Chronological feed of everything captured from Yasunobu Nakamura.
paper / nakamuralab / Apr 7
Proposes a theoretical scheme for a deterministic photon-photon √SWAP gate using a three-level lambda system in reflection geometry with single photons. The lambda system acts as a passive temporary memory for photonic qubits, requiring no preparation of its initial state or auxiliary control fields during gate operations. This approach supports scalable quantum computation by enabling deterministic entangling gates without active manipulation.
quantum-physicsphoton-photon-gateswap-gatelambda-systemquantum-computationarxiv-paper
“The gate realizes a deterministic photon-photon (SWAP)^{1/2} gate”
paper / nakamuralab / Apr 7
Measurements of decoherence in Josephson-junction flux qubits at varying bias conditions isolate 1/f flux noise as a primary dephasing source, evidenced by Gaussian decay of echo signals. The 1/f flux noise spectral density is (10^{-6} Φ_0)^2/Hz at 1 Hz. At optimal bias, where noise sources are decoupled, coherence is limited by qubit energy relaxation.
flux-qubitsdecoherence1/f-noisejosephson-junctionsquantum-coherencesuperconducting-qubits
“Echo signal in flux qubits exhibits Gaussian decay due to 1/f flux noise dephasing”
paper / nakamuralab / Apr 7
Proposes a design coupling two flux qubits using the quantum inductance of a third high-frequency qubit, enabling a microwave-induced parametric coupling scheme. Qubits operate continuously at their optimal symmetry points for maximal coherence. The coupling exhibits its own optimal point insensitive to low-frequency flux noise, facilitating robust two-qubit gates extensible to multiqubit systems.
flux-qubitstunable-couplingquantum-inductancesuperconducting-qubitsmesoscale-physicssuperconductivityarxiv-paper
“Tunably couples a pair of flux qubits via quantum inductance of a third high-frequency qubit”
paper / nakamuralab / Apr 7
Researchers apply a spin-echo-type pulse sequence using gate-voltage pulses to a charge-based two-level system in a Cooper-pair box, enabling refocused echo signals that mitigate inhomogeneity in ensemble measurements. The observed echo decay time aligns with estimated decoherence times, indicating low-frequency 1/f charge noise as the primary dephasing source. This demonstrates coherent control techniques for superconducting charge qubits despite dominant charge noise.
superconductivitycooper-pair-boxcharge-echospin-echodecoherence1/f-noisemesoscale-physics
“A spin-echo-type technique using gate-voltage pulses was applied to a charge-based artificial two-level system in a small superconducting electrode.”
paper / nakamuralab / Apr 7
Researchers demonstrated coherent quantum-state evolution in a single-Cooper-pair box, a macroscopic superconducting two-level system where charge states differing by 2e are coupled via Cooper-pair tunneling. A short voltage pulse nonadiabatically tunes the energy levels to control superposition dynamics, with the resulting state read out via tunneling current through a probe junction. This achieves coherent operation and measurement of a solid-state qubit candidate.
quantum-computingqubitsuperconductivitycooper-pair-boxjosephson-junctioncoherent-controlmesoscale-physics
“A single-Cooper-pair box forms an artificial two-level quantum system with charge states differing by 2e coupled by Cooper-pair tunneling.”
paper / nakamuralab / Apr 7
Researchers developed a focused-ion-beam (FIB) etching process from multiple directions to fabricate nanoscale Nb/(Al-)Al2O3/Nb tunnel junctions. Applied to a single-electron transistor (SET), the device exhibited superconducting gap energy and transition temperature matching bulk Nb values, confirming junction quality. The SET's single-electron charging energy exceeds 1 K, with characterization spanning 0.04-40 K.
josephson-junctionssingle-electron-transistorniobium-junctionsnanoscale-fabricationsuperconducting-gapmesoscale-physicsmaterials-science
“The fabrication process uses focused-ion-beam etching from different directions on Nb/(Al-)Al2O3/Nb trilayer.”
paper / nakamuralab / Mar 11
Researchers developed focusing surface-acoustic-wave (SAW) resonators on thin-film lithium niobate on sapphire, using films thinner than the SAW wavelength to confine modes to the surface. Contoured electrodes shaped as 2D Gaussian beams achieve near-diffraction-limited focusing, verified by optical imaging. Apodization of interdigitated transducer electrodes suppresses higher-order transverse modes, enabling single-mode operation critical for hybrid quantum systems.
quantum-physicssaw-resonatorslithium-niobatesurface-acoustic-wavestransverse-mode-suppressionoptics
“Focusing SAW resonators on thin-film lithium niobate enable strong interactions in hybrid quantum systems by reducing mode volume and suppressing diffraction losses”
paper / nakamuralab / Mar 11
Researchers demonstrate generation of single microwave photons in four orthogonal temporal modes using photon-shaping with a fixed-frequency transmon qubit in a waveguide. They achieve mode-selective absorption via time-reversed emission, with efficiencies exceeding 0.89 for matched modes and below 0.13 for orthogonal modes. Rejected photons maintain orthogonality, supporting cascaded selective absorption in multi-node quantum networks and enabling higher-dimensional encoding for enhanced capacity.
temporal-modesmicrowave-photonsquantum-communicationsuperconducting-qubitsphoton-shapingmode-selective-transferquantum-networks
“Single microwave photons are generated in four orthogonal temporal modes using a fixed-frequency transmon qubit.”
paper / nakamuralab / Feb 5
Extended haloscope search recovered prior HEMT dataset, identifying an excess near 1.036 GHz that met candidate criteria but failed validation via independent cross-checks and re-examination. Search expanded over 20-MHz band (1.026-1.045 GHz) using quantum-noise-limited amplifier, achieving sensitivity near DFSZ benchmark. No axion signal confirmed; sets new 90% CL upper limits on axion-photon coupling. Emphasizes robust validation as sensitivity nears discovery thresholds.
axion-searchhaloscopedark-matterparticle-physicshep-exaxion-photon-coupling
“An excess power near 1.036 GHz satisfied candidate-selection criteria in recovered HEMT data.”
paper / nakamuralab / Dec 9
Researchers demonstrate deterministic quantum state transfer and remote entanglement between fixed-frequency superconducting qubits on separate chips using itinerant microwave photons. A frequency-tunable photon-generation technique compensates for sender-receiver mismatches without tunable circuit elements, paired with broadband transfer resonators of two coupled coplanar-waveguide resonators providing over 100 MHz bandwidth. This achieves state transfer fidelities around 79% and Bell-state fidelities around 73% across a 30-MHz photon frequency range, avoiding control complexity for scalable quantum networks.
quantum-communicationsuperconducting-qubitsquantum-networksfixed-frequency-qubitsbroadband-resonatorsstate-transferremote-entanglement
“Deterministic quantum state transfer and remote entanglement generation is achieved between fixed-frequency superconducting qubits on separate chips.”
paper / nakamuralab / Nov 21
Reanalysis of CAPP-12T MC axion haloscope data at 5.311 GHz detects no monochromatic high-frequency gravitational waves (HFGW), yielding 90% CL strain exclusion limits of h_0 ≈ 3.9 × 10^{-21} in optimal sky regions. In the black hole superradiance axion cloud model, this excludes M_BH ≃ 1.22 × 10^{-6} M_⊙ black holes within ~10^{-2} AU of Earth under benchmark assumptions. Electromagnetic resonant cavities emerge as viable, sensitive detectors for both persistent and transient HFGW signals.
gravitational-waveshigh-frequency-gwaxion-haloscopeblack-hole-superradiancehep-experimentcavity-detection
“No rescan candidates for monochromatic HFGW detected in 2 MHz band centered at 5.311 GHz”
paper / nakamuralab / Nov 3
Researchers demonstrate an all-microwave CZ gate using a fixed-frequency transmon coupler and multi-path coupling to minimize residual ZZ interactions between data transmons while boosting data-coupler coupling for faster gate times. The gate induces state-dependent geometric phases by driving at the midpoint of dispersively shifted resonances in the |gf⟩–|eg⟩ transition. Post-gate coupler state measurement detects a subset of decoherence errors as erasures, supporting erasure-aware quantum error correction.
quantum-computingcz-gatetransmon-couplermicrowave-gateerror-detectionsuperconducting-qubitsquantum-error-correction
“All-microwave CZ gate achieves high fidelity by suppressing residual ZZ interactions using multi-path coupling with a fixed-frequency transmon coupler.”
paper / nakamuralab / Aug 14
Researchers demonstrate generation of frequency-bin-encoded dual-rail cluster states using a superconducting circuit with a fixed-frequency transmon qubit, resonator, and Purcell filter via time-frequency multiplexing of microwave photons. The dual-rail encoding supports erasure detection via photon occupancy, yielding state fidelities over 50% for up to four logical qubits pre-correction and eight post-erasure error discarding. Localizable entanglement persists across chains of seven qubits initially and eleven after correction, outperforming single-rail schemes in photon loss robustness and paving the way for scalable microwave photonic quantum processing.
cluster-statesfrequency-bin-encodingdual-rail-qubitsmicrowave-photonssuperconducting-circuitsquantum-entanglementquantum-physics
“State fidelity exceeds 50% for cluster states of up to four logical qubits”
paper / nakamuralab / Jul 6
An 8-cell microwave resonator haloscope searched for KSVZ axion dark matter near 5.9 GHz, extending frequency range multifold over single-cell designs while preserving detection volume. Sensitivity was boosted by a quantum-limited flux-driven Josephson parametric amplifier and sideband-summing technique. No excess signal detected in 5.83-5.94 GHz, excluding axion-photon couplings down to 1.2e-14 GeV^-1 at 90% CL, approaching KSVZ benchmark.
axion-dark-matterksvz-axionhaloscopemicrowave-resonatorjosephson-parametric-amplifierhep-exparticle-physics-experiment
“Experiment scanned axion dark matter in 5.83-5.94 GHz using 8-cell cavity haloscope”
paper / nakamuralab / Jun 26
Researchers demonstrate projected ensembles on a 16-qubit superconducting quantum processor to probe chaotic many-body dynamics, observing Haar-distributed steady states in a charge-conserved sector as direct evidence of deep thermalization. They introduce ensemble-averaged entropy as a metric to quantify information leakage to the environment. This approach advances quantum simulation by providing a general framework beyond traditional density matrix measures like expectation values or entanglement.
quantum-simulationmany-body-dynamicssuperconducting-qubitsprojected-ensemblesdeep-thermalizationquantum-chaosquantum-processor
“Projected ensembles on a 16-qubit superconducting processor with square lattice experimentally show Haar-distributed steady states in charge-conserved sector, evidencing deep thermalization.”
paper / nakamuralab / May 8
Researchers utilized the TM020 mode in a cylindrical cavity haloscope with an innovative tuning mechanism to search for axion dark matter at masses around 21 μeV, extending beyond the conventional <10 μeV range. This approach delivered sensitivity 1.7 times better than the KSVZ benchmark across a 100 MHz bandwidth. The results mark a methodological advance for higher-mass axion searches, addressing theoretical predictions favoring masses above prior experimental limits.
axion-dark-mattercavity-haloscopetm020-modeksvz-sensitivityhigh-energy-physicshep-exarxiv-paper
“Cavity haloscopes are the most sensitive method for axion detection.”
paper / nakamuralab / May 5
Fluxonium qubits offer large anharmonicity and high coherence at their sweet spot but require precise DC magnetic flux bias, complicating large-scale integration due to wiring overhead, crosstalk, heating, and decoherence. The proposed flux-trapping fluxonium qubit leverages fluxoid quantization to achieve optimal phase biasing internally at operating temperature, eliminating the need for external flux control lines. The design's working principle is introduced, with experimental demonstration of phase biasing via fluxoid quantization.
fluxonium-qubitflux-trappingsuperconducting-qubitsquantum-computingfluxoid-quantizationqubit-design
“Fluxonium qubits require precise external magnetic flux bias to achieve high performance at their sweet spot.”
paper / nakamuralab / Apr 23
Researchers introduce a three-stage impedance-transformer scheme using high-kinetic-inductance NbTiN films to achieve nonlinear resonator impedances up to tens of ohms, overcoming the sub-10 Ω limit of prior designs. This enables kinetic-inductance parametric amplifiers (KIMPA) with 17 dB gain over 450 MHz bandwidth at 8.4 GHz, quantum-limited noise of 0.5-1.3 quanta, and saturation power of -68 dBm—30 dB higher than JJ-based amplifiers. The approach simplifies fabrication, supports higher temperatures/magnetic fields, and extends to other three-wave-mixing devices.
parametric-amplifierskinetic-inductanceimpedance-engineeringnbtinquantum-physicssuperconductorsmicrowave-amplifiers
“Impedance-engineered PAs limited to Z_NR below 10 Ω require large capacitance”
paper / nakamuralab / Apr 22
Researchers demonstrate non-demolition fluorescence readout and high-fidelity unconditional reset for fluxonium qubits using dissipation engineering, bypassing dispersive resonator interactions. A planar filter protects the qubit from energy relaxation while enhancing readout transition decay. Strategic selection of the readout transition boosts quantum non-demolition fidelity, enabling fast all-microwave reset without resonators.
fluxonium-qubitnon-demolition-readoutqubit-resetdissipation-engineeringsuperconducting-qubitsquantum-computation
“Non-demolition fluorescence readout of fluxonium qubits is achieved without a resonator using dissipation engineering.”
paper / nakamuralab / Dec 6
Researchers demonstrate on-demand generation of microwave time-bin qubits using superconducting circuit QED, enabling scalable encoding in temporal modes for quantum networks. Wigner tomography is performed with a single heterodyne detector by dynamically adjusting measurement quadratures via a phase-sensitive amplifier for each time bin. Phase information is preserved without shared phase reference between generation and measurement hardware, confirming qubit robustness.
quantum-physicsmicrowave-qubittime-bin-qubitsuperconducting-circuitswigner-tomographyquantum-electrodynamics
“Microwave time-bin qubits are generated on-demand using superconducting circuit quantum electrodynamics architecture.”
paper / nakamuralab / Oct 21
Researchers demonstrate single-shot detection of a single magnon in a millimeter-sized ferromagnetic crystal using a superconducting qubit as a quantum sensor. Detection relies on entanglement between a magnetostatic mode and the qubit, followed by qubit state measurement, achieving quantum efficiency up to 0.71. This establishes a magnonic analog to single-photon detectors, advancing quantum magnonics for magnetism studies and quantum technologies.
quantum-physicssuperconducting-qubitmagnon-detectionquantum-magnonicshybrid-quantum-systemssingle-shot-detectionentanglement
“A superconducting qubit detects a single magnon in a millimeter-sized ferromagnetic crystal.”
paper / nakamuralab / Sep 14
Superconducting qubits suffer radiative decay through control lines, limiting coherence. The Josephson quantum filter (JQF) uses a strongly coupled auxiliary qubit to induce subradiance, suppressing data qubit decay into the line. This passive element maintains fast gate speeds and supports scalable integration in waveguide QED architectures.
quantum-physicssuperconducting-qubitssubradiancejosephson-quantum-filterqubit-protectionwaveguide-qed
“Coupling between a superconducting qubit and control line causes radiative decay of the qubit into the line.”
paper / nakamuralab / May 12
Researchers demonstrate real-space imaging of optical heterodyne signals from Brillouin light scattering by coherently driven magnons in magnetostatic modes. This technique characterizes surface Damon-Eshbach modes in a 1D magnonic crystal formed by aluminum strips on a ferromagnetic film, with band structures deduced from Fourier transforms of the images. The method offers a simple approach to probe magnons in structured films, enabling studies of Anderson localization and topological magnon transport.
magnonic-crystalsoptical-heterodynebrillouin-scatteringmagnetostatic-modesdamon-eshbachcondensed-matterarxiv-paper
“Optical heterodyne imaging visualizes heterodyne signals produced by Brillouin light scattering from coherently driven magnons in magnetostatic modes.”
paper / nakamuralab / Apr 18
SVQS leverages SSVQE to identify low-lying eigensubspaces of static Hamiltonians, then simulates dynamics within those subspaces on NISQ hardware with reduced overhead versus prior methods. Experimental validation on H2 demonstrates time-evolution operators with subspace process fidelities of 0.88-0.98. This approach bypasses full error correction, targeting chemistry and materials science applications.
quantum-simulationnisc-devicesvariational-quantumsubspace-methodsquantum-eigensolverquantum-dynamics
“SVQS simulates quantum dynamics driven by static Hamiltonians on NISQ devices without quantum error correction”
paper / nakamuralab / Mar 17
Continuous translational deformations of periodic potentials produce localized boundary states, distinct from defect perturbations or terminations. A rigorous theoretical proof establishes their emergence, with experimental validation in microwave photonic crystals. The mechanism manifests topological phase windings in reflected waves from translated crystals.
topological-boundary-modestranslational-deformationsphotonic-crystalsmesoscale-physicsmicrowave-experimentsarxiv-paper
“Localized states emerge from continuous translational deformations of periodic potentials.”
paper / nakamuralab / Feb 8
Hybrid quantum systems leverage collective spin excitations (magnons) in ferromagnetic materials, with microwave cavity modes serving as the foundational interface for coherent interactions. Magnons extend to couplings with optical photons (cavity optomagnonics), phonons (cavity magnomechanics), and superconducting qubits (quantum magnonics). These platforms support quantum optics in solid-state magnetic systems and enable quantum information processing and sensing.
hybrid-quantum-systemsmagnonicsspin-wavesquantum-magnonicscavity-optomagnonicsquantum-sensingquantum-information
“Coherent interaction between microwave cavity modes and collective spin-wave modes (magnons) forms the backbone of magnonics-based hybrid quantum systems”
paper / nakamuralab / Oct 30
VQGO constructs target multi-qubit gates by variationally optimizing parameterized circuits of high-fidelity single-qubit gates and fixed, low-controllability multi-qubit gates like cross-resonance. Numerical simulations demonstrate high-fidelity CNOT gates using cross-resonance gates with finite crosstalk in superconducting qubit models. The method also enables fast, high-fidelity four-qubit syndrome extraction via simultaneous cross-resonance drives despite non-commutative crosstalk, offering a scalable path for quantum gate design.
quantum-physicsvariational-quantum-gate-optimizationvqgosuperconducting-qubitscnot-gatecross-resonance-gatessyndrome-extraction
“VQGO constructs high-fidelity CNOT gates using cross-resonance gates affected by finite crosstalk”
paper / nakamuralab / Sep 26
Researchers demonstrate structural imaging of magnetostatic spin-wave modes in millimeter-sized ferromagnetic spheres using resonant magnetic induction tomography at microwave frequencies. This technique identifies non-trivial modes by resolving their azimuthal and polar dependencies, filling a gap for bulk solids of revolution lacking prior imaging methods. It enables fundamental magnonic studies and hybrid systems beyond uniform precession.
spin-wavesmagnetic-induction-tomographyferromagnetic-spheremagnonicsmicrowave-imagingmaterials-sciencecond-mat
“Resonant magnetic induction tomography in the microwave range images magnetostatic spin-wave modes in a millimeter-sized ferromagnetic sphere.”
paper / nakamuralab / Aug 9
Researchers demonstrate synthetic radiation pressure from microwave photons on phonons in a surface acoustic wave (SAW) resonator using a superconducting Josephson-junction circuit's strong second-order nonlinearity. This enhances interaction strength by an order of magnitude, enabling the single-photon quantum regime where a single off-resonant photon significantly alters the mechanical quantum state. The approach advances cavity optomechanics and supports quantum interfaces between electromagnetic and mechanical systems.
quantum-optomechanicsradiation-pressuresurface-acoustic-wavesingle-photon-regimesuperconducting-josephsonphonon-photon-interaction
“Electromagnetic radiation pressure from photons has been used in cavity optomechanics to control macroscopic mechanical motions at the quantum limit.”
paper / nakamuralab / Aug 9
Researchers propose generating intracavity Schrödinger cat states deterministically in a Kerr-nonlinear parametric oscillator (KPO) via quantum adiabatic evolution, then releasing them on-demand into traveling fields by dynamically controlling the parametric pump amplitude. The method enables high-fidelity traveling cat states, with quality enhanced using shortcut-to-adiabaticity techniques to mitigate non-adiabatic errors. This advances scalable quantum optics by providing controllable macroscopic quantum superpositions in free-space modes.
quantum-physicsschrodinger-cat-statesparametric-oscillatorkerr-nonlinearityquantum-adiaabatic-evolutionsuperconductivityarxiv-paper
“Traveling Schrödinger cat states can be generated on-demand using a Kerr-nonlinear parametric oscillator (KPO).”
paper / nakamuralab / Apr 9
Gebhard-Ruckenstein hopping between microwave resonators in a superconducting device produces linear energy dispersion, enabling chiral microwave propagation. This mechanism supports nonreciprocal microwave transmission, allowing the device to function as an on-chip circulator. Attaching transmission lines extends its operational bandwidth significantly.
arxiv-papernonreciprocal-transmissionmicrowave-devicesgebhard-ruckenstein-hoppingsuperconducting-resonatorschiral-propagationquantum-circulator
“Gebhard-Ruckenstein hopping causes linear energy dispersion in the superconducting device”
paper / nakamuralab / Mar 12
Researchers replace the single Josephson junction in a transmon qubit with a full Josephson junction array coupled to a superconducting microwave cavity, enabling dispersive readout of the array's resonance frequency shift at 10 mK using single-photon-level coherent states to minimize heating. The array maps to a frustrated XY model under external magnetic fields, exhibiting vortex dynamics including Mott insulator and BKT transitions. They observe signatures of ordered vortex lattices specifically at rational flux fillings per plaquette.
circuit-qedjosephson-junction-arrayvortex-latticesuperconductivityquantum-phase-transitionsbkt-transitionmesoscale-physics
“Josephson junction array coupled to a microwave cavity maps to the XY model, showing Mott insulator and Berezinskii-Kosterlitz-Thouless (BKT) transitions analogous to type-II superconductors”
paper / nakamuralab / Oct 2
A hybrid quantum system integrates a SAW resonator, microwave resonator, and superconducting qubit to achieve ultra-sensitive measurement of SAW fluctuations. The driven qubit's nonlinearity induces parametric coupling that up-converts SAW excitations to the microwave domain. This enables noise spectroscopy revealing thermal fluctuations in the SAW resonator approaching the quantum limit.
quantum-physicssuperconducting-qubitsurface-acoustic-wavessaw-resonatorquantum-measurementhybrid-quantum-systemarxiv-paper
“Ultra-sensitive measurement of SAW resonator fluctuations is achieved using a hybrid system of SAW resonator, MW resonator, and superconducting qubit.”
paper / nakamuralab / Sep 2
Researchers implemented Maxwell's demon on a superconducting qubit using circuit-QED techniques, featuring quantum nondemolition projective measurements and coherent feedback control. They verified generalized integral fluctuation theorems and achieved conversion of measurement information into work amid quantum fluctuations. This establishes superconducting circuits as a platform for quantum information thermodynamics, with ties to quantum error correction.
maxwells-demonsuperconducting-qubitquantum-thermodynamicsinformation-to-workcircuit-qedfluctuation-theoremsfeedback-control
“Maxwell's demon was implemented using coherent control and quantum nondemolition projective measurements on a superconducting qubit.”
paper / nakamuralab / Jul 4
Numerical analysis of driven nonlinear oscillator networks with dissipation reveals a Boltzmann-like output probability distribution, where the energy corresponds to the combinatorial optimization cost function. This generalizes quantum heating from single to coupled oscillators, explaining the thermal-like behavior. The approach enables hardware implementations for Boltzmann sampling, applicable to AI tasks like Boltzmann machine learning.
quantum-bifurcationdissipative-oscillatorsquantum-optimizationboltzmann-samplingquantum-heatingcombinatorial-optimization
“The output probability distribution of the dissipative quantum bifurcation machine is Boltzmann-like, with energy matching the optimization problem's cost function.”
paper / nakamuralab / Jun 2
Researchers demonstrate electro-mechano-optical detection of NMR by up-converting radio-frequency signals to the optical regime via a high-stress silicon nitride membrane. The membrane couples the electrical NMR detection circuit to an optical cavity through electro-mechanical and opto-mechanical interactions, preserving traditional nuclear induction versatility. Signal-to-noise ratio can surpass conventional electrical methods by enhancing electro-mechanical coupling, despite current limits from Brownian and technical noise. This approach enables mechanical parametric amplification and potential integration with laser cooling of nuclear spins.
nmr-detectionelectro-mechano-opticalsilicon-nitride-membranequantum-physicschemical-physicsoptomechanical-coupling
“NMR signals are up-converted from radio-frequency to optical regime using a high-stress silicon nitride membrane.”
paper / nakamuralab / May 12
Develops an electro-optomechanical system using surface acoustic waves (SAW) in piezoelectric materials with high optoelastic susceptibility for coupling radio waves and optical light to SAW. Exploits tensorial optoelastic effect for polarization-dependent photon-SAW interaction, proposes 2D SAW focusing circuits and optical cavities to boost coupling. Estimates single-photon optomechanical coupling rate g0, identifying paths for strength improvement.
quantum-physicscavity-optomechanicssurface-acoustic-wavesoptoelastic-effectphoton-phonon-couplingarxiv-paper
“Piezoelectric material with large optoelastic susceptibility couples both radio waves and optical light to surface acoustic waves (SAW).”
paper / nakamuralab / Dec 27
Researchers demonstrate a stochastic error suppression method using control pulses to dynamically shape the noise environment in superconducting qubits by pumping out quasiparticles. This approach reduces unpaired electron density near the device by 70%, yielding a threefold increase in qubit relaxation times (T1) and reduced coherence variability. Unlike dynamical decoupling, it targets irreversible relaxation processes rather than reversible dephasing.
superconducting-qubitsquasiparticle-pumpingquantum-coherenceerror-suppressionqubit-relaxationquantum-noise
“Pumping sequence reduces quasiparticle density by 70%”
paper / nakamuralab / Oct 4
Researchers demonstrate resolution of individual magnon number states in a millimeter-sized ferromagnet coupled to a transmon qubit in the strong dispersive regime. Spectroscopic measurements enable detection of a single spin flip among over 10^19 spins through changes in the ferromagnet's magnetic dipole. This hybrid system opens pathways for encoding qubits into non-classical magnon states, interfacing superconducting processors with optical photons.
quantum-magnonicsmagnon-statessuperconducting-qubitsstrong-dispersive-regimehybrid-quantum-systemsferromagnet-magnons
“Individual magnons are observed in a millimeter-sized ferromagnet coherently coupled to a superconducting qubit.”
paper / nakamuralab / Sep 28
Measurements on five flux-driven Josephson parametric amplifiers (JPAs) reveal hysteretic resonant frequency dependence on applied magnetic flux in three devices, modeled via numerical simulations of the 2D dc-SQUID potential landscape for finite screening parameter β_L > 0. The model achieves excellent agreement with experimental data. Nondegenerate gain responses across JPAs are accurately described by the same theoretical framework.
josephson-parametric-amplifiersflux-driven-jpasuperconducting-circuitsdc-squidquantum-amplifiershysteretic-responsenondegenerate-gain
“Three out of five flux-driven JPAs exhibit hysteretic dependence of resonant frequency on applied magnetic flux.”
paper / nakamuralab / May 4
Researchers propose a superconducting circuit that undergoes a super-radiant phase transition (SRPT) in thermal equilibrium. Analytically confirmed for infinite artificial atoms, with numerical diagonalization showing asymptotic convergence for finite atom numbers. Classical analysis provides intuitive interpretation of the quantum effect.
quantum-physicssuper-radiant-phase-transitionsuperconducting-circuitsrptthermal-equilibriumartificial-atomsarxiv-paper
“A superconducting circuit is proposed that displays super-radiant phase transition in thermal equilibrium.”
paper / nakamuralab / Feb 4
Researchers developed a multimode electromechanical system coupling several membrane vibrational modes to a 3D loop-gap superconducting microwave cavity via a narrow-gap capacitor. Tight electric field confinement enables quantum strong coupling under red-sideband pumping. Two-tone parametric drives induce strong coupling between two mechanical modes mediated by a virtual cavity photon, enabling tunable inter-mode coupling for mechanical entanglement generation.
quantum-electromechanicsstrong-couplingmultimode-systemscavity-electromechanicssuperconducting-cavityquantum-physicsarxiv-paper
“Multiple membrane vibrational modes couple to a three-dimensional loop-gap superconducting microwave cavity.”
paper / nakamuralab / Jan 21
Researchers demonstrate single microwave photon detection using an impedance-matched artificial Λ-type three-level system formed by dressed states of a driven superconducting qubit coupled to a microwave resonator. The detector achieves 0.66 ± 0.06 detection efficiency for photons propagating through a waveguide, with a reset time of ~400 ns. This advances microwave quantum optics by overcoming the challenge of low-energy microwave quanta, enabling applications in quantum sensing, communication, and information processing.
quantum-physicssingle-photon-detectionmicrowave-photonsuperconducting-qubitlambda-systemarxiv-paper
“Single microwave photon detection is demonstrated using a waveguide.”
paper / nakamuralab / Jan 15
Researchers demonstrate bidirectional coherent conversion between microwave and optical photons using hybridized modes of a microwave cavity and ferromagnetic Kittel magnon mode. Microwave photons couple to the hybrid via the cavity, while optical fields interface through Faraday and inverse Faraday effects on the Kittel mode. This enables quantum-noise-limited amplification and supports long-distance quantum communication, with theoretical and experimental efficiency analysis provided.
microwave-optical-conversionferromagnetic-magnonsquantum-photon-conversionkittel-modefaraday-effecthybrid-cavity-magnon
“Bidirectional coherent conversion between microwave and optical photons is achieved at the single-quantum level using ferromagnetic collective spin excitations.”
paper / nakamuralab / Sep 19
Proposes a circuit QED device where a superconducting qubit couples dispersively to two resonators: one forms an impedance-matched Λ system for deterministic capture of incoming itinerant microwave photons, the other enables continuous monitoring. Achieves 0.9 detection efficiency with ~10 MHz bandwidth using realistic parameters. Advances continuous, non-demolition detection in quantum optics.
quantum-physicsmicrowave-photonscircuit-qedsuperconducting-qubitphoton-detectionarxiv-paper
“Superconducting qubit couples dispersively to two resonators for microwave photon detection”
paper / nakamuralab / Aug 21
Microwave quantum optics techniques enable strong coupling between a single magnon in a ferromagnetic insulator sphere and a microwave cavity mode. A superconducting qubit is integrated into the cavity, mediating coupling to the magnon via virtual photons, resulting in observable magnon-vacuum-induced Rabi splitting. This hybrid system supports generation and characterization of non-classical magnon quantum states.
quantum-magnonicssuperconducting-qubitmagnon-couplingquantum-hybrid-systemsmicrowave-cavityrabi-splittingarxiv-paper
“Strong coupling is demonstrated between a magnetostatic mode and a microwave cavity mode in the single-magnon limit.”
paper / nakamuralab / Jan 16
Driving a qubit-resonator in the strong dispersive regime creates an impedance-matched Λ system in dressed states, where a resonant single microwave photon triggers a deterministic Raman transition exciting the qubit. Detection combines this with fast dispersive qubit readout, achieving near-unity efficiency without precise pulse shaping under conservative parameters. The system supports rapid reset via microwave pulses for short dead times and high repetition rates.
quantum-physicsmicrowave-photon-detectionqubit-resonatorlambda-systemsingle-photon-detectorarxiv-paper
“A resonant single photon deterministically induces a Raman transition and excites the qubit in the impedance-matched Λ system”
paper / nakamuralab / Oct 14
Researchers demonstrate coherent coupling between a magnon in a millimeter-sized ferromagnetic sphere and a superconducting qubit, mediated by virtual photons in a microwave cavity. The coupling strength exceeds damping rates, achieving the strong coupling regime essential for quantum coherence. A parametric microwave drive enables tunable magnon-qubit interaction, advancing quantum control in magnonic systems.
quantum-magnonicssuperconducting-qubitcoherent-couplingmagnon-qubit-hybridferromagnetic-magnonstrong-coupling-regimequantum-physics
“Coherent coupling is demonstrated between a magnon excitation in a millimetre-sized ferromagnetic sphere and a superconducting qubit.”
paper / nakamuralab / May 6
Researchers achieve strong coupling between the Kittel magnon mode in a yttrium iron garnet sphere and a microwave cavity, evidenced by large normal-mode splitting. This coupling occurs in the quantum regime with average thermal or external excitations below one magnon/photon. Coupling strength scales with the square root of spin number; Kittel-mode linewidth shows nonmonotonic temperature dependence below 1 K due to two-level system dissipation.
quantum-physicsferromagnetic-magnonsmicrowave-photonsstrong-couplingkittel-modeyttrium-iron-garnetarxiv-paper
“Large normal-mode splitting occurs between the Kittel mode in YIG and a microwave cavity mode.”
paper / nakamuralab / Feb 6
Researchers measured high-frequency flux noise in a superconducting flux qubit by analyzing Rabi oscillation decay under strong microwave driving up to 1.7 GHz, enabled by the qubit's large anharmonicity and inductive coupling. This regime exceeds the rotating-wave approximation validity, given the 4.8 GHz level splitting. The flux noise spectral density decreases with frequency up to 300 MHz, closely aligning with 1/f extrapolation from free-induction-decay data; surface electron spins are proposed as a potential source.
flux-qubitnoise-spectroscopyrabi-oscillationssuperconducting-qubitflux-noise1/f-noisequantum-decoherence
“Rabi frequencies up to 1.7 GHz were achieved in the flux qubit without significant additional decoherence.”