223 research outputs found
Photonic chip based optical frequency comb using soliton induced Cherenkov radiation
By continuous wave pumping of a dispersion engineered, planar silicon nitride
microresonator, continuously circulating, sub-30fs short temporal dissipative
solitons are generated, that correspond to pulses of 6 optical cycles and
constitute a coherent optical frequency comb in the spectral domain. Emission
of soliton induced Cherenkov radiation caused by higher order dispersion
broadens the spectral bandwidth to 2/3 of an octave, sufficient for self
referencing, in excellent agreement with recent theoretical predictions and the
broadest coherent microresonator frequency comb generated to date. In a further
step, this frequency comb is fully phase stabilized. The ability to preserve
coherence over a broad spectral bandwidth using soliton induced Cherenkov
radiation marks a critical milestone in the development of planar optical
frequency combs, enabling on one hand application in e.g. coherent
communications, broadband dual comb spectroscopy and Raman spectral imaging,
while on the other hand significantly relaxing dispersion requirements for
broadband microresonator frequency combs and providing a path for their
generation in the visible and UV. Our results underscore the utility and
effectiveness of planar microresonator frequency comb technology, that offers
the potential to make frequency metrology accessible beyond specialized
laboratories.Comment: Changes: - Added data (new Fig.4) on the first full phase
stabilization of a dissipative Kerr soliton (or dissipative cavity soliton)
in a microresonator - Extended Fig. 8 in the SI - Introduced nomenclature of
dissipative Kerr solitons - Minor other change
Biharmonic wave maps into spheres
A global weak solution of the biharmonic wave map equation in the energy space for spherical targets is constructed. The equation is reformulated as a conservation law and solved by a suitable Ginzburg-Landau type approximation
More cost-sharing, less cost? Evidence on reference price drugs
This paper evaluates the causal effects of changes in reference prices (RP) on prices, copayments, and overall expenditures for off-patent pharmaceuticals. With reference pricing, firms set prices freely and the health plan covers the expenses only up to a certain threshold. We use quarterly data of the German market for anti-epileptics at the package level and at the active substance level and exploit that the RP has been adjusted in some of the active substances but not in others in a difference-in-differences framework. At the product level, we find that a lower RP reduces prices for both brand-name drugs and generics, but leads to higher copayments, especially for brand-name drugs. At the aggregate level, we find that a lower RP leads to savings for the public health insurer since revenues decrease substantially for brand-name firms and, to a lesser extent, also for generic firms. Overall expenditures (payments by the health insurer and the patients) for brand-name drugs decrease in proportion to the decrease in the RP, while the adjustment does not significantly influence overall expenditures for generics
Octave Spanning Frequency Comb on a Chip
Optical frequency combs have revolutionized the field of frequency metrology
within the last decade and have become enabling tools for atomic clocks, gas
sensing and astrophysical spectrometer calibration. The rapidly increasing
number of applications has heightened interest in more compact comb generators.
Optical microresonator based comb generators bear promise in this regard.
Critical to their future use as 'frequency markers', is however the absolute
frequency stabilization of the optical comb spectrum. A powerful technique for
this stabilization is self-referencing, which requires a spectrum that spans a
full octave, i.e. a factor of two in frequency. In the case of mode locked
lasers, overcoming the limited bandwidth has become possible only with the
advent of photonic crystal fibres for supercontinuum generation. Here, we
report for the first time the generation of an octave-spanning frequency comb
directly from a toroidal microresonator on a silicon chip. The comb spectrum
covers the wavelength range from 990 nm to 2170 nm and is retrieved from a
continuous wave laser interacting with the modes of an ultra high Q
microresonator, without relying on external broadening. Full tunability of the
generated frequency comb over a bandwidth exceeding an entire free spectral
range is demonstrated. This allows positioning of a frequency comb mode to any
desired frequency within the comb bandwidth. The ability to derive octave
spanning spectra from microresonator comb generators represents a key step
towards achieving a radio-frequency to optical link on a chip, which could
unify the fields of metrology with micro- and nano-photonics and enable
entirely new devices that bring frequency metrology into a chip scale setting
for compact applications such as space based optical clocks
eXplainable AI for Quantum Machine Learning
Parametrized Quantum Circuits (PQCs) enable a novel method for machine
learning (ML). However, from a computational point of view they present a
challenge to existing eXplainable AI (xAI) methods. On the one hand,
measurements on quantum circuits introduce probabilistic errors which impact
the convergence of these methods. On the other hand, the phase space of a
quantum circuit expands exponentially with the number of qubits, complicating
efforts to execute xAI methods in polynomial time. In this paper we will
discuss the performance of established xAI methods, such as Baseline SHAP and
Integrated Gradients. Using the internal mechanics of PQCs we study ways to
speed up their computation
Sideband Injection Locking in Microresonator Frequency Combs
Frequency combs from continuous-wave-driven Kerr-nonlinear microresonators
have evolved into a key photonic technology with applications from optical
communication to precision spectroscopy. Essential to many of these
applications is the control of the comb's defining parameters, i.e.,
carrier-envelope offset frequency and repetition rate. An elegant and
all-optical approach to controlling both degrees of freedom is the suitable
injection of a secondary continuous-wave laser into the resonator onto which
one of the comb lines locks. Here, we study experimentally such sideband
injection locking in microresonator soliton combs across a wide optical
bandwidth and derive analytic scaling laws for the locking range and repetition
rate control. As an application example, we demonstrate optical frequency
division and repetition rate phase-noise reduction to three orders of magnitude
below the noise of a free-running system. The presented results can guide the
design of sideband injection-locked, parametrically generated frequency combs
with opportunities for low-noise microwave generation, compact optical clocks
with simplified locking schemes and more generally, all-optically stabilized
frequency combs from Kerr-nonlinear resonators.Comment: 13 pages, 6 figure
Quantum Control of the Hyperfine Spin of a Cs Atom Ensemble
We demonstrate quantum control of a large spin-angular momentum associated
with the F=3 hyperfine ground state of 133Cs. A combination of time dependent
magnetic fields and a static tensor light shift is used to implement
near-optimal controls and map a fiducial state to a broad range of target
states, with yields in the range 0.8-0.9. Squeezed states are produced also by
an adiabatic scheme that is more robust against errors. Universal control
facilitates the encoding and manipulation of qubits and qudits in atomic ground
states, and may lead to improvement of some precision measurements.Comment: 4 pages, 4 figures (color
Biharmonic wave maps: local wellposedness in high regularity
We show a local wellposedness result for biharmonic wave maps with initial data of sufficiently high Sobolev regularity. Moreover, we obtain a blow-up criterion for these solutions. In contrast to the wave maps equation we use a vanishing viscosity argument and an appropriate parabolic regularization in order to obtain the existence result. The geometric nature of the equation is exploited to prove convergence of the approximate solutions and uniqueness of the limit
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