30 research outputs found
Probing quantum coherence in qubit arrays
We discuss how the observation of population localization effects in
periodically driven systems can be used to quantify the presence of quantum
coherence in interacting qubit arrays. Essential for our proposal is the fact
that these localization effects persist beyond tight-binding Hamiltonian
models. This result is of special practical relevance in those situations where
direct system probing using tomographic schemes becomes infeasible beyond a
very small number of qubits. As a proof of principle, we study analytically a
Hamiltonian system consisting of a chain of superconducting flux qubits under
the effect of a periodic driving. We provide extensive numerical support of our
results in the simple case of a two-qubits chain. For this system we also study
the robustness of the scheme against different types of noise and disorder. We
show that localization effects underpinned by quantum coherent interactions
should be observable within realistic parameter regimes in chains with a larger
number o
Coulomb-assisted braiding of Majorana fermions in a Josephson junction array
We show how to exchange (braid) Majorana fermions in a network of
superconducting nanowires by control over Coulomb interactions rather than
tunneling. Even though Majorana fermions are charge-neutral quasiparticles
(equal to their own antiparticle), they have an effective long-range
interaction through the even-odd electron number dependence of the
superconducting ground state. The flux through a split Josephson junction
controls this interaction via the ratio of Josephson and charging energies,
with exponential sensitivity. By switching the interaction on and off in
neighboring segments of a Josephson junction array, the non-Abelian braiding
statistics can be realized without the need to control tunnel couplings by gate
electrodes. This is a solution to the problem how to operate on topological
qubits when gate voltages are screened by the superconductor
Quantum dynamics of local phase differences between reservoirs of driven interacting bosons separated by simple aperture arrays
We present a derivation of the effective action for the relative phase of
driven, aperture-coupled reservoirs of weakly-interacting condensed bosons from
a (3+1)-D microscopic model with local U(1) gauge symmetry. We show that
inclusion of local chemical potential and driving velocity fields as a gauge
field allows derivation of the hydrodynamic equations of motion for the driven
macroscopic phase differences across simple aperture arrays. For a single
aperture, the current-phase equation for driven flow contains sinusoidal,
linear, and current-bias contributions. We compute the renormalization group
(RG) beta function of the periodic potential in the effective action for small
tunneling amplitudes and use this to analyze the temperature dependence of the
low-energy current-phase relation, with application to the transition from
linear to sinusoidal current-phase behavior observed in experiments by
Hoskinson et al. \cite{packard} for liquid He driven through nanoaperture
arrays. Extension of the microscopic theory to a two-aperture array shows that
interference between the microscopic tunneling contributions for individual
apertures leads to an effective coupling between apertures which amplifies the
Josephson oscillations in the array. The resulting multi-aperture current-phase
equations are found to be equivalent to a set of equations for coupled pendula,
with microscopically derived couplings.Comment: 16 pages, 5 figures v2: typos corrected, RG phase diagram correcte
Static flux bias of a flux qubit using persistent current trapping
Qubits based on the magnetic flux degree of freedom require a flux bias,
whose stability and precision strongly affect the qubit performance, up to a
point of forbidding the qubit operation. Moreover, in the perspective of
multiqubit systems, it must be possible to flux-bias each qubit independently,
hence avoiding the traditional use of externally generated magnetic fields in
favour of on-chip techniques that minimize cross-couplings. The solution
discussed in this paper exploits a persistent current, trapped in a
superconducting circuit integrated on chip that can be inductively coupled with
an individual qubit. The circuit does not make use of resistive elements that
can be detrimental for the qubit coherence. The trapping procedure allows to
control and change stepwise the amount of stored current; after that, the
circuit can be completely disconnected from the external sources. We show in a
practical case how this works and how to drive the bias circuit at the required
value.Comment: 5 figures submitted to Superconductor Science and Technolog
Many-particle confinement by constructed disorder and quantum computing
Many-particle confinement (localization) is studied for a 1D system of
spinless fermions with nearest-neighbor hopping and interaction, or
equivalently, for an anisotropic Heisenberg spin-1/2 chain. This system is
frequently used to model quantum computers with perpetually coupled qubits. We
construct a bounded sequence of site energies that leads to strong
single-particle confinement of all states on individual sites. We show that
this sequence also leads to a confinement of all many-particle states in an
infinite system for a time that scales as a high power of the reciprocal
hopping integral. The confinement is achieved for strong interaction between
the particles while keeping the overall bandwidth of site energies
comparatively small. The results show viability of quantum computing with
time-independent qubit coupling.Comment: An invited paper for the topical issue of J. Opt. B on quantum
contro
Top-transmon: hybrid superconducting qubit for parity-protected quantum computation
Qubits constructed from uncoupled Majorana fermions are protected from
decoherence, but to perform a quantum computation this topological protection
needs to be broken. Parity-protected quantum computation breaks the protection
in a minimally invasive way, by coupling directly to the fermion parity of the
system --- irrespective of any quasiparticle excitations. Here we propose to
use a superconducting charge qubit in a transmission line resonator (a socalled
transmon) to perform parity-protected rotations and read-out of a topological
(top) qubit. The advantage over an earlier proposal using a flux qubit is that
the coupling can be switched on and off with exponential accuracy, promising a
reduced sensitivity to charge noise.Comment: 7 pages, 5 figure
Photodetection of propagating quantum microwaves in circuit QED
We develop the theory of a metamaterial composed of an array of discrete
quantum absorbers inside a one-dimensional waveguide that implements a
high-efficiency microwave photon detector. A basic design consists of a few
metastable superconducting nanocircuits spread inside and coupled to a
one-dimensional waveguide in a circuit QED setup. The arrival of a {\it
propagating} quantum microwave field induces an irreversible change in the
population of the internal levels of the absorbers, due to a selective
absorption of photon excitations. This design is studied using a formal but
simple quantum field theory, which allows us to evaluate the single-photon
absorption efficiency for one and many absorber setups. As an example, we
consider a particular design that combines a coplanar coaxial waveguide with
superconducting phase qubits, a natural but not exclusive playground for
experimental implementations. This work and a possible experimental realization
may stimulate the possible arrival of "all-optical" quantum information
processing with propagating quantum microwaves, where a microwave photodetector
could play a key role.Comment: 27 pages, submitted to Physica Scripta for Nobel Symposium on "Qubits
for Quantum Information", 200
Characterization of a fabrication process for the integration of superconducting qubits and RSFQ circuits
In order to integrate superconducting qubits with rapid-single-flux-quantum
(RSFQ) control circuitry, it is necessary to develop a fabrication process that
fulfills at the same time the requirements of both elements: low critical
current density, very low operating temperature (tens of milliKelvin) and
reduced dissipation on the qubit side; high operation frequency, large
stability margins, low dissipated power on the RSFQ side. For this purpose, VTT
has developed a fabrication process based on Nb trilayer technology, which
allows the on-chip integration of superconducting qubits and RSFQ circuits even
at very low temperature. Here we present the characterization (at 4.2 K) of the
process from the point of view of the Josephson devices and show that they are
suitable to build integrated superconducting qubits