### Browsing by Author "Maisi, Ville F."

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Item Andreev tunneling and quasiparticle excitations in mesoscopic normal metal - superconductor structures(Centre for Metrology and Accreditation (MIKES), 2014) Maisi, Ville F.; Manninen, Antti, Centre for Metrology and Accreditation, Finland; O.V. Lounasmaa -laboratorio; O.V. Lounasmaa Laboratory; PICO group; Perustieteiden korkeakoulu; School of Science; Pekola, Jukka, Professor, Aalto University, O.V. Lounasmaa Laboratory, FinlandMesoscopic physics deals with systems whose size is between everyday macroscopic scale and the microscopic scale of individual atoms. With mesoscopic structures the flow of single electrons can be controlled. This thesis focuses on the control of single electrons with normal metal - superconductor structures. The emphasis is put on understanding the limitations of the control in the so-called SINIS turnstile, which is a device transporting one electron at a time. By repeating the drive with frequency f, the resulting electrical current in ideal operation I = ef could be utilized as the new definition in the SI unit system. Here e is the elementary charge. In the first part of the thesis, we review the physics of tunnel-coupled normal metals and superconductors and present the operation principle of the SINIS turnstile. We then show parallel operation of ten such devices. This allows one to reach larger currents required for high accuracy measurements. In addition we show that the experimental setup needs to be carefully designed in order to avoid spurious effects due to environmentally assisted tunneling. The second part of the thesis focuses on Andreev tunneling. In this process two electrons tunnel at once in form of a Cooper pair. Andreev tunneling leads to transfer errors, when the tunneling of a single electron is preferred. We discuss the experimental detection techniques of Andreev tunneling based on direct current measurements as well as on electron counting. Furthermore, we show experimentally that by having large enough energy cost for charging the structures, achieved by decreasing the size of the system, Andreev tunneling is suppressed and the accuracy of the turnstile improves. The electron counting techniques allows us to study nontrivial statistics of Andreev tunneling. In the last part of the thesis, excitations in a superconductor are considered. At low temperatures, the number of excitations of a superconductor should diminish exponentially. However, excess excitations in form of broken Cooper pairs are typically present limiting the performance of superconducting circuits. We discuss ways of probing the excitations in the normal metal - superconductor based structures. We investigate the diffusion of the quasiparticles and their relaxation to normal metallic traps or due to recombination into Cooper pairs via electron-phonon interaction.Item Extreme reductions of entropy in an electronic double dot(American Physical Society, 2019-03-18) Singh, Shilpi; Roldan, Edgar; Neri, Izaak; Khaymovich, Ivan M.; Golubev, Dmitry S.; Maisi, Ville F.; Peltonen, Joonas T.; Juelicher, Frank; Pekola, Jukka P.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Phenomena and Devices; Center for Advancing Electronics Dresden; Max Planck Institute for the Physics of Complex Systems; Lund UniversityWe experimentally study negative fluctuations of stochastic entropy production in an electronic double dot operating in nonequilibrium steady-state conditions. We record millions of random electron tunneling events at different bias points, thus collecting extensive statistics. We show that for all bias voltages, the experimental average values of the minima of stochastic entropy production lie above -k(B), where k(B) is the Boltzmann constant, in agreement with recent theoretical predictions for nonequilibrium steady states. Furthermore, we also demonstrate that the experimental cumulative distribution of the entropy production minima is bounded, at all times and for all bias voltages, by a universal expression predicted by the theory. We also extend our theory by deriving a general bound for the average value of the maximum heat absorbed by a mesoscopic system from the environment and compare this result with experimental data. Finally, we show by numerical simulations that these results are not necessarily valid under nonstationary conditions.Item Full Counting Statistics of Andreev Tunneling(American Physical Society (APS), 2014) Maisi, Ville F.; Kambly, Dania; Flindt, Christian; Pekola, Jukka P.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceWe employ a single-charge counting technique to measure the full counting statistics of Andreev events in which Cooper pairs are either produced from electrons that are reflected as holes at a superconductor–normal-metal interface or annihilated in the reverse process. The full counting statistics consists of quiet periods with no Andreev processes, interrupted by the tunneling of a single electron that triggers an avalanche of Andreev events giving rise to strongly super-Poissonian distributions.Item Interplay of the Inverse Proximity Effect and Magnetic Field in Out-of-Equilibrium Single-Electron Devices(2017-05-26) Nakamura, Shuji; Pashkin, Yuri A.; Taupin, Mathieu; Maisi, Ville F.; Khaymovich, Ivan M.; Mel'Nikov, Alexander S.; Peltonen, Joonas T.; Pekola, Jukka P.; Okazaki, Yuma; Kashiwaya, Satoshi; Kawabata, Shiro; Vasenko, Andrey S.; Tsai, Jaw-Shen; Kaneko, Nobu Hisa; National Institute of Advanced Industrial Science and Technology; Lancaster University; Vienna University of Technology; Department of Applied Physics; Max Planck Institute for the Physics of Complex Systems; Russian Academy of Sciences; Higher School of Economics; RIKENWe show that a weak external magnetic field affects significantly nonequilibrium quasiparticle (QP) distributions under the conditions of the inverse proximity effect, using the single-electron hybrid turnstile as a generic example. Inverse proximity suppresses the superconducting gap in superconducting leads in the vicinity of turnstile junctions, thus, trapping hot QPs in this region. An external magnetic field creates additional QP traps in the leads in the form of vortices or regions with a reduced superconducting gap resulting in the release of QPs away from the junctions. We present clear experimental evidence of the interplay of the inverse proximity effect and magnetic field revealing itself in the superconducting gap enhancement and significant improvement of the turnstile characteristics. The observed interplay and its theoretical explanation in the context of QP overheating are important for various superconducting and hybrid nanoelectronic devices, which find applications in quantum computation, photon detection, and quantum metrology.Item Local and Nonlocal Two-Electron Tunneling Processes in a Cooper Pair Splitter(American Physical Society, 2022-11-11) Ranni, Antti; Mannila, Elsa T.; Eriksson, Axel; Golubev, Dmitry S.; Pekola, Jukka P.; Maisi, Ville F.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Phenomena and Devices; Lund UniversityWe measure the rates and coupling coefficients for local Andreev, nonlocal Andreev, and elastic cotunneling processes. The nonlocal Andreev process, giving rise to Cooper pair splitting, exhibits the same coupling coefficient as the elastic cotunneling whereas the local Andreev process is more than 2 orders of magnitude stronger than the corresponding nonlocal one. Theory estimates describe the findings and explain the large difference in the nonlocal and local coupling arising from competition between electron diffusion in the superconductor and tunnel junction transparency.Item Parallel pumping of electrons(IOP Publishing, 2009) Maisi, Ville F.; Pashkin, Yuri A.; Kafanov, Sergey; Tsai, Jaw-Shen; Pekola, Jukka P.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceWe present the simultaneous operation of ten single-electron turnstiles leading to one order of magnitude increase in current level up to 100 pA. Our analysis of device uniformity and background charge stability implies that the parallelization can be made without compromising the strict requirements of accuracy and current level set by quantum metrology. In addition, we discuss how offset charge instability limits the integration scale of single-electron turnstiles.Item Real-time observation of Cooper pair splitting showing strong non-local correlations(Nature Publishing Group, 2021-11-04) Ranni, Antti; Brange, Fredrik; Mannila, Elsa T.; Flindt, Christian; Maisi, Ville F.; Department of Applied Physics; Quantum Transport; Centre of Excellence in Quantum Technology, QTF; Lund UniversityControlled generation and detection of quantum entanglement between spatially separated particles constitute an essential prerequisite both for testing the foundations of quantum mechanics and for realizing future quantum technologies. Splitting of Cooper pairs from a superconductor provides entangled electrons at separate locations. However, experimentally accessing the individual split Cooper pairs constitutes a major unresolved issue as they mix together with electrons from competing processes. Here, we overcome this challenge with the first real-time observation of the splitting of individual Cooper pairs, enabling direct access to the time-resolved statistics of Cooper pair splitting. We determine the correlation statistics arising from two-electron processes and find a pronounced peak that is two orders of magnitude larger than the background. Our experiment thereby allows to unambiguously pinpoint and select split Cooper pairs with 99% fidelity. These results open up an avenue for performing experiments that tap into the spin-entanglement of split Cooper pairs.Item Single-electron current sources(2013-10-02) Pekola, Jukka P.; Saira, Olli Pentti; Maisi, Ville F.; Kemppinen, Antti; Möttönen, Mikko; Pashkin, Yuri A.; Averin, Dmitri V.; Department of Applied PhysicsThe control of electrons at the level of the elementary charge e was demonstrated experimentally already in the 1980s. Ever since, the production of an electrical current ef, or its integer multiple, at a drive frequency f has been a focus of research for metrological purposes. This review discusses the generic physical phenomena and technical constraints that influence single-electron charge transport and presents a broad variety of proposed realizations. Some of them have already proven experimentally to nearly fulfill the demanding needs, in terms of transfer errors and transfer rate, of quantum metrology of electrical quantities, whereas some others are currently "just" wild ideas, still often potentially competitive if technical constraints can be lifted. The important issues of readout of single-electron events and potential error correction schemes based on them are also discussed. Finally, an account is given of the status of single-electron current sources in the bigger framework of electric quantum standards and of the future international SI system of units, and applications and uses of single-electron devices outside the metrological context are briefly discussed.Item Single-electron current sources: Toward a refined definition of the ampere(American Physical Society (APS), 2013) Pekola, Jukka P.; Saira, Olli-Pentti; Maisi, Ville F.; Kemppinen, Antti; Möttönen, Mikko; Pashkin, Yuri A.; Averin, Dmitri V.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceThe control of electrons at the level of the elementary charge e was demonstrated experimentally already in the 1980s. Ever since, the production of an electrical current ef, or its integer multiple, at a drive frequency f has been a focus of research for metrological purposes. This review discusses the generic physical phenomena and technical constraints that influence single-electron charge transport and presents a broad variety of proposed realizations. Some of them have already proven experimentally to nearly fulfill the demanding needs, in terms of transfer errors and transfer rate, of quantum metrology of electrical quantities, whereas some others are currently “just” wild ideas, still often potentially competitive if technical constraints can be lifted. The important issues of readout of single-electron events and potential error correction schemes based on them are also discussed. Finally, an account is given of the status of single-electron current sources in the bigger framework of electric quantum standards and of the future international SI system of units, and applications and uses of single-electron devices outside the metrological context are briefly discussed.Item Tunneling and relaxation of single quasiparticles in a normal-superconductor-normal single-electron transistor(American Physical Society (APS), 2014) Heimes, Andreas; Maisi, Ville F.; Golubev, Dmitri S.; Marthaler, Michael; Schön, Gerd; Pekola, Jukka P.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceWe investigate the properties of a hybrid single-electron transistor, involving a small superconducting island sandwiched between normal metal leads, which is driven by dc plus ac voltages. In order to describe its properties we derive from the microscopic theory a set of coupled equations. They consist of a master equation for the probability to find excess charges on the island, with rates depending on the distribution of nonequilibrium quasiparticles. Their dynamics follows from a kinetic equation which accounts for the excitation by single-electron tunneling as well as the relaxation and eventual recombination due to the interaction with phonons. Our low-temperature results compare well with recent experimental findings obtained for ac-driven hybrid single-electron turnstiles.