724 research outputs found
Optimal control for one-qubit quantum sensing
Quantum systems can be exquisite sensors thanks to their sensitivity to
external perturbations. This same characteristic also makes them fragile to
external noise. Quantum control can tackle the challenge of protecting quantum
sensors from environmental noise, while leaving their strong coupling to the
target field to be measured. As the compromise between these two conflicting
requirements does not always have an intuitive solution, optimal control based
on numerical search could prove very effective. Here we adapt optimal control
theory to the quantum sensing scenario, by introducing a cost function that,
unlike the usual fidelity of operation, correctly takes into account both the
unknown field to be measured and the environmental noise. We experimentally
implement this novel control paradigm using a Nitrogen Vacancy center in
diamond, finding improved sensitivity to a broad set of time varying fields.
The demonstrated robustness and efficiency of the numerical optimization, as
well as the sensitivity advantaged it bestows, will prove beneficial to many
quantum sensing applications
Quantum sensing
"Quantum sensing" describes the use of a quantum system, quantum properties
or quantum phenomena to perform a measurement of a physical quantity.
Historical examples of quantum sensors include magnetometers based on
superconducting quantum interference devices and atomic vapors, or atomic
clocks. More recently, quantum sensing has become a distinct and rapidly
growing branch of research within the area of quantum science and technology,
with the most common platforms being spin qubits, trapped ions and flux qubits.
The field is expected to provide new opportunities - especially with regard to
high sensitivity and precision - in applied physics and other areas of science.
In this review, we provide an introduction to the basic principles, methods and
concepts of quantum sensing from the viewpoint of the interested
experimentalist.Comment: 45 pages, 13 figures. Submitted to Rev. Mod. Phy
Simulations of Information Transport in Spin Chains
Transport of quantum information in linear spin chains has been the subject
of much theoretical work. Experimental studies by nuclear spin systems in
solid-state by NMR (a natural implementation of such models) is complicated
since the dipolar Hamiltonian is not solely comprised of nearest-neighbor
XY-Heisenberg couplings. We present here a similarity transformation between
the XY-Heisenberg Hamiltonian and the grade raising Hamiltonian, an interaction
which is achievable with the collective control provided by radio-frequency
pulses in NMR. Not only does this second Hamiltonian allows us to simulate the
information transport in a spin chain, but it also provides a means to observe
its signature experimentally
Detection of a light echo from SN1998bu
About 500d after explosion the light curve of the Type Ia SN1998bu suddenly
flattened and at the same time the spectrum changed from the typical nebular
emission to a blue continuum with broad absorption and emission features
reminiscent of the SN spectrum at early phases. We show that in analogy to
SN1991Tbu (Schmidt et al. 1994), this can be explained by the emergence of a
light echo from a foreground dust cloud. Based on a simple model we argue that
the amount of dust required can consistently explain the extinction which has
been estimated by completely independent methods. Because of the similar echo
luminosity but much higher optical depth of the dust in SN1998bu compared with
SN1991T, we expect that the echo ring size of SN1998bu grows faster than in
SN1991T. HST observations have indeed confirmed this prediction.Comment: 5 pages (including 3 figures) - Accepted for pubblication in ApJ
Letter
Nebular Spectra of SN 1998bw Revisited: Detailed Study by One and Two Dimensional Models
Refined one- and two-dimensional models for the nebular spectra of the
hyper-energetic Type Ic supernova (SN) 1998bw, associated with the gamma-ray
burst GRB980425, from 125 to 376 days after B-band maximum are presented. One
dimensional, spherically symmetric spectrum synthesis calculations show that
reproducing features in the observed spectra, i.e., the sharply peaked [OI]
6300\AA doublet and MgI] 4570\AA emission, and the broad [FeII] blend around
5200\AA, requires the existence of a high-density O-rich core expanding at low
velocities (\lsim 8,000 km s) and of Fe-rich material moving faster
than the O-rich material. Synthetic spectra at late phases from aspherical
(bipolar) explosion models are also computed with a two-dimensional spectrum
synthesis code. The above features are naturally explained by the aspherical
model if the explosion is viewed from a direction close to the axis of symmetry
(), since the aspherical model yields a high-density O-rich
region confined along the equatorial axis. By examining a large parameter space
(in energy and mass), our best model gives following physical quantities: the
kinetic energy ergs \gsim 8 - 12 and the
main-sequence mass of the progenitor star M_{\rm ms} \gsim 30 - 35 \Msun. The
temporal spectral evolution of SN 1998bw also indicates mixing among Fe-, O-,
and C-rich regions, and highly clumpy structure.Comment: 38 pages, 22 figures. ApJ, 640 (01 April 2006 issue), in pres
On the Redshift Distribution of Gamma Ray Bursts in the Swift Era
A simple physical model for long-duration gamma ray bursts (GRBs) is used to
fit the redshift (z) and the jet opening-angle distributions measured with
earlier GRB missions and with Swift. The effect of different sensitivities for
GRB triggering is sufficient to explain the difference in the z distributions
of the pre-Swift and Swift samples, with mean redshifts of ~1.5 and
~2.7, respectively. Assuming that the emission properties of GRBs do not
change with time, we find that the data can only be fitted if the comoving
rate-density of GRB sources exhibits positive evolution to z >~ 3-5. The mean
intrinsic beaming factor of GRBs is found to range from ~34-42, with the Swift
average opening half-angle ~10 degree, compared to the pre-Swift
average of ~7 degree. Within the uniform jet model, the GRB
luminosity function is proportional to L^{-3.25}_*, as inferred from our best
fit to the opening angle distribution. Because of the unlikely detection of
several GRBs with z <~ 0.25, our analysis indicates that low redshift GRBs
represent a different population of GRBs than those detected at higher
redshifts. Neglecting possible metallicity effects on GRB host galaxies, we
find that ~1 GRB occurs every 600,000 yrs in a local L_* spiral galaxy like the
Milky Way. The fraction of high-redshift GRBs is estimated at 8-12% and 2.5-6%
at z >= 5 and z >= 7, respectively, assuming continued positive evolution of
the GRB rate density to high redshifts.Comment: Accepted for publication in ApJ. The paper contains 29 pages and 24
figure
The Rates of Hypernovae and Gamma-Ray Bursts: Implications for their Progenitors
A critical comparison of estimates for the rates of hypernovae (HNe) and
gamma-ray bursts (GRBs) is presented. Within the substantial uncertainties, the
estimates are shown to be quite comparable and give a Galactic rate of
-- yr for both events. These rates are several
orders of magnitude lower than the rate of core-collapse supernovae, suggesting
that the evolution leading to a HN/GRB requires special circumstances, very
likely due to binary interactions. Various possible binary channels are
discussed, and it is shown that these are generally compatible with the
inferred rates.Comment: Accepted by Astrophysical Journal Letters. 12 page
Strong magnetic coupling between an electronic spin qubit and a mechanical resonator
We describe a technique that enables a strong, coherent coupling between a
single electronic spin qubit associated with a nitrogen-vacancy impurity in
diamond and the quantized motion of a magnetized nano-mechanical resonator tip.
This coupling is achieved via careful preparation of dressed spin states which
are highly sensitive to the motion of the resonator but insensitive to
perturbations from the nuclear spin bath. In combination with optical pumping
techniques, the coherent exchange between spin and motional excitations enables
ground state cooling and the controlled generation of arbitrary quantum
superpositions of resonator states. Optical spin readout techniques provide a
general measurement toolbox for the resonator with quantum limited precision
Chandra Detection of Massive Black Holes in Galactic Cooling Flows
Anticipating forthcoming observations with the Chandra X-ray telescope, we
describe the continuation of interstellar cooling flows deep into the cores of
elliptical galaxies. Interstellar gas within about r = 50 parsecs from the
massive black hole is heated to T > 1 keV and should be visible unless thermal
heating is diluted by non-thermal pressure. Since our flows are subsonic near
the massive black holes, distributed cooling continues within 300 pc from the
center. Dark, low mass stars formed in this region may be responsible for some
of the mass attributed to central black holes.Comment: 6 pages with 3 figures; accepted by Astrophysical Journal Letter
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