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Register a new userNoise suppression in transmission of single flux quantum signals
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Added by
Andrey L. Pankratov
Publication date
2005
fields
Physics
and research's language is
English
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We present the analysis of the mean switching time and its standard deviation of an overdamped Josephson junction, driven by a direct current and a single flux quantum (SFQ) pulse. The performed analysis allows to find the optimal value of the bias current of the clock generator, responsible for the shape of SFQ pulse, which minimizes noise-induced switching errors.
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Magnetic flux noise is a dominant source of dephasing and energy relaxation in superconducting qubits. The noise power spectral density varies with frequency as $1/f^alpha$ with $alpha sim 1$ and spans 13 orders of magnitude. Recent work indicates that the noise is from unpaired magnetic defects on the surfaces of the superconducting devices. Here, we demonstrate that adsorbed molecular O$_2$ is the dominant contributor to magnetism in superconducting thin films. We show that this magnetism can be suppressed by appropriate surface treatment or improvement in the sample vacuum environment. We observe a suppression of static spin susceptibility by more than an order of magnitude and a suppression of $1/f$ magnetic flux noise power spectral density by more than a factor of 5. These advances open the door to realization of superconducting qubits with improved quantum coherence.
We study the low frequency current correlations of an individual single-walled carbon nanotube at liquid He temperature. We have distinguished two physical regimes -- zero dimensional quantum dot and one dimensional quantum wire -- in terms of an energy spacing from the finite tube length in both differential conductance and shot noise measurements. In a one dimensional wire regime, we observed a highly suppressed shot noise from all measured tube devices, suggesting that electron-electron interactions play an important role.
We study a superconducting transmission line (TL) formed by distributed LC oscillators and excited by external magnetic fluxes which are aroused from random magnetization (A) placed in substrate or (B) distributed at interfaces of a two-wire TL. Low-frequency dynamics of a random magnetic field is described based on the diffusion Langevin equation with a short-range source caused by (a) random amplitude or (b) gradient of magnetization. For a TL modeled as a two-port network with open and shorted ends, the effective magnetic flux at the open end has non-local dependency on noise distribution along the TL. The flux-flux correlation function is evaluated and analyzed for the regimes (Aa), (Ab). (Ba), and (Bb). Essential frequency dispersion takes place around the inverse diffusion time of random flux along the TL. Typically, noise effect increases with size faster than the area of TL. The flux-flux correlator can be verified both via the population relaxation rate of the qubit, which is formed by the Josephson junction shunted by the TL with flux noises, and via random voltage at the open end of the TL.
This article addresses the question whether the magnetic flux of stationary vortices or of half flux quanta generated by frustrated superconducting rings is noisy. It is found that the flux noise generated intrinsically by a superconductor is, in good approximation, not enhanced by stationary vortices. Half flux quanta generated by $pi$-rings are characterized by considerably larger noise.
Focused ion beam (FIB) technology has been used to fabricate miniature Nb DC SQUIDs which incorporate resistively-shunted microbridge junctions and a central loop with a hole diameter ranging from 1058 nm to 50 nm. The smallest device, with a 50 nm hole diameter, has a white flux noise level of 2.6 microphy_{0}/Hz^{0.5} at 10^{4} Hz. The scaling of the flux noise properties and focusing effect of the SQUID with the hole size were examined. The observed low-frequency flux noise of different devices were compared with the contribution due to the spin fluctuation of defects during FIB processing and the thermally activated flux hopping in the SQUID washer.
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