36 research outputs found
Decoherence in Solid State Qubits
Interaction of solid state qubits with environmental degrees of freedom
strongly affects the qubit dynamics, and leads to decoherence. In quantum
information processing with solid state qubits, decoherence significantly
limits the performances of such devices. Therefore, it is necessary to fully
understand the mechanisms that lead to decoherence. In this review we discuss
how decoherence affects two of the most successful realizations of solid state
qubits, namely, spin-qubits and superconducting qubits. In the former, the
qubit is encoded in the spin 1/2 of the electron, and it is implemented by
confining the electron spin in a semiconductor quantum dot. Superconducting
devices show quantum behavior at low temperatures, and the qubit is encoded in
the two lowest energy levels of a superconducting circuit. The electron spin in
a quantum dot has two main decoherence channels, a (Markovian) phonon-assisted
relaxation channel, due to the presence of a spin-orbit interaction, and a
(non-Markovian) spin bath constituted by the spins of the nuclei in the quantum
dot that interact with the electron spin via the hyperfine interaction. In a
superconducting qubit, decoherence takes place as a result of fluctuations in
the control parameters, such as bias currents, applied flux, and bias voltages,
and via losses in the dissipative circuit elements.Comment: review article, 66 pages, 10 figure
Magnetic tilting and emergent Majorana spin connection in topological superconductors
Due to the charge neutral and localized nature of surface Majorana modes,
detection schemes usually rely on local spectroscopy or interference through
the Josephson effect. Here, we theoretically study the magnetic response of a
two-dimensional cone of Majorana fermions localized at the surface of class
DIII Topological Superconductors. For a field parallel to the surface the
Zeeman term vanishes and the orbital term induces a Doppler shift of the
Andreev levels resulting in a tilting of the surface Majorana cone. For fields
larger than a critical threshold field the system undergoes a transition
from type I to type II Dirac-Majorana cone. In a spherical geometry the surface
curvature leads to the emergence of the Majorana spin connection in the tilting
term via an interplay between orbital and Zeeman, that generates a finite
non-trivial coupling between negative and positive energy states. Majorana
modes are thus expected to show a finite response to the applied field, that
acquires a universal character in finite geometries and opens the way to
detection of Majorana modes via time-dependent magnetic fields.Comment: 5 pages Main text, 5 pages Appendix, 3 figures. arXiv admin note:
substantial text overlap with arXiv:1802.0920
Chiral Majorana Interference as a Source of Quantum Entanglement
Interferometry is a powerful tool for entanglement production and detection
in multiterminal mesoscopic systems. Here we propose a setup to produce,
manipulate and detect entanglement in the electron-hole degree of freedom by
exploiting Andreev reflection on chiral one-dimensional channels via
interferometry. We study the best possible case in which two-particle
interferometry produces superpositions of maximally entangled states. This is
achieved by mixing chiral Dirac channels through chiral Majorana modes. We show
that it is possible to extract entanglement witnesses through current
cross-correlation measurements.Comment: 5 pages, 1 figur
Time-reversal and rotation symmetry breaking superconductivity in Dirac materials
We consider mixed symmetry superconducting phases in Dirac materials in the
odd parity channel, where pseudoscalar and vector order parameters can coexist
due to their similar critical temperatures when attractive interactions are of
finite range. We show that the coupling of these order parameters to unordered
magnetic dopants favors the condensation of novel time-reversal symmetry
breaking (TRSB) phases, characterized by a condensate magnetization, rotation
symmetry breaking, and simultaneous ordering of the dopant moments. We find a
rich phase diagram of mixed TRSB phases characterized by peculiar bulk
quasiparticles, with Weyl nodes and nodal lines, and distinctive surface
states. These findings are consistent with recent experiments on
NbBiSe that report evidence of point nodes, nematicity, and TRSB
superconductivity induced by Nb magnetic moments.Comment: 11 pages, 2 figure
Interactions in Electronic Mach-Zehnder Interferometers with Copropagating Edge Channels
We study Coulomb interactions in the finite bias response of Mach-Zehnder
interferometers, which exploit copropagating edge states in the integer quantum
Hall effect. Here, interactions are particularly important since the coherent
coupling of edge channels is due to a resonant mechanism that is spoiled by
inelastic processes. We find that interactions yield a saturation, as a
function of bias voltage, of the period-averaged interferometer current, which
gives rise to unusual features, such as negative differential conductance,
enhancement of the visibility of the current, and nonbounded or even diverging
visibility of the differential conductance.Comment: 5 pages, 2 figure
Full control of qubit rotations in a voltage-biased superconducting flux qubit
We study a voltage-controlled version of the superconducting flux qubit
[Chiorescu et al., Science 299, 1869 (2003)] and show that full control of
qubit rotations on the entire Bloch sphere can be achieved. Circuit graph
theory is used to study a setup where voltage sources are attached to the two
superconducting islands formed between the three Josephson junctions in the
flux qubit. Applying a voltage allows qubit rotations about the y axis, in
addition to pure x and z rotations obtained in the absence of applied voltages.
The orientation and magnitude of the rotation axis on the Bloch sphere can be
tuned by the gate voltages, the external magnetic flux, and the ratio alpha
between the Josephson energies via a flux-tunable junction. We compare the
single-qubit control in the known regime alpha<1 with the unexplored range
alpha>1 and estimate the decoherence due to voltage fluctuations.Comment: 12 pages, 12 figures, 1 tabl
Superconducting resonators as beam splitters for linear-optics quantum computation
A functioning quantum computer will be a machine that builds up, in a
programmable way, nonclassical correlations in a multipartite quantum system.
Linear optics quantum computation (LOQC) is an approach for achieving this
function that requires only simple, reliable linear optical elements, namely
beam splitters and phase shifters. Nonlinear optics is only required in the
form of single-photon sources for state initialization, and detectors. However,
the latter remain difficult to achieve with high fidelity. A new setting for
quantum optics has arisen in circuit quantum electrodynamics (cQED) using
superconducting (SC) quantum devices, and opening up the way to LOQC using
microwave, rather than visible photons. Much progress is being made in SC
qubits and cQED: high-fidelity Fock state generation and qubit measurements
provide single photon sources and detection. Here we show that the LOQC toolkit
in cQED can be completed with high-fidelity (>99.92%) linear optical elements.Comment: 4 pages, 3 figure