expressed in the front propagation velocity. The contribution of the density anisotropy to the textural energy of the superfluid in rotation is measured from the high to the zero-temperature limit. Comparison with theory allows determination of the superfluid energy gap. We have mapped the superfluid order parameter flare out textures in terms of applied flow and temperature. A quartz tuning fork with a high quality factor has been studied in superfluid ³He-B. The device is found to be an excellent tool to measure temperature, pressure and viscosity. In the zero-temperature limit, where other temperature measurement devices start to saturate, the fork's sensitivity increases due to the exponential dependence on the quasiparticle density.
  • Fluctuations, relaxation and proximity effect in superconducting circuits
    (2011) Peltonen, Joonas
     Perustieteiden korkeakoulu | Doctoral dissertation (article-based)
    Mesoscopic physics investigates structures smaller than the everyday macroscopic scale but larger than the scale of individual atoms, with properties that can often only be explained in terms of the laws of quantum mechanics. A typical mesoscopic electrical component is a tunnel junction, formed by a thin insulating oxide layer separating two metallic electrodes. In this thesis, various mesoscopic circuits containing sub-micron tunnel junctions between normal (N) and superconducting (S) metals are studied experimentally at sub-kelvin temperatures. An emphasis is placed on the influence of electrical fluctuations on the systems, as well as the strong connection between electrical and thermal transport in them. We first demonstrate that a Josephson tunnel junction between two S electrodes functions as an on-chip detector of current fluctuations in a wide band of frequencies, potentially useful for studying charge transport in various mesoscopic systems. The lifetime of the zero-voltage state in a current-biased junction is very sensitive to the fluctuations in the bias current. We are able to observe the non-Gaussian nature of the shot noise generated by electrons tunneling across another nearby tunnel junction coupled to the detector junction. Several of the experiments in this thesis probe how the transport close to a transparent interface between a superconducting and a normal conducting electrode is modified by the phenomenon of superconducting proximity effect. We present direct measurements of electron overheating in a normal metal weak link between two superconductors, explaining the routinely observed hysteretic current-voltage characteristic. Electronic temperature is probed locally by contacting the N island to an additional S electrode via an oxide barrier (I), thereby forming an NIS tunnel junction. Connecting the S electrodes of such a proximity SNS weak link into a closed loop, we further demonstrate use of the structure as a sensitive magnetometer with low dissipation. We probe also the electronic thermal conduction of short S wires between two N terminals. Due to the inverse proximity effect, the thermal conductance is found to be strongly enhanced beyond the value for a bulk superconductor. We consider theoretically the prospects for rectifying thermal fluctuations by an NIS junction in a suitable electromagnetic environment, thereby realizing a Brownian refrigerator. Finally, we report the observation of increased cooling power in a voltage-biased NIS junction in small applied magnetic fields. This is attributed to enhanced relaxation of the hot electrons injected into the S lead of the junction.
  • Thin film bulk acoustic wave devices : performance optimization and modeling
    (2011) Pensala, Tuomas
     Perustieteiden korkeakoulu | Doctoral dissertation (article-based)
    Thin film bulk acoustic wave (BAW) resonators and filters operating in the GHz range are used in mobile phones for the most demanding filtering applications and complement the surface acoustic wave (SAW) based filters. Their main advantages are small size and high performance at frequencies above 2 GHz. This work concentrates on the characterization, performance optimization, and modeling techniques of thin film BAW devices. Laser interferometric vibration measurements together with plate wave dispersion modeling are used to extract the full set of elastic material parameters for sputter deposited ZnO, demonstrating a method for obtaining material data needed for accurate simulation of the devices. The effectiveness of the acoustic interference reflector used to isolate the vibration from the substrate is studied by 1-D modeling, 2-D finite element method and by electrical and laser interferometric measurements. It is found that the Q-value of reflector-based BAW resonators operating at 2 GHz is limited to approximately 2000 by mechanisms other than leakage through the reflector. Suppression of spurious resonances in ZnO resonators is studied in depth by modeling and measurements. It is verified that the approximate mode orthogonality is behind the suppression in boundary frame type ZnO devices operating in the piston mode, but also another narrow band mode suppression mechanism is found. A plate wave dispersion based 2-D simulation scheme for laterally acoustically coupled BAW resonator filters is developed and employed in designing of experimental devices, which show both good agreement with the model predictions and a remarkable 4.9 % relative bandwidth.
  • Tunnel junction thermometry and thermalisation of electrons
    (2010) Holmqvist, Tommy
     Aalto-yliopiston teknillinen korkeakoulu | Doctoral dissertation (article-based)
    In this work, normal metal and superconducting tunnel junctions have been studied. In particular applications in low temperature electronic thermometry are investigated. The double oxidation method for achieving high resistance junctions is described and shown to produce high quality devices. A new invention, the single junction thermometer (SJT), is described, and proof of the concept experiments demonstrating the functionality of the device are described. A key feature of the SJT is a well controlled electromagnetic environment surrounding a single junction. It is shown that embedding a single junction within arrays of other junctions is a superior way for controlling the environment as compared to previously proposed methods. Improved thermalisation of electrons is shown to be beneficial for the performance of superconducting RSFQ (Rapid Single Flux Quantum) devices. The so called grey zone of the most basic of these devices, the balanced comparator, is investigated. Improved performance based on metallic cooling fins has been achieved. In the same spirit improved thermalisation of the Coulomb blockade thermometers extends their operation towards lower temperatures. A method employing thick metallic islands was developed and tested to this end.