### Browsing by Author "Vadimov, Vasilii"

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Item Exceptional-point-assisted entanglement, squeezing, and reset in a chain of three superconducting resonators(American Physical Society, 2023-07) Teixeira, Wallace S.; Vadimov, Vasilii; Mörstedt, Timm; Kundu, Suman; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTFThe interplay between coherent and dissipative dynamics required in various control protocols of quantum technology has motivated studies of open-system degeneracies, referred to as exceptional points (EPs). Here, we introduce a scheme for fast quantum-state synthesis using exceptional-point engineering in a lossy chain of three superconducting resonators. We theoretically find that the rich physics of EPs can be used to identify regions in the parameter space that favor a fast and quasistable transfer of squeezing and entanglement or a fast reset of the system. For weakly interacting resonators with the coupling strength g, the obtained quasistabilization timescales are identified as 1/(22g), and reset infidelities below 10-5 are obtained with a waiting time of roughly 6/g in the case of weakly squeezed resonators. Our results shed light on the role of EPs in multimode Gaussian systems and pave the way for optimized distribution of squeezing and entanglement between different nodes of a photonic network using dissipation as a resource.Item Many-excitation removal of a transmon qubit using a single-junction quantum-circuit refrigerator and a two-tone microwave drive(Nature Portfolio, 2024-06-14) Teixeira, Wallace; Mörstedt, Timm; Viitanen, Arto; Kivijärvi, Heidi; Gunyhó, András; Tiiri, Maaria; Kundu, Suman; Sah, Aashish; Vadimov, Vasilii; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; Department of Applied Physics; Quantum Computing and DevicesAchieving fast and precise initialization of qubits is a critical requirement for the successful operation of quantum computers. The combination of engineered environments with all-microwave techniques has recently emerged as a promising approach for the reset of superconducting quantum devices. In this work, we experimentally demonstrate the utilization of a single-junction quantum-circuit refrigerator (QCR) for an expeditious removal of several excitations from a transmon qubit. The QCR is indirectly coupled to the transmon through a resonator in the dispersive regime, constituting a carefully engineered environmental spectrum for the transmon. Using single-shot readout, we observe excitation stabilization times down to roughly 500 ns, a 20-fold speedup with QCR and a simultaneous two-tone drive addressing the e–f and f0–g1 transitions of the system. Our results are obtained at a 48-mK fridge temperature and without postselection, fully capturing the advantage of the protocol for the short-time dynamics and the drive-induced detrimental asymptotic behavior in the presence of relatively hot other baths of the transmon. We validate our results with a detailed Liouvillian model truncated up to the three-excitation subspace, from which we estimate the performance of the protocol in optimized scenarios, such as cold transmon baths and fine-tuned driving frequencies. These results pave the way for optimized reset of quantum-electric devices using engineered environments and for dissipation-engineered state preparation.Item Multimode physics of the unimon circuit(American Physical Society, 2024-07-01) Tuohino, Sasu; Vadimov, Vasilii; Teixeira, Wallace; Malmelin, Tommi; Silveri, Matti; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; Quantum Dynamics; Quantum Computing and Devices; Quantum Dynamics; University of OuluWe consider a superconducting half-wavelength resonator that is grounded at its both ends and contains a single Josephson junction. Previously this circuit was considered as a unimon qubit in the single-mode approximation where dc-phase-biasing the junction to π leads to increased anharmonicity and 99.9% experimentally observed single-qubit gate fidelity. Inspired by the promising first experimental results, we develop here a theoretical and numerical model for the detailed understanding of the multimode physics of the unimon circuit. To this end, first, we consider the high-frequency modes of the unimon circuit and find that even though these modes are at their ground state, they imply a significant renormalization to the Josephson energy. We introduce an efficient method to fully account for the relevant modes and show that unexcited high-lying modes lead to corrections in the qubit energy and anharmonicity. Interestingly, provided that the junction is offset from the middle of the circuit, we find strong cross-Kerr coupling strengths between a few low-lying modes. This observation paves the way for the utilization of the multimode structure, for example, as several qubits embedded into a single unimon circuit.Item Persistence of correlations in many-body localized spin chains(American Physical Society, 2020-10-29) Vadimov, Vasilii; Ala-Nissila, Tapio; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; Multiscale Statistical and Quantum PhysicsWe study the evolution and persistence of quantum and classical correlations between spatially separated sites in the disordered XXZ spin chain in its ergodic and many-body-localized phases using exact diagonalization and matrix product state simulations. We show that in the many-body-localized phase, quantum entanglement survives in long-time unitary dynamics of the system, even though it is more fragile than the classical correlations. We study the dependence of the residual entanglement and correlations on the system size and on the disorder strength. Finally, we demonstrate the robustness of entanglement and correlations with respect to local dissipation in the bulk of the system.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; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; VTT Technical Research Centre of Finland; University of OuluThe 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; Department of Applied Physics; Quantum Computing and Devices; 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; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; Multiscale Statistical and Quantum Physics; IQM; Forschungszentrum Jülich; University of OuluThe 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 Single-shot readout of a superconducting qubit using a thermal detector(Springer, 2024-04) Gunyho, Andras; Kundu, Suman; Ma, Jian; Liu, Wei; Niemelä, Sakari; Catto, Giacomo; Vadimov, Vasilii; Vesterinen, Visa; Singh, Priyank; Chen, Qiming; Möttönen, Mikko; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Computing and Devices; Quantum Computing and DevicesMeasuring the state of a qubit is a key fundamental operation of a quantum computer. High-fidelity single-shot readout of superconducting qubits can be achieved using parametric amplifiers at millikelvin temperatures. However, scaling parametric amplifiers beyond hundreds of qubits is challenging due to practical size and power limitations. Nanobolometers can, in contrast, offer scalability, sensitivity and speed suitable for qubit readout. Here we show that a bolometer can provide single-shot qubit readout with a readout duration of 13.9 μs and a single-shot fidelity of 0.618. The fidelity is mainly limited by the energy relaxation time of the qubit (28 μs), and a fidelity of 0.927 is found after removing errors arising from this relaxation. In the future, higher-fidelity single-shot readout may be achieved through improvements in chip design and experimental setup, as well as a change in the bolometer absorber material to reduce the readout time to the level of hundreds of nanoseconds and below.Item Theoretical description of the voltage-driven unimon(2022-12-22) Malmelin, Tommi; Vadimov, Vasilii; Perustieteiden korkeakoulu; Möttönen, MikkoItem Unimon qubit(Nature Publishing Group, 2022-11-12) Hyyppä, Eric; Kundu, Suman; Chan, Chun Fai; Gunyhó, András; Hotari, Juho; Janzso, David; Juliusson, Kristinn; Kiuru, Olavi; Kotilahti, Janne; Landra, Alessandro; Liu, Wei; Marxer, Fabian; Mäkinen, Akseli; Orgiazzi, Jean Luc; Palma, Mario; Savytskyi, Mykhailo; Tosto, Francesca; Tuorila, Jani; Vadimov, Vasilii; Li, Tianyi; Ockeloen-Korppi, Caspar; Heinsoo, Johannes; Tan, Kuan Yen; Hassel, Juha; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; IQM; Department of Applied PhysicsSuperconducting qubits seem promising for useful quantum computers, but the currently wide-spread qubit designs and techniques do not yet provide high enough performance. Here, we introduce a superconducting-qubit type, the unimon, which combines the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. In agreement with our quantum models, we measure the qubit frequency, ω01/(2π), and increased anharmonicity α/(2π) at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13 ns single-qubit gates on two qubits with (ω01, α) = (4.49 GHz, 434 MHz) × 2π and (3.55 GHz, 744 MHz) × 2π, respectively. The energy relaxation seems to be dominated by dielectric losses. Thus, improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible useful quantum advantage with noisy systems.Item Validity of Born-Markov master equations for single- And two-qubit systems(American Physical Society, 2021-06-17) Vadimov, Vasilii; Tuorila, Jani; Orell, Tuure; Stockburger, Jürgen; Ala-Nissila, Tapio; Ankerhold, Joachim; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; Multiscale Statistical and Quantum Physics; University of Oulu; Ulm UniversityThe urgent need for reliable simulation tools to match the extreme accuracy needed to control tailored quantum devices highlights the importance of understanding open quantum systems and their modeling. To this end, we compare here the commonly used Redfield and Lindblad master equations against numerically exact results in the case of one and two resonant qubits transversely coupled at a single point to a Drude-cut ohmic bath. All the relevant parameters are varied over a broad range, which allows us to give detailed predictions about the validity and physically meaningful applicability of the weak-coupling approaches. We characterize the accuracy of the approximate approaches by comparing the maximum difference of their system evolution superoperators with numerically exact results. After optimizing the parameters of the approximate models to minimize the difference, we also explore if and to what extent the weak-coupling equations can be applied at least as phenomenological models. Optimization may lead to an accurate reproduction of experimental data, but yet our results are important to estimate the reliability of the extracted parameter values such as the bath temperature. Our findings set general guidelines for the range of validity of the usual Born-Markov master equations and indicate that they fail to accurately describe the physics in a surprisingly broad range of parameters, in particular, at low temperatures. Since quantum-technological devices operate there, their accurate modeling calls for a careful choice of methods.