28 research outputs found
Imaging spontaneous currents in superconducting arrays of pi-junctions
Superconductors separated by a thin tunneling barrier exhibit the Josephson
effect that allows charge transport at zero voltage, typically with no phase
shift between the superconductors in the lowest energy state. Recently,
Josephson junctions with ground state phase shifts of pi proposed by theory
three decades ago have been demonstrated. In superconducting loops,
pi-junctions cause spontaneous circulation of persistent currents in zero
magnetic field, analogous to spin-1/2 systems. Here we image the spontaneous
zero-field currents in superconducting networks of temperature-controlled
pi-junctions with weakly ferromagnetic barriers using a scanning SQUID
microscope. We find an onset of spontaneous supercurrents at the 0-pi
transition temperature of the junctions Tpi = 3 K. We image the currents in
non-uniformly frustrated arrays consisting of cells with even and odd numbers
of pi-junctions. Such arrays are attractive model systems for studying the
exotic phases of the 2D XY-model and achieving scalable adiabatic quantum
computers.Comment: Pre-referee version. Accepted to Nature Physic
Noiseless nonreciprocity in a parametric active device
Nonreciprocal devices such as circulators and isolators belong to an
important class of microwave components employed in applications like the
measurement of mesoscopic circuits at cryogenic temperatures. The measurement
protocols usually involve an amplification chain which relies on circulators to
separate input and output channels and to suppress backaction from different
stages on the sample under test. In these devices the usual reciprocal symmetry
of circuits is broken by the phenomenon of Faraday rotation based on magnetic
materials and fields. However, magnets are averse to on-chip integration, and
magnetic fields are deleterious to delicate superconducting devices. Here we
present a new proposal combining two stages of parametric modulation emulating
the action of a circulator. It is devoid of magnetic components and suitable
for on-chip integration. As the design is free of any dissipative elements and
based on reversible operation, the device operates noiselessly, giving it an
important advantage over other nonreciprocal active devices for quantum
information processing applications.Comment: 17 pages, 4 figures + 12 pages Supplementary Informatio
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
Fermi surface instability at the hidden-order transition of URu2Si2
Solids with strong electron correlations generally develop exotic phases of
electron matter at low temperatures. Among such systems, the heavy-fermion
semi-metal URu2Si2 presents an enigmatic transition at To = 17.5 K to a `hidden
order' state whose order parameter remains unknown after 23 years of intense
research. Various experiments point to the reconstruction and partial gapping
of the Fermi surface when the hidden-order establishes. However, up to now, the
question of how this transition affects the electronic spectrum at the Fermi
surface has not been directly addressed by a spectroscopic probe. Here we show,
using angle-resolved photoemission spectroscopy, that a band of heavy
quasi-particles drops below the Fermi level upon the transition to the
hidden-order state. Our data provide the first direct evidence of a large
reorganization of the electronic structure across the Fermi surface of URu2Si2
occurring during this transition, and unveil a new kind of Fermi-surface
instability in correlated electron systemsComment: 15 pages, 5 figure
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