746 research outputs found
What Powered the Optical Transient AT2017gfo Associated with GW170817?
The groundbreaking discovery of the optical transient AT2017gfo associated with GW170817 opens a unique opportunity to study the physics of double neutron star (NS) mergers. We argue that the standard interpretation of AT2017gfo as being powered by radioactive decay of r-process elements faces the challenge of simultaneously accounting for the peak luminosity and peak time of the event, as it is not easy to achieve the required high mass, and especially the low opacity of the ejecta required to fit the data. A plausible solution would be to invoke an additional energy source, which is probably provided by the merger product. We consider energy injection from two types of the merger products: (1) a post-merger black hole powered by fallback accretion; and (2) a long-lived NS remnant. The former case can only account for the early emission of AT2017gfo, with the late emission still powered by radioactive decay. In the latter case, both early- and late-emission components can be well interpreted as due to energy injection from a spinning-down NS, with the required mass and opacity of the ejecta components well consistent with known numerical simulation results. We suggest that there is a strong indication that the merger product of GW170817 is a long-lived (supramassive or even permanently stable), low magnetic field NS. The result provides a stringent constraint on the equations of state of NSs
Time-optimal universal quantum gates on superconducting circuits
Decoherence is inevitable when manipulating quantum systems. It decreases the
quality of quantum manipulations and thus is one of the main obstacles for
large-scale quantum computation, where high-fidelity quantum gates are needed.
Generally, the longer a gate operation is, the more decoherence-induced gate
infidelity will be. Therefore, how to shorten the gate time becomes an urgent
problem to be solved. To this end, time-optimal control based on solving the
quantum brachistochrone equation is a straightforward solution. Here, based on
time-optimal control, we propose a scheme to realize universal quantum gates on
superconducting qubits in a two-dimensional square lattice configuration, and
the two-qubit gate fidelity approaches 99.9\%. Meanwhile, we can further
accelerate the Z-axis gate considerably by adjusting the detuning of the
external driving. Finally, in order to reduce the influence of the dephasing
error, decoherence-free subspace encoding is also incorporated in our physical
implementation. Therefore, we present a fast quantum scheme which is promising
for large-scale quantum computation.Comment: v2 accepte
Hybrid quantum device based on NV centers in diamond nanomechanical resonators plus superconducting waveguide cavities
We propose and analyze a hybrid device by integrating a microscale diamond
beam with a single built-in nitrogen-vacancy (NV) center spin to a
superconducting coplanar waveguide (CPW) cavity. We find that under an ac
electric field the quantized motion of the diamond beam can strongly couple to
the single cavity photons via dielectric interaction. Together with the strong
spin-motion interaction via a large magnetic field gradient, it provides a
hybrid quantum device where the dia- mond resonator can strongly couple both to
the single microwave cavity photons and to the single NV center spin. This
enables coherent information transfer and effective coupling between the NV
spin and the CPW cavity via mechanically dark polaritons. This hybrid
spin-electromechanical de- vice, with tunable couplings by external fields,
offers a realistic platform for implementing quantum information with single NV
spins, diamond mechanical resonators, and single microwave photons.Comment: Accepted by Phys. Rev. Applie
Studying newborn neutron stars by the transient emission after stellar collapses and compact binary mergers
The formation of neutron stars (NSs), both from collapses of massive stars
and mergers of compact objects, can be usually indicated by bright transients
emitted from explosively-ejected material. In particular, if the newborn NSs
can rotate at a millisecond period and have a sufficiently high magnetic field,
then the spin-down of the NSs would provide a remarkable amount of energy to
the emitting material. As a result, super-luminous supernovae could be produced
in the massive stellar collapse cases, while some unusual fast evolving and
luminous optical transients could arise from the cases of NS mergers and
accretion-induced collapses of white dwarfs. In all cases, if the dipolar
magnetic fields of the newborn NSs can be amplified to be as high as
G, a relativistic jet could be launched and then a gamma-ray burst can be
produced as the jet successfully breaks out from the surrounding
nearly-isotropic ejected material.Comment: 10 pages, 9 pictures, to appear in the AIP Proceedings of the
Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era
of Gravitational Wave Astronomy, Jan. 3-7, Xiamen, Chin
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