arXiv subjects

Tsuyoshi Yamamoto

Publications and source records attributed to Tsuyoshi Yamamoto.

At least 19 recordsLinked to original sources

arXiv2026

Engineered two-photon dissipative confinement of a Kerr-cat qubit using SISIS quantum circuit refrigerator

Kerr-cat qubits realized in periodically driven superconducting nonlinear resonators are a promising platform for quantum information processing with biased noise. Pure dephasing in such systems induces leakage out of the qubit subspace, motivating the use of quantum circuit refrigeration (QCR) to remove excess excitations. While conventional superconductor--insulator--normal-metal--insulator--superconductor (SINIS)-based QCRs can suppress leakage via single-photon absorption, they also enhance QCR-induced phase-flip errors. Here we investigate a QCR based on a superconductor--insulator--superconductor--insulator--superconductor (SISIS) junction coupled to a Kerr parametric oscillator (KPO). We show that a SISIS-based QCR can operate in a regime where single-photon processes are suppressed while two-photon absorption dominates. As a result, the proposed SISIS-based QCR achieves strong suppression of dephasing-induced leakage while substantially reducing the increase in phase-flip errors associated with QCR operation. These results demonstrate that the proposed SISIS-based QCR provides an effective approach for mitigating leakage while limiting QCR-induced phase-flip errors in Kerr-cat qubits.

quant-ph↗

arXiv2026

Revisiting Thermal Scalability for Large-Scale Superconducting Quantum Systems

The readout amplification chain imposes a critical thermal scalability bottleneck in large-scale superconducting quantum systems. This happens through three mechanisms: amplifier dissipation, passive conduction through bias wiring and Joule heating within that same wiring. These terms are absent or only partially represented in several prior system-level thermal-scalability models, leading to bottleneck misidentification and scalability overestimation. In this work, we improve upon previous system-level heat estimation models by fully accounting for the major heat sources in modern cryogenic quantum systems including the active dissipation, passive conduction, and Joule heating in the readout amplifier module. Our analysis demonstrates that amplifier-associated heat emerges as the dominant thermal bottleneck that fundamentally alters the thermal landscape of modern large-scale cryogenic systems. We explore various technology options and their tradeoffs to identify configurations that reduce this critical heat load and improve scalability. Finally, we evaluate forward-looking system configurations, including larger refrigeration platforms and optical approaches, and analyze forward-looking pathways toward single-fridge 10k-qubit cryogenic systems.

quant-ph↗

arXiv2026

Heat rectification through a quantum two-level system

We study heat rectification through a quantum two-level system asymmetrically coupled to two thermal baths, as described by the Ohmic spin-boson model. We evaluate the steady-state heat current using a tensor-network approach, which enables us to access the strongly correlated regime, and benchmark the results against analytical formulas in several limiting regimes, including the weak-coupling and incoherent-tunneling regimes. We identify a scaling regime where the studied system flows from an ultraviolet regime, at temperatures larger than the Kondo temperature, to an infrared regime, at temperatures lower than the Kondo temperature. By applying perturbation theory near the infrared fixed point, we find that the rectification ratio follows a universal power law. Our numerical results agree well with this analytical prediction. Our results provide a fundamental understanding of how dissipation-induced many-body physics affects heat transport.

cond-mat.mes-hall↗

arXiv2026

Change in bit-flip times of Kerr parametric oscillators caused by their interactions

We experimentally investigate how interactions between Kerr parametric oscillators (KPOs) degrade their bit-flip times, where a bit flip is defined as a transition between the two degenerate ground states of a KPO. Interactions between KPOs cause quantum states of KPOs to leak outside the computational subspace, leading to bit flips. Bit flips degrade fidelity and pose a significant problem for KPO-based quantum information processing. We performed an experiment in which a weak microwave signal is injected into one KPO to emulate photon injection from another KPO, and find that the bit-flip time decreases by an order of magnitude due to induced excitations, depending on the frequency and power of the injected signal. Methods to mitigate the decrease in bit-flip times caused by interactions between KPOs are discussed, including adjusting the pump frequencies, coherent-state amplitudes, and couplings between KPOs. These findings provide valuable insights for scaling up KPO-based quantum computers.

quant-ph↗

arXiv2026

Fluxon Time-Delay Readout of a Superconducting Qubit Protected by a Spectral Gap in a Josephson Transmission Line

We theoretically investigate a readout scheme of the quantum state of a superconducting qubit based on time delay of a single flux quantum (SFQ), also known as a fluxon, propagating in a Josephson transmission line (JTL). We concretely study the time-delay readout based on capacitive coupling between a transmon qubit and a JTL, and we evaluate the time delay depending on the qubit state. We also reveal a feature of the absence of fluxon pinning and exponential suppression of nonadiabatic transitions caused by the propagating fluxon, which is advantageous for the time-delay readout. We extend the analysis to a multi-level transmon as well. Owing to the spectral gap in the JTL, the radiative decay of the qubit mediated by the JTL is exponentially suppressed, and thus the transmission line itself also serves as a filter protecting the qubit. The readout scheme requires neither complicated wiring to low-temperature stages nor bulky microwave components, which are bottlenecks for integration of a large-scale superconducting quantum computer.

quant-ph↗

arXiv2025

Four-body interactions in Kerr parametric oscillator circuits

We theoretically present new unit circuits of Kerr parametric oscillators (KPOs) with four-body interactions, which enable the scalable embedding of all-to-all connected logical Ising spins using the Lechner-Hauke-Zoller (LHZ) scheme. These unit circuits enable four-body interactions using linear couplers, making the circuit fabrication and characterization much simpler than those of conventional unit circuits with nonlinear couplers. Numerical calculations indicate that the magnitudes of the coupling constants can be comparable to those in conventional circuits. On the basis of this theory, we designed a four-KPO circuit and experimentally confirmed the four-body correlation by measuring the pump-phase dependence of the parity of the four-KPO states. We show that the choice of the pump frequencies are important not only to enable the four-body interaction, but to cancel the effects of other unwanted interactions. Using the circuit, we demonstrated the quantum annealing based on the LHZ scheme, where the strength of the interaction between the logical Ising spins is mapped to the local field and controlled by a coherent drive applied to each KPO.

quant-ph↗

arXiv2025

Theoretical Analysis of Photonic Resonances in Spectroscopic Measurements of a Kerr Nonlinear Resonator

The Kerr parametric oscillator (KPO) has recently attracted considerable attention from the perspective of its applications to quantum information processing, and understanding its properties is an important challenge. Spectroscopic measurements serve as an effective means of elucidating detailed information about the system, such as the energy-level structure and the transition matrix elements of the KPO. Conventional spectroscopy requires the drive frequency to match an energy spacing with a nonzero transition matrix element. In recent years, a phenomenon called photonic resonance (PR) has been theoretically predicted in KPO spectroscopy. Specifically, resonance occurs under the condition that the detuning is set to $n/2$ times the Kerr nonlinearity, where $n$ is a natural number. However, under this condition the transition matrix element vanishes, and thus the mechanism by which photonic resonance (PR) arises has remained unclear. In this work, we aim to elucidate the physical origin of PR observed in KPO spectroscopy. We first performed theoretical calculations and experiments of spectroscopic measurements, confirming that PR can indeed be observed and that the theoretical and experimental results are in qualitative agreement. We then carried out an analytical study under the assumption of an ideal noiseless environment. Our analysis revealed that, although the transition matrix element of the external field expressed in the system's energy eigenbasis is zero, higher-order perturbative effects induce Rabi oscillations between the ground and excited states. Furthermore, numerical simulations in a time domain including the effect of decoherence demonstrated that coherent oscillations decay, leading to the appearance of PR.

quant-ph↗

arXiv2025

Thermal conductance at the superradiant phase transition in the quantum Rabi model

The quantum Rabi model exhibits a superradiant phase transition when the coupling becomes strong, even though it involves only two components: a two-level atom and a single bosonic mode. This phase transition is referred to as a finite-component quantum phase transition, in contrast to conventional phase transitions in many-body systems. In this Letter, we investigate heat transport across an atom embedded in bosonic modes, modeled by the quantum Rabi model, between two thermal baths. We found a manifestation of the superradiant phase transition in the thermal conductance, which represents the linear response to a temperature bias. Our Letter can be helpful for the development of quantum heat devices utilizing controllable finite-component quantum phase transitions.

cond-mat.mes-hall↗

arXiv2025

Four-body coupler for superconducting qubits based on Josephson parametric oscillators

We theoretically propose a circuit of the four-body coupler for superconducting qubits based on Josephson parametric oscillators (JPOs). Our coupler for the four-body interaction has a superconducting loop, similar to a capacitively shunted flux qubit, where an external magnetic flux set to half a flux quantum is threaded. This coupler circuit is a specific setup of the circuit called superconducting nonlinear asymmetric inductive elements (SNAIL) and also is a generalization of the previously proposed one for the four-body interaction of JPOs. We clarify roles of circuit parameters in the four-body interaction and, in particular, show that the four-body coupling constant in our circuit can be significantly increased by tuning capacitance of the coupler or the area ratio of the Josephson junctions of the coupler.

quant-ph↗

arXiv2025

Energy exchange and fluctuations between a dissipative qubit and a monitor under continuous measurement and feedback

Continuous quantum measurement and feedback induce energy exchange between a dissipative qubit and a monitor even in the steady state, as a measurement backaction. Using the Lindblad equation, we identified the maximum and minimum values of the steady-state energy flow as the measurement and feedback states vary, and we demonstrate the qubit cooling induced by these processes. Turning our attention to quantum trajectories under continuous measurement and feedback, we observe that the energy flow fluctuates around the steady-state values. We reveal that the fluctuations are strongly influenced by the measurement backaction, distinguishing them from the standard Poisson noise typically observed in electronic circuits. Our results offer potential application in the development of quantum refrigerators controlled by continuous measurement and feedback, and provide deep insight into quantum thermodynamics from the perspective of fluctuation.

quant-ph↗

arXiv2025

Stabilization of Kerr-cat qubits with quantum circuit refrigerator

A periodically-driven superconducting nonlinear resonator can implement a Kerr-cat qubit, which provides a promising route to a quantum computer with a long lifetime. However, the system is vulnerable to pure dephasing, which causes unwanted excitations outside the qubit subspace. Therefore, we require a refrigeration technology which confines the system in the qubit subspace. We theoretically study on-chip refrigeration for Kerr-cat qubits based on photon-assisted electron tunneling at tunneling junctions, called quantum circuit refrigerator (QCR). Rates of QCR-induced deexcitations of the system can be changed by more than four orders of magnitude by tuning a bias voltage across the tunneling junctions. Unwanted QCR-induced bit flips are greatly suppressed due to quantum interference in the tunneling process, and thus the long lifetime is preserved. The QCR can serve as a tunable dissipation source which stabilizes Kerr-cat qubits.

quant-ph↗

arXiv2025

Theoretical study of the Spectroscopic measurements of Kerr non-linear resonators with four-body interaction

Quantum annealing provides a promising way to solve combinational optimization problems where the solutions correspond to the ground state of the Ising Hamiltonian. We can implement quantum annealing using the Kerr non-linear resonators, with bifurcation phenomena emerging when subjected to a parametric drive. These bifurcated states can function as bases of qubits. Moreover, integrating four-body interactions between physical qubits enables the establishment of effective all-to-all long-range interactions between logical qubits, which is essential for practical quantum annealing. While theoretical proposals exist for creating four-body interactions within Kerr non-linear resonators, there has not been experimental verification through their spectroscopic signatures. In this paper, we theoretically investigate the spectroscopic measurements of Kerr non-linear resonators featuring four-body interaction. We identify six distinct frequencies exhibiting population changes by employing resonant driving on one resonator and weak driving on another. Analytical and numerical calculations validate these findings. Our study demonstrates the potential of spectroscopy in characterizing systems with four-body interactions, offering insights for realizing quantum annealing with Kerr parametric oscillators.

quant-ph↗

arXiv2025

Resonance fluorescence spectra of a driven Kerr nonlinear resonator

Resonance fluorescence spectra of a driven Kerr nonlinear resonator is investigated both theoretically and experimentally. When the Kerr nonlinear resonator is driven strongly such that the induced Rabi frequency is comparable to or larger than the Kerr nonlinearity, the system cannot be approximated as a two-level system. We theoretically derive characteristic features in the fluorescence spectra such as the decrease of the center-peak intensity and the asymmetric sideband peaks in the presence of finite dephasing. Those features are consistently explained by the population of the initial dressed state and its transition matrix element to the final dressed state of the transition corresponding to each peak. Finally, we experimentally measure the resonance fluorescence spectra of a driven superconducting Kerr nonlinear resonator and find a quantitative agreement with our theory.

quant-ph↗

arXiv2024

Single-qubit rotations on a binomial code without ancillary qubits

Great attention has been paid to binomial codes utilizing bosonic systems as logical qubits with error correction capabilities. However, implementing single-qubit rotation operations on binomial codes has proven challenging, requiring an ancillary qubit in previous approaches. Here, we propose a method for performing logical qubit rotation on binomial codes without requiring an ancillary qubit. Specifically, we explain how to implement $X$-axis rotations by simultaneously applying two-frequency parametric drives to resonators with nonlinearity. Furthermore, we show that $Z$-axis rotations could be realized with the detuning. Due to the reduction of the need for the ancillary qubit for the logical qubit rotation, our proposed approach is advantageous for quantum computation in the NISQ era, where the number of qubits is limited.

quant-ph↗

arXiv2024

Microwave-multiplexed qubit controller using adiabatic superconductor logic

Cryogenic qubit controllers (QCs) are the key to build large-scale superconducting quantum processors. However, developing scalable QCs is challenging because the cooling power of a dilution refrigerator is too small (~10 $μ$W at ~10 mK) to operate conventional logic families, such as complementary metal-oxide-semiconductor logic and superconducting single-flux-quantum logic, near qubits. Here we report on a scalable QC using an ultra-low-power superconductor logic family, namely adiabatic quantum-flux-parametron (AQFP) logic. The AQFP-based QC, referred to as the AQFP-multiplexed QC (AQFP-mux QC), produces multi-tone microwave signals for qubit control with an extremely small power dissipation of 81.8 pW per qubit. Furthermore, the AQFP-mux QC adopts microwave multiplexing to reduce the number of coaxial cables for operating the entire system. As a proof of concept, we demonstrate an AQFP-mux QC chip that produces microwave signals at two output ports through microwave multiplexing and demultiplexing. Experimental results show an output power of approximately $-$80 dBm and on/off ratio of ~40 dB at each output port. Basic mixing operation is also demonstrated by observing sideband signals.

physics.app-ph↗

arXiv2024

Thermal transport across a Josephson junction in a dissipative environment

At zero temperature, a Josephson junction coupled to an ohmic environment displays a quantum phase transition between superconducting and insulating phases, depending whether the resistance of the environment is below or above the resistance quantum. At finite temperature, this so-called Schmid transition turns into a crossover. We determine the conditions under which the temperature dependence of the thermal conductance, which characterizes heat flow from a hot to cold resistor across the Josephson junction, displays universal scaling characteristic of the Schmid transition. We also discuss conditions for heat rectification to happen in the circuit. Our work can serve as a guide for identifying signatures of the Schmid transition in heat transport experiments.

cond-mat.supr-con↗

arXiv2024

Heat flow from a measurement apparatus monitoring a dissipative qubit

We investigate the heat flow of a qubit coupled to heat baths under continuous quantum measurement. In the steady-state limit, we show that heat always flows from the measurement apparatus into the qubit regardless of the measured qubit state and derive lower and upper bounds for the heat current between the qubit and the measurement apparatus. Furthermore, we study the transient dynamics of the heat current and the excess heat during the transient regime.

quant-ph↗

arXiv2023

Spectroscopy of flux-driven Kerr parametric oscillators by reflection coefficient measurement

We report the spectroscopic characterization of a Kerr parametric oscillator (KPO) based on the measurement of its reflection coefficient under a two-photon drive induced by flux modulation. The measured reflection spectra show good agreement with numerical simulations in term of their dependence on the two-photon drive amplitude. The spectra can be interpreted as changes in system's eigenenergies, transition matrix elements, and the population of the eigenstates, although the linewidth of the resonance structure is not fully explained. We also show that the drive-amplitude dependence of the spectra can be explained analytically by using the concepts of Rabi splitting and the Stark shift. By comparing the experimentally obtained spectra with theory, we show that the two-photon drive amplitude at the device can be precisely determined, which is important for the application of KPOs in quantum information processing.

quant-ph↗