Browsing by Author "Masuda, Shumpei"

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    Broadband Lamb shift in an engineered quantum system
    (NATURE PUBLISHING GROUP, 2019-06) Silveri, Matti; Masuda, Shumpei; Sevriuk, Vasilii; Tan, Kuan Y.; Jenei, Máté; Hyyppä, Eric; Hassler, Fabian; Partanen, Matti; Goetz, Jan; Lake, Russell E.; Grönberg, Leif; Möttönen, Mikko; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Computing and Devices; Helsinki School of Economics; RWTH Aachen University; VTT Technical Research Centre of Finland
    The shift of the energy levels of a quantum system owing to broadband electromagnetic vacuum fluctuations—the Lamb shift—has been central for the development of quantum electrodynamics and for the understanding of atomic spectra 1–6 . Identifying the origin of small energy shifts is still important for engineered quantum systems, in light of the extreme precision required for applications such as quantum computing 7,8 . However, it is challenging to resolve the Lamb shift in its original broadband case in the absence of a tuneable environment. Consequently, previous observations 1–5 , 9 in non-atomic systems are limited to environments comprising narrowband modes 10–12 . Here, we observe a broadband Lamb shift in high-quality superconducting resonators, a scenario also accessing static shifts inaccessible in Lamb’s experiment 1,2 . We measure a continuous change of several megahertz in the fundamental resonator frequency by externally tuning the coupling strength to the engineered broadband environment, which is based on hybrid normal-metal–insulator–superconductor tunnel junctions 13–15 . Our results may lead to improved control of dissipation in high-quality engineered quantum systems and open new possibilities for studying synthetic open quantum matter 16–18 using this hybrid experimental platform.
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    Fast forward of adiabatic control of tunneling states
    (2017-06-09) Nakamura, Katsuhiro; Khujakulov, Anvar; Avazbaev, Sanat; Masuda, Shumpei; Department of Applied Physics; Quantum Computing and Devices; National University of Uzbekistan named after Mirzo Ulugbek; Humboldt University of Berlin; Tashkent State Pedagogical University
    By developing the preceding work on the fast forward of transient phenomena of quantum tunneling by Khujakulov and Nakamura [Phys. Rev. A 93, 022101 (2016)2469-992610.1103/PhysRevA.93.022101], we propose a scheme of the exact fast forward of adiabatic control of stationary tunneling states with use of the electromagnetic field. The idea allows the acceleration of both the amplitude and phase of wave functions throughout the fast-forward time range. The scheme realizes the fast-forward observation of the transport coefficients under the adiabatically changing barrier with the fixed energy of an incoming particle. As typical examples, we choose systems with (1) Eckart's potential with tunable asymmetry and (2) double δ-function barriers under tunable relative height. We elucidate the driving electric field to guarantee the stationary tunneling state during a rapid change of the barrier and evaluate both the electric-field-induced temporary deviation of transport coefficients from their stationary values and the modulation of the phase of complex scattering coefficients.
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    Observation of microwave absorption and emission from incoherent electron tunneling through a normal-metal-insulator-superconductor junction
    (2018-12-01) Masuda, Shumpei; Tan, Kuan Y.; Partanen, Matti; Lake, Russell E.; Govenius, Joonas; Silveri, Matti; Grabert, Hermann; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; University of Freiburg
    We experimentally study nanoscale normal-metal-insulator-superconductor junctions coupled to a superconducting microwave resonator. We observe that bias-voltage-controllable single-electron tunneling through the junctions gives rise to a direct conversion between the electrostatic energy and that of microwave photons. The measured power spectral density of the microwave radiation emitted by the resonator exceeds at high bias voltages that of an equivalent single-mode radiation source at 2.5 K although the phonon and electron reservoirs are at subkelvin temperatures. Measurements of the generated power quantitatively agree with a theoretical model in a wide range of bias voltages. Thus, we have developed a microwave source which is compatible with low-temperature electronics and offers convenient in-situ electrical control of the incoherent photon emission rate with a predetermined frequency, without relying on intrinsic voltage fluctuations of heated normal-metal components or suffering from unwanted losses in room temperature cables. Importantly, our observation of negative generated power at relatively low bias voltages provides a novel type of verification of the working principles of the recently discovered quantum-circuit refrigerator.
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    Quantum-circuit Refrigerator
    (2017-05-08) Tan, Kuan; Partanen, Matti; Lake, Russell; Govenius, Joonas; Masuda, Shumpei; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices
    Quantum technology promises revolutionizing applications in information processing, communications, sensing and modelling. However, efficient on-demand cooling of the functional quantum degrees of freedom remains challenging in many solid-state implementations, such as superconducting circuits. Here we demonstrate direct cooling of a superconducting resonator mode using voltage-controllable electron tunnelling in a nanoscale refrigerator. This result is revealed by a decreased electron temperature at a resonator-coupled probe resistor, even for an elevated electron temperature at the refrigerator. Our conclusions are verified by control experiments and by a good quantitative agreement between theory and experimental observations at various operation voltages and bath temperatures. In the future, we aim to remove spurious dissipation introduced by our refrigerator and to decrease the operational temperature. Such an ideal quantum-circuit refrigerator has potential applications in the initialization of quantum electric devices. In the superconducting quantum computer, for example, fast and accurate reset of the quantum memory is needed.
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    Spin-selective electron transfer in a quantum dot array
    (2018-01-17) Masuda, Shumpei; Tan, Kuan Yen; Nakahara, Mikio; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; Shanghai University
    We propose a spin-selective coherent electron transfer in a silicon quantum dot array. Oscillating magnetic fields and temporally controlled gate voltages are utilized to separate the electron wave function into different quantum dots depending on the spin state. We introduce a nonadiabatic protocol based on π pulses and an adiabatic protocol which offer fast electron transfer and robustness against the error in the control-field pulse area, respectively. We also study a shortcut-to-adiabaticity protocol which compromises these two protocols. We show that this scheme can be extended to multielectron systems straightforwardly and used for nonlocal manipulations of electrons.
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    Theoretical Study on Spin-Selective Coherent Electron Transfer in a Quantum Dot Array
    (MDPI AG, 2019-12-22) Masuda, Shumpei; Tan, Kuan; Nakahara, Mikio; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; Tokyo Medical and Dental University; Shanghai University
    Recently, we proposed the spin-selective coherent electron transfer in a silicon-quantum-dot array. It requires temporal tuning of two pulses of an oscillating magnetic field and gate voltage control. This paper proposes a simpler method that requires a single pulse of oscillating magnetic field and gate voltage control. We examined the robustness of the control against the error in the pulse amplitude and the effect of the excited states relaxation to the control efficiency. In addition, we propose a novel control method based on a shortcuts-to-adiabaticity protocol, which utilizes two pulses but requires temporal control of the pulse amplitude for only one of them. We compared their efficiencies under the effect of realistic pulse amplitude errors and relaxation.
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    Theory of quantum-circuit refrigeration by photon-assisted electron tunneling
    (2017-09-22) Silveri, Matti; Grabert, Hermann; Masuda, Shumpei; Tan, Kuan Yen; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; University of Freiburg
    We focus on a recently experimentally realized scenario of normal-metal-insulator-superconductor tunnel junctions coupled to a superconducting resonator. We develop a first-principles theory to describe the effect of photon-assisted electron tunneling on the quantum state of the resonator. Our results are in very good quantitative agreement with the previous experiments on refrigeration and heating of the resonator using the photon-assisted tunneling, thus providing a stringent verification of the developed theory. Importantly, our results provide simple analytical estimates of the voltage-tunable coupling strength and temperature of the thermal reservoir formed by the photon-assisted tunneling. Consequently, they are used to introduce optimization principles for initialization of quantum devices using such a quantum-circuit refrigerator. Thanks to the first-principles nature of our approach, extension of the theory to the full spectrum of quantum electric devices seems plausible.