6 research outputs found
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
Superradiance and Phase Multistability in Circuit Quantum Electrodynamics
By modeling the coupling of multiple superconducting qubits to a single
cavity in the circuit-quantum electrodynamics (QED) framework we find that it
should be possible to observe superradiance and phase multistability using
currently available technology. Due to the exceptionally large couplings
present in circuit-QED we predict that superradiant microwave pulses should be
observable with only a very small number of qubits (just three or four), in the
presence of energy relaxation and non-uniform qubit-field coupling strengths.
This paves the way for circuit-QED implementations of superradiant state
readout and decoherence free subspace state encoding in subradiant states. The
system considered here also exhibits phase multistability when driven with
large field amplitudes, and this effect may have applications for collective
qubit readout and for quantum feedback protocols.Comment: Published Versio
Quantum Computation by Communication
We present a new approach to scalable quantum computing--a ``qubus
computer''--which realises qubit measurement and quantum gates through
interacting qubits with a quantum communication bus mode. The qubits could be
``static'' matter qubits or ``flying'' optical qubits, but the scheme we focus
on here is particularly suited to matter qubits. There is no requirement for
direct interaction between the qubits. Universal two-qubit quantum gates may be
effected by schemes which involve measurement of the bus mode, or by schemes
where the bus disentangles automatically and no measurement is needed. In
effect, the approach integrates together qubit degrees of freedom for
computation with quantum continuous variables for communication and
interaction.Comment: final published versio