95 research outputs found
A Simple Language Model based on PMI Matrix Approximations
In this study, we introduce a new approach for learning language models by
training them to estimate word-context pointwise mutual information (PMI), and
then deriving the desired conditional probabilities from PMI at test time.
Specifically, we show that with minor modifications to word2vec's algorithm, we
get principled language models that are closely related to the well-established
Noise Contrastive Estimation (NCE) based language models. A compelling aspect
of our approach is that our models are trained with the same simple negative
sampling objective function that is commonly used in word2vec to learn word
embeddings.Comment: Accepted to EMNLP 201
Photon-statistics force in ultrafast electron dynamics
In strong-field physics and attosecond science, intense light induces
ultrafast electron dynamics. Such ultrafast dynamics of electrons in matter is
at the core of phenomena such as high harmonic generation (HHG), where these
dynamics lead to emission of extreme UV bursts with attosecond duration. So
far, all ultrafast dynamics of matter were understood to originate purely from
the classical vector potential of the driving light, disregarding the influence
of the quantum nature of light. Here we show that dynamics of matter driven by
bright (intense) light significantly depend on the quantum state of the driving
light, which induces an effective photon-statistics force. To provide a unified
framework for the analysis & control over such a force, we extend the
strong-field approximation (SFA) theory to account for non-classical driving
light. Our quantum SFA (qSFA) theory shows that in HHG, experimentally feasible
squeezing of the driving light can shift & shape electronic trajectories and
attosecond pulses at the scale of hundreds of attoseconds. Our work presents a
new degree-of-freedom for attosecond spectroscopy, by relying on nonclassical
electromagnetic fields, and more generally, introduces a direct connection
between attosecond science and quantum optics
Generation of squeezed high-order harmonics
For decades, most research on high harmonic generation (HHG) considered
matter as quantum but light as classical, leaving the quantum-optical nature of
the harmonics an open question. Here we explore the quantum properties of high
harmonics. We derive a formula for the quantum state of the high harmonics,
when driven by arbitrary quantum light states, and then explore specific cases
of experimental relevance. Specifically, for a moderately squeezed pump, HHG
driven by squeezed coherent light results in squeezed high harmonics. Harmonic
squeezing is optimized by syncing ionization times with the pump's squeezing
phase. Beyond this regime, as pump squeezing is increased, the harmonics
initially acquire squeezed thermal photon statistics, and then occupy an
intricate quantum state which strongly depends on the semi-classical nonlinear
response function of the interacting system. Our results pave the way for the
generation of squeezed extreme-ultraviolet ultrashort pulses, and, more
generally, quantum frequency conversion into previously inaccessible spectral
ranges, which may enable ultrasensitive attosecond metrology
High harmonic generation driven by quantum light
High harmonic generation (HHG) is an extreme nonlinear process where intense
pulses of light drive matter to emit high harmonics of the driving frequency,
reaching the extreme ultraviolet (XUV) and x-ray spectral ranges. So far, the
HHG process was always generated by intense laser pulses that are well
described as a classical electromagnetic field. Advances in the generation of
intense squeezed light motivate us to revisit the fundamentals of HHG and ask
how the photon statistics of light may alter this process, and more generally
alter the field of extreme nonlinear optics. The role of photon statistics in
non-perturbative interactions of intense light with matter has remained
unexplored in both experiments and theory. Here we show that the defining
spectral characteristics of HHG, such as the plateau and cutoff, are sensitive
to the photon statistics of the driving light. While coherent (classical) and
Fock light states induce the established HHG cutoff law, thermal and squeezed
states substantially surpass it, extending the cutoff compared to classical
light of the same intensity. Hence, shaping the photon statistics of light
enables producing far higher harmonics in HHG. We develop the theory of extreme
nonlinear optics driven by squeezed light, and more generally by arbitrary
quantum states of light. Our work introduces quantum optical concepts to
strong-field physics as new degrees of freedom in the creation and control of
HHG, and finally shows that experiments in this field are feasible. Looking
forward, HHG driven by quantum light creates quantum states of XUV and X-rays,
enabling applications of quantum optics in new spectral regimes
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