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Register a new userDephasing of a superconducting qubit induced by photon noise
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We have studied the dephasing of a superconducting flux-qubit coupled to a DC-SQUID based oscillator. By varying the bias conditions of both circuits we were able to tune their effective coupling strength. This allowed us to measure the effect of such a controllable and well-characterized environment on the qubit coherence. We can quantitatively account for our data with a simple model in which thermal fluctuations of the photon number in the oscillator are the limiting factor. In particular, we observe a strong reduction of the dephasing rate whenever the coupling is tuned to zero. At the optimal point we find a large spin-echo decay time of $4 mu s$.
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We theoretically study measurement induced-dephasing of a superconducting qubit in the circuit QED architecture and compare the results to those obtained experimentally by Schuster {it et al.}, [Phys. Rev. Lett. 94, 123602 (2005)]. Strong coupling of the qubit to the resonator leads to a significant ac-Stark shift of the qubit transition frequency. As a result, quantum fluctuations in the photon number populating the resonator cause dephasing of the qubit. We find good agreement between the predicted line shape of the qubit spectrum and the experimental results. Furthermore, in the strong dispersive limit, where the Stark shift per photon is large compared to the cavity decay rate and the qubit linewidth, we predict that the qubit spectrum will be split into multiple peaks, with each peak corresponding to a different number of photons in the cavity.
The proximity effect (PE) between superconductor and confined electrons can induce the effective pairing phenomena of electrons in nanowire or quantum dot (QD). Through interpreting the PE as an exchange of virtually quasi-excitation in a largely gapped superconductor, we found that there exists another induced dynamic process. Unlike the effective pairing that mixes the QD electron states coherently, this extra process leads to dephasing of the QD. In a case study, the dephasing time is inversely proportional to the Coulomb interaction strength between two electrons in the QD. Further theoretical investigations imply that this dephasing effect can decrease the quality of the zero temperature mesoscopic electron transportation measurements by lowering and broadening the corresponding differential conductance peaks.
We evaluate the rates of energy and phase relaxation of a superconducting qubit caused by stray photons with energy exceeding the threshold for breaking a Cooper pair. All channels of relaxation within this mechanism are associated with the change in the charge parity of the qubit, enabling the separation of the photon-assisted processes from other contributions to the relaxation rates. Among the signatures of the new mechanism is the same order of rates of the transitions in which a qubit looses or gains energy.
We suggest and demonstrate a protocol which suppresses dephasing due to the low-frequency noise by qubit motion, i.e., transfer of the logical qubit of information in a system of $n geq 2$ physical qubits. The protocol requires only the nearest-neighbor coupling and is applicable to different qubit structures. We further analyze its effectiveness against noises with arbitrary correlations. Our analysis, together with experiments using up to three superconducting qubits, shows that for the realistic uncorrelated noises, qubit motion increases the dephasing time of the logical qubit as $sqrt{n}$. In general, the protocol provides a diagnostic tool to measure the noise correlations.
The loss and noise mechanisms of superconducting resonators are useful tools for understanding decoherence in superconducting circuits. While the loss mechanisms have been heavily studied, noise in superconducting resonators has only recently been investigated. In particular, there is an absence of literature on noise in the single photon limit. Here, we measure the loss and noise of an aluminium on silicon quarter-wavelength ($lambda/4$) resonator in the single photon regime.
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