36 research outputs found
Stochastic Master Equation Analysis of Optimized Three-Qubit Nondemolition Parity Measurement
We analyze a direct parity measurement of the state of three superconducting
qubits in circuit quantum electrodynamics. The parity is inferred from a
homodyne measurement of the reflected/transmitted microwave radiation and the
measurement is direct in the sense that the parity is measured without the need
for any quantum circuit operations or for ancilla qubits. Qubits are coupled to
two resonant cavity modes, allowing the steady state of the emitted radiation
to satisfy the necessary conditions to act as a pointer state for the parity.
However, the transient dynamics violates these conditions and we analyze this
detrimental effect and show that it can be overcome in the limit of weak
measurement signal. Our analysis shows that, with a moderate degree of
post-selection, it is possible to achieve post-measurement states with fidelity
of order 95%. We believe that this type of measurement could serve as a
benchmark for future error-correction protocols in a scalable architecture
Dispersive Qubit Measurement by Interferometry with Parametric Amplifiers
We perform a detailed analysis of how an amplified interferometer can be used
to enhance the quality of a dispersive qubit measurement, such as one performed
on a superconducting transmon qubit, using homodyne detection on an amplified
microwave signal. Our modeling makes a realistic assessment of what is possible
in current circuit-QED experiments; in particular, we take into account the
frequency-dependence of the qubit-induced phase shift for short microwaves
pulses. We compare the possible signal-to-noise ratios obtainable with
(single-mode) SU(1,1) interferometers with the current coherent measurement and
find a considerable reduction in measurement error probability in an
experimentally-accessible range of parameters
Cooling of a Nanomechanical Resonator in the Presence of a Single Diatomic Molecule
We propose a theoretical scheme for coupling a nanomechanical resonator to a
single diatomic molecule via microwave cavity mode of a driven LC resonator. We
describe the diatomic molecule by a Morse potential and find the corresponding
equations of motion of the hybrid system by using Fokker-Planck formalism.
Analytical expressions for the effective frequency and the effective damping of
the nanomechanical resonator are obtained. We analyze the ground state cooling
of the nanomechanical resonator in presence of the diatomic molecule. The
results confirm that presence of the molecule improves the cooling process of
the mechanical resonator. Finally, the effect of molecule's parameters on the
cooling mechanism is studied.Comment: 10 pages, 8 figure
Reversible optical to microwave quantum interface
We describe a reversible quantum interface between an optical and a microwave
field using a hybrid device based on their common interaction with a
micro-mechanical resonator in a superconducting circuit. We show that, by
employing state-of-the-art opto-electro-mechanical devices, one can realise an
effective source of (bright) two-mode squeezing with an optical idler (signal)
and a microwave signal, which can be used for high-fidelity transfer of quantum
states between optical and microwave fields by means of continuous variable
teleportation.Comment: 5 + 3 pages, 5 figure