72 research outputs found
Testing the Clauser-Horne-Shimony-Holt inequality using observables with arbitrary spectrum
The Clauser-Horne-Shimony and Holt inequality applies when measurements with
binary outcomes are performed on physical systems under the assumption of local
realism. Testing such inequalities in the quantum realm usually involves either
measurements of two--valued quantum observables or pre-defining a context
dependent binning procedure. Here we establish the conditions to test the
Clauser-Horne-Shimony and Holt inequality using any quantum observable. Our
result applies to observables with an arbitrary spectrum and no prior knowledge
of their underlying Hilbert space's dimension is required. Finally, we
demonstrate the proposed general measurement strategy, that can be seen as
positive operator valued measurements performed on the system, using the
formalism of modular variables applied to the transverse degrees of freedom of
single photons.Comment: 10 pages, 4 figures, including a new Sec.
Finite temperature reservoir engineering and entanglement dynamics
We propose experimental methods to engineer reservoirs at arbitrary
temperature which are feasible with current technology. Our results generalize
to mixed states the possibility of quantum state engineering through controlled
decoherence. Finite temperature engineered reservoirs can lead to the
experimental observation of thermal entanglement --the appearance and increase
of entanglement with temperature-- to the study of the dependence of finite
time disentanglement and revival with temperature, quantum thermodynamical
effects, among many other applications, enlarging the comprehension of
temperature dependent entanglement properties
Quantum search with non--orthogonal entangled states
We propose a classical to quantum information encoding system using
non--orthogonal states and apply it to the problem of searching an element in a
quantum list. We show that the proposed encoding scheme leads to an exponential
gain in terms of quantum resources and, in some cases, to an exponential gain
in the number of runs of the protocol. In the case where the output of the
search algorithm is a quantum state with some particular physical property, the
searched state is found with a single query to the introduced oracle. If the
obtained quantum state must be converted back to classical information, our
protocol demands a number of repetitions that scales polynomially with the
number of qubits required to encode a classical string
Quantum search with modular variables
We give a dimension independent formulation of the quantum search algorithm
introduced in [L. K. Grover, Phys. Rev. Lett. {\bf 79}, 325 (1997)]. This
algorithm provides a quadratic gain when compared to its classical counterpart
by manipulating quantum two--level systems, qubits. We show that this gain,
already known to be optimal, is preserved, irrespectively of the dimension of
the system used to encode quantum information. This is shown by adapting the
protocol to Hilbert spaces of any dimension using the same sequence of
operations/logical gates as its original qubit formulation. Our results are
detailed and illustrated for a system described by continuous variables, where
qubits can be encoded in infinitely many distinct states using the modular
variable formalism
Generation of bright squeezed light at 1.06 using cascaded non-linearities in a triply resonant c.w. PPLN OPO
We have used an ultra-low threshold continuous-wave Optical Parametric
Oscillator (OPO) to reduce the quantum fluctuations of the reflected pump beam
below the shot noise limit. The OPO consisted of a triply resonant cavity
containing a Periodically-Poled Lithium Niobate crystal pumped by a Nd:YAG
laser and giving signal and idler wavelengths close to 2.12 microns and a
threshold as low as 300 microwatts. We detected the quantum fluctuations of the
pump beam reflected by the OPO using a slightly modified homodyne detection
technique. The measured noise reduction was 30 % (inferred noise reduction at
the output of the OPO 38 %).Comment: 9 pages, 6 figures, accepted in Phys. Rev. A, uses REVTeX
Quantum information processing in phase space: A modular variables approach
Binary quantum information can be fault tolerantly encoded in states defined
in infinite dimensional Hilbert spaces. Such states define a computational
basis, and permit a perfect equivalence between continuous and discrete
universal operations. The drawback of this encoding is that the corresponding
logical states are unphysical, meaning infinitely localized in phase space. We
use the modular variables formalism to show that, in a number of protocols
relevant for quantum information and for the realization of fundamental tests
of quantum mechanics, it is possible to loosen the requirements on the logical
subspace without jeopardizing their usefulness or their successful
implementation. Such protocols involve measurements of appropriately chosen
modular observables that permit the readout of the encoded discrete quantum
information from the corresponding logical states. Finally, we demonstrate the
experimental feasibility of our approach by applying it to the transverse
degrees of freedom of single photons.Comment: 15 pages, 4 figure
Quantum information with modular variables
We introduce a novel strategy, based on the use of modular variables, to
encode and deterministically process quantum information using states described
by continuous variables. Our formalism leads to a general recipe to adapt
existing quantum information protocols, originally formulated for finite
dimensional quantum systems, to infinite dimensional systems described by
continuous variables. This is achieved by using non unitary and non-gaussian
operators, obtained from the superposition of gaussian gates, together with
adaptative manipulations in qubit systems defined in infinite dimensional
Hilbert spaces. We describe in details the realization of single and two qubit
gates and briefly discuss their implementation in a quantum optical set-up
An alternative representation for pure symmetric states of qubits and its applications to entanglement classification
We prove that the vast majority of symmetric states of qubits can be
decomposed in a unique way into a superposition of spin 1/2 coherent states.
For the case of two qubits, the proposed decomposition reproduces the Schmidt
decomposition and therefore, in the case of a higher number of qubits, can be
considered as its generalization. We analyze the geometrical aspects of the
proposed representation and its invariant properties under the action of local
unitary and local invertible transformations. As an application, we identify
the most general classes of entanglement and representative states for any
number of qubits in a symmetric state.Comment: A qubit "flip" on the equations (13), (14) has been corrected.
Equations (20), (21) have been modified in accordanc
Quantum Communication Between Remote Mechanical Resonators
Mechanical resonators represent one of the most promising candidates to
mediate the interaction between different quantum technologies, bridging the
gap between efficient quantum computation and long-distance quantum
communication. In this letter, we introduce a novel interferometric scheme
where the interaction of a mechanical resonator with input/output quantum
pulses is controlled by an independent classical drive. We design protocols for
state teleportation and direct quantum state transfer, between distant
mechanical resonators. The proposed device, feasible with state-of-the-art
technology, can serve as building block for the implementation of long-distance
quantum networks of mechanical resonators
Continuous discretization of infinite dimensional Hilbert spaces
In quantum theory, observables with a continuous spectrum are known to be
fundamentally different from those with a discrete and finite spectrum. While
some fundamental tests and applications of quantum mechanics originally
formulated for discrete variables have been translated to continuous ones, this
is not the case in general. For instance, despite their importance, no
experimental demonstration of nonlocality exists in the continuous variables
regime. Attempts to bridge this gap and put continuous variables on a closer
footing to discrete ones used dichotomization. However, this approach considers
only discrete properties of the continuum, and its infinitesimal properties are
not fully exploited. Here we show that it is possible to manipulate, detect and
classify continuous variable states using observables with a continuous
spectrum revealing properties and symmetries which are analogous to finite
discrete systems. Our approach leads to an operational way to define and adapt,
to arbitrary continuous quantum systems, quantum protocols and algorithms
typical to discrete systems.Comment: 5 pages + Supplementary Informatio
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