Chronological feed of everything captured from Yasunobu Nakamura.
paper / nakamuralab / Apr 23
Researchers developed a superconducting acousto-optic phase modulator on lithium niobate, integrating a titanium-diffused optical waveguide with a surface acoustic wave (SAW) resonator using superconducting niobium titanium nitride electrodes. At 8K, it achieves a length-Vπ product of 1.78 V·cm, comparable to electro-optic modulators in size and fiber coupling. Simulations and optical cavity enhancements project Vπ reduction to 0.27 V through simple extensions.
quantum-physicssuperconducting-deviceacousto-optic-modulatorlithium-niobatecryogenic-operationarxiv-paperoptical-waveguide
“The device operates at T=8K with a measured length-Vπ product of 1.78 V·cm.”
paper / nakamuralab / Feb 25
Researchers introduce inductive quantum circuits that generate exact unitary t-designs for any t on arbitrary qubits, overcoming prior limitations to approximate designs only. They apply 2t-designs to define t-th order randomized benchmarking (t-RB), with 2-RB specifically probing self-adjointness of quantum noise—a metric tied to quantum error correction feasibility. Numerical simulations confirm 2-RB viability in 1-2 qubit systems, while experiments on a superconducting qubit reveal inter-qubit interactions as a key noise source hindering QEC.
quantum-circuitsunitary-t-designsrandomized-benchmarkingquantum-noisequantum-error-correctionsuperconducting-qubits
“Quantum circuits exist that generate exact unitary t-designs for any t on an arbitrary number of qubits”
paper / nakamuralab / Jan 21
A flux-driven Josephson parametric amplifier (JPA) operating at 2.3 GHz delivers added noise approaching the quantum limit, serving as the first-stage amplifier in cryogenic signal chains for axion search experiments. When operated at 19 dB gain and cascaded with two 4 K cryogenic amplifiers, the full chain reaches a noise temperature of 120 mK. This performance addresses a key bottleneck in parameter scanning speed for millikelvin-temperature haloscope detectors reliant on low-noise readout.
josephson-parametric-amplifierquantum-limited-noiseaxion-searchflux-driven-jpasuperconductivitycryogenic-amplificationhaloscope-detectors
“Flux-driven JPA operates at around 2.3 GHz with added noise approaching the quantum limit.”
paper / nakamuralab / Nov 2
Quantum mechanics imposes a standard quantum limit (SQL) of 0.5 added noise photons (quantum efficiency of 0.5) for phase-preserving amplification of narrowband signals. This limit is surpassed using nondegenerate parametric amplification of broadband signals, theoretically reaching efficiency 1. Experimentally, a flux-driven Josephson parametric amplifier with broadband thermal signals achieves 0.69 ± 0.02 efficiency, exceeding SQL.
quantum-amplificationstandard-quantum-limitjosephson-parametric-amplifierquantum-efficiencymicrowave-signalssuperconducting-devicesarxiv-paper
“Standard quantum limit for phase-preserving amplification of narrowband signals adds at least half a photon of input noise, equivalent to 0.5 quantum efficiency.”
paper / nakamuralab / Jun 25
Researchers demonstrate magnon-exciton proximity coupling at the interface between a YIG thin film hosting long-lived magnons and a MoSe2 flake with strongly-bound excitons. Magnons induce a dynamical valley Zeeman effect on excitons through interfacial exchange interactions. This hybrid system enables coherent information transduction between microwave and optical domains.
magnon-exciton-couplingvan-der-waals-heterointerfacesspin-exciton-interactionyig-mose2mesoscale-physicsquantum-materialsoptics-physics
“Magnon-exciton coupling occurs at the interface between a magnetic thin film and an atomically-thin semiconductor.”
paper / nakamuralab / May 19
Researchers demonstrate quantum sensing of steady-state magnon population in a ferrimagnetic crystal's magnetostatic mode using dispersive coupling to a superconducting qubit. Detection via Ramsey interferometry achieves a sensitivity of 10^{-3} magnons/√Hz. The method relies on magnon-induced dephasing, where fluctuations reduce qubit coherence proportional to magnon number.
quantum-sensingsuperconducting-qubitmagnonsdissipation-dephasingferrimagnetic-crystalramsey-interferometryarxiv-paper
“Quantum sensing of steady-state magnon population in a ferrimagnetic crystal's magnetostatic mode is experimentally demonstrated.”
paper / nakamuralab / May 6
A parity-violated superconducting qubit with an asymmetric Josephson-junction loop under magnetic flux bias provides second-order nonlinearity for first-order inter-qubit sideband transitions to a transmon qubit, achieving Rabi frequencies up to 30 MHz. Unwanted static ZZ interaction is eliminated via ac Stark shifts from a continuous microwave drive near-resonant to the sideband. Randomized benchmarking yields average two-qubit gate fidelities of 0.967 for CZ, 0.951 for iSWAP, and 0.956 for SWAP.
quantum-physicssuperconducting-qubitstwo-qubit-gatesarxiv-paperjosephson-junctionquantum-computing
“Parity-violated superconducting qubit enables first-order inter-qubit sideband transitions with Rabi frequencies up to 30 MHz”
paper / nakamuralab / Apr 4
An artificial molecule of two qubits spaced a quarter-wavelength apart in a waveguide achieves dynamically selective bidirectional emission and absorption of single itinerant microwave photons via input-output theory simulations. Directional emission results from destructive interference when the qubits are prepared in an appropriate entangled state, suppressing photon propagation in one direction. The device also supports on-demand absorption and transmission of incoming photons, enabling fully interconnected one-dimensional quantum networks for node-to-node quantum information exchange.
quantum-physicsmicrowave-photonswaveguide-qubitsdirectional-emissionquantum-networksuperconducting-qubitsarxiv-paper
“Two qubits coupled to a waveguide a quarter-wavelength apart form an artificial molecule capable of dynamically selective bidirectional emission of a single itinerant microwave photon.”
paper / nakamuralab / Mar 14
Researchers demonstrate RF-to-optical signal conversion in a dielectric sphere using overlapping acoustic and optical whispering gallery modes (WGMs) along the equator. Triple-resonance phase-matching enhances Brillouin scattering selectively in one sideband, enabling efficient coupling from RF-excited acoustic WGMs to optical output. The electro-optomechanical system leverages high-Q WGMs for both wave types to achieve observed conversion.
whispering-gallery-modesbrillouin-scatteringrf-optical-conversionelectro-optomechanicsopticsquantum-physics
“Whispering gallery modes exhibit high quality factors for both acoustic and electromagnetic waves”
paper / nakamuralab / Feb 5
Superconducting qubits face a fundamental trade-off between fast control via microwave pulses and long coherence times due to radiative decay into control lines. Researchers introduce a Josephson quantum filter (JQF), a strongly coupled second qubit along the control line, which blocks qubit photon emission to suppress relaxation while saturating to transmit large-amplitude control pulses. Experiments demonstrate improved qubit relaxation time without reducing Rabi frequency, mitigating heating and crosstalk in scalable quantum processors.
superconducting-qubitsjosephson-quantum-filterqubit-coherencequantum-controlradiative-decayquantum-physics
“Radiative decay into the control line imposes a trade-off between fast qubit control and long qubit lifetime.”
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.”
paper / nakamuralab / Jun 28
Researchers propose implementing an impedance-matched Λ-type three-level system using dressed states from a driven superconducting qubit coupled to a resonator in a one-dimensional waveguide. This setup achieves perfect absorption of input microwave photons, which are deterministically down-converted to other frequency modes via Raman transition without reflection. The configuration supports single-photon-level microwave detection by leveraging enhanced light-matter interference.
quantum-physicslambda-systemsuperconducting-qubitdressed-statessingle-photon-detectionmicrowave-photonarxiv-paper
“A single photon induces a deterministic Raman transition in an impedance-matched Λ-type three-level system, switching the electronic state.”
paper / nakamuralab / Nov 9
Consecutive positive and negative π pulses on a superconducting qubit amplify rotations caused by microwave pulse distortions, enabling measurement of the perpendicular rotation axis as a function of pulse period. This reconstructs the actual pulse shape arriving at the qubit. Predistorting the input signal with the extracted response improves pulse fidelity, achieving average single-qubit gate fidelity above 99.8%.
quantum-physicssuperconducting-qubitsquantum-gatespulse-distortiongate-fidelityarxiv-paper
“Consecutive positive and negative π pulses amplify qubit evolution due to microwave pulse distortion, causing rotation around a perpendicular axis.”
paper / nakamuralab / Apr 28
Researchers demonstrate dynamical decoupling in a superconducting flux qubit coupled to a microscopic two-level system (TLS), using rapid qubit frequency shifts as refocusing pulses to suppress dephasing from transition frequency fluctuations. A single pulse reduces dephasing and improves entangled state coherence time, with multiple pulses yielding further enhancements matching a 1/f noise model. The approach applies to transverse-coupled two-qubit systems, promising improved gate fidelities for fault-tolerant quantum computing.
quantum-physicsdynamical-decouplingdephasingsuperconducting-qubitstwo-level-systems1f-noisequantum-computing
“Dynamical decoupling via rapid qubit transition frequency changes acts as a refocusing pulse reducing dephasing in qubit-TLS entangled states.”
paper / nakamuralab / Jan 30
In a superconducting flux qubit tunably coupled to a microwave resonator, off-resonant driving of the resonator generates an oscillating field that strongly modifies the qubit's Rabi frequency. This introduces low-frequency noise in the coupling parameter, reducing coherence time during driven evolution. A rotary-echo pulse sequence, analogous to Hahn echo for driven systems, effectively mitigates this noise.
superconducting-qubitflux-qubitquantum-resonatorrabi-frequencycoherence-timerotary-echoarxiv-paper
“Off-resonant driving of the resonator mediates an oscillating field experienced by the qubit”
paper / nakamuralab / Jan 26
Researchers measured low-frequency noise (0.01-100 Hz) in a superconducting flux qubit using Ramsey interferometry with Fourier-transform spectroscopy, accessing frequencies up to the repetition rate. Both effective flux noise and effective critical-current/charge noise follow 1/f power laws consistent with higher-frequency (0.2-20 MHz) measurements. No temperature dependence observed over 65-200 mK, and no correlations between noise types, with implications for dephasing in all superconducting qubits.
superconducting-qubitlow-frequency-noisenoise-spectroscopyflux-qubit1/f-noisequbit-dephasingquantum-noise
“Low-frequency noise spectrum (0.01-100 Hz) in superconducting flux qubit measured via Ramsey free-induction interference and single-shot readout.”
paper / nakamuralab / Apr 27
Researchers measured low-frequency flux-noise correlations in a superconducting flux qubit with independent loops controlling energy splitting and tunnel coupling. Dephasing rate measurements at varied bias points enabled extraction of noise amplitude and sign, revealing anti-correlated fluctuations between loops. This matches a model attributing noise to randomly oriented unpaired spins on the metal surface.
flux-qubitsuperconducting-qubitflux-noisenoise-correlationsqubit-dephasingsuperconductivityquantum-physics
“The flux qubit device has two independent loops controlling energy splitting and tunnel coupling”
paper / nakamuralab / Jan 25
Dynamical decoupling via CPMG sequences with up to 200 π-pulses suppresses low-frequency dephasing noise in a superconducting flux qubit, extending baseline T2 by 50-fold to 23 μs, approaching the 12 μs T1 limit. This achieves Gaussian pure dephasing times exceeding 100 μs. The method's filtering enables reconstruction of the environmental noise power spectral density using Rabi and relaxation data.
dynamical-decouplingsuperconducting-qubitnoise-spectroscopydecoherence-mitigationflux-qubitquantum-noisecpMg-sequence
“Superconducting flux qubit has energy-relaxation time T1 = 12 μs”
paper / nakamuralab / Sep 12
Quantum mechanics' counterintuitive properties, such as superposition and entanglement, underpin quantum information science, which exploits these for advantages in storing, transmitting, and processing information. Quantum computers promise exponential computational speedups for specific tasks, driving global research efforts. Multiple physical platforms—including photons and superconducting circuits—are under development, though no clear frontrunner has emerged amid ongoing challenges.
quantum-computingquantum-informationquantum-physicsarxiv-papersuperconducting-qubitsphotonic-qubits
“Quantum mechanics accurately predicts phenomena like particles being in two places simultaneously and instantaneous entanglement between remote particles.”
paper / nakamuralab / Jul 7
Researchers studied dephasing in two inductively coupled Josephson-junction flux qubits sharing superconducting loops. Dephasing rates of the first excited state were tuned via flux bias to control flux noise sensitivities, showing enhancement or suppression based on sensitivity amplitudes and signs. Quantification revealed 1/f flux noise with dominant local fluctuations over correlated components.
flux-qubitsdecoherenceflux-noisesuperconductivityjosephson-junctionsquantum-physicsinductive-coupling
“Dephasing rate of the first excited state in coupled flux qubits is enhanced or suppressed by tuning flux bias sensitivities to flux noises”