### Browsing by Author "Silveri, Matti"

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Item 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; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; Helsinki School of Economics; RWTH Aachen University; VTT Technical Research Centre of FinlandThe 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.Item Charge dynamics in quantum-circuit refrigeration: Thermalization and microwave gain(AIP Publishing, 2021-12-01) Hsu, Hao; Silveri, Matti; Sevriuk, Vasilii; Möttönen, Mikko; Catelani, Gianluigi; Forschungszentrum Jülich; University of Oulu; Centre of Excellence in Quantum Technology, QTF; Quantum Computing and Devices; Department of Applied PhysicsPrevious studies of photon-assisted tunneling through normal-metal–insulator–superconductor junctions have exhibited potential for providing a convenient tool to control the dissipation of quantum-electric circuits in situ. However, the current literature on such a quantum-circuit refrigerator (QCR) does not present a detailed description for the charge dynamics of the tunneling processes or the phase coherence of the open quantum system. Here, we derive a master equation describing both quantum-electric and charge degrees of freedom, and discover that typical experimental parameters of low temperature and yet lower charging energy yield a separation of time scales for the charge and quantum dynamics. Consequently, the minor effect of the different charge states can be taken into account by averaging over the charge distribution. We also consider applying an ac voltage to the tunnel junction, which enables control of the decay rate of a superconducting qubit over four orders of magnitude by changing the driveamplitude; we find an order-of-magnitude drop in the qubit excitation in 40 ns and a residual reset infidelity below 10−4. Furthermore, for the normal island, we consider the case of charging energy and single-particle level spacing large compared to the superconducting gap, i.e., a quantum dot. Although the decay rates arising from such a dot QCR appear low for use in qubit reset, the device can provide effective negative damping (gain) to the coupled microwave resonator. The Fano factor of such a millikelvin microwave source may be smaller than unity, with the latter value being reached close to the maximum attainable power.Item Exceptional points in tunable superconducting resonators(American Physical Society, 2019-10-07) Partanen, Matti; Goetz, Jan; Tan, Kuan Yen; Kohvakka, Kassius; Sevriuk, Vasilii; Lake, Russell E.; Kokkoniemi, Roope; Ikonen, Joni; Hazra, Dibyendu; Makinen, Akseli; Hyyppa, Eric; Gronberg, Leif; Vesterinen, Visa; Silveri, Matti; Mottonen, Mikko; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Computing and Devices; Department of Applied Physics; VTT Technical Research Centre of FinlandSuperconducting quantum circuits are potential candidates to realize a large-scale quantum computer. The envisioned large density of integrated components, however, requires a proper thermal management and control of dissipation. To this end, it is advantageous to utilize tunable dissipation channels and to exploit the optimized heat flow at exceptional points (EPs). Here, we experimentally realize an EP in a superconducting microwave circuit consisting of two resonators. The EP is a singularity point of the effective Hamiltonian, and corresponds to critical damping with the most efficient heat transfer between the resonators without back and forth oscillation of energy. We observe a crossover from underdamped to overdamped coupling across the EP by utilizing photon-assisted tunneling as an in situ tunable dissipative element in one of the resonators. These methods can be used to obtain fast dissipation, for example, for initializing qubits to their ground states. In addition, these results pave the way for thorough investigation of parity-time symmetry and the spontaneous symmetry breaking at the EP in superconducting quantum circuits operating at the level of single energy quanta.Item Non-Hermitian topological quantum states in a reservoir-engineered transmon chain(American Physical Society, 2023-03-15) Brzezicki, Wojciech; Silveri, Matti; Płodzień, Marcin; Massel, Francesco; Hyart, Timo; Department of Applied Physics; Correlated Quantum Materials (CQM); Jagiellonian University; University of Oulu; Institute of Physics of the Polish Academy of Sciences; University of South-Eastern NorwayDissipation in open systems enriches the possible symmetries of the Hamiltonians beyond the Hermitian framework, allowing the possibility of novel non-Hermitian topological phases which exhibit long-living end states that are protected against disorder. So far, non-Hermitian topology has been explored in settings where probing genuine quantum effects has been challenging. We theoretically show that a non-Hermitian topological quantum phase can be realized in a reservoir-engineered transmon chain. The spatial modulation of dissipation is obtained by coupling each transmon to a quantum circuit refrigerator, allowing in situ tuning of dissipation strength in a wide range. By solving the many-body Lindblad master equation using a combination of the density matrix renormalization group and Prosen-Seligman third quantization approaches, we show that the topological end modes and the associated phase transition are visible in simple reflection measurements with experimentally realistic parameters. Finally, we demonstrate that genuine quantum effects are observable in this system via robust and slowly decaying long-range quantum entanglement of the topological end modes, which can be generated passively starting from a locally excited transmon.Item 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; University of FreiburgWe 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.Item Photon-number-dependent effective Lamb shift(American Physical Society, 2021-09) Viitanen, Arto; Silveri, Matti; Jenei, Mate; Sevriuk, Vasilii; Tan, Kuan Y.; Partanen, Matti; Goetz, Jan; Gronberg, Leif; Vadimov, Vasilii; Lahtinen, Valtteri; Mottonen, Mikko; Quantum Computing and Devices; University of Oulu; Centre of Excellence in Quantum Technology, QTF; Department of Applied Physics; VTT Technical Research Centre of FinlandThe Lamb shift, an energy shift arising from the presence of the electromagnetic vacuum, has been observed in various quantum systems and established as part of the energy shift independent of the environmental photon number. However, typical studies are based on simplistic bosonic models which may be challenged in practical quantum devices. We demonstrate a hybrid bosonic-fermionic environment for a linear resonator mode and observe that the photon number in the environment can dramatically increase both the dissipation and the effective Lamb shift of the mode. Our observations are quantitatively described by a first-principles model, which we develop here also to guide device design for future quantum-technological applications. The device demonstrated here can be utilized as a fully rf-operated quantum-circuit refrigerator to quickly reset superconducting qubits.Item Quantum error correction under numerically exact open-quantum-system dynamics(American Physical Society, 2023) Babu, Aravind; Orell, Tuure; Vadimov, Vasilii; Teixeira, Wallace; Möttönen, Mikko; Silveri, Matti; University of Oulu; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTFThe known quantum error-correcting codes are typically built on approximative open-quantum-system models such as Born-Markov master equations. However, it is an open question how such codes perform in actual physical systems that, to some extent, necessarily exhibit phenomena beyond the limits of these models. To this end, we employ numerically exact open-quantum-system dynamics to analyze the performance of a five-qubit error-correction code where each qubit is coupled to its own bath. We first focus on the performance of a single error-correction cycle covering timescales and coupling strengths beyond those of Born-Markov models. We observe distinct power-law behavior of the error-corrected channel infidelity ∝t2a: a≲2 in the ultrashort times t<3/ωc and a≈1/2 in the short-time range 3/ωcItem Recent Developments in Quantum-Circuit Refrigeration(WILEY-VCH VERLAG, 2022-07) Mörstedt, Timm Fabian; Viitanen, Arto; Vadimov, Vasilii; Sevriuk, Vasilii; Partanen, Matti; Hyyppä, Eric; Catelani, Gianluigi; Silveri, Matti; Tan, Kuan Yen; Möttönen, Mikko; Quantum Computing and Devices; Multiscale Statistical and Quantum Physics; IQM; Forschungszentrum Jülich; University of Oulu; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsThe recent progress in direct active cooling of the quantum-electric degrees of freedom in engineered circuits, or quantum-circuit refrigeration is reviewed. In 2017, the discovery of a quantum-circuit refrigerator (QCR) based on photon-assisted tunneling of quasiparticles through normal-metal–insulator–superconductor junctions inspired a series of experimental studies demonstrating the following main properties: i) the direct-current (dc) bias voltage of the junction can change the QCR-induced damping rate of a superconducting microwave resonator by orders of magnitude and give rise to nontrivial Lamb shifts, ii) the damping rate can be controlled in nanosecond time scales, and ii) the dc bias can be replaced by a microwave excitation, the amplitude of which controls the induced damping rate. Theoretically, it is predicted that state-of-the-art superconducting resonators and qubits can be reset with an infidelity lower than 10−4 in tens of nanoseconds using experimentally feasible parameters. A QCR-equipped resonator has also been demonstrated as an incoherent photon source with an output temperature above 1 K yet operating at millikelvin. This source has been used to calibrate cryogenic amplification chains. In the future, the QCR may be experimentally used to quickly reset superconducting qubits, and hence assist in the great challenge of building a practical quantum computer.Item Stark Effect and Generalized Bloch-Siegert Shift in a Strongly Driven Two-Level System(American Physical Society (APS), 2010) Tuorila, Jani; Silveri, Matti; Sillanpää, Mika A.; Thuneberg, Erkki; Makhlin, Yuriy; Hakonen, Pertti; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceA superconducting qubit was driven in an ultrastrong fashion by an oscillatory microwave field, which was created by coupling via the nonlinear Josephson energy. The observed Stark shifts of the “atomic” levels are so pronounced that corrections even beyond the lowest-order Bloch-Siegert shift are needed to properly explain the measurements. The quasienergies of the dressed two-level system were probed by resonant absorption via a cavity, and the results are in agreement with a calculation based on the Floquet approach.Item Stückelberg interference in a superconducting qubit under periodic latching modulation(2015) Silveri, Matti; Kumar, K. S.; Tuorila, J.; Li, J.; Vepsäläinen, A.; Thuneberg, E.V.; Paraoanu, G. S.; Department of Applied Physics; O.V.Lounasmaa-laboratorio; Superconducting Qubits and Circuit QED; Multiscale Statistical and Quantum PhysicsItem 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 FreiburgWe 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.Item Tunable refrigerator for nonlinear quantum electric circuits(American Physical Society, 2020-06-11) Hsu, Hao; Silveri, Matti; Gunyho, Andras; Goetz, Jan; Catelani, Gianluigi; Mottonen, Mikko; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Computing and DevicesThe emerging quantum technological applications call for fast and accurate initialization of the corresponding devices to low-entropy quantum states. To this end, we theoretically study a recently demonstrated quantum-circuit refrigerator in the case of nonlinear quantum electric circuits such as superconducting qubits. The maximum refrigeration rate of transmon and flux qubits is observed to be roughly an order of magnitude higher than that of usual linear resonators, increasing flexibility in the design. We find that for typical experimental parameters, the refrigerator is suitable for resetting different qubit types to fidelities above 99.99% in a few or a few tens of nanoseconds depending on the scenario. Thus the refrigerator appears to be a promising tool for quantum technology and for detailed studies of open quantum systems.