55 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
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
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
Quantum-Assisted Telescope Arrays
Quantum networks provide a platform for astronomical interferometers capable
of imaging faint stellar objects. In a recent work [arXiv:1809.01659], we
presented a protocol that circumvents transmission losses with efficient use of
quantum resources and modest quantum memories. Here we analyze a number of
extensions to that scheme. We show that it can be operated as a truly broadband
interferometer and generalized to multiple sites in the array. We also analyze
how imaging based on the quantum Fourier transform provides improved
signal-to-noise ratio compared to classical processing. Finally, we discuss
physical realizations including photon-detection-based quantum state transfer.Comment: 10 pages, 8 figures; v2 - clarifications and references; v3 - close
to published versio
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
Topological Quantum Optics in Two-Dimensional Atomic Arrays
We demonstrate that two-dimensional atomic emitter arrays with subwavelength
spacing constitute topologically protected quantum optical systems where the
photon propagation is robust against large imperfections while losses
associated with free space emission are strongly suppressed. Breaking
time-reversal symmetry with a magnetic field results in gapped photonic bands
with non-trivial Chern numbers and topologically protected, long-lived edge
states. Due to the inherent nonlinearity of constituent emitters, such systems
provide a platform for exploring quantum optical analogues of interacting
topological systems.Comment: 11 pages and 9 figures; paper updated to match published versio
Optical Interferometry with Quantum Networks
We propose a method for optical interferometry in telescope arrays assisted
by quantum networks. In our approach, the quantum state of incoming photons
along with an arrival time index is stored in a binary qubit code at each
receiver. Nonlocal retrieval of the quantum state via entanglement-assisted
parity checks at the expected photon arrival rate allows for direct extraction
of the phase difference, effectively circumventing transmission losses between
nodes. Compared to prior proposals, our scheme (based on efficient quantum data
compression) offers an exponential decrease in required entanglement bandwidth.
Experimental implementation is then feasible with near-term technology,
enabling optical imaging of astronomical objects akin to well-established radio
interferometers and pushing resolution beyond what is practically achievable
classically.Comment: 7 + 6 pages, 3 + 1 figures; v2 - clarifications and further
discussion of implementation; v3 - close to published versio
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