33 research outputs found
Photonic quantum memory in two-level ensembles based on modulating the refractive index in time: equivalence to gradient echo memory
We present a quantum memory protocol that allows to store light in ensembles
of two-level atoms, e.g. rare-earth ions doped into a crystal, by modulating
the refractive index of the host medium of the atoms linearly in time. We show
that under certain conditions the resulting dynamics is equivalent to that
underlying the gradient echo memory protocol, which relies on a spatial
gradient of the atomic resonance frequencies. We discuss the prospects for an
experimental implementation.Comment: 5 pages, 2 figure
Coherent storage and manipulation of broadband photons via dynamically controlled Autler-Townes splitting
The coherent control of light with matter, enabling storage and manipulation
of optical signals, was revolutionized by electromagnetically induced
transparency (EIT), which is a quantum interference effect. For strong
electromagnetic fields that induce a wide transparency band, this quantum
interference vanishes, giving rise to the well-known phenomenon of
Autler-Townes splitting (ATS). To date, it is an open question whether ATS can
be directly leveraged for coherent control as more than just a case of "bad"
EIT. Here, we establish a protocol showing that dynamically controlled
absorption of light in the ATS regime mediates coherent storage and
manipulation that is inherently suitable for efficient broadband quantum memory
and processing devices. We experimentally demonstrate this protocol by storing
and manipulating nanoseconds-long optical pulses through a collective spin
state of laser-cooled Rb atoms for up to a microsecond. Furthermore, we show
that our approach substantially relaxes the technical requirements intrinsic to
established memory schemes, rendering it suitable for broad range of platforms
with applications to quantum information processing, high-precision
spectroscopy, and metrology.Comment: 14 pages with 6 figures; 3 pages supplementary info with 2
supplementary figure
Beyond transcoherent states: Field states for effecting optimal coherent rotations on single or multiple qubits
Semiclassically, laser pulses can be used to implement arbitrary
transformations on atomic systems; quantum mechanically, residual atom-field
entanglement spoils this promise. Transcoherent states are field states that
fix this problem in the fully quantized regime by generating perfect coherence
in an atom initially in its ground or excited state. We extend this fully
quantized paradigm in four directions: First, we introduce field states that
transform an atom from its ground or excited state to any point on the Bloch
sphere without residual atom-field entanglement. The best strong pulses for
carrying out rotations by angle are are squeezed in photon-number
variance by a factor of . Next, we investigate implementing
rotation gates, showing that the optimal Gaussian field state for enacting a
pulse on an atom in an arbitrary, unknown initial state is number
squeezed by less: . Third, we extend these
investigations to fields interacting with multiple atoms simultaneously,
discovering once again that number squeezing by is optimal for
enacting pulses on all of the atoms simultaneously, with small
corrections on the order of the ratio of the number of atoms to the average
number of photons. Finally, we find field states that best perform arbitrary
rotations by through nonlinear interactions involving -photon
absorption, where the same optimal squeezing factor is found to be
. Backaction in a wide variety of atom-field interactions can
thus be mitigated by squeezing the control fields by optimal amounts.Comment: Updated formatting following acceptance in Quantu
Measuring the quadrature coherence scale on a cloud quantum computer
Coherence underlies quantum phenomena, yet it is manifest in classical
theories; delineating coherence's role is a fickle business. The quadrature
coherence scale (QCS) was invented to remove such ambiguity, quantifying
quantum features of any single-mode bosonic system without choosing a preferred
orientation of phase space. The QCS is defined for any state, reducing to
well-known quantities in appropriate limits, including Gaussian and pure
states, and perhaps most importantly for a coherence measure, it is highly
sensitive to decoherence. Until recently, it was unknown how to measure the
QCS; we here report on an initial measurement of the QCS for squeezed light and
thermal states of light. This is performed using Xanadu's machine Borealis,
accessed through the cloud, which offers the configurable beam splitters and
photon-number-resolving detectors essential for measuring the QCS. The data and
theory match well, certifying the usefulness of interferometers and
photon-counting devices in certifying quantumness.Comment: 11 pages including 4 figures and 1 appendix; close to published
versio
Ultrafast slow-light: Raman-induced delay of THz-bandwidth pulses
We propose and experimentally demonstrate a scheme to generate
optically-controlled delays based on off-resonant Raman absorption. Dispersion
in a transparency window between two neighboring, optically-activated Raman
absorption lines is used to reduce the group velocity of broadband 765 nm
pulses. We implement this approach in a potassium titanyl phosphate (KTP)
waveguide at room temperature, and demonstrate Raman-induced delays of up to
140 fs for a 650-fs duration, 1.8-THz bandwidth, signal pulse; the available
delay-bandwidth product is . Our approach is applicable to single
photon signals, offers wavelength tunability, and is a step toward processing
ultrafast photons.Comment: 5+4 pages, 4+2 figure