### Browsing by Author "Tan, Kuan Yen"

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Item Broadband tunable phase shifter for microwaves(AMER INST PHYSICS, 2020-06-01) Zhang, Jinli; Li, Tianyi; Kokkoniemi, Roope; Yan, Chengyu; Liu, Wei; Partanen, Matti; Tan, Kuan Yen; He, Ming; Ji, Lu; Grönberg, Leif; Möttönen, Mikko; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Computing and Devices; Nankai University; VTT Technical Research Centre of FinlandWe implement a broadly tunable phase shifter for microwaves based on superconducting quantum interference devices (SQUIDs) and study it both experimentally and theoretically. At different frequencies, a unit transmission coefficient, |S21| = 1, can be theoretically achieved along a curve where the phase shift is controllable by the magnetic flux. The fabricated device consists of three equidistant SQUIDs interrupting a transmission line. We model each SQUID embedded at different positions along the transmission line with two parameters, capacitance and inductance, the values of which we extract from the experiments. In our experiments, the tunability of the phase shift varies from 0.07 × π to 0.14 × π radians along the full-transmission curve with the input frequency ranging from 6.00 GHz to 6.28 GHz. The reported measurements are in good agreement with simulations, which is promising for future design work of phase shifters for different applications.Item Electron counting in a silicon single-electron pump(2015-10-16) Tanttu, Tuomo; Rossi, Alessandro; Tan, Kuan Yen; Huhtinen, Kukka-Emilia; Chan, Kok Wai; Möttönen, Mikko; Dzurak, Andrew S.; Department of Applied Physics; University of New South WalesWe report electron counting experiments in a silicon metal-oxide-semiconductor quantum dot architecture which has been previously demonstrated to generate a quantized current in excess of 80 pA with uncertainty below 30 parts per million. Single-shot detection of electrons pumped into a reservoir dot is performed using a capacitively coupled single-electron transistor. We extract the full probability distribution of the transfer of n electrons per pumping cycle for We find that the probabilities extracted from the counting experiment are in agreement with direct current measurements in a broad range of dc electrochemical potentials of the pump. The electron counting technique is also used to confirm the improving robustness of the pumping mechanism with increasing electrostatic confinement of the quantum dot.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 Long-Distance Transmon Coupler with cz -Gate Fidelity above 99.8 %(American Physical Society, 2023-01) Marxer, Fabian; Vepsäläinen, Antti; Jolin, Shan W.; Tuorila, Jani; Landra, Alessandro; Ockeloen-Korppi, Caspar; Liu, Wei; Ahonen, Olli; Auer, Adrian; Belzane, Lucien; Bergholm, Ville; Chan, Chun Fai; Chan, Kok Wai; Hiltunen, Tuukka; Hotari, Juho; Hyyppä, Eric; Ikonen, Joni; Janzso, David; Koistinen, Miikka; Kotilahti, Janne; Li, Tianyi; Luus, Jyrgen; Papic, Miha; Partanen, Matti; Räbinä, Jukka; Rosti, Jari; Savytskyi, Mykhailo; Seppälä, Marko; Sevriuk, Vasilii; Takala, Eelis; Tarasinski, Brian; Thapa, Manish J.; Tosto, Francesca; Vorobeva, Natalia; Yu, Liuqi; Tan, Kuan Yen; Hassel, Juha; Möttönen, Mikko; Heinsoo, Johannes; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Computing and Devices; IQMTunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate a tunable qubit-qubit coupler based on a floating transmon device, which allows us to place qubits at least 2 mm apart from each other while maintaining over 50-MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter, which is needed for fast high-fidelity readout. In addition, simulations show that the large qubit-qubit distance significantly lowers unwanted non-nearest-neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-Z gate with (99.81±0.02)% fidelity.Item Microwave Admittance of Gold-Palladium Nanowires with Proximity-Induced Superconductivity(2017) Lake, Russell E.; Govenius, Joonas; Kokkoniemi, Roope; Tan, Kuan Yen; Partanen, Matti; Virtanen, Pauli; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and DevicesQuantitative electrical admittance measurements of diffusive superconductor-normal-metal-superconductor (SNS) junctions at gigahertz frequencies and millikelvin temperatures are reported. The gold-palladium-based SNS junctions are arranged into a chain of superconducting quantum interference devices. The chain is coupled strongly to a multimode microwave resonator with a mode spacing of approximately 0.6 GHz. By measuring the resonance frequencies and quality factors of the resonator modes, the dissipative and reactive parts of the admittance of the chain are extracted. The phase and temperature dependence of the admittance near 1 GHz are compared with theory based on the time-dependent Usadel equations. This comparison allows the identification of important discrepancies between theory and experiment that are not resolved by including inelastic scattering or elastic spin-flip scattering in the theory.Item Pauli Blockade in Silicon Quantum Dots with Spin-Orbit Control(American Physical Society, 2021-01-07) Seedhouse, Amanda E.; Tanttu, Tuomo; Leon, Ross C. C.; Zhao, Ruichen; Tan, Kuan Yen; Hensen, Bas; Hudson, Fay E.; Itoh, Kohei M.; Yoneda, Jun; Yang, Chih Hwan; Morello, Andrea; Laucht, Arne; Coppersmith, Susan N.; Saraiva, Andre; Dzurak, Andrew S.; Department of Applied Physics; Quantum Computing and DevicesQuantum computation relies on accurate measurements of qubits not only for reading the output of the calculation, but also to perform error correction. Most proposed scalable silicon architectures utilize Pauli blockade of triplet states for spin-to-charge conversion. In recent experiments there have been instances when instead of conventional triplet blockade readout, Pauli blockade is sustained only between parallel spin configurations, with vertical bar T-0 > relaxing quickly to the singlet state and leaving vertical bar T+> and vertical bar T-> states blockaded-which we call parity readout. Both types of blockade can be used for readout in quantum computing, but it is crucial to maximize the fidelity and understand in which regime the system operates. We devise and perform an experiment in which the crossover between parity and singlet-triplet readout can be identified by investigating the underlying physics of the vertical bar T-0 > relaxation rate. This rate is tunable over 4 orders of magnitude by controlling the Zeeman energy difference between the dots induced by spin-orbit coupling, which in turn depends on the direction of the applied magnetic field. We suggest a theoretical model incorporating charge noise and relaxation effects that explains quantitatively our results. Investigating the model both analytically and numerically, we identify strategies to obtain on demand either singlet-triplet or parity readout consistently across large arrays of dots. We also discuss how parity readout can be used to perform full two-qubit state tomography and its impact on quantum error-detection schemes in large-scale silicon quantum computers.Item 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-spin qubits in isotopically enriched silicon at low magnetic field(NATURE PUBLISHING GROUP, 2019-12-03) Zhao, Ruichen; Tanttu, Tuomo; Tan, Kuan Yen; Hensen, Bas; Chan, Kok Wai; Hwang, Jason; Leon, Ross; Yang, Chi Herng; Gilbert, Will; Hudson, Fay; Itoh, Kohei; Kiselev, Andrey; Ladd, Thaddeus; Morello, Andrea; Laucht, Arne; Dzurak, Andrew; Department of Applied Physics; Quantum Computing and Devices; Centre of Excellence in Quantum Technology, QTF; University of New South Wales; Keio University; HRL LaboratoriesSingle-electron spin qubits employ magnetic fields on the order of 1 Tesla or above to enable quantum state readout via spin-dependent-tunnelling. This requires demanding microwave engineering for coherent spin resonance control, which limits the prospects for large scale multi-qubit systems. Alternatively, singlet-triplet readout enables high-fidelity spin-state measurements in much lower magnetic fields, without the need for reservoirs. Here, we demonstrate low-field operation of metal-oxide-silicon quantum dot qubits by combining coherent single-spin control with high-fidelity, single-shot, Pauli-spin-blockade-based ST readout. We discover that the qubits decohere faster at low magnetic fields with T"Rabi=18.6 μs and T2*=1.4 μs at 150 mT. Their coherence is limited by spin flips of residual 29Si nuclei in the isotopically enriched 28Si host material, which occur more frequently at lower fields. Our finding indicates that new trade-offs will be required to ensure the frequency stabilization of spin qubits, and highlights the importance of isotopic enrichment of device substrates for the realization of a scalable silicon-based quantum processor.Item 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 UniversityWe 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.Item 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 Three-waveform bidirectional pumping of single electrons with a silicon quantum dot(2016-11-08) Tanttu, Tuomo; Rossi, Alessandro; Tan, Kuan Yen; Mäkinen, Akseli; Chan, Kok Wai; Dzurak, Andrew S.; Möttönen, Mikko; Department of Applied Physics; Quantum Computing and Devices; University of Cambridge; University of New South WalesSemiconductor-based quantum dot single-electron pumps are currently the most promising candidates for the direct realization of the emerging quantum standard of the ampere in the International System of Units. Here, we discuss a silicon quantum dot single-electron pump with radio frequency control over the transparencies of entrance and exit barriers as well as the dot potential. We show that our driving protocol leads to robust bidirectional pumping: one can conveniently reverse the direction of the quantized current by changing only the phase shift of one driving waveform with respect to the others. We anticipate that this pumping technique may be used in the future to perform error counting experiments by pumping the electrons into and out of a reservoir island monitored by a charge sensor.Item 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.