Frequency cavity pulling induced by a single semiconducting artificial atom

Artificial atoms inserted in optical cavities are higly promising systems for quantum information processing. However, unlike real atoms, artificial ones are coupled to the solid-state environment, whether vibrationnal or electrostatic. This irreducible dephasing opens a whole new field of investigation for cavity quantum electrodynamics where the emitter linewidth overcomes the resonator one. Here, we show that phonon induced dephasing leads to spectacular cavity pulling effects in the emission of a semiconductor quantum dot deterministically coupled to a pillar cavity. A strong pulling, broadening and narrowing of the cavity mode emission is observed when changing the spectral detuning between the emitter and the cavity. These features, observed with a single emitter, are highly reminiscent of the well known cavity pulling effects observed in lasers where thousand of atoms act as an inhomogeneous gain medium.

Analia Zwick

Weizmann Institute of Science

Optimal quantum state transfer in noisy qubit-channels

Quantum communication between remote registers is one of the key building blocks for building quantum computers. Spin chains can act as quantum channels for the transmission of information with an accuracy that depends of the interaction between the spins. Perfect state transfer channels can be obtained by accurately engineering each of those interactions to well specified values. In the eagerness to avoid such demanding and challenging requirement, we show that, by tailoring only the boundary qubit interactions of the simplest homogeneous channel, a high fidelity transfer can be achieved. The method work even for room temperature and it is also robust against static perturbations. We find an optimal regime for fast transfer and we observe a power-law scaling of the fidelity depending on the chain length and the disorder strength. In general, improving the fidelity and/or the robutness of the channel requires longer times for transferring the information. By adding also dynamical control on the boundary interactions, we can increase by orders of magnitude the information transfer speed and the fidelity while keeping a comparable robustness against perturbations. This is done by using a general optimization method for dynamical control of quantum open systems.

Andrew White

University of Queensland

TBA

Ciara Morgan

Institut für Theoretische Physik, Leibniz Universität Hannover

“Pretty strong” converse for the quantum capacity of degradable channels

We exhibit a possible road towards a strong converse for the quantum capacity of degradable channels. In particular, we show that all degradable channels obey what we call a “pretty strong” converse: When the code rate increases above the quantum capacity, the ﬁdelity makes a discontinuous jump from 1 to at most 1/√2, asymptotically. A similar result can be shown for the private (classical) capacity. Furthermore, we can show that if the strong converse holds for symmetric channels (which have quantum capacity zero), then degradable channels obey the strong converse: The above mentioned asymptotic jump of the ﬁdelity at the quantum capacity is then from 1 down to 0. This work is in collaboration with Andeas Winter.

Daniel Cavalcanti

Centre for Quantum Technologies, National University of Singapore

TBA

Daniel Felinto

UFPE

Dynamics of the reading process of a quantum memory based in cold atoms

The mechanism of extraction of information stored in a DLCZ-type quantum memory is studied in detail. We consider then memories containing a single excitation of a collective atomic state, which is mapped into a single photon during the reading process. A theory is developed for the wavepacket of the extracted photon, leading to a simple analytical expression depending on the key parameters of the problem, like detuning and intensity of the read field and the number of atoms in the atomic ensemble. This theory is then compared to a large set of experimental situations and a satisfactory quantitative agreement is obtained. In this way, we are able to systematically study the saturation and spectrum of the reading process, as well as clarify the role of superradiance in the system.We also report the direct experimental observation of Rabi oscillations of the collective excitation in the system, and discuss applications of such memories to quantum information protocols.

Daniel Jonathan

Universidade Federal Fluminense

Direct measurement of the local invariants of quantum evolutions

We present a family of polynomials that generate all the local unitary invariants of any trace-preserving quantum evolution. Explicit circuits are constructed that allow these quantities to be directly measured in an experiment, without the use of quantum process tomography. As an application, we identify a complete, minimal-rank set of polynomial invariants for two-qubit unitary gates, which moreover have well-defined interpretations in terms of the gates' abilities to create entanglement or communicate classical information.

Diego Wisniacki

Universidad de Buenos Aires

Control of open quantum systems and the quantum speed limit

Unitary control and decoherence appear to be irreconcilable in quantum mechanics. When a quantum system interacts with an environment, control strategies usually fail due to decoherence. Recently, we propose a time-optimal unitary control protocol suitable for quantum open systems. The method is based on the succesive diabatic and sudden switch transitions in the avoided crossings of the energy spectra of the closed systems. We show that the speed of this control protocol meets the fundamental bounds imposed by the quantum speed limit, thus making this scheme ideal for application where decoherence needs to be avoided. We show that our method can achieve complex control strategies with high accuracy in open quantum systems.

Elsi-Mari Laine

University of Turku

Nonlocal memory effects and noisy quantum teleportation

We explore the possibility to generate a nonlocal quantum channel from a local interaction. It is shown that for a generic dephasing process initial correlations between the environments can generate a nonlocal quantum process giving rise to dynamics where the system globally exhibits strong memory effects although the local dynamics is Markovian. In a model of two entangled photons interacting with two dephasing environments we find a direct connection between the degree of memory effects and the amount of correlations in the initial environmental state.
We further show that perfect quantum teleportation can be achieved with mixed photon polarization states when nonlocal memory effects influence the dynamics of the quantum system. The protocol is carried out with a pair of photons, whose initial maximally entangled state is destroyed by local decoherence prior to teleportation. It is demonstrated that the presence of strong nonlocal memory effects, which arise from initial correlations between the environments of the photons, allow to restore perfect teleportation. We further analyze how the amount of initial correlations within the environment affects the fidelity of the protocol, and find that for a moderate amount of correlations the fidelity exceeds the one of the previously known optimal teleportation protocol without memory effects. Our results show that memory effects can be exploited in harnessing noisy quantum systems for quantum communication and that non-Markovianity is a resource for quantum information tasks.

Ernesto F. Galvão

Universidade Federal Fluminense

Bunching and boson sampling in integrated photonic chips

Integrated photonic chip technologies enable the implementation of stable, large-scale interferometers with applications ranging from quantum metrology to quantum computation. I will discuss two recent applications that we have recently demonstrated: a restricted photonic quantum computer known as a boson sampling computer; and a systematic study of photonic bunching in multimode chips. These two sets of experiments have recently verified the permanent formula that governs the dynamics of non-interacting bosons, and also confirmed a new theoretical result on bosonic bunching that I will present.
This is joint work with N. Spagnolo, C. Vitelli, L. Sansoni, E. Maiorino, P. Mataloni, F. Sciarrino, D. J. Brod, A. Crespi, R. Ramponi, and R. Osellame.

Felipe Fernandes Fanchini

UNESP - Bauru

Multipartite Quantum Correlations in Open Quantum Systems

In this paper we present a measure of quantum correlation for a multipartite system, defined as the sum of the correlations for all possible partitions. Our measure can be defined for quantum discord (QD), geometric quantum discord (GQD), or even by the entanglement of formation (EOF). For tripartite pure states, we show that the multipartite measure for the QD and the EOF are equivalent, which gives a way to compare the distribution and the robustness of these correlations in open quantum systems. We study dissipative dynamics for two distinct families of entanglement: a W state and a GHZ state. We show that, while for the W state the QD is more robust than the entanglement, for the GHZ state this is not true. It turns out that the initial genuine multipartite entanglement present in the GHZ state makes the EOF more robust than the QD.

Fernando de Melo

CWI

The Power of Noisy Fermionic Quantum Computation

We consider the realization of universal quantum computation through braiding of Majorana fermions supplemented by unprotected preparation of noisy ancillae. It has been shown by Bravyi (2006 Phys. Rev. A 73 042313) that under the assumption of perfect braiding operations, universal quantum computation is possible if the noise rate on a particular four-fermion ancilla is below 40%. We show that beyond a noise rate of 89% on this ancilla the quantum computation can be efficiently simulated classically: we explicitly show that the noisy ancilla is a convex mixture of Gaussian fermionic states in this region, while for noise rates below 53% we prove that the state is not a mixture of Gaussian states. These results are obtained by generalizing concepts in entanglement theory to the setting of Gaussian states and their convex mixtures. In particular, we develop a complete set of criteria, namely the existence of a Gaussian-symmetric extension, which determine whether a state is a convex mixture of Gaussian states.

Gabriela Lemos

Institut fuer Quantenoptik und Quanteninformation (IQOQI)

Experimental Observation of Quantum Chaos in a Beam of Light

We implement the quantum Kicked Harmonic Oscillator (KHO) in the spatial degrees of freedom of light. We measure the time-evolved Wigner function directly, and observe signatures of quantum non-linear dynamics. Our setup is decoherence-free and we can control experimental parameters to explore the transition from regular to essentially chaotic dynamics, as well as the quantum-classical transition. Due to this robustness and versatility, our scheme can be used to experimentally explore a variety of non-linear quantum phenomena. As an example, we couple this system to a quantum bit and experimentally investigate the decoherence produced by regular or chaotic dynamics.

George Knee

University of Oxford

Is weak-value amplification useful for parameter estimation?

Quantum sensors that harness exclusively quantum phenomena (such as entanglement) can achieve superior performance compared to those employing only classical principles. Recently, a technique based on postselected, weakly performed measurements has emerged as a method of overcoming technical noise in the detection and estimation of small interaction parameters, particularly in optical systems. But are such schemes, known collectively as 'Weak-Value amplification', really any better than standard, direct approaches? I try to answer these questions through the use of realistic examples.

Gerardo Adesso

University of Nottingham

Characterizing nonclassical correlations via local quantum uncertainty

Quantum mechanics predicts that measurements of incompatible observables carry a minimum uncertainty which is independent of technical deficiencies of the measurement apparatus or incomplete knowledge of the state of the system. Nothing yet seems to prevent a single physical quantity, such as one spin component, from being measured with arbitrary precision. Here we show that an intrinsic quantum uncertainty on a single observable is ineludible in a number of physical situations. When revealed on local observables of a bipartite system, such uncertainty defines an entire class of bona fide measures of nonclassical correlations. For the case of 2 × d systems, we find that a unique measure is defined, which we evaluate in closed form. We then discuss the role that these correlations, which are of the ‘discord’ type, can play in the context of quantum metrology. We show in particular that the amount of discord present in a bipartite mixed probe state guarantees a minimum precision, as quantified by the quantum Fisher information, in the optimal phase estimation protocol.

Gonzalo Alvarez

Weizmann Institute, Israel

TBA

Ladislav Mista

Department of Optics, Palacky University Olomouc

Gaussian intrinsic entanglement

Secret classical correlations are the analog of quantum entanglement, and the latter can be mapped onto the former. This offers the possibility of quantifying the amount of entanglement in a quantum state as a maximum amount of secret correlations one can obtain from the mapping of the state onto a classical probability distribution.
We use this approach to propose a new quantifier of Gaussian entanglement that is restricted to Gaussian states and operations. We show that the proposed quantity is an entanglement monotone under Gaussian local operations and classical communication. In the two-mode case a closed expression of the quantifier can be derived for certain classes of states with a three-mode purification. Furthermore, the quantifier can be evaluated numerically for some other important classes of states. The introduced quantity is operationally associated with the secret key agreement protocol of classical cryptography and can be viewed as a classical analog to squashed entanglement in a Gaussian scenario.

Juan Pablo Paz

Universidad de Buenos Aires

TBA

Luis Sanchez Soto

Universidad Complutense de Madrid

TBA

Luiz Davidovich

Universidade Federal do Rio de Janeiro, Brazil

TBA

Marcelo Martinelli

IF-USP

Expanding the entangled space in continuous variables

In the continuous variables domain, full reconstruction of the quantum state of optical fields relies in the high quantum efficiency of the photodetectors associated with powerful electronics to eliminate spurious sources of noise. We will show how these tools, applied to an old and useful source of quantum states (the optical parametric oscillator) can give us an expanded, fully entangled six mode state. Immediate implications to quantum teleportation protocols will be presented.

Marcelo Terra Cunha

UFMG

State-independent contextuality with identical particles

It has been recently conjectured that the state-independency of quantum contextuality may be lost when the indistinguishability of identical particles is taken into account. We show that quantum state-independent contextuality exists for any system of more than one identical bosonic qudits, and for most systems of fermionic qudits. The only exception is the case of d fermionic qudits, since there the dimension of the antisymmetric subspace is 1, which is insufficient for contextuality. For all the other cases, either the symmetry precludes the existence of physical states, or we provide an explicit method to produce quantum state-independent contextual correlations.

Marco Quintino

Université de Genève (Universidade de Genebra)

Genuine Hidden Nonlocality

Entanglement and nonlocality (Bell inequality violation) play a
central role in quantum information and thus been extensively studied.
Nevertheless, the precise relation between these concepts its not yet
understood. While entanglement and nonlocality are equivalent for
pure states the situation is more subtle for mixed states. In a seminal
paper, Werner showed that there exist entangled states which are local,
that is, their correlations under local projective measurements cannot
violate a Bell inequality. Some years later, Popescu showed that some
of these local entangled states can have their nonlocality revealed
after a local filtering process, introducing the idea of hidden nonlocality.
However, the states considered by Popescu are guaranteed to be local
only for projective measurements, leaving open the question of what
would happen in a general POVM measurement scenario. That is, it
could be the case that when POVMs are considered, local filters are
not necessary to reveal the nonlocality of a quantum state. Hence in
this case, there would be no hidden nonlocality.
Here however we show that there exist genuine hidden nonlocality.
Specifically, we present a two-qubit state for which we construct a
local model for general POVMs that violates a Bell inequality when
local filters are applied. Our results bring new insight to the problem of understanding the exact relation between entanglement and
nonlocality.

Mateus Araujo

Universität Wien

Decrease in query complexity for quantum computers with indefinite causal structure

Inspired by quantum gravity considerations, a new model of quantum computation was introduced by Chiribella et al. in arXiv:0912.0195, in which superposition of circuits with different causal structure was allowed. It left open a natural question about it: is it more powerful than standard quantum computation? Here we answer this question in the affirmative, by showing a promise problem which can be solved in time $O(n)$ by a quantum computer with indefinite causal structure, whereas the best known quantum algorithm has query complexity $O(n^2)$.

Max Stillmann

TBA

Paulo A. Nussenzveig

IF-USP

TBA

Roberto M. Serra

Universidade Federal do ABC (UFABC)

Coherent enhancement in noisy quantum metrology

Quantum mechanical systems can be used to out perform classical ones in several tasks. For instance, quantum correlations can be employed to beat the shot-noise (standard) limit in metrology protocols. Such parameter estimation methods are crucial for both advances in science and the development of technologies. Almost all quantum technologies operate with some level of noise and how quantum-enhancement fares in the presence of noise is still unclear. Here we show that, when a system in mixed multipartite quantum state is used to estimate a parameter, the information must be decoded by coherent interactions between the parts of the probe. On the other hand, noiseless quantum metrology protocols are adaptively additive for any pure entangled states, that is, no coherent processing is necessary in this case. This leads to an operational interpretation for the ability of performing coherent interactions in noisy parameter estimation and highlights a cornerstone difference between mixed and pure states. Operating a molecular ensemble quantum simulator driven by nuclear magnetic resonance techniques (NMR), we experimentally demonstrate that processing a quantum probe by coherent interactions lead to a quantum advantage. This reveals a sharper non-equivalence between noise and noiseless settings for high precision measurements.

Raúl Rossignoli

Depto. de Fisica-IFLP-CIC and Depto. de Cs. Bs., Facultad de Ingeniería UNLP

Quantum discord and parity in spin 1 systems

We consider the evaluation of the quantum discord, the geometric discord and the information deficit in a system formed by a spin-1 and a complementary spin system. A characterization of general projective
measurements in such system in terms of spin averages is also discussed, which allows one to easily visualize their deviation from standard spin measurements. It is shown that the measurement optimizing these measures
corresponds in general to a nonspin measurement. The important case of states that commute with the total Sz spin-parity is discussed in detail, and the general stationary measurements for such states (parity preserving
measurements) are identified. Numerical and analytical results for the quantum discord, the geometric discord, and the one-way information deficit in the relevant case of a mixture of two aligned spin-1 states are also presented.

Seiji Armstrong

Australian National University

Generation of ultra-large-scale entangled graph states

Quantum entanglement is a fundamental and defining feature of quantum physics. Remarkably, it is also the key ingredient in quantum information processing (QIP).
In this presentation I will discuss the experimental generation of ultra-large-
scale quantum entangled states with light. The entangled states are
created unconditionally, and employ multiplexing in the time domain. The entangled graph states comprise of more than 10,000 fully inseparable wave-packets existing in two optical continuous-wave beams. I will also propose an experimentally feasible implementation of measurement-based quantum-computation on the graph state.

Sebastião José Nascimento de Pádua

Departamento de Física - UFMG

Measurement of entanglement witnesses and demonstration of concentration protocol in a two qutrit system

We first present an experimental technique for a complete characterization
of entanglement in a two-qutrit state generated using transverse
spatial correlations of two parametric down-converted photons. We verify
entanglement for a particular case via entanglement witness operators
which are decomposed into a sum of local observables of single path and
superposition projection operators [1]. Experimentally, these operators are accomplished by using a spatial light modulator and a polarizing beam splitter which allow to modulate the amplitude of individually chosen path states.
The quantification of entanglement is computed by the negativity obtained
from the expectation values of the entanglement witnesses implemented.
In the second part of the talk, we report on a technique that allows for the entanglement manipulation of qudits states encoded in
the linear transverse momentum of photon pairs generated in the process of spontaneous parametric downconversion. It relies on the use of spatial light modulators and interferometric techniques for implementing the desired local measurements. As an application of our method we perform the optimal entanglement concentration of entangled qutrits states, where maximally entangled states are obtained with the maximal success probability [2]. The procedure of entanglement concentration
is fundamental for many quantum information tasks, thus, showing the potential of our technique for future experimental investigations considering high-dimensional Hilbert spaces.
[1] A. J. Gutiérrez-Esparza, W. M. Pimenta, B. Marques, A. A.
Matoso, J. L. Lucio M., and S. Pádua, Optics Express 20, 26351 (2012).
[2] B. Marques, A. A. Matoso, W. M. Pimenta, A. J. Gutiérrez-Esparza, G. Lima, Leonardo Neves, A. Delgado, C. Saavedra and S. Pádua, Phys. Rev. A 87, 052327 (2013 (2013).

Stephen Walborn

Universidade Federal do Rio de Janeiro, Brazil

TBA

Thiago de Oliveira

UFF

Nonviolation of Bell's Inequality in Translation Invariant Systems

The nature of quantum correlations in strongly correlated systems has been a subject of intense research. In particular, it has been realized that entanglement and quantum discord are present at quantum phase transitions and able to characterize it. Surprisingly, it has been shown for a number of different systems that qubit pairwise states, even when highly entangled, do not violate Bell's inequalities, being in this sense local. Here we show that such a local character of quantum correlations is in fact general for translation invariant systems and has its origins in the monogamy trade-off obeyed by tripartite Bell correlations. We illustrate this result in a quantum spin chain with a soft breaking of translation symmetry. In addition, we extend the monogamy inequality to the $N$-qubit scenario, showing that the bound increases with $N$ and providing examples of its saturation through uniformly generated random pure states.

Vladimir Buzek

Slovak Academy of Sciences, Slovakia

TBA

Zdenek Hradil

Department of Optics, Palacky University Olomouc

Uncertainty relations for improving the quantum tomography

Quantum Tomography is a method for retrieving full information about quantum systems. However, due to the limited resources restricting our skills to prepare and to control the states, to do the measurement on demand and subsequently to use this information, the capacity of the tomography cannot be fully exploited. Alternatively, rules of quantum mechanics can be formulated by means of moments of measurable quantities. Uncertainties and corresponding minimum uncertainty states then represent borders in the configuration space, which can be used for fundamental calibration and improving the sensitivity of reconstruction for particular states. We will demonstrate this on the examples of angular momentum and unitary angle operator and photon-number and quadrature operators.