Browsing by Author "Hakonen, Pertti, Prof., Aalto University, O. V. Lounasmaa Laboratory, Finland"

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    Microwave Experiments and Noise in Mesoscopic Devices
    (Aalto University, 2015) Sarkar, Jayanta; Hakonen, Pertti, Prof., Aalto University, O. V. Lounasmaa Laboratory, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; O. V. Lounasmaa Laboratory, Nano; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, O. V. Lounasmaa Laboratory, Finland
    This thesis is a compilation of several works in the area of condensed matter physics, and with devices based on superconducting Josephson junctions (JJ) as the common denominator for the presented projects apart from the work on diffusive systems. Microwave measurements were conducted on a Superconducting Cooper pair transistor to explore its current-phase relationship. Measurements on a JJ-based qubit coupled to a LC resonator revealed the vibronic transitions obeying the Franck-Condon principle. The main body of the thesis is the work done on the Bloch Oscillating Transistor (BOT), an ultra low noise quantum amplifier. In the present work, we investigated the dynamics of the BOT near the bifurcation threshold as well as implemented differential BOTs to check its capability to reject common mode signals. To account for our studies of quantum features in mesoscopic systems other than JJs, we performed an experiment similar to the Hanbury-Brown and Twiss interferometry in optics. For this mesoscopic interference experiment we selected a multiterminal diffusive system. We developed a low temperature noise measurement scheme to study current-current correlations in the GHz frequency range. In our experiments we found a small positive HBT exchange correction factor in the non-interacting limit at low bias voltage in the presence of quantum interference. We found negative exchange correction factor in the hot electron case for similar structures, which agrees well with the theory. Altogether, our experiments demonstrated the theoretically predicted HBT exchange effects in non-interacting and interacting regime of electron transport in a diffusive mesoscopic system.
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    Nonlinearities and quantum phenomena in nanoelectromechanical systems
    (Aalto University, 2015) Khan, Raphaël; Heikkilä, Tero, Prof., Aalto University, O.V. Lounasmaa laboratory, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; O. V. Lounasmaa Laboratory; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, O. V. Lounasmaa Laboratory, Finland
    This dissertation addresses different types of nonlinear phenomena in nanoelectromechanical systems (NEMS) with an aim to find quantum behavior in them. Quantum mechanics is a theory that describes physics at the atomic scale. Without it many phenomena such as the electronic properties of crystalline matter would not be properly understood. However, the way the world works on quantum scale is nothing like how it appears in the classical perception of human beings. Although all the objects around us are made of atoms, none of the objects show quantum behavior since perturbations coming from their surroundings destroy the quantum states. During the last decade, NEMS have caught the interest of the scientific community since they are promising candidates to study and test quantum mechanics in the macroscopic scale.  In recent experiments, optomechanical systems have been used to cool nanomechanical resonators to their ground state. Ground state cooling is a requirement for the observation of the quantum nature of the mechanical resonator. But in addition to being close to the ground state, nonlinearities are needed to distinguish the quantum behavior from the classical one. Therefore, great effort is spent studying nonlinearities either within the mechanical resonator or due to an external system coupled to the nanomechanical resonator.  This dissertation is composed of an introduction and four research articles published in high-level physics journals. The introduction starts by discussing the basic theory of  NEMS. Then it specifies some relevant nonlinearities that occur in nanoelectromechanical systems, and how one can use them for the observation of quantum phenomena on macroscopic scale. The details of such nonlinearities are described in the research articles presented in the thesis.
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    Quantum noise correlations and amplifiers in mesoscopic systems
    (Aalto University, 2016) Lähteenmäki, Pasi; Teknillisen fysiikan laitos; Department of Applied Physics; Low Temperature Laboratory, Nano; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, O. V. Lounasmaa Laboratory, Finland
    This thesis work deals with a range of superconducting devices operated near the fundamental performance limits set by quantum mechanics along with the nature of quantum vacuum itself when perturbed by such devices. These devices are operated in a dilution refrigerator cooled down to sub-Kelvin temperatures. Low temperature is absolutely essential for quantum behavior of these devices and the minimal inherent noise contribution. Biased, selectively damped superconducting tunnel junction, (also known as the Josephson junction) displaying negative differential resistance, was employed to construct a device capable of amplifying weak microwave signal reflections from it. This device can be operated near the intrinsic precision limits set by the Heisenberg's uncertainty principle. Arrays of Superconducting Quantum Interference Devices (SQUIDs), operated below the critical current, can be used for metamaterial resonators whose resonant frequency can be quickly modulated by magnetic flux. These devices are operated as parametric amplifiers and they are capable of amplifying weak microwave signals near the quantum limit of noise. But more importantly, they are capable of perturbing the vacuum itself, which creates correlated photon pairs seeded by vacuum fluctuations. This effect is known as the dynamical Casimir effect (DCE) and is demonstrated in this thesis work by using power and correlation measurements at microwave frequencies. Furthermore, investigations on the DCE have been extended using a double modulation scheme yielding vacuum-induced coherence and tripartite correlations. Nanocarbon devices have also been studied in this thesis, in particular a wideband low noise nanotube electrometer constructed from a nanotube with proximity induced superconductivity. This device has certain unique benefits when operated in a microwave environment which are discussed in the thesis. Another nanocarbon device, a double quantum dot constructed from graphene and superconducting leads has been used to demonstrate Cooper-pair splitting. This device operates as a tunable source of entangled electrons.