[Show abstract][Hide abstract] ABSTRACT: The scheme recently proposed in [M. Scala et al., Phys Rev Lett 111, 180403
(2013)], where a gravity-dependent phase shift is induced on the spin of a
nitrogen-vacancy (NV) center in a trapped nanodiamond by the interaction
between its magnetic moment and the quantized motion of the particle, provides
a way to detect spatial quantum superpositions by means of spin measurements
only. Here, the effect of unwanted coupling with other motional degrees of
freedom is considered and we show that it does not affect the validity of the
scheme. Both this coupling and the additional error source due to misalignment
between the quantization axis of the NV center spin and the trapping axis are
shown not to change the qualitative behavior of the system, so that a proof-of-
principle experiment can be neatly performed. Our analysis, which shows that
the scheme retains the important features of not requiring ground state cooling
and of being resistant to thermal fluctuations, can be useful for the several
schemes which have been proposed recently for testing macroscopic
superpositions in trapped microsystems.
[Show abstract][Hide abstract] ABSTRACT: Spin-chain models have been widely studied in terms of quantum information processes, for instance for the faithful transmission of quantum states. Here, we investigate the limitations of mapping this process to an equivalent one through a bosonic chain. In particular, we keep in mind experimental implementations, which the progress in integrated waveguide circuits could make possible in the very near future. We consider the feasibility of exploiting the higher dimensionality of the Hilbert space of the chain elements for the transmission of a larger amount of information, and the effects of unwanted excitations during the process. Finally, we exploit the information-flux method to provide bounds to the transfer fidelity.
Physical Review A 08/2015; 92(2):022350. DOI:10.1103/PhysRevA.92.022350 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Quantum theory is based on a mathematical structure totally different from
conventional arithmetic. Due to the symmetric nature of bosonic particles,
annihilation or creation of single particles translates a quantum state
depending on how many bosons are already in the given quantum system. This
proportionality results in a variety of non-classical features of quantum
mechanics including the bosonic commutation relation. The annihilation and
creation operations have recently been implemented in photonic systems.
However, this feature of quantum mechanics does not preclude the possibility of
realizing conventional arithmetic in quantum systems. We implement conventional
addition and subtraction of single phonons for a trapped \Yb ion in a harmonic
potential. In order to realize such operations, we apply the transitionless
adiabatic passage scheme on the anti-Jaynes-Cummings coupling between the
internal energy states and external motion states of the ion. By performing the
operations on superpositions of Fock states, we realize the hybrid computation
of classical arithmetic in quantum parallelism, and show that our operations
are useful to engineer quantum states. Our single-phonon operations are nearly
deterministic and robust against parameter changes, enabling handy repetition
of the operations independently from the initial state of the atomic motion. We
demonstrate the transform of a classical state to a nonclassical one of highly
sub-Poissonian phonon statistics and a Gaussian state to a non-Gaussian state,
by applying a sequence of the operations. The operations implemented here are
the Susskind-Glogower phase operators, whose non-commutativity is also
demonstrated.
[Show abstract][Hide abstract] ABSTRACT: Bone Marrow Transplantation is a high quality, peer-reviewed journal covering all aspects of clinical and basic haemopoietic stem cell transplantation.
Bone marrow transplantation 06/2015; DOI:10.1038/bmt.2015.145 · 3.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Utilizing the tools of quantum optics to prepare and manipulate quantum
states of motion of a mechanical resonator is currently one of the most
promising routes to explore non-classicality at a macroscopic scale. An
important quantum optomechanical tool yet to be experimentally demonstrated is
the ability to perform complete quantum state reconstruction. Here, after
providing a brief introduction to quantum states in phase space, we review and
contrast the current proposals for state reconstruction of mechanical motional
states and discuss experimental progress. Furthermore, we show that mechanical
quadrature tomography using back-action-evading interactions gives an
$s$-parameterized Wigner function where the numerical parameter $s$ is directly
related to the optomechanical measurement strength. We also discuss the effects
of classical noise in the optical probe for both state reconstruction and state
preparation by measurement.
Annalen der Physik 01/2015; 527(1-2). DOI:10.1002/andp.201400124 · 3.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is a topic of fundamental and practical importance how a quantum
correlated state can be reliably distributed through a noisy channel for
quantum information processing. The concept of quantum steering recently
defined in a rigorous manner is relevant to study it under certain
circumstances and we here address quantum steerability of Gaussian states to
this aim. In particular, we attempt to reformulate the criterion for Gaussian
steering in terms of local and global purities and show that it is sufficient
and necessary for the case of steering a 1-mode system by a $N$-mode system. It
subsequently enables us to reinforce a strong monogamy relation under which
only one party can steer a local system of 1-mode. Moreover, we show that only
a negative partial-transpose state can manifest quantum steerability by
Gaussian measurements in relation to the Peres conjecture. We also discuss our
formulation for the case of distributing a two-mode squeezed state via one-way
quantum channels making dissipation and amplification effects, respectively.
Journal of Physics A Mathematical and Theoretical 11/2014; 48(13). DOI:10.1088/1751-8113/48/13/135301 · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Phase estimation, at the heart of many quantum metrology and communication schemes, can be strongly affected by noise, whose amplitude may not be known, or might be subject to drift. Here we investigate the joint estimation of a phase shift and the amplitude of phase diffusion at the quantum limit. For several relevant instances, this multiparameter estimation problem can be effectively reshaped as a two-dimensional Hilbert space model, encompassing the description of an interferometer phase probed with relevant quantum states-split single-photons, coherent states or N00N states. For these cases, we obtain a trade-off bound on the statistical variances for the joint estimation of phase and phase diffusion, as well as optimum measurement schemes. We use this bound to quantify the effectiveness of an actual experimental set-up for joint parameter estimation for polarimetry. We conclude by discussing the form of the trade-off relations for more general states and measurements.
[Show abstract][Hide abstract] ABSTRACT: We study quantum non-Markovianity in the early stage of the emission process
of a two-level atom coupled to a semi-infinite waveguide, where the waveguide
termination behaves as a perfect mirror. Specifically, we restrict to the
analysis of the process for times shorter than twice the time delay t_d, where
t_d is the duration of a round trip along the atom-mirror path. We show the
emergence of a threshold in the parameters space separating the Markovian and
non-Markovian regions.
Physica Scripta 09/2014; T160(T160). DOI:10.1088/0031-8949/2014/T160/014043 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Wheeler's delayed-choice experiment illustrates vividly that the observer plays a central role in quantum physics by demonstrating that complementarity or wave-particle duality can be enforced even after the photon has already entered the interferometer. The delayed-choice quantum eraser experiment further demonstrates that complementarity can be enforced even after detection of a quantum system, elucidating the foundational nature of complementarity in quantum physics. However, the applicability of the delayed-choice method for practical quantum information protocols continues to be an open question. Here, we introduce and experimentally demonstrate the delayed-choice decoherence suppression protocol, in which the decision to suppress decoherence on an entangled two-qubit state is delayed until after the decoherence and even after the detection of a qubit. Our result suggests a new way to tackle Markovian decoherence in a delayed manner, applicable for practical entanglement distribution over a dissipative channel.
[Show abstract][Hide abstract] ABSTRACT: We investigate the emission of a polaritonic system, where the coupling
between a large number of two-level emitters and a single-mode cavity field is
non-adiabatically switched on. Counter-rotating terms as well as the so-called
$A^2$ term are included in the light-matter interaction, where ${\bf A }$ is
the vector potential. We find that the Thomas-Reiche-Kuhn sum rule enforces
qualitative constraints on the quantum statistics of the system radiation,
which consists of two spectrally resolved output modes. For ideal two-level
emitters the populations of the two modes are always found equal. This result
cannot be recovered if $A^2$ is neglected, or even if it is included
perturbatively via renormalization of the cavity frequency. We then extend our
study to imperfect two-level emitters, featuring residual couplings to higher
levels, and find that a naive application of the two-level approximation alters
these predictions incorrectly. We discuss how a refined two-level approximation
may be obtained by rescaling the $A^2$ term.
Physical Review A 07/2014; 91(6). DOI:10.1103/PhysRevA.91.063840 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We address the problem of continuous-variable quantum phase estimation in the
presence of linear disturbance at the Hamiltonian level, by means of Gaussian
probe states. In particular we discuss both unitary and random disturbance, by
considering the parameter which characterizes the unwanted linear term present
in the Hamiltonian as fixed (unitary disturbance) or random with a given
probability distribution (random disturbance). We derive the optimal input
Gaussian states at fixed energy, maximizing the quantum Fisher information over
the squeezing angle and the squeezing energy fraction, and we discuss the
scaling of the quantum Fisher information in terms of the output number of
photons $n_{out}$. We observe that in the case of unitary disturbance the
optimal state is a squeezed vacuum state and the quadratic scaling is
conserved. As regards the random disturbance, we observe that the optimal
squeezing fraction may not be equal to one, and, for any non-zero value of the
noise parameter, the quantum Fisher information scales linearly with the
average number of photons. We finally discuss the performance of homodyne
measurement, comparing the achievable precision with the ultimate limit posed
by the quantum Cram\'er-Rao bound.
Physical Review A 07/2014; 90(4). DOI:10.1103/PhysRevA.90.042119 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The superposition principle is at the heart of quantum mechanics and at the
root of many paradoxes arising when trying to extend its predictions to our
everyday world. Schroedinger's cat is the prototype of such paradoxes and here,
in contrast to many others, we choose to investigate it from the operational
point of view. We experimentally demonstrate a universal strategy for producing
an unambiguously distinguishable type of superposition, that of an arbitrary
pure state and its orthogonal. It relies on only a limited amount of
information about the input state to first generate its orthogonal one. Then, a
simple change in the experimental parameters is used to produce arbitrary
superpositions of the mutually orthogonal states. Constituting a sort of
Schroedinger's black box, able to turn a whole zoo of input states into
coherent superpositions, our scheme can produce arbitrary continuous-variable
optical qubits, which may prove practical for implementing quantum technologies
and measurement tasks.
[Show abstract][Hide abstract] ABSTRACT: We propose a general framework to effectively "open" a high-Q resonator, that is, to release the quantum state initially prepared in it in the form of a traveling electromagnetic wave. This is achieved by employing a mediating mode that scatters coherently the radiation from the resonator into a one-dimensional continuum of modes such as a waveguide. The same mechanism may be used to "feed" a desired quantum field to an initially empty cavity. Switching between an open and "closed" resonator may then be obtained by controlling either the detuning of the scatterer or the amount of time it spends in the resonator. First, we introduce the model in its general form, identifying (i) the traveling mode that optimally retains the full quantum information of the resonator field and (ii) a suitable figure of merit that we study analytically in terms of the system parameters. Then, we discuss two feasible implementations based on ensembles of two-level atoms interacting with cavity fields. In addition, we discuss how to integrate traditional cavity QED in our proposal using three-level atoms.
[Show abstract][Hide abstract] ABSTRACT: We address detection of quantum non-Gaussian states, i.e. nonclassical states
that cannot be expressed as a convex mixture of Gaussian states, and present a
method to derive a new family of criteria based on generic linear functionals.
We then specialise this method to derive witnesses based on $s$-parametrized
quasiprobability functions, generalising previous criteria based on the Wigner
function. In particular we discuss in detail and analyse the properties of
Husimi Q-function based witnesses and prove that they are often more effective
than previous criteria in detecting quantum non-Gaussianity of various kinds of
non-Gaussian states evolving in a lossy channel.
Physical Review A 03/2014; 90(1). DOI:10.1103/PhysRevA.90.013810 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report direct evidence of the bosonic nature of surface plasmon polaritons
(SPPs) in a scattering-based beamsplitter. A parametric down-conversion source
is used to produce two indistinguishable photons, each of which is converted
into a SPP on a metal-stripe waveguide and then made to interact through a
semi-transparent Bragg mirror. In this plasmonic analog of the Hong-Ou-Mandel
experiment, we measure a coincidence dip with a visibility of 72%, a key
signature that SPPs are bosons and that quantum interference is clearly
involved.
[Show abstract][Hide abstract] ABSTRACT: We consider the integration of quantum emitters into a negative permeability
metamaterial design in order to introduce tunability as well as nonlinear
behavior. The unit cell of our metamaterial is a ring of metamolecules, each
consisting of a metal nanoparticle and a two-level semiconductor quantum dot
(QD). Without the QDs, the ring of the unit cell is known to act as an
artificial optical magnetic resonator. By adding the QDs we show that a Fano
interference profile is introduced into the magnetic field scattered from the
ring. This induced interference is shown to cause an appreciable effect in the
collective magnetic resonance of the unit cell. We find that the interference
provides a means to tune the response of the negative permeability
metamaterial. The exploitation of the QD's inherent nonlinearity is proposed to
modulate the metamaterial's magnetic response with a separate control field.
Physical Review A 01/2014; 89(1). DOI:10.1103/PhysRevA.89.013822 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We consider a quantum emitter ("atom") radiating in a one-dimensional (1D)
photonic waveguide in the presence of a single mirror. This setup can be
implemented in a variety of platforms, such as quantum dots emitting in a
terminated 1D photonic crystal/nanowire, or even in free space by means of a
standard mirror and an ion trapped at the focus of high-numerical-aperture
lenses. It is known that the feedback mechanism introduced by the mirror gives
rise to a complex emission process that can feature significant memory effects.
Here, we carry out a systematic analysis of the non-Markovian (NM) character of
such process in terms of refined, recently developed notions of quantum
non-Markovianity such as indivisibility and information back-flow. NM effects
are quantified as a function of the time delay and phase shift associated with
the atom-mirror optical path. We find, in particular, that unless an
atom-photon bound state is formed a finite time delay is always required in
order for NM effects to be exhibited. This identifies a finite threshold in
parameter space, separating the Markovian and non-Markovian regimes.
Physical Review A 12/2013; 90(1). DOI:10.1103/PhysRevA.90.012113 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We show how the interference between spatially separated states of the center of mass (c.m.) of a mesoscopic harmonic oscillator can be evidenced by coupling it to a spin and performing solely spin manipulations and measurements (Ramsey interferometry). We propose to use an optically levitated diamond bead containing a nitrogen-vacancy center spin. The nanoscale size of the bead makes the motional decoherence due to levitation negligible. The form of the spin-motion coupling ensures that the scheme works for thermal states so that moderate feedback cooling suffices. No separate control or observation of the c.m. state is required and thereby one dispenses with cavities, spatially resolved detection, and low-mass-dispersion ensembles. The controllable relative phase in the Ramsey interferometry stems from a gravitational potential difference so that it uniquely evidences coherence between states which involve the whole nanocrystal being in spatially distinct locations.
[Show abstract][Hide abstract] ABSTRACT: Highly quantum non-linear interactions between different bosonic modes lead
to the generation of quantum non-Gaussian states, i.e. states that cannot be
written as mixtures of Gaussian states. A paradigmatic example is given by
Schr\"odinger's cat states, that is coherent superpositions of coherent states
with opposite amplitude. We here consider a novel quantum non-Gaussianity
criterion recently proposed in the literature and prove its effectiveness on
Schr\"odinger cat states evolving in a lossy bosonic channel. We prove that
quantum non-Gaussianity can be effectively detected for high values of losses
and for large coherent amplitudes of the cat states.
Physica Scripta 09/2013; T160(T160). DOI:10.1088/0031-8949/2014/T160/014035 · 1.13 Impact Factor