4 research outputs found
Supercurrent reversal in quantum dots
When two superconductors become electrically connected by a weak link a
zero-resistance supercurrent can flow. This supercurrent is carried by Cooper
pairs of electrons with a combined charge of twice the elementary charge, e.
The 2e charge quantum is clearly visible in the height of Shapiro steps in
Josephson junctions under microwave irradiation and in the magnetic flux
periodicity of h/2e in superconducting quantum interference devices. Several
different materials have been used to weakly couple superconductors, such as
tunnel barriers, normal metals, or semiconductors. Here, we study supercurrents
through a quantum dot created in a semiconductor nanowire by local
electrostatic gating. Due to strong Coulomb interaction, electrons only tunnel
one-by-one through the discrete energy levels of the quantum dot. This
nevertheless can yield a supercurrent when subsequent tunnel events are
coherent. These quantum coherent tunnelling processes can result in either a
positive or a negative supercurrent, i.e. in a normal or a pi-junction,
respectively. We demonstrate that the supercurrent reverses sign by adding a
single electron spin to the quantum dot. When excited states of the quantum dot
are involved in transport, the supercurrent sign also depends on the character
of the orbital wavefunctions
Dynamical decoupling and noise spectroscopy with a superconducting flux qubit
The characterization and mitigation of decoherence in natural and artificial
two-level systems (qubits) is fundamental to quantum information science and
its applications. Decoherence of a quantum superposition state arises from the
interaction between the constituent system and the uncontrolled degrees of
freedom in its environment. Within the standard Bloch-Redfield picture of
two-level system dynamics, qubit decoherence is characterized by two rates: a
longitudinal relaxation rate Gamma1 due to the exchange of energy with the
environment, and a transverse relaxation rate Gamma2 = Gamma1/2 + Gamma_phi
which contains the pure dephasing rate Gamma_phi. Irreversible energy
relaxation can only be mitigated by reducing the amount of environmental noise,
reducing the qubit's internal sensitivity to that noise, or through multi-qubit
encoding and error correction protocols (which already presume ultra-low error
rates). In contrast, dephasing is in principle reversible and can be refocused
dynamically through the application of coherent control pulse methods. In this
work we demonstrate how dynamical-decoupling techniques can moderate the
dephasing effects of low-frequency noise on a superconducting qubit with
energy-relaxation time T1 = 1/Gamma1 = 12 us. Using the CPMG sequence with up
to 200 pi-pulses, we demonstrate a 50-fold improvement in the transverse
relaxation time T2 over its baseline value. We observe relaxation-limited times
T2(CPMG) = 23 us = 2 T1 resulting from CPMG-mediated Gaussian pure-dephasing
times in apparent excess of 100 us. We leverage the filtering property of this
sequence in conjunction with Rabi and energy relaxation measurements to
facilitate the spectroscopy and reconstruction of the environmental noise power
spectral density.Comment: 21 pages (incl. 11-page appendix); 4 (+7) figure
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference