Chronological feed of everything captured from Yasunobu Nakamura.
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”