213 research outputs found
Micrometre-scale refrigerators
A superconductor with a gap in the density of states or a quantum dot with
discrete energy levels is a central building block in realizing an electronic
on-chip cooler. They can work as energy filters, allowing only hot
quasiparticles to tunnel out from the electrode to be cooled. This principle
has been employed experimentally since the early 1990s in investigations and
demonstrations of micrometre-scale coolers at sub-kelvin temperatures. In this
paper, we review the basic experimental conditions in realizing the coolers and
the main practical issues that are known to limit their performance. We give an
update of experiments performed on cryogenic micrometre-scale coolers in the
past five years
Cooling of suspended nanostructures with tunnel junctions
We have investigated electronic cooling of suspended nanowires with SINIS
tunnel junction coolers. The suspended samples consist of a free standing
nanowire suspended by four narrow ( 200 nm) bridges. We have compared two
different cooler designs for cooling the suspended nanowire. We demonstrate
that cooling of the nanowire is possible with a proper SINIS cooler design
Electronic cooling of a submicron-sized metallic beam
We demonstrate electronic cooling of a suspended AuPd island using
superconductor-insulator-normal metal tunnel junctions. This was achieved by
developing a simple fabrication method for reliably releasing narrow submicron
sized metal beams. The process is based on reactive ion etching and uses a
conducting substrate to avoid charge-up damage and is compatible with e.g.
conventional e-beam lithography, shadow-angle metal deposition and oxide tunnel
junctions. The devices function well and exhibit clear cooling; up to factor of
two at sub-kelvin temperatures.Comment: 4 pages, 3 figure
Correlated adiabatic and isocurvature CMB fluctuations in the wake of the WMAP
In general correlated models, in addition to the usual adiabatic component
with a spectral index n_ad1 there is another adiabatic component with a
spectral index n_ad2 generated by entropy perturbation during inflation. We
extend the analysis of a correlated mixture of adiabatic and isocurvature CMB
fluctuations of the WMAP group, who set the two adiabatic spectral indices
equal. Allowing n_ad1 and n_ad2 to vary independently we find that the WMAP
data favor models where the two adiabatic components have opposite spectral
tilts. Using the WMAP data only, the 2-sigma upper bound for the isocurvature
fraction f_iso of the initial power spectrum at k_0=0.05 Mpc^{-1} increases
somewhat, e.g., from 0.76 of n_ad2 = n_ad1 models to 0.84 with a prior n_iso <
1.84 for the isocurvature spectral index. We also comment on a possible
degeneration between the correlation component and the optical depth tau.
Moreover, the measured low quadrupole in the TT angular power could be achieved
by a strong negative correlation, but then one needs a large tau to fit the TE
spectrum.Comment: 5 pages, 7 figures. V2: Added 2 figures and revised a bit the results
section. This is a slightly longer version than the published one in PR
Magnetic-field-induced stabilization of nonequilibrium superconductivity in a normal-metal/insulator/superconductor junction
A small magnetic field is found to enhance relaxation processes in a superconductor, thus stabilizing superconductivity in nonequilibrium conditions. In a normal-metal (N)/insulator/superconductor (S) tunnel junction, applying a field of the order of 100μT leads to significantly improved cooling of the N island by quasiparticle (QP) tunneling. These findings are attributed to faster QP relaxation within the S electrodes as a result of enhanced QP drain through regions with a locally suppressed energy gap due to magnetic vortices in the S leads at some distance from the junction.Peer reviewe
High-fidelity adiabatic inversion of a electron spin qubit in natural silicon
The main limitation to the high-fidelity quantum control of spins in
semiconductors is the presence of strongly fluctuating fields arising from the
nuclear spin bath of the host material. We demonstrate here a substantial
improvement in single-qubit gate fidelities for an electron spin qubit bound to
a P atom in natural silicon, by applying adiabatic inversion instead of
narrow-band pulses. We achieve an inversion fidelity of 97%, and we observe
signatures in the spin resonance spectra and the spin coherence time that are
consistent with the presence of an additional exchange-coupled donor. This work
highlights the effectiveness of adiabatic inversion techniques for spin control
in fluctuating environments.Comment: 4 pages, 2 figure
Bell's inequality violation with spins in silicon
Bell's theorem sets a boundary between the classical and quantum realms, by
providing a strict proof of the existence of entangled quantum states with no
classical counterpart. An experimental violation of Bell's inequality demands
simultaneously high fidelities in the preparation, manipulation and measurement
of multipartite quantum entangled states. For this reason the Bell signal has
been tagged as a single-number benchmark for the performance of quantum
computing devices. Here we demonstrate deterministic, on-demand generation of
two-qubit entangled states of the electron and the nuclear spin of a single
phosphorus atom embedded in a silicon nanoelectronic device. By sequentially
reading the electron and the nucleus, we show that these entangled states
violate the Bell/CHSH inequality with a Bell signal of 2.50(10). An even higher
value of 2.70(9) is obtained by mapping the parity of the two-qubit state onto
the nuclear spin, which allows for high-fidelity quantum non-demolition
measurement (QND) of the parity. Furthermore, we complement the Bell inequality
entanglement witness with full two-qubit state tomography exploiting QND
measurement, which reveals that our prepared states match the target maximally
entangled Bell states with 96\% fidelity. These experiments demonstrate
complete control of the two-qubit Hilbert space of a phosphorus atom, and show
that this system is able to maintain its simultaneously high initialization,
manipulation and measurement fidelities past the single-qubit regime.Comment: 10 pages, 3 figures, 1 table, 4 extended data figure
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