### Browsing by Department "Quantum Phenomena and Devices"

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Item Active Quasiparticle Suppression in a Non-Equilibrium Superconductor(AMERICAN CHEMICAL SOCIETY, 2020-07-08) Marin Suarez, Marco; Peltonen, Joonas; Pekola, J.P.; Quantum Phenomena and Devices; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsQuasiparticle (qp) poisoning is a major issue that impairs the operation of various superconducting devices. Even though these devices are often operated at temperatures well below the critical point where the number density of excitations is expected to be exponentially suppressed, their bare operation and stray microwave radiation excite the non-equilibrium qp’s. Here we use voltage-biased superconducting junctions to demonstrate and quantify qp extraction in the turnstile operation of a superconductor–insulator–normal metal–insulator–superconductor single-electron transistor. In this operation regime, excitations are injected into the superconducting leads at a rate proportional to the driving frequency. We reach a reduction of density by an order of magnitude even for the highest injection rate of 2.4 × 108 qp’s per second when extraction is turned on.Item Control of the surface plasmon dispersion and Purcell effect at the metamaterial-dielectric interface(Nature Publishing Group, 2020-11-30) Ivanov, Konstantin A.; Morozov, Konstantin M.; Pozina, Galia; Gubaydullin, Azat R.; Girshova, Elizaveta I.; Kaliteevski, Mikhail A.; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Linköping University; Quantum Phenomena and Devices; Department of Applied PhysicsThe use of metamaterial as a way to mitigate the negative effects of absorption in metals on the Purcell effect in metal-dielectric structures is investigated. A layered metal-dielectric structure is considered as an anisotropic medium in the long-wavelength limit. The dispersion of the surface plasmon appearing at the boundary between such a structure and a different dielectric material, as well as the position of the peak in the local density of states are studied for various combinations of materials and filling factors of the periodic structure. The calculated frequency dependence of the Purcell factor demonstrates an increase in peak value compared to the conventional plasmonic structure. The results obtained using effective media approach are compared to the results of numerical modelling.Item Correlated versus uncorrelated noise acting on a quantum refrigerator(2017-09-05) Karimi, Bayan; Pekola, Jukka P.; Quantum Phenomena and Devices; Department of Applied PhysicsTwo qubits form a quantum four-level system. The golden-rule based transition rates between these states are determined by the coupling of the qubits to noise sources. We demonstrate that depending on whether the noise acting on the two qubits is correlated or not, these transitions are governed by different selection rules. In particular, we find that for fully correlated or anticorrelated noise, there is a protected state, and the dynamics of the system depends then on its initialization. For nearly (anti)correlated noise, there is a long time scale determining the temporal evolution of the qubits. We apply our results to a quantum Otto refrigerator based on two qubits coupled to hot and cold baths. The steady-state power does not scale with the number (=2 here) of the qubits when there is a strong correlation of noise acting on them; under driven conditions the highest cooling power of the refrigerator is achieved for fully uncorrelated baths.Item Determining the parameters of a random telegraph signal by digital low pass filtering(2018-06-11) Singh, Shilpi; Mannila, Elsa T.; Golubev, Dmitry S.; Peltonen, Joonas T.; Pekola, Jukka P.; Centre of Excellence in Quantum Technology, QTF; Quantum Phenomena and Devices; Department of Applied PhysicsWe propose a method to determine the switching rates of a random telegraph signal. We apply digital low pass filtering with varying bandwidths to the raw signal, evaluate the cumulants of the resulting distributions, and compare them with the analytical prediction. This technique is useful in the case of a slow detector with response time comparable to the time interval between the switching events. We demonstrate the efficiency of this method by analyzing random telegraph signals generated by individual charge tunneling events in metallic single-electron transistors.Item Electron-phonon coupling of epigraphene at millikelvin temperatures measured by quantum transport thermometry(AMER INST PHYSICS, 2021-03-08) Karimi, Bayan; He, Hans; Chang, Yu Cheng; Wang, Libin; Pekola, Jukka P.; Yakimova, Rositsa; Shetty, Naveen; Peltonen, Joonas T.; Lara-Avila, Samuel; Kubatkin, Sergey; Centre of Excellence in Quantum Technology, QTF; Chalmers University of Technology; Department of Applied Physics; Linköping University; Quantum Phenomena and DevicesWe investigate the basic charge and heat transport properties of charge neutral epigraphene at sub-kelvin temperatures, demonstrating a nearly logarithmic dependence of electrical conductivity over more than two decades in temperature. Using graphene's sheet conductance as an in situ thermometer, we present a measurement of electron-phonon heat transport at mK temperatures and show that it obeys the T4 dependence characteristic for a clean two-dimensional conductor. Based on our measurement, we predict the noise-equivalent power of ∼ 10 - 22 W / Hz of the epigraphene bolometer at the low end of achievable temperatures.Item Experimental Observation of the Role of Mutual Information in the Nonequilibrium Dynamics of a Maxwell Demon(2014-07-14) Koski, J.V.; Maisi, Ville; Sagawa, T.; Pekola, J.P.; O.V.Lounasmaa-laboratorio; Quantum Phenomena and DevicesWe validate experimentally a fluctuation relation known as generalized Jarzynski equality governing the work distribution in a feedback-controlled system. The feedback control is performed on a single electron box analogously to the original Szilard engine. In the generalized Jarzynski equality, mutual information is treated on an equal footing with the thermodynamic work. Our measurements provide the first evidence of the role of mutual information in the fluctuation theorem and thermodynamics of irreversible processes.Item Fast and accurate Cooper pair pump(American Physical Society, 2019-12-16) Erdman, Paolo A.; Taddei, Fabio; Peltonen, Joonas T.; Fazio, Rosario; Pekola, Jukka P.; Scuola Normale Superiore di Pisa; Consiglio Nazionale delle Ricerche (CNR); Quantum Phenomena and Devices; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsWe propose a method to perform accurate and fast charge pumping in superconducting nanocircuits. Combining topological properties and quantum control techniques based on shortcuts to adiabaticity, we show that it is theoretically possible to achieve perfectly quantized charge pumping at any finite-speed driving. Model-specific errors may still arise due the difficulty of implementing the exact control. We thus assess this and other practical issues in a specific system comprised of three Josephson junctions. Using realistic system parameters, we show that our scheme can improve the pumping accuracy of this device by various orders of magnitude. Possible metrological perspectives are discussed.Item Fast Electron Thermometry for Ultrasensitive Calorimetric Detection(2015) Gasparinetti, S.; Viisanen, Klaara; Saira, Olli-Pentti; Faivre, T.; Arzeo, M.; Meschke, M.; Pekola, J.P.; Quantum Phenomena and Devices; Department of Applied PhysicsWe demonstrate radio-frequency thermometry on a micrometer-sized metallic island below 100 mK. Our device is based on a normal-metal–insulator–superconductor tunnel junction coupled to a resonator with transmission readout. In the first generation of the device, we achieve 90 μK/√Hz noise-equivalent temperature with 10 MHz bandwidth. We measure the thermal relaxation time of the electron gas in the island, which we find to be of the order of 100 μs. Such a calorimetric detector, upon optimization, can be seamlessly integrated into superconducting circuits, with immediate applications in quantum-thermodynamics experiments down to single quanta of energy.Item Full Counting Statistics of Andreev Tunneling(2014-01-23) Maisi, Ville; Kambly, Dania; Flindt, Christian; Pekola, Jukka P.; Quantum Phenomena and Devices; University of Geneva; O.V.Lounasmaa-laboratorioWe 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 A Josephson radiation comb generator(2015) Solinas, P.; Gasparinetti, S.; Golubev, D.; Giazotto, F.; Quantum Phenomena and Devices; Department of Applied PhysicsWe propose the implementation of a Josephson Radiation Comb Generator (JRCG) based on a dc superconducting quantum interference device (SQUID) driven by an external magnetic field. When the magnetic flux crosses a diffraction node of the critical current interference pattern, the superconducting phase undergoes a jump of π and a voltage pulse is generated at the extremes of the SQUID. Under periodic drive this allows one to generate a sequence of sharp, evenly spaced voltage pulses. In the frequency domain, this corresponds to a comb-like structure similar to the one exploited in optics and metrology. With this device it is possible to generate up to several hundreds of harmonics of the driving frequency. For example, a chain of 50 identical high-critical-temperature SQUIDs driven at 1 GHz can deliver up to a 0.5 nW at 200 GHz. The availability of a fully solid-state radiation comb generator such as the JRCG, easily integrable on chip, may pave the way to a number of technological applications, from metrologyto sub-millimeter wave generation.Item Joule heating effects in high-Transparency Josephson junctions(American Physical Society, 2021-10-21) Tomi, Matti; Samatov, Mikhail R.; Vasenko, Andrey S.; Laitinen, Antti; Hakonen, Pertti; Golubev, Dmitry S.; Centre of Excellence in Quantum Technology, QTF; Higher School of Economics; Quantum Phenomena and Devices; Department of Applied PhysicsWe study, both theoretically and experimentally, the features on the current-voltage characteristic of a highly transparent Josephson junction caused by transition of the superconducting leads to the normal state. These features appear due to the suppression of the Andreev excess current. We show that by tracing the dependence of the voltage, at which the transition occurs, on the bath temperature and by analyzing the suppression of the excess current by the bias voltage one can recover the temperature dependence of the heat flow out of the junction. We verify theory predictions by fabricating two highly transparent superconductor-graphene-superconductor (SGS) Josephson junctions with suspended and nonsuspended graphene as a nonsuperconducting section between Al leads. Applying the above mentioned technique we show that the cooling power of the suspended junction depends on the bath temperature as â Tbath3.1 close to the superconducting critical temperature.Item Learning to Measure: Adaptive Informationally Complete Generalized Measurements for Quantum Algorithms(American Physical Society, 2021-11-29) Garcia-Perez, Guillermo; Rossi, Matteo A. C.; Sokolov, Boris; Tacchino, Francesco; Barkoutsos, Panagiotis Kl; Mazzola, Guglielmo; Tavernelli, Ivano; Maniscalco, Sabrina; Quantum Phenomena and Devices; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsMany prominent quantum computing algorithms with applications in fields such as chemistry and materials science require a large number of measurements, which represents an important roadblock for future real-world use cases. We introduce a novel approach to tackle this problem through an adaptive measurement scheme. We present an algorithm that optimizes informationally complete positive operator-valued measurements (POVMs) on the fly in order to minimize the statistical fluctuations in the estimation of relevant cost functions. We show its advantage by improving the efficiency of the variational quantum eigensolver in calculating ground-state energies of molecular Hamiltonians with extensive numerical simulations. Our results indicate that the proposed method is competitive with state-of-the-art measurement-reduction approaches in terms of efficiency. In addition, the informational completeness of the approach offers a crucial advantage, as the measurement data can be reused to infer other quantities of interest. We demonstrate the feasibility of this prospect by reusing ground-state energy-estimation data to perform high-fidelity reduced state tomography.Item Optimized proximity thermometer for ultrasensitive detection : Role of an ohmic electromagnetic environment(American Physical Society, 2023-07-01) Nikolić, Danilo; Karimi, Bayan; Rengel, Diego Subero; Pekola, Jukka P.; Belzig, Wolfgang; Universität Konstanz; Centre of Excellence in Quantum Technology, QTF; Quantum Phenomena and Devices; Department of Applied PhysicsWe propose a mesoscopic thermometer for ultrasensitive detection based on the proximity effect in superconductor-normal metal (SN) heterostructures. The device is based on the zero-bias anomaly due to the inelastic Cooper-pair tunneling in an SNIS junction (I stands for an insulator) coupled to an ohmic electromagnetic (EM) environment. The theoretical model is done in the framework of the quasiclassical Usadel Green's formalism and the dynamical Coulomb blockade. The usage of an ohmic EM environment makes the thermometer highly sensitive down to very low temperatures, Formula Presented. Moreover, defined in this way, the thermometer is stable against small but nonvanishing voltage amplitudes typically used for measuring the zero-bias differential conductance in experiments. Finally, we propose a simplified view, based on an analytic treatment, which is in very good agreement with numerical results and can serve as a tool for the development, calibration, and optimization of such devices in future experimentsin quantum calorimetry.Item Photonic heat transport across a Josephson junction(American Physical Society, 2019-09-05) Thomas, George; Pekola, Jukka P.; Golubel, Dmitry S.; Quantum Phenomena and Devices; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsWe present a detailed study of photonic heat transport across a Josephson junction coupled to two arbitrary linear circuits having different temperatures. First, we consider the linear approximation, in which a nonlinear Josephson potential is replaced by a quadratic one and the junction acts as an inductor. Afterwards, we discuss the effects of junction anharmonicity. We separately consider the weak-coupling limit, in which the Bloch band structure of the junction energy spectrum plays an important role, and the opposite strong-coupling regime. We apply our general results to two specific models: a Josephson junction coupled to two Ohmic resistors and two resonators. We derive simple analytical approximations for the photonic heat flux in many limiting cases. We demonstrate that electric circuits with embedded Josephson junctions provide a useful platform for quantum thermodynamics experiments.Item Photonic heat transport from weak to strong coupling(American Physical Society, 2023-03-01) Tam, Minh; Thomas, George; Golubev, Dmitry S.; Aalto University; Quantum Phenomena and Devices; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsSuperconducting circuits provide a favorable platform for quantum thermodynamic experiments. An important component for such experiments is a heat valve, i.e., a device which allows one to control the heat power flowing through the system. Here we theoretically study the heat valve based on a superconducting quantum interference device (SQUID) coupled to two heat baths via two resonators. The heat current in such a system can be tuned by magnetic flux. We investigate how the heat current modulation depends on the coupling strength g between the SQUID and the resonators. In the weak coupling regime the heat current modulation grows as g2, but, surprisingly, at the intermediate coupling it can be strongly suppressed. This effect is linked to the resonant nature of the heat transport at weak coupling, where the heat current dependence on the magnetic flux is a periodic set of narrow peaks. At the intermediate coupling the peaks become broader and overlap, thus reducing the heat modulation. At very strong coupling the heat modulation grows again and finally saturates at a constant value.Item Radio-Frequency Coulomb-Blockade Thermometry(American Physical Society, 2022-01-26) Blanchet, Florian; Chang, Yu Cheng; Karimi, Bayan; Peltonen, Joonas T.; Pekola, Jukka P.; Quantum Computing and Devices; Quantum Phenomena and Devices; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsWe present a scheme and demonstrate measurements of a Coulomb-blockade thermometer (CBT) in a microwave-transmission setup. The sensor is embedded in an LCR resonator, where R is determined by the conductance of the junction array of the CBT. A transmission measurement yields a signal that is directly proportional to the conductance of the CBT, thus enabling the calibration-free operation of the thermometer. This is verified by measuring an identical sensor simultaneously in the usual dc setup. The important advantage of the rf measurement is its speed: the whole bias dependence of the CBT conductance can now be measured in a time of about 100 ms, which is 1000 times faster than in a standard dc measurement. The achieved noise-equivalent temperature of this rf primary measurement is about 1mK/Hz at the bath temperature T=200mK.Item Suppression of Back-Tunneling Events in Hybrid Single-Electron Turnstiles by Source-Drain Bias Modulation(American Physical Society, 2023-04) Marín-Suárez, Marco; Pashkin, Yuri A.; Peltonen, Joonas T.; Pekola, Jukka P.; Quantum Phenomena and Devices; Lancaster University; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsThe accuracy of single-electron currents produced in hybrid turnstiles at high operation frequencies is, among other errors, limited by electrons tunneling in the wrong direction. Increasing the barrier transparency between the island and the leads, together with the source-drain bias, helps to suppress these events in a larger frequency range, although they lead to some additional errors. We experimentally demonstrate a driving scheme that suppresses tunneling in the wrong direction, thus extending the range of frequencies for generating accurate single-electron currents. The main feature of this approach is an additional ac signal applied to the bias with frequency twice that applied to the gate electrode. This allows additional modulation of the island chemical potential. By using this approach under certain parameters, we improve the single-electron current accuracy by one order of magnitude. Finally, we show through experimentally contrasted calculations that our method can improve accuracy even in devices for which the usual gate driving gives errors of the order of 10-3 at high frequencies and can bring them under 5×10-4.Item Ultimate Accuracy of Frequency to Power Conversion by Single-Electron Injection(American Physical Society, 2022-07-15) Pekola, Jukka P.; Marín-Suárez, Marco; Pyhäranta, Tuomas; Karimi, Bayan; Centre of Excellence in Quantum Technology, QTF; Quantum Phenomena and Devices; Department of Applied PhysicsWe analyze theoretically the properties of the recently introduced and experimentally demonstrated converter of frequency to power. The system is composed of a hybrid single-electron box with normal island and superconducting lead, and the detector of the energy flow using a thermometer on a normal metal bolometer. Here, we consider its potential for metrology. The errors in power arise mainly from inaccuracy of injecting electrons at the precise energy equal to the energy gap of the superconductor. We calculate the main systematic error in the form of the excess average energy of the injected electrons and its cumulants, and that due to subgap leakage. We demonstrate by analytic and numerical calculations that the systematic error in detection can, in principle, be made much smaller than the injection errors, which also, with proper choice of system parameters, can be very small, <1%, at low enough temperature. Finally, we propose a simplified configuration for metrological purposes.Item Utilization of the superconducting transition for characterizing low-quality-factor superconducting resonators(AMER INST PHYSICS, 2019-07-08) Chang, Yu Cheng; Karimi, Bayan; Senior, Jorden; Ronzani, Alberto; Peltonen, Joonas T.; Goan, Hsi Sheng; Chen, Chii Dong; Pekola, Jukka P.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Phenomena and Devices; National Taiwan UniversityCharacterizing superconducting microwave resonators with highly dissipative elements is a technical challenge, but a requirement for implementing and understanding the operation of hybrid quantum devices involving dissipative elements, e.g., for thermal engineering and detection. We present experiments on λ/4 superconducting niobium coplanar waveguide resonators, terminating at the antinode by a dissipative copper microstrip via aluminum leads, such that the resonator response is difficult to measure in a typical microwave environment. By measuring the transmission both above and below the superconducting transition of aluminum, we are able to isolate the resonance. We then experimentally verify this method with copper microstrips of increasing thicknesses, from 50 nm to 150 nm, and measure quality factors in the range of 10-67 in a consistent way.