31 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
Evading noise in multiparameter quantum metrology with indefinite causal order
Quantum theory allows the traversing of multiple channels in a superposition
of different orders. When the order in which the channels are traversed is
controlled by an auxiliary quantum system, various unknown parameters of the
channels can be estimated by measuring only the control system, even when the
state of the probe alone would be insensitive. Moreover, increasing the
dimension of the control system increases the number of simultaneously
estimable parameters, which has important metrological ramifications. We
demonstrate this capability for simultaneously estimating both unitary and
noise parameters, including multiple parameters from the same unitary such as
rotation angles and axes and from noise channels such as depolarization,
dephasing, and amplitude damping in arbitrary dimensions. We identify regimes
of unlimited advantages, taking the form of smaller variances in
estimation when the noise probability is , for both single and
multiparameter estimation when using our schemes relative to any comparable
scheme whose causal order is definite.Comment: 18 pages, 7 figure
Entanglement over global distances via quantum repeaters with satellite links
We study entanglement creation over global distances based on a quantum
repeater architecture that uses low-earth orbit satellites equipped with
entangled photon sources, as well as ground stations equipped with quantum
non-demolition detectors and quantum memories. We show that this approach
allows entanglement creation at viable rates over distances that are
inaccessible via direct transmission through optical fibers or even from very
distant satellites.Comment: 5+3 pages, 3+2 figure
Quantum Memory with a controlled homogeneous splitting
We propose a quantum memory protocol where a input light field can be stored
onto and released from a single ground state atomic ensemble by controlling
dynamically the strength of an external static and homogeneous field. The
technique relies on the adiabatic following of a polaritonic excitation onto a
state for which the forward collective radiative emission is forbidden. The
resemblance with the archetypal Electromagnetically-Induced-Transparency (EIT)
is only formal because no ground state coherence based slow-light propagation
is considered here. As compared to the other grand category of protocols
derived from the photon-echo technique, our approach only involves a
homogeneous static field. We discuss two physical situations where the effect
can be observed, and show that in the limit where the excited state lifetime is
longer than the storage time, the protocols are perfectly efficient and
noise-free. We compare the technique to other quantum memories, and propose
atomic systems where the experiment can be realized.Comment: submitted to New Journal of Physics, Focus on Quantum Memor
Entanglement between more than two hundred macroscopic atomic ensembles in a solid
We create a multi-partite entangled state by storing a single photon in a
crystal that contains many large atomic ensembles with distinct resonance
frequencies. The photon is re-emitted at a well-defined time due to an
interference effect analogous to multi-slit diffraction. We derive a lower
bound for the number of entangled ensembles based on the contrast of the
interference and the single-photon character of the input, and we
experimentally demonstrate entanglement between over two hundred ensembles,
each containing a billion atoms. In addition, we illustrate the fact that each
individual ensemble contains further entanglement. Our results are the first
demonstration of entanglement between many macroscopic systems in a solid and
open the door to creating even more complex entangled states.Comment: 10 pages, 8 figures; see also parallel submission by Frowis et a