69 research outputs found
Pure dephasing in flux qubits due to flux noise with spectral density scaling as
For many types of superconducting qubits, magnetic flux noise is a source of
pure dephasing. Measurements on a representative dc superconducting quantum
interference device (SQUID) over a range of temperatures show that , where is the flux noise spectral density,
is of the order of 1 and ; is the flux quantum. For a qubit with an energy level
splitting linearly coupled to the applied flux, calculations of the dependence
of the pure dephasing time of Ramsey and echo pulse sequences on
for fixed show that decreases rapidly as is
reduced. We find that is relatively insensitive to the noise
bandwidth, , for all provided the ultraviolet
cutoff frequency . We calculate the ratio of the echo () and Ramsey () sequences, and the dependence
of the decay function on and . We investigate the case in which
is fixed at the "pivot frequency" Hz while
is varied, and find that the choice of can greatly influence the
sensitivity of and to the value of .
Finally, we present calculated values of in a qubit corresponding
to the values of and measured in our SQUID.Comment: 7 pages, 8 figures, 1 tabl
Two-Level Systems in Nucleated and Non-Nucleated Epitaxial alpha-Tantalum films
Building usefully coherent superconducting quantum processors depends on
reducing losses in their constituent materials. Tantalum, like niobium, has
proven utility as the primary superconducting layer within highly coherent
qubits. But, unlike Nb, high temperatures are typically used to stabilize the
desirable body-centered-cubic phase, alpha-Ta, during thin film deposition. It
has long been known that a thin Nb layer permits the room-temperature
nucleation of alpha-Ta, although neither an epitaxial process nor few-photon
microwave loss measurements have been reported for Nb-nucleated Ta films prior
to this study. We compare resonators patterned from Ta films grown at high
temperature (500 {\deg}C) and films nucleated at room temperature, in order to
understand the impact of crystalline order on quantum coherence. In both cases,
films grew with Al2O3 (001) || Ta (110) indicating that the epitaxial
orientation is independent of temperature and is preserved across the Nb/Ta
interface. We use conventional low-power spectroscopy to measure two level
system (TLS) loss, as well as an electric-field bias technique to measure the
effective dipole moments of TLS in the surfaces of resonators. In our
measurements, Nb-nucleated Ta resonators had greater loss tangent (1.5 +/- 0.1
x 10^-5) than non-nucleated (5 +/- 1 x 10^-6) in approximate proportion to
defect densities as characterized by X-ray diffraction (0.27 {\deg} vs 0.18
{\deg} [110] reflection width) and electron microscopy (30 nm vs 70 nm domain
size). The dependence of the loss tangent on domain size indicates that the
development of more ordered Ta films is likely to lead to improvements in qubit
coherence times. Moreover, low-temperature alpha-Ta epitaxy may enable the
growth of new, microstate-free heterostructures which would not withstand high
temperature processing
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Implications of heavy-ion-induced satellite x-ray emission. II. Production of K and L x rays by 0. 9 to 2. 6 MeV/u Ar ions in thick targets of V, Cu, Nb, Ta, and Pt
Cross sections are reported for x-ray production in targets of /sup 23/V, /sup 29/Cu, /sup 41/Nb, /sup 73/Ta, and /sup 78/Pt by /sup 40/Ar ions of 36.0, 56.4, 76.6, and 103 MeV. Because the targets were relatively thick, approx. 1 mg/cm/sup 2/, the data were corrected, using a novel approach, for projectile energy loss and x-ray attenuation in the targets. The cross sections so analyzed are compared with the predictions of the first Born approximation as well as with those of a more extensive treatment which includes energy loss, Coulomb deflection, perturbed stationary-state, and relativistic effects. The significant discrepancies between the data and this latter theory are atrributed primarily to the influence of multiple ionization on the x-ray emission probabilities
Multiple outer-shell ionization effect in inner-shell x-ray production by light ions
This article discusses multiple outer-shell ionization effect in inner-shell x-ray production by light ions
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Detection and measurement of nuclear radiation
The technique of radiation characterization is reviewed, with particular emphasis on new methods and their practi-cal aspects. Each type of detector is discussed in terms of its principle of operation and its applicability to various problems in counting and spectrometry. Auxiliary electronic instrumentation and the function of each instrument are described in general terms. Other topics discussed include low-level counting, absolute counting, and the mounting of radioactive sources. (137 references.) (C.H.
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Atomic Energy Commission Report MTA-31
From abstract: An automatic gamma-ray spectrometer for routine assays in radiochemical investigations has been constructed. Circuits are described for the automatic control of the pulse analyzer bias and automatic recording of the counting data. Typical gamma-ray spectra are shown as an indication of the perfomance obtained
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THE SPECTROMETRIC DETERMINATION OF SOME BETA PARTICLE AND CONVERSION ELECTRON ENERGIES
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Detection and Measurement of Nuclear Radiation
The technique of radiation characterization is reviewed, with particular emphasis on new methods and their practi-cal aspects. Each type of detector is discussed in terms of its principle of operation and its applicability to various problems in counting and spectrometry. Auxiliary electronic instrumentation and the function of each instrument are described in general terms. Other topics discussed include low-level counting, absolute counting, and the mounting of radioactive sources. (137 references.) (C.H.
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RADIOCHEMICAL AND SPECTROMETER STUDIES OF SEVERAL NEUTRON-DEFICIENT ZIRCONIUM ISOTOPES AND THEIR DECAY PRODUCTS
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