539 research outputs found
Efficient atomic clocks operated with several atomic ensembles
Atomic clocks are typically operated by locking a local oscillator (LO) to a
single atomic ensemble. In this article we propose a scheme where the LO is
locked to several atomic ensembles instead of one. This results in an
exponential improvement compared to the conventional method and provides a
stability of the clock scaling as with being the number
of atoms in each of the ensembles and is a constant depending on
the protocol being used to lock the LOComment: 10 pages, 8 figure
Elementary test for non-classicality based on measurements of position and momentum
We generalise a non-classicality test described by Kot et al. [Phys. Rev.
Lett. 108, 233601 (2010)], which can be used to rule out any classical
description of a physical system. The test is based on measurements of
quadrature operators and works by proving a contradiction with the classical
description in terms of a probability distribution in phase space. As opposed
to the previous work, we generalise the test to include states without
rotational symmetry in phase space. Furthermore, we compare the performance of
the non-classicality test with classical tomography methods based on the
inverse Radon transform, which can also be used to establish the quantum nature
of a physical system. In particular, we consider a non-classicality test based
on the so-called filtered back-projection formula. We show that the general
non-classicality test is conceptually simpler, requires less assumptions on the
system and is statistically more reliable than the tests based on the filtered
back-projection formula. As a specific example, we derive the optimal test for
a quadrature squeezed single photon state and show that the efficiency of the
test does not change with the degree of squeezing
Hybrid Quantum Repeater Protocol With Fast Local Processing
We propose a hybrid quantum repeater protocol combining the advantages of
continuous and discrete variables. The repeater is based on the previous work
of Brask et al. [Phys. Rev. Lett. 105, 160501 (2010)] but we present two ways
of improving this protocol. In the previous protocol entangled single-photon
states are produced and grown into superpositions of coherent states, known as
two-mode cat states. The entanglement is then distributed using homodyne
detection. To improve the protocol, we replace the time-consuming non-local
growth of cat states with local growth of single-mode cat states, eliminating
the need for classical communication during growth. Entanglement is generated
in subsequent connection processes. Furthermore the growth procedure is
optimized. We review the main elements of the original protocol and present the
two modifications. Finally the two protocols are compared and the modified
protocol is shown to perform significantly better than the original protocol.Comment: 14 pages, 7 figure
Super sensitivity and super resolution with quantum teleportation
We propose a method for quantum enhanced phase estimation based on continuous
variable (CV) quantum teleportation. The phase shift probed by a coherent state
can be enhanced by repeatedly teleporting the state back to interact with the
phase shift again using a supply of two-mode squeezed vacuum states. In this
way, both super resolution and super sensitivity can be obtained due to the
coherent addition of the phase shift. The protocol enables Heisenberg limited
sensitivity and super- resolution given sufficiently strong squeezing. The
proposed method could be implemented with current or near-term technology of CV
teleportation.Comment: 5 pagers, 3 figure
One- and two-axis squeezing of atomic ensembles in optical cavities
The strong light-matter coupling attainable in optical cavities enables the
generation of highly squeezed states of atomic ensembles. It was shown in
[Phys. Rev. A 66, 022314 (2002)] how an effective one-axis twisting Hamiltonian
can be realized in a cavity setup. Here, we extend this work and show how an
effective two-axis twisting Hamiltonian can be realized in a similar cavity
setup. We compare the two schemes in order to characterize their advantages. In
the absence of decoherence, the two-axis Hamiltonian leads to more squeezing
than the one-axis Hamiltonian. If limited by decoherence from spontaneous
emission and cavity decay, we find roughly the same level of squeezing for the
two schemes scaling as (NC)^(1/2) where C is the single atom cooperativity and
N is the total number of atoms. When compared to an ideal squeezing operation,
we find that for specific initial states, a dissipative version of the one-axis
scheme attains higher fidelity than the unitary one-axis scheme or the two-axis
scheme. However, the unitary one-axis and two-axis schemes perform better for
general initial states.Comment: 13 pages, 6 figure
Near Heisenberg limited atomic clocks in the presence of decoherence
The ultimate stability of atomic clocks is limited by the quantum noise of
the atoms. To reduce this noise it has been suggested to use entangled atomic
ensembles with reduced atomic noise. Potentially this can push the stability
all the way to the limit allowed by the Heisenberg uncertainty relation, which
is denoted the Heisenberg limit. In practice, however, entangled states are
often more prone to decoherence, which may prevent reaching this performance.
Here we present an adaptive measurement protocol that in the presence of a
realistic source of decoherence enables us to get near Heisenberg limited
stability of atomic clocks using entangled atoms. The protocol may thus realize
the full potential of entanglement for quantum metrology despite the
detrimental influence of decoherence.Comment: 13 pages, 9 figures. Note that new reference: Y. Matsuzaki, S. C.
Benjamin, and J. Fitzsimons, Phys. Rev. A 84, 012103 (2011) is adde
Quantum Networks with Deterministic Spin-Photon Interfaces
We consider how recent experimental progress on deterministic solid state
spin-photon interfaces enable the construction of a number of key elements of
quantum networks. After reviewing some of the recent experimental achievements,
we discuss their integration into Bell state analyzers, quantum non-demolition
detection, and photonic cluster state generation. Finally, we outline how these
elements can be used for long-distance entanglement generation and quantum key
distribution in a quantum network.Comment: 13 pages, 7 figure
Photonic Controlled-Phase Gates Through Rydberg Blockade in Optical Cavities
We propose a novel scheme for high fidelity photonic controlled phase gates
using Rydberg blockade in an ensemble of atoms in an optical cavity. The gate
operation is obtained by first storing a photonic pulse in the ensemble and
then scattering a second pulse from the cavity, resulting in a phase change
depending on whether the first pulse contained a single photon. We show that
the combination of Rydberg blockade and optical cavities effectively enhances
the optical non-linearity created by the strong Rydberg interaction and thereby
reduces the requirements for photonic quantum gates. The resulting gate can be
implemented with cavities of moderate finesse which allows for highly efficient
processing of quantum information encoded in photons. As a particular example
of this, we show how the gate can be employed to increase the communication
rate of quantum repeaters based on atomic ensembles.Comment: main manuscript 5 pages with 11 pages of supplementary informatio
Distributed quantum sensing in a continuous variable entangled network
Networking plays a ubiquitous role in quantum technology. It is an integral
part of quantum communication and has significant potential for upscaling
quantum computer technologies that are otherwise not scalable. Recently, it was
realized that sensing of multiple spatially distributed parameters may also
benefit from an entangled quantum network. Here we experimentally demonstrate
how sensing of an averaged phase shift among four distributed nodes benefits
from an entangled quantum network. Using a four-mode entangled continuous
variable (CV) state, we demonstrate deterministic quantum phase sensing with a
precision beyond what is attainable with separable probes. The techniques
behind this result can have direct applications in a number of primitives
ranging from biological imaging to quantum networks of atomic clocks
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