25 research outputs found
Near-Field Scanning Microwave Microscopy in the Single Photon Regime
The microwave properties of nano-scale structures are important in a wide
variety of applications in quantum technology. Here we describe a low-power
cryogenic near-field scanning microwave microscope (NSMM) which maintains
nano-scale dielectric contrast down to the single microwave photon regime, up
to times lower power than in typical NSMMs. We discuss the remaining
challenges towards developing nano-scale NSMM for quantum coherent interaction
with two-level systems as an enabling tool for the development of quantum
technologies in the microwave regime
Electrostatic effects in coupled quantum dot-point contact-single electron transistor devices
We study the operation of a system where quantum dot (QD) and point contact (PC) defined in a two-dimensional electron gas of a high-mobility GaAs/AlGaAs heterostructure are capacitively coupled to each other and to metallic single electron transistor (SET). The charge state of the quantum dot can be probed by the point contact or single electron transistor. These can be used for sensitive detection of terahertz radiation. In this work, we explore an electrostatic model of the system. From the model, we determine the sensitivity of the point contact and the single electron transistor to the charge excitation of the quantum dot. Nearly periodic oscillations of the point contact conductance are observed in the vicinity of pinch-off voltage. They can be attributed to Coulomb blockade effect in a quasi-1D channel because of unintentional formation of small quantum dot. The latter can be a result of fluctuations in GaAs quantum well thickness
Andreev Probe of Persistent Current States in Superconducting Quantum Circuits
Using the extraordinary sensitivity of Andreev interferometers to the
superconducting phase difference associated with currents, we measure the
persistent current quantum states in superconducting loops interrupted by
Josephson junctions. Straightforward electrical resistance measurements of the
interferometers give continuous read-out of the states, allowing us to
construct the energy spectrum of the quantum circuit. The probe is estimated to
be more precise and faster than previous methods, and can measure the local
phase difference in a wide range of superconducting circuits.Comment: Changes made in light of referees comments; to appear in PR
Charge control of blockade of Cooper pair tunneling in highly disordered TiN nanowires in an inductive environment
Low-energy quasiparticle transport through Andreev levels
We measure the resistance of a normal mesoscopic sample with two Superconducting mirrors and find two regimes with qualitatively different behavior. At temperatures below 90 mK peaks in the conductance were found when the phase difference between the two superconductors is an odd multiple of rr. The peak heights increase with decreasing temperature. Above 100 mK the observed peaks give way to dips in the conductance. While the high-temperature behavior can be explained in terms of the thermal effect [Phys. Rev. Lett, 76, 823 (1996)], we propose that the low-temperature behavior is: a manifestation of resonant transmission of low-energy quasiparticles through Andreev states. [S0163-1829(99)11641-2]
Direct demonstration of circulating currents in a controllable -SQUID generated by a 0 to transition of the weak links
A controllable -SQUID is a DC SQUID with two controllable
-junctions as weak links. A controllable -junction consists of a
superconducting - normal metal - superconducting Josephson junction with two
additional contacts to the normal region of the junction. By applying a voltage
over these contacts it is possible to control the sate of the junction,
i.e. a conventional (0) state or a -state, depending on the magnitude of
. We demonstrate experimentally that, by putting one junction into a
-state, a screening current is generated around the SQUID loop at integer
external flux. To be able to do this, we have fabricated controllable
-junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at
1.4 K only the Nb-Ag device shows the transition to a -state as a function
of consistent with theoretical predictions. In a controllable SQUID
based on Nb-Ag we observe, a part from a screening current at integer external
flux, a phase shift of of the oscillations under suitable
current bias, depending on the magnitude of .Comment: 11 pages, 12 figures, subm. to Phys. Rev.
Mixing of coherent waves in a single three-level artificial atom
We report coherent frequency conversion in the gigahertz range via three-wave
mixing on a single artificial atom in open space. All frequencies involved are
in vicinity of transition frequencies of the three-level atom. A cyclic
configuration of levels is therefore essential, which we have realised with an
artificial atom based on the flux qubit geometry. The atom is continuously
driven at two transition frequencies and we directly measure the coherent
emission at the sum or difference frequency. Our approach enables coherent
conversion of the incoming fields into the coherent emission at a designed
frequency in prospective devices of quantum electronics.Comment: 5 pages, 4 figure
Observation of a controllable PI-junction in a 3-terminal Josephson device
Recently Baselmans et al. [Nature, 397, 43 (1999)] showed that the direction
of the supercurrent in a superconductor/normal/superconductor Josephson
junction can be reversed by applying, perpendicularly to the supercurrent, a
sufficiently large control current between two normal reservoirs. The novel
behavior of their 4-terminal device (called a controllable PI-junction) arises
from the nonequilibrium electron energy distribution established in the normal
wire between the two superconductors. We have observed a similar supercurrent
reversal in a 3-terminal device, where the control current passes from a single
normal reservoir into the two superconductors. We show theoretically that this
behavior, although intuitively less obvious, arises from the same
nonequilibrium physics present in the 4-terminal device. Moreover, we argue
that the amplitude of the PI-state critical current should be at least as large
in the 3-terminal device as in a comparable 4-terminal device.Comment: 4 pages, 4 figures, to appear in Physical Review B Rapid
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