652 research outputs found
Enhancement and state tomography of a squeezed vacuum with circuit quantum electrodynamics
We study the dynamics of a general quartic interaction Hamiltonian under the
influence of dissipation and non-classical driving. We show that this scenario
could be realised with a cascaded superconducting cavity-qubit system in the
strong dispersive regime in a setup similar to recent experiments. In the
presence of dissipation, we find that an effective Hartree-type decoupling with
a Fokker-Planck equation yields a good approximation. We find that the
stationary state is approximately a squeezed vacuum, which is enhanced by the
-factor of the cavity but conserved by the interaction. The qubit
non-linearity, therefore, does not significantly influence the highly squeezed
intracavity microwave field but, for a range of realistic parameters, enables
characterisation of itinerant squeezed fields
Hybrid teleportation via entangled coherent states in circuit quantum electrodynamics
We propose a deterministic scheme for teleporting an unknown qubit through
continuous-variable entangled states in superconducting circuits. The qubit is
a superconducting two-level system and the bipartite quantum channel is a
photonic entangled coherent state between two cavities. A Bell-type measurement
performed on the hybrid state of solid and photonic states brings a
discrete-variable unknown electronic state to a continuous-variable photonic
cat state in a cavity mode. This scheme further enables applications for
quantum information processing in the same architecture of circuit-QED such as
verification and error-detection schemes for entangled coherent states.
Finally, a dynamical method of a self-Kerr tunability in a cavity state has
been investigated for minimizing self-Kerr distortion and all essential
ingredients are shown to be experimentally feasible with the state of the art
superconducting circuits.Comment: 9 pages and 5 figure
Differentiating Majorana from Andreev Bound States in a Superconducting Circuit
We investigate the low-energy theory of a one-dimensional finite capacitance
topological Josephson junction. Charge fluctuations across the junction couple
to resonant microwave fields and can be used to probe microscopic excitations
such as Majorana and Andreev bound states. This marriage between localized
microscopic degrees of freedom and macroscopic dynamics of the superconducting
phase, leads to unique spectroscopic patterns which allow us to reveal the
presence of Majorana fermions among the low-lying excitations.Comment: 9 pages, 3 figure
Protected ground states in short chains of coupled spins in circuit quantum electrodynamics
The two degenerate ground states of the anisotropic Heisenberg (XY) spin
model of a chain of qubits (pseudo-spins) can encode quantum information, but
their degree of protection against local perturbations is known to be only
partial. We examine the properties of the system in the presence of non-local
spin-spin interactions, possibly emerging from the quantum electrodynamics of
the device. We find a phase distinct from the XY phase admitting two ground
states which are highly protected against all local field perturbations,
persisting across a range of parameters. In the context of the XY chain we
discuss how the coupling between two ground states can be used to observe
signatures of topological edge states in a small controlled chain of
superconducting transmon qubits.Comment: 10 pages, 11 figure
Fermion parity measurement and control in Majorana circuit quantum electrodynamics
We investigate the quantum electrodynamics of a device based on a topological
superconducting circuit embedded in a microwave resonator. The device stores
its quantum information in coherent superpositions of fermion parity states
originating from Majorana fermion hybridization. This generates a highly
isolated qubit whose coherence time could be greatly enhanced. We extend the
conventional semiclassical method and obtain analytical derivations for strong
transmon-photon coupling. Using this formalism, we develop protocols to
initialize, control, and measure the parity states. We show that, remarkably,
the parity eigenvalue can be detected via dispersive shifts of the optical
cavity in the strong-coupling regime and its state can be coherently
manipulated via a second-order sideband transition.Comment: 7 pages, 3 figures (published version
Designing Kerr interactions using multiple superconducting qubit types in a single circuit
The engineering of Kerr interactions has great potential for quantum
information processing applications in multipartite quantum systems and for
investigation of many-body physics in a complex cavity-qubit network. We study
how coupling multiple different types of superconducting qubits to the same
cavity modes can be used to modify the self- and cross-Kerr effects acting on
the cavities and demonstrate that this type of architecture could be of
significant benefit for quantum technologies.
Using both analytical perturbation theory results and numerical simulations,
we first show that coupling two superconducting qubits with opposite
anharmonicities to a single cavity enables the effective self-Kerr interaction
to be diminished, while retaining the number splitting effect that enables
control and measurement of the cavity field. We demonstrate that this reduction
of the self-Kerr effect can maintain the fidelity of coherent states and
generalised Schr\"{o}dinger cat states for much longer than typical coherence
times in realistic devices. Next, we find that the cross-Kerr interaction
between two cavities can be modified by coupling them both to the same pair of
qubit devices. When one of the qubits is tunable in frequency, the strength of
entangling interactions between the cavities can be varied on demand, forming
the basis for logic operations on the two modes. Finally, we discuss the
feasibility of producing an array of cavities and qubits where intermediary and
on-site qubits can tune the strength of self- and cross-Kerr interactions
across the whole system. This architecture could provide a way to engineer
interesting many-body Hamiltonians and a useful platform for quantum simulation
in circuit quantum electrodynamics
Extended Hubbard model for mesoscopic transport in donor arrays in silicon
Arrays of dopants in silicon are promising platforms for the quantum
simulation of the Fermi-Hubbard model. We show that the simplest model with
only on-site interaction is insufficient to describe the physics of an array of
phosphorous donors in silicon due to the strong intersite interaction in the
system. We also study the resonant tunneling transport in the array at low
temperature as a mean of probing the features of the Hubbard physics, such as
the Hubbard bands and the Mott gap. Two mechanisms of localization which
suppresses transport in the array are investigated: The first arises from the
electron-ion core attraction and is significant at low filling; the second is
due to the sharp oscillation in the tunnel coupling caused by the intervalley
interference of the donor electron's wavefunction. This disorder in the tunnel
coupling leads to a steep exponential decay of conductance with channel length
in one-dimensional arrays, but its effect is less prominent in two-dimensional
ones. Hence, it is possible to observe resonant tunneling transport in a
relatively large array in two dimensions
Semiconductor Microstructure in a Squeezed Vacuum: Electron-Hole Plasma Luminescence
We consider a semiconductor quantum-well placed in a wave guide microcavity
and interacting with the broadband squeezed vacuum radiation, which fills one
mode of the wave guide with a large average occupation. The wave guide modifies
the optical density of states so that the quantum well interacts mostly with
the squeezed vacuum. The vacuum is squeezed around the externally controlled
central frequency \om_0, which is tuned above the electron-hole gap ,
and induces fluctuations in the interband polarization of the quantum-well. The
power spectrum of scattered light exhibits a peak around \om_0, which is
moreover non-Lorentzian and is a result of both the squeezing and the
particle-hole continuum. The squeezing spectrum is qualitatively different from
the atomic case. We discuss the possibility to observe the above phenomena in
the presence of additional non-radiative (e-e, phonon) dephasing.Comment: 6 pages, 3 figure
Optimal control of two qubits via a single cavity drive in circuit quantum electrodynamics
Optimization of the fidelity of control operations is of critical importance
in the pursuit of fault-tolerant quantum computation. We apply optimal control
techniques to demonstrate that a single drive via the cavity in circuit quantum
electrodynamics can implement a high-fidelity two-qubit all-microwave gate that
directly entangles the qubits via the mutual qubit-cavity couplings. This is
performed by driving at one of the qubits' frequencies which generates a
conditional two-qubit gate, but will also generate other spurious interactions.
These optimal control techniques are used to find pulse shapes that can perform
this two-qubit gate with high fidelity, robust against errors in the system
parameters. The simulations were all performed using experimentally relevant
parameters and constraints.Comment: Final published versio
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