Chronological feed of everything captured from Yasunobu Nakamura.
paper / nakamuralab / 4d ago
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 / 4d ago
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 / 4d ago
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 / 4d ago
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 / 4d ago
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 / 4d ago
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 / Mar 10
This JTWPA uses a coplanar lumped-element waveguide with open-stub capacitors and Manhattan-pattern junctions to reach insertion loss below 1 dB up to 12 GHz. Windowed sinusoidal impedance modulation, specifically Tukey-windowed with 8% variation, suppresses gain ripples for 20-23 dB gain over 5 GHz under ideal conditions and 17-20 dB over 4.8 GHz with mismatches. It delivers 0.18 quanta added noise above quantum limit at 20 dB gain, -99 dBm saturation, and zero/negative backward gain below band-gap.
josephson-junctiontraveling-wave-amplifierparametric-amplifierquantum-amplifiersuperconducting-waveguidelow-noise-amplifierarxiv-paper
“Insertion loss below 1 dB up to 12 GHz”
paper / nakamuralab / Mar 7
Researchers demonstrate generation of frequency-tunable, time-symmetric shaped single microwave photons using a fixed-frequency superconducting qubit and resonator as a bandpass filter. An off-resonant drive signal stimulates photon emission into a transmission line, with frequency and shape controlled by modulating drive amplitude and frequency. The method achieves 40 MHz tunability with ~95% state and process fidelities, avoiding tunable circuit elements for scalable quantum communication.
superconducting-qubitmicrowave-photonquantum-communicationfrequency-tunablephoton-shapingquantum-physics
“Frequency tunability of 40 MHz is achieved for shaped microwave photons while maintaining time-symmetric mode shape.”
paper / nakamuralab / Mar 4
Researchers propose and demonstrate a capacitively shunted double-transmon coupler (DTC) that achieves bias-free idling at zero flux, eliminating DC current requirements and associated wiring/heat issues. The design delivers CZ gate fidelity comparable to prior DTCs, with limitations identified as coupler decoherence via error budget analysis using randomized benchmarking. It also offers a wide operational flux range, mitigating remnant flux problems despite magnetic shielding.
superconducting-qubitscz-gatedouble-transmon-couplerflux-biasquantum-gate-fidelityshunt-capacitancequantum-decoherence
“Capacitively shunted DTC realizes high-fidelity CZ gates without DC flux bias.”
paper / nakamuralab / Jan 18
Researchers introduce selective-excitation-pulse (SEP) technique for frequency-multiplexed control of superconducting qubits sharing a single microwave line. SEP shapes drive pulses to nullify excitation at non-target qubit frequencies, mitigating crosstalk when detuning is smaller than pulse bandwidth. In a three fixed-frequency transmon qubit experiment, SEP achieves gate fidelities matching Gaussian pulses while suppressing unwanted excitations.
superconducting-qubitspulse-shapingselective-excitationfrequency-multiplexingtransmon-qubitsquantum-controlarxiv-paper
“Conventional superconducting qubit architectures require dedicated control lines per qubit, scaling microwave lines linearly with qubit count.”
paper / nakamuralab / Jan 6
Strong driving of Kerr nonlinear resonators, where Rabi frequency exceeds Kerr nonlinearity, invalidates two-level system approximations. Theoretical analysis predicts reduced center-peak intensity and asymmetric sidebands under finite dephasing, attributed to dressed-state populations and transition matrix elements. Experiments with superconducting Kerr resonators confirm these features quantitatively.
quantum-physicsresonance-fluorescencekerr-nonlinear-resonatorsuperconducting-resonatorrabi-frequencyarxiv-paper
“Strong driving of Kerr nonlinear resonator with Rabi frequency comparable to or larger than Kerr nonlinearity prevents two-level system approximation”
youtube / nakamuralab / Nov 12
This content details significant advancements in superconducting qubit technology, emphasizing improvements in coherence times from nanoseconds to milliseconds through enhanced understanding and fabrication. It covers the evolution of circuit quantum electrodynamics (cQED) and waveguide QED for strong atom-photon interaction. The core insight revolves around the development of scalable qubit architectures, specifically focusing on fixed-frequency transmons with innovative wiring and readout schemes to enable larger-scale, high-fidelity quantum computing.
superconducting-qpuquantum-computingquantum-error-correctionquantum-hardwarequantum-networkingqubit-coherencequantum-control
“Superconducting qubit coherence times have improved by approximately six orders of magnitude since 2000, reaching milliseconds from nanoseconds.”
paper / nakamuralab / Oct 4
Researchers introduce an efficient tomography method using matrix-product-operator formalism to reconstruct exponentially large density matrices of many-body photonic states. They demonstrate full characterization of cluster states up to 35 microwave photonic qubits, revealing photon source degradation only in large-scale generation. The approach applies broadly to entangled qubit systems for high-fidelity benchmarking.
quantum-tomographymicrowave-photonscluster-statesmany-body-entanglementmatrix-product-operatorquantum-state-reconstructionphotonic-qubits
“Efficient tomography reconstructs the full 2^35 × 2^35 density matrix of a 35-qubit microwave photonic cluster state.”
paper / nakamuralab / Sep 8
Researchers demonstrate a compact notch-filter circuit using capacitively and inductively coupled readout and filter resonators, achieving destructive interference that eliminates the Purcell decay channel. This allows linewidths up to 42 MHz for 56-ns simultaneous readout of four superconducting qubits. Average assignment fidelity reaches 99.77%, with the top qubit exceeding 99.9%, marking a breakthrough in multiplexed readout speed and accuracy.
superconducting-qubitsqubit-readoutpurcell-filteringquantum-measurementmultiplexed-readoutquantum-circuits
“Readout resonator and filter resonator coupled capacitively and inductively produce a notch-filter eliminating Purcell decay via destructive interference”
paper / nakamuralab / Jun 12
Researchers at CAPP have developed and characterized flux-driven Josephson Parametric Amplifiers (JPAs) as quantum noise-limited amplifiers for axion dark matter searches. The work details characterization techniques, bandwidth improvement methods, and achievement of ultra-low noise temperatures to maximize detection sensitivity. JPAs are positioned as critical enablers for advancing axion search experiments probing fundamental physics.
josephson-parametric-amplifierjpaquantum-noise-limitedaxion-searchcapphep-exinstrumentation-detectors
“Flux-driven Josephson Parametric Amplifiers (JPAs) are quantum noise limited amplifiers”
paper / nakamuralab / Apr 29
The Nelder-Mead heuristic search dynamically optimizes multidimensional parameters of flux-driven Josephson parametric amplifiers (JPAs) to maximize gain and minimize noise temperature in axion haloscope experiments. This addresses environmental variability and the need for broadband tuning, ensuring quantum-limited performance for weak axion signal detection via microwave photon conversion in strong magnetic fields. Analysis demonstrates the algorithm's effectiveness in enhancing signal-to-noise ratios across experimental setups.
josephson-parametric-amplifierparameter-optimizationnelder-meadaxion-detectiondark-mattercavity-haloscopequantum-amplifiers
“Josephson parametric amplifiers (JPAs) provide quantum-limited noise for detecting axion signals in cavity haloscopes”
paper / nakamuralab / Feb 29
Researchers implemented a double-transmon coupler (DTC) in superconducting quantum processors, achieving 99.90% fidelity for CZ gates and 99.98% for single-qubit gates using state-of-the-art fabrication and reinforcement learning-based pulse optimization. The DTC suppresses residual qubit interactions while enabling fast two-qubit operations, particularly for highly detuned qubits. This advances NISQ devices and fault-tolerant quantum computing via quantum error correction.
quantum-computingcz-gatesuperconducting-qubitstransmon-couplergate-fidelityquantum-error-correctionreinforcement-learning-pulses
“DTC scheme attains 99.90% fidelity for CZ gates”
paper / nakamuralab / Feb 20
CAPP's Main Axion eXperiment conducted a high-sensitivity search for axion dark matter in the 1.025–1.185 GHz (4.24–4.91 μeV) range using a 37-liter copper cavity in a 12 T Nb3Sn/NbTi magnet. Innovations include a dilution refrigerator achieving <40 mK core temperature and quantum-limited readout yielding ≤200 mK system noise, equivalent to ~4×10^3 thermal photons/s in the axion bandwidth, against an expected ~10^2 axion signal photons/s for DFSZ coupling. This setup delivers unprecedented sensitivity, imposing the world's strictest limits on axion-photon coupling in this regime.
axion-dark-matteraxion-haloscopecavity-experimentdark-matter-searchhigh-sensitivity-detectionparticle-physics
“Search conducted over 1.025–1.185 GHz (4.24–4.91 μeV) mass range”
paper / nakamuralab / Feb 14
Path signature, a mathematical tool for stochastic time series analysis, enhances qubit state assignment fidelity by extracting features from raw measurement records beyond simple integration. Demonstrated on five hardware setups—including multiplexed and non-multiplexed with/without parametric amplifiers—it consistently improves readout performance. The method also detects mid-measurement state transitions, enabling accurate prediction of the final qubit state.
superconducting-qubitsqubit-readoutpath-signaturequantum-non-demolitionquantum-measurementstochastic-analysis
“Path signature method improves assignment fidelity in superconducting qubit readout across all tested setups”
paper / nakamuralab / Dec 18
Theoretical models from multiple groups converge on dark matter axion masses between 20-30 μeV. A multi-cell cavity haloscope in a 12 T magnetic field searched for axion-induced microwave signals in the 21.86-22.00 μeV range. The experiment ruled out KSVZ axions as dark matter at 90% confidence level, validating targeted searches based on cosmological predictions.
dark-matter-axionsaxion-searchhaloscope-experimenthigh-energy-physicsksvz-modelmicrowave-detection
“Theoretical approaches from independent groups constrain axion mass to 20-30 μeV overlap”
paper / nakamuralab / Sep 25
Researchers propose semi-analytically optimized pulses based on perturbative analysis to enable ZZ-interaction-free single-qubit gates on superconducting transmon qubits. These gates remain robust against slow qubit-frequency fluctuations spanning a few MHz, countering ZZ-induced shifts from neighboring qubits. Experimental demonstration confirms viability without extra hardware, aiding scalable fixed-frequency qubit integration.
quantum-physicssuperconducting-qubitszz-interactionsingle-qubit-gatepulse-optimizationtransmon-qubitsarxiv-paper
“Static ZZ interaction causes qubit frequency shifts depending on neighboring qubit states in fixed-frequency transmon qubits.”
paper / nakamuralab / Sep 8
A nonlinear Purcell filter mitigates photon-noise-induced dephasing in superconducting qubit dispersive readout by dynamically narrowing the readout resonator's linewidth during pulses, enhancing qubit sensitivity without performance loss. Fabricated devices show 3x improved noise tolerance over linear filters and another 3x faster measurement rates via bifurcation. Achieves 99.4% readout fidelity and 99.2% QND fidelity in 40 ns pulses, preserving qubit coherence.
superconducting-qubitsdispersive-readoutpurcell-filterquantum-noisephoton-noisenonlinear-filterqubit-coherence
“Nonlinear Purcell filter reduces effective linewidth of readout resonator during readout pulse”
paper / nakamuralab / Jul 6
Researchers experimentally demonstrate a virtual two-qubit gate, decomposing it into single-qubit unitaries and projection gates implemented via mid-circuit measurements for expectation-value estimation. A deterministic sampling scheme minimizes required circuit evaluations, while quantum error mitigation counters measurement errors. The approach yields an average gate fidelity of 0.9938 ± 0.0002 for a virtual controlled-Z gate, enabling efficient simulation with fewer qubits or distant qubit operations.
quantum-computingtwo-qubit-gatequantum-process-tomographyvirtual-gatequantum-error-mitigationexperimental-demonstration
“Virtual two-qubit gates are decomposed into single-qubit unitary gates and projection gates for expectation-value estimation.”
paper / nakamuralab / May 9
Proposes a systematic method using a look-up table (LUT) and online fine-tuning to control resonance frequency, pump frequency, and power in flux-driven Josephson Parametric Amplifiers (JPAs). Demonstrates 20 dB gain at 5.9 GHz with 100 MHz bandwidth tunable in 20 kHz steps. Applied successfully in a haloscope axion experiment, enhancing stochastic small-signal detection at millikelvin temperatures.
josephson-parametric-amplifiersjpa-tuningflux-driven-jpamicrowave-amplificationquantum-detectorsaxion-haloscopemillikelvin-experiments
“Flux-driven JPA achieves 20 dB gain centered at 5.9 GHz.”
paper / nakamuralab / Apr 26
Researchers at CAPP-IBS developed a 4-channel system for characterizing Josephson parametric amplifiers (JPAs) and cryogenic low-noise amplifiers (cLNAs based on HEMTs) in a 20 mK dry dilution refrigerator. The setup uses a wideband noise source (NS) thermally isolated via plastic spacers, superconducting wires, and coaxial cables, supporting continuous months-long axion haloscope experiments in 12 T fields and 40-liter cavities. It measures gain and noise temperature for 4 HEMTs and 2 JPAs in a single cool-down, optimizing scanning rate and quantum-limited sensitivity for 1-6 GHz frequencies.
josephson-parametric-amplifierssuperconductivitycryogenic-measurementquantum-detectorsaxion-searchhemt-amplifiersinstrumentation
“Axion haloscope experiments at CAPP operate from 1 GHz to 6 GHz in 12 T magnetic fields using ~40-liter cavities”
paper / nakamuralab / Apr 13
CAPP's axion haloscope experiments target 1-6 GHz frequencies, operating cavities at mK temperatures to maximize sensitivity to weak axion signals. Low-noise amplifiers were developed and deployed across 5 haloscopes covering distinct frequency bands. These amplifiers achieve noise temperatures approaching the quantum limit, enabling quantum noise-limited performance.
axion-searchcavity-haloscopelow-noise-amplifiersquantum-limitdark-mattersuperconductivitylow-temperature-physics
“Axion solves the strong CP problem in QCD and is a leading dark matter candidate.”
paper / nakamuralab / Apr 10
Researchers deploy a flux-driven Josephson parametric amplifier (JPA) as the first-stage amplifier in axion haloscope experiments, targeting noise levels near the half-quantum limit at millikelvin temperatures. Measurements demonstrate added noise below 110 mK across 0.938–0.963 GHz, supporting axion dark matter searches and strong CP problem resolution. This advances cryogenic amplification for faint signal detection.
josephson-parametric-amplifieraxion-detectiondark-mattersuperconductivitylow-noise-amplifiermillikelvin-electronics
“Axion is a candidate for dark matter and solves the strong CP problem.”
paper / nakamuralab / Feb 14
Researchers demonstrate all-microwave control of fixed-frequency superconducting qubits using a transmon coupler's third-order nonlinearity under microwave drive to induce swap interactions between data transmons. This enables a controlled-Z (CZ) gate with high drive efficiency and minimal residual ZZ interaction across wide detunings. Analytical and numerical models validate the approach, reducing noise and wiring complexity in quantum circuits.
quantum-physicssuperconducting-qubitstransmon-couplermicrowave-controlcontrolled-z-gatearxiv-paper
“Swap interaction between two data transmons is induced by the third-order nonlinearity of a coupler transmon under microwave drive”
paper / nakamuralab / Feb 3
CAPP-12TB haloscope data from March 2022 yields the first search for axion dark matter in the Sagittarius tidal stream around 4.55 μeV. The analysis excludes DFSZ axion densities ρ_a ≳ 0.184 GeV/cm³ and KSVZ axion densities ρ_a ≳ 0.025 GeV/cm³ over the mass range 4.51–4.59 μeV at 90% CL. No signal detected, setting new limits on enhanced local DM density in galactic substructure.
axion-dark-mattersagittarius-tidal-streamhaloscope-experimentdark-matter-searchhep-excapp-12tbarxiv-paper
“First search for Sagittarius tidal stream axion dark matter using CAPP-12TB haloscope”
paper / nakamuralab / Nov 14
Experiments on 2D JJAs show no sharp superconductor-insulator transition but a gradual crossover to the insulating phase as EJ/EC decreases, evidenced by temperature-dependent resistance. Low-T I-V curves follow I = cV + bV^a, modeled via BKT with finite-range logarithmic Cooper-pair interactions, enabling extraction of crossover temperatures and insulating phase diagram. At ultra-low T, negative differential conductance emerges from coherent single-Cooper-pair tunneling.
josephson-junction-arraysinsulating-phasenonlinear-transportberezinskii-kosterlitz-thoulesssuperconductivitycooper-pair-tunnelingquantum-physics
“2D JJAs exhibit a gradual crossover to the insulating phase rather than a sharp phase transition”
paper / nakamuralab / Oct 20
The CAPP-12TB haloscope conducted an axion dark matter search at Dine-Fischler-Srednicki-Zhitnitskii (DFSZ) sensitivity, excluding the axion-photon coupling g_{aγγ} down to 6.2×10^{-16} GeV^{-1} over the mass range 4.51–4.59 μeV at 90% CL, assuming axions comprise 100% of local dark matter. This sensitivity also excludes Kim-Shifman-Vainshtein-Zakharov (KSVZ) axion dark matter constituting just 13% of local dark matter density. The experiment plans broader mass range searches ahead.
axion-dark-matterhaloscope-experimentcapp-12tbaxion-photon-couplingdark-matter-searchhep-experiment
“CAPP-12TB excludes g_{aγγ} down to 6.2×10^{-16} GeV^{-1} for axion masses 4.51–4.59 μeV at 90% CL assuming 100% local DM”
paper / nakamuralab / Sep 13
In deep-strong coupling regimes, hybrid qubit-resonator systems feature an entangled quantum vacuum with nonzero average virtual photons that break the parity symmetry of a dispersively coupled ancillary probe qubit. Researchers experimentally demonstrated this effect using a flux qubit deep-strongly coupled to a lumped-element superconducting resonator, with an Xmon artificial atom as the probe. This observation enables direct exploration of quantum vacuum phenomena inaccessible via photon detection.
quantum-physicssymmetry-breakinghybrid-quantum-systemsdeep-strong-couplingsuperconducting-qubitsquantum-vacuumarxiv-paper
“Qubit-resonator systems in deep-strong coupling have an entangled quantum vacuum with nonzero average photon number in the resonator”
paper / nakamuralab / Jul 27
CAPP's axion dark matter experiment reached a system noise temperature of 200 mK using a flux-driven Josephson parametric amplifier in phase-insensitive mode, the lowest among published cavity haloscope searches. A two-stage scanning method increased scan speed by 26%. This enabled exclusion of QCD axion two-photon couplings with an order-of-magnitude higher sensitivity than prior limits over the 9.39-9.51 μeV mass range.
axion-dark-matterhep-exjosephson-parametric-amplifiercapp-experimentquantum-noiseqcd-axioninstrumentation
“Axion search conducted over mass range 9.39-9.51 μeV”
paper / nakamuralab / Jul 13
Experiment demonstrates quantum discord's asymptotic robustness to thermal noise in propagating two-mode squeezed microwave states generated by Josephson parametric amplifiers, contrasting with entanglement's sudden death under asymmetric noise injection. A crossover between discord and entanglement occurs at low noise photon numbers due to tripartite noise effects, with their difference linked to locally inaccessible information flow. This disparity underpins security advantages in quantum key distribution protocols.
quantum-correlationstwo-mode-squeezed-statesquantum-discordquantum-entanglementmicrowave-statesjosephson-parametric-amplifiersnoise-robustness
“Quantum discord persists against thermal noise in two-mode squeezed microwave states while entanglement undergoes sudden death.”
paper / nakamuralab / Feb 13
Researchers suppress resonator-mediated qubit decay by over two orders of magnitude across a 600 MHz bandwidth using the distributed-element, multi-mode nature of the resonator as an intrinsic Purcell filter. This allows increasing the resonator's external decay rate without compromising qubit lifetime, enabling 40-ns readout at 99.1% fidelity and 100-ns reset with under 1.7% residual excitation. The approach overcomes the fundamental speed-lifetime tradeoff in dispersive qubit operations.
superconducting-qubitpurcell-filterquantum-readoutresonator-couplingqubit-resetquantum-physics
“Resonator-mediated qubit decay is suppressed by more than two orders of magnitude over a 600 MHz bandwidth”
paper / nakamuralab / Jun 17
Cavity magnonics explores interactions between magnons in magnetic materials and confined microwave photons, enabling strong coupling and nonlinear regimes. Enhancements via Brillouin light scattering in optical resonators allow magnon distribution control. Cavity-mediated coupling to superconducting qubits supports single-magnon detection, positioning the field for quantum technology integration.
cavity-magnonicsmagnon-photon-couplingmesoscale-physicscondensed-matterquantum-technologiesmicrowave-cavitiesarxiv-paper
“Cavity magnonics involves interaction of magnons and confined electromagnetic fields.”
paper / nakamuralab / May 18
Researchers demonstrate optical polarimetry for measuring surface acoustic waves (SAW) by converting the slope of the periodically tilting surface into polarization rotation angle. This method provides local measurements of SAW amplitude and phase with resolution set by the optical beam spot size. Calibration is straightforward in the shot-noise-limited regime, offering quantitative benefits for SAW-based technologies over traditional path modulation or diffraction techniques.
surface-acoustic-wavesoptical-polarimetrysaw-measurementapplied-physicsopticsquantum-physics
“SAW measurement via optical polarimetry translates surface slope into polarization rotation.”