arXiv subjects

John H. Reina

Publications and source records attributed to John H. Reina.

At least 37 records · Page 2Linked to original sources

arXiv2016

Quantum Locality in Game Strategy

Game theory is a well established branch of mathematics whose formalism has a vast range of applications from the social sciences, biology, to economics. Motivated by quantum information science, there has been a leap in the formulation of novel game strategies that lead to new (quantum Nash) equilibrium points whereby players in some classical games are always outperformed if sharing and processing joint information ruled by the laws of quantum physics is allowed. We show that, for a bipartite non zero-sum game, input local quantum correlations, and separable states in particular, suffice to achieve an advantage over any strategy that uses classical resources, thus dispensing with quantum nonlocality, entanglement, or even discord between the players' input states. This highlights the remarkable key role played by pure quantum coherence at powering some protocols. Finally, we propose an experiment that uses separable states and basic photon interferometry to demonstrate the locally-correlated quantum advantage.

quant-ph↗

arXiv2014

Extracting Information from Qubit-Environment Correlations

Most works on open quantum systems generally focus on the reduced physical system by tracing out the environment degrees of freedom. Here we show that the qubit distributions with the environment are essential for a thorough analysis, and demonstrate that the way that quantum correlations are distributed in a quantum register is constrained by the way in which each subsystem gets correlated with the environment. For a two-qubit system coupled to a common dissipative environment $\mathcal{E}$, we show how to optimise interqubit correlations and entanglement via a quantification of the qubit-environment information flow, in a process that, perhaps surprisingly, does not rely on the knowledge of the state of the environment. To illustrate our findings, we consider an optically-driven bipartite interacting qubit $AB$ system under the action of $\mathcal{E}$. By tailoring the light-matter interaction, a relationship between the qubits early stage disentanglement and the qubit-environment entanglement distribution is found. We also show that, under suitable initial conditions, the qubits energy asymmetry allows the identification of physical scenarios whereby qubit-qubit entanglement minima coincide with the extrema of the $A\mathcal{E}$ and $B\mathcal{E}$ entanglement oscillations.

quant-ph↗

arXiv2013

Correlations in emitters coupled to plasmonic waveguides

We report on quantum, classical, and total correlations in a set of distant quantum emitters coupled via their interaction with the plasmon modes of a one-dimensional waveguide driven by an external laser field. The coupling of the emitters with the plasmonic modes and its influence on the collective decay rate suggest that entanglement does not play a significant role in the qubit dynamics. Rather, discord is the quantity that matters and should be harnessed as a resource.

quant-ph↗

arXiv2012

Correlations in optically-controlled quantum emitters

We address the problem of optically controlling and quantifying the dissipative dynamics of quantum and classical correlations in a set-up of individual quantum emitters under external laser excitation. We show that both types of correlations, the former measured by the quantum discord, are present in the system's evolution even though the emitters may exhibit an early stage disentanglement. In the absence of external laser pumping,we demonstrate analytically, for a set of suitable initial states, that there is an entropy bound for which quantum discord and entanglement of the emitters are always greater than classical correlations, thus disproving an early conjecture that classical correlations are greater than quantum correlations. Furthermore, we show that quantum correlations can also be greater than classical correlations when the system is driven by a laser field. For scenarios where the emitters' quantum correlations are below their classical counterparts, an optimization of the evolution of the quantum correlations can be carried out by appropriately tailoring the amplitude of the laser field and the emitters' dipole-dipole interaction. We stress the importance of using the entanglement of formation, rather than the concurrence, as the entanglement measure, since the latter can grow beyond the total correlations and thus give incorrect results on the actual system's degree of entanglement.

quant-ph↗

arXiv2010

Full Additivity of the Entanglement of Formation

We present a general strategy that allows a more flexible method for the construction of fully additive multipartite entanglement monotones than the ones so far reported in the literature of axiomatic entanglement measures. Within this framework we give a proof of a conjecture of outstanding implications in information theory: the full additivity of the Entanglement of Formation.

quant-ph↗

arXiv2010

Nonlocal fluctuations and control of dimer entanglement dynamics

We report on the dissipative dynamics of an entangled, bipartite interacting system. We show how to induce and control the so-called early stage disentanglement (and the delayed entanglement generation) dynamics by means of a driving laser field. We demonstrate that some of the features currently associated with pure non-Markovian effects in such entanglement behavior can actually take place in Markovian environments if background noise QED fluctuations are considered. We illustrate this for the case of a dimer interacting molecular system for which emission rates, interaction strength, and radiative corrections have been previously measured. We also show that even in the absence of collective decay mechanisms and qubit-qubit interactions, the entanglement still exhibits collapse-revival behavior. Our results indicate that zero point energy fluctuations should be taken into account when formulating precise entanglement dynamics statements.

quant-ph↗

arXiv2009

Enhanced quantum entanglement in the non-Markovian dynamics of biomolecular excitons

We show that quantum coherence of biomolecular excitons is maintained over exceedingly long times due to the constructive role of their non-Markovian protein-solvent environment. Using a numerically exact approach, we demonstrate that a slow quantum bath helps to sustain quantum entanglement of two pairs of Forster coupled excitons, in contrast to a Markovian environment. We consider the crossover from a fast to a slow bath and from weak to strong dissipation and show that a slow bath can generate robust entanglement. This persists to surprisingly high temperatures, even higher than the excitonic gap and is absent for a Markovian bath.

cond-mat.mes-hall↗

arXiv2009

Geometric multipartite entanglement measures

Within the framework of constructions for quantifying entanglement, we build a natural scenario for the assembly of multipartite entanglement measures based on Hopf bundle-like mappings obtained through Clifford algebra representations. Then, given the non-factorizability of an arbitrary two-qubit density matrix, we give an alternate quantity that allows the construction of two types of entanglement measures based on their arithmetical and geometrical averages over all pairs of qubits in a register of size N, and thus fully characterize its degree and type of entanglement. We find that such an arithmetical average is both additive and strongly super additive.

quant-ph↗

arXiv2009

Molecular architectures based on pi-conjugated block copolymers for global quantum computation

We propose a molecular setup for the physical implementation of a barrier global quantum computation scheme based on the electron-doped pi-conjugated copolymer architecture of nine blocks PPP-PDA-PPP-PA-(CCH-acene)-PA-PPP-PDA-PPP (where each block is an oligomer). The physical carriers of information are electrons coupled through the Coulomb interaction, and the building block of the computing architecture is composed by three adjacent qubit systems in a quasi-linear arrangement, each of them allowing qubit storage, but with the central qubit exhibiting a third accessible state of electronic energy far away from that of the qubits' transition energy. The third state is reached from one of the computational states by means of an on-resonance coherent laser field, and acts as a barrier mechanism for the direct control of qubit entanglement. Initial estimations of the spontaneous emission decay rates associated to the energy level structure allow us to compute a damping rate of order 10^{-7} s, which suggest a not so strong coupling to the environment. Our results offer an all-optical, scalable, proposal for global quantum computing based on semiconducting pi-conjugated polymers.

cond-mat.mes-hall↗

arXiv2009

Non-equilibrium entanglement in a driven Dicke model

We study the entanglement dynamics in the externally-driven single-mode Dicke model in the thermodynamic limit, when the field is in resonance with the atoms. We compute the correlations in the atoms-field ground state by means of the density operator that represents the pure state of the universe and the reduced density operator for the atoms, which results from taking the partial trace over the field coordinates. As a measure of bipartite entanglement, we calculate the linear entropy, from which we analyze the entanglement dynamics. In particular, we found a strong relation between the stability of the dynamical parameters and the reported entanglement.

cond-mat.mes-hall↗

arXiv2009

Dissipative dynamics of superconducting hybrid qubit systems

We perform a theoretical study of composite superconducting qubit systems for the case of a coupled qubit configuration based on a hybrid qubit circuit made of both charge and phase qubits, which are coupled via a sigma(x)xsigma(z) interaction. We compute the system's eigen-energies in terms of the qubit transition frequencies and the strength of the inter-qubit coupling, and describe the sensitivity of the energy crossing/anti-crossing features to such coupling. We compute the hybrid system's dissipative dynamics for the cases of i) collective and ii) independent decoherence, whereby the system interacts with one common and two different baths of harmonic oscillators, respectively. The calculations have been performed within the Bloch-Redfield formalism and we report the solutions for the populations and the coherences of the system's reduced density matrix. The dephasing and relaxation rates are explicitly calculated as a function of the heat bath temperature.

cond-mat.supr-con↗

arXiv2007

Total correlations as fully additive entanglement monotones

We generalize the strategy presented in Refs. [1, 2], and propose general conditions for a measure of total correlations to be an entanglement monotone using its pure (and mixed) convex-roof extension. In so doing, we derive crucial theorems and propose a concrete candidate for a total correlations measure which is a fully additive entanglement monotone.

quant-ph↗

arXiv2007

A pairwise additive strategy for quantifying multipartite entanglement

Based on the idea of measuring the factorizability of a given density matrix, we propose a pairwise analysis strategy for quantifying and understanding multipartite entanglement. The methodology proves very effective as it immediately guarantees, in addition to the usual entanglement properties, additivity and strong super additivity. We give a specific set of quantities that fulfill the protocol and which, according to our numerical calculations, make the entanglement measure an LOCC non-increasing function. The strategy allows a redefinition of the structural concept of global entanglement.

quant-ph↗

arXiv2005

Anticrossings in Foerster Coupled Quantum Dots

We consider two coupled generic quantum dots, each modelled by a simple potential which allows the derivation of an analytical expression for the inter-dot Foerster coupling, in the dipole-dipole approximation. We investigate the energy level behaviour of this coupled two-dot system under the influence of an external applied electric field and predict the presence of anticrossings in the optical spectra due to the Foerster interaction.

quant-ph↗

arXiv2005

Radiative corrections and quantum gates in molecular systems

We propose a method for quantum information processing using molecules coupled to an external laser field. This utilizes molecular interactions, control of the external field and an effective energy shift of the doubly-excited state of two coupled molecules. Such a level shift has been seen in the two-photon resonance experiments recently reported in Ref. [1]. Here we show that this can be explained in terms of the QED Lamb shift. We quantify the performance of the proposed quantum logic gates in the presence of dissipative mechanisms. The unitary transformations required for performing one- and two-qubit operations can be implemented with present day technology.

quant-ph↗

arXiv2005

Optical Quantum Computation with Perpetually Coupled Spins

The possibility of using strongly and continuously interacting spins for quantum computation has recently been discussed. Here we present a simple optical scheme that achieves this goal while avoiding the drawbacks of earlier proposals. We employ a third state, accessed by a classical laser field, to create an effective barrier to information transfer. The mechanism proves to be highly efficient both for continuous and pulsed laser modes; moreover it is very robust, tolerating high decay rates for the excited states. The approach is applicable to a broad range of systems, in particular dense structures such as solid state self-assembled (e.g., molecular) devices. Importantly, there are existing structures upon which `first step' experiments could be immediately performed.

quant-ph↗

arXiv2003

Resonant Transfer of Excitons and Quantum Computation

Resonant energy transfer mechanisms have been observed in the sensitized luminescence of solids, in quantum dots and in molecular nanostructures, and they also play a central role in light harvesting processes in photosynthetic organisms. We demonstrate that such mechanisms, together with the exciton-exciton binding energy shift typical of these nanostructures, can be used to perform universal quantum logic. In particular, we show how to generate controlled exciton entanglement and identify two different regimes of quantum behaviour.

quant-ph↗

arXiv2001

Decoherence of quantum registers

The dynamical evolution of a quantum register of arbitrary length coupled to an environment of arbitrary coherence length is predicted within a relevant model of decoherence. The results are reported for quantum bits (qubits) coupling individually to different environments (`independent decoherence') and qubits interacting collectively with the same reservoir (`collective decoherence'). In both cases, explicit decoherence functions are derived for any number of qubits. The decay of the coherences of the register is shown to strongly depend on the input states: we show that this sensitivity is a characteristic of $both$ types of coupling (collective and independent) and not only of the collective coupling, as has been reported previously. A non-trivial behaviour ("recoherence") is found in the decay of the off-diagonal elements of the reduced density matrix in the specific situation of independent decoherence. Our results lead to the identification of decoherence-free states in the collective decoherence limit. These states belong to subspaces of the system's Hilbert space that do not get entangled with the environment, making them ideal elements for the engineering of ``noiseless'' quantum codes. We also discuss the relations between decoherence of the quantum register and computational complexity based on the new dynamical results obtained for the register density matrix.

quant-ph↗