1,089 research outputs found
Liquid relaxation: A new Parodi-like relation for nematic liquid crystals
We put forward a hydrodynamic theory of nematic liquid crystals that includes
both anisotropic elasticity and dynamic relaxation. Liquid remodeling is
encompassed through a continuous update of the shear-stress free configuration.
The low-frequency limit of the dynamical theory reproduces the classical
Ericksen-Leslie theory, but it predicts two independent identities between the
six Leslie viscosity coefficients. One replicates Parodi's relation, while the
other-which involves five Leslie viscosities in a nonlinear way-is new. We
discuss its significance, and we test its validity against evidence from
physical experiments, independent theoretical predictions, and
molecular-dynamics simulations.Comment: 6 pages, 1 figure, 2 table
Digital Quantum Rabi and Dicke Models in Superconducting Circuits
We propose the analog-digital quantum simulation of the quantum Rabi and
Dicke models using circuit quantum electrodynamics (QED). We find that all
physical regimes, in particular those which are impossible to realize in
typical cavity QED setups, can be simulated via unitary decomposition into
digital steps. Furthermore, we show the emergence of the Dirac equation
dynamics from the quantum Rabi model when the mode frequency vanishes. Finally,
we analyze the feasibility of this proposal under realistic superconducting
circuit scenarios.Comment: 5 pages, 3 figures. Published in Scientific Report
Initialization by measurement of a two-qubit superconducting circuit
We demonstrate initialization by joint measurement of two transmon qubits in
3D circuit quantum electrodynamics. Homodyne detection of cavity transmission
is enhanced by Josephson parametric amplification to discriminate the two-qubit
ground state from single-qubit excitations non-destructively and with 98.1%
fidelity. Measurement and postselection of a steady-state mixture with 4.7%
residual excitation per qubit achieve 98.8% fidelity to the ground state, thus
outperforming passive initialization.Comment: 5 pages, 4 figures, and Supplementary Information (7 figures, 1
table
General method for extracting the quantum efficiency of dispersive qubit readout in circuit QED
We present and demonstrate a general three-step method for extracting the
quantum efficiency of dispersive qubit readout in circuit QED. We use active
depletion of post-measurement photons and optimal integration weight functions
on two quadratures to maximize the signal-to-noise ratio of the
non-steady-state homodyne measurement. We derive analytically and demonstrate
experimentally that the method robustly extracts the quantum efficiency for
arbitrary readout conditions in the linear regime. We use the proven method to
optimally bias a Josephson traveling-wave parametric amplifier and to quantify
different noise contributions in the readout amplification chain.Comment: 10 pages, 6 figure
Implementing optimal control pulse shaping for improved single-qubit gates
We employ pulse shaping to abate single-qubit gate errors arising from the
weak anharmonicity of transmon superconducting qubits. By applying shaped
pulses to both quadratures of rotation, a phase error induced by the presence
of higher levels is corrected. Using a derivative of the control on the
quadrature channel, we are able to remove the effect of the anharmonic levels
for multiple qubits coupled to a microwave resonator. Randomized benchmarking
is used to quantify the average error per gate, achieving a minimum of
0.007+/-0.005 using 4 ns-wide pulse.Comment: 4 pages, 4 figure
Selective darkening of degenerate transitions for implementing quantum controlled-NOT gates
We present a theoretical analysis of the selective darkening method for
implementing quantum controlled-NOT (CNOT) gates. This method, which we
recently proposed and demonstrated, consists of driving two
transversely-coupled quantum bits (qubits) with a driving field that is
resonant with one of the two qubits. For specific relative amplitudes and
phases of the driving field felt by the two qubits, one of the two transitions
in the degenerate pair is darkened, or in other words, becomes forbidden by
effective selection rules. At these driving conditions, the evolution of the
two-qubit state realizes a CNOT gate. The gate speed is found to be limited
only by the coupling energy J, which is the fundamental speed limit for any
entangling gate. Numerical simulations show that at gate speeds corresponding
to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and
increases further for lower gate speeds. In addition, the effect of
higher-lying energy levels and weak anharmonicity is studied, as well as the
scalability of the method to systems of multiple qubits. We conclude that in
all these respects this method is competitive with existing schemes for
creating entanglement, with the added advantages of being applicable for qubits
operating at fixed frequencies (either by design or for exploitation of
coherence sweet-spots) and having the simplicity of microwave-only operation.Comment: 25 pages, 5 figure
Quantum Hall conductance of two-terminal graphene devices
Measurement and theory of the two-terminal conductance of monolayer and
bilayer graphene in the quantum Hall regime are compared. We examine features
of conductance as a function of gate voltage that allow monolayer, bilayer, and
gapped samples to be distinguished, including N-shaped distortions of quantum
Hall plateaus and conductance peaks and dips at the charge neutrality point.
Generally good agreement is found between measurement and theory. Possible
origins of discrepancies are discussed
Noise Correlations in a Coulomb Blockaded Quantum Dot
We report measurements of current noise auto- and cross-correlation in a
tunable quantum dot with two or three leads. As the Coulomb blockade is lifted
at finite source-drain bias, the auto-correlation evolves from super-Poissonian
to sub-Poissonian in the two-lead case, and the cross-correlation evolves from
positive to negative in the three-lead case, consistent with transport through
multiple levels. Cross-correlations in the three-lead dot are found to be
proportional to the noise in excess of the Poissonian value in the limit of
weak output tunneling
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