112 research outputs found
Magnetic field tuning of coplanar waveguide resonators
We describe measurements on microwave coplanar resonators designed for
quantum bit experiments. Resonators have been patterned onto sapphire and
silicon substrates, and quality factors in excess of a million have been
observed. The resonant frequency shows a high sensitivity to magnetic field
applied perpendicular to the plane of the film, with a quadratic dependence for
the fundamental, second and third harmonics. Frequency shift of hundreds of
linewidths can be obtained.Comment: Accepted for publication in AP
On the properties of superconducting planar resonators at mK temperatures
Planar superconducting resonators are now being increasingly used at mK
temperatures in a number of novel applications. They are also interesting
devices in their own right since they allow us to probe the properties of both
the superconductor and its environment. We have experimentally investigated
three types of niobium resonators - including a lumped element design -
fabricated on sapphire and SiO_2/Si substrates. They all exhibit a non-trivial
temperature dependence of their centre frequency and quality factor. Our
results shed new light on the interaction between the electromagnetic waves in
the resonator and two-level fluctuators in the substrate.Comment: V2 includes some minor corrections/changes. Submitted to PR
Circuit QED with a Flux Qubit Strongly Coupled to a Coplanar Transmission Line Resonator
We propose a scheme for circuit quantum electrodynamics with a
superconducting flux-qubit coupled to a high-Q coplanar resonator. Assuming
realistic circuit parameters we predict that it is possible to reach the strong
coupling regime. Routes to metrological applications, such as single photon
generation and quantum non-demolition measurements are discussed.Comment: 8 pages, 5 figure
Anomalously strong pinning of the filling factor nu=2 in epitaxial graphene
We explore the robust quantization of the Hall resistance in epitaxial
graphene grown on Si-terminated SiC. Uniquely to this system, the dominance of
quantum over classical capacitance in the charge transfer between the substrate
and graphene is such that Landau levels (in particular, the one at exactly zero
energy) remain completely filled over an extraordinarily broad range of
magnetic fields. One important implication of this pinning of the filling
factor is that the system can sustain a very high nondissipative current. This
makes epitaxial graphene ideally suited for quantum resistance metrology, and
we have achieved a precision of 3 parts in 10^10 in the Hall resistance
quantization measurements
Coupling of a locally implanted rare-earth ion ensemble to a superconducting micro-resonator
We demonstrate the coupling of rare-earth ions locally implanted in a
substrate (Gd in AlO) to a superconducting NbN
lumped-element micro-resonator. The hybrid device is fabricated by a controlled
ion implantation of rare-earth ions in well-defined micron-sized areas, aligned
to lithographically defined micro-resonators. The technique does not degrade
the internal quality factor of the resonators which remain above .
Using microwave absorption spectroscopy we observe electron-spin resonances in
good agreement with numerical modelling and extract corresponding coupling
rates of the order of MHz and spin linewidths of MHz.Comment: 4 pages, 2 Figure
Influence of impurity spin dynamics on quantum transport in epitaxial graphene
Experimental evidence from both spin-valve and quantum transport measurements
points towards unexpectedly fast spin relaxation in graphene. We report
magnetotransport studies of epitaxial graphene on SiC in a vector magnetic
field showing that spin relaxation, detected using weak-localisation analysis,
is suppressed by an in-plane magnetic field, , and thereby
proving that it is caused at least in part by spinful scatterers. A
non-monotonic dependence of effective decoherence rate on
reveals the intricate role of scatterers' spin dynamics in forming the
interference correction to conductivity, an effect that has gone unnoticed in
earlier weak localisation studie
Disorder induced Dirac-point physics in epitaxial graphene from temperature-dependent magneto-transport measurements
We report a study of disorder effects on epitaxial graphene in the vicinity
of the Dirac point by magneto-transport. Hall effect measurements show that the
carrier density increases quadratically with temperature, in good agreement
with theoretical predictions which take into account intrinsic thermal
excitation combined with electron-hole puddles induced by charged impurities.
We deduce disorder strengths in the range 10.2 31.2 meV, depending on
the sample treatment. We investigate the scattering mechanisms and estimate the
impurity density to be cm for our samples.
An asymmetry in the electron/hole scattering is observed and is consistent with
theoretical calculations for graphene on SiC substrates. We also show that the
minimum conductivity increases with increasing disorder potential, in good
agreement with quantum-mechanical numerical calculations.Comment: 6 pages, 3 figure
Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system
We demonstrate quantum Hall resistance measurements with metrological
accuracy in a small cryogen-free system operating at a temperature of around
3.8K and magnetic fields below 5T. Operating this system requires little
experimental knowledge or laboratory infrastructure, thereby greatly advancing
the proliferation of primary quantum standards for precision electrical
metrology. This significant advance in technology has come about as a result of
the unique properties of epitaxial graphene on SiC.Comment: 15 pages, 9 figure
Pound-locking for characterization of superconducting microresonators
We present a new application and implementation of the so-called Pound
locking technique for the interrogation of superconducting microresonators. We
discuss how by comparing against stable frequency sources this technique can be
used to characterize properties of resonators that can not be accessed using
traditional methods. Specifically, by analyzing the noise spectra and the Allan
deviation we obtain valuable information about the nature of the noise in
superconducting planar resonators. This technique also greatly improves the
read-out accuracy and measurement throughput compared to conventional methods.Comment: 5 page
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