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Sep 22, 2013
09/13

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Vittorio Giovannetti

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We derive a collection of separability conditions for bipartite systems of dimensions d X d which is based on the entropic version of the uncertainty relations. A detailed analysis of the two-qubit case is given by comparing the new separability conditions with existing criteria.

Source: http://arxiv.org/abs/quant-ph/0307171v3

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Sep 18, 2013
09/13

by
Vittorio Giovannetti

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A dynamical model for quantum channel is introduced which allows one to pass continuously from the memoryless case to the case in which memory effects are present. The quantum and classical communication rates of the model are defined and explicit expression are provided in some limiting case. In this context we introduce noise attenuation strategies where part of the signals are sacrificed to modify the channel environment. The case of qubit channel with phase damping noise is analyzed in...

Source: http://arxiv.org/abs/quant-ph/0509016v2

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Sep 18, 2013
09/13

by
Vittorio Giovannetti

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Hong-Ou-Mandel interferometry allows one to detect the presence of entanglement in two-photon input states. The same result holds for two-particles input states which obey to Fermionic statistics. In the latter case however anti-bouncing introduces qualitative differences in the interferometer response. This effect is analyzed in a Gedankenexperiment where the particles entering the interferometer are assumed to belong to a one-parameter family of quons which continuously interpolate between...

Source: http://arxiv.org/abs/quant-ph/0605223v2

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Jul 19, 2013
07/13

by
Vittorio Giovannetti

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The information capacities and ``distillability'' of a quantum channel are studied in the presence of auxiliary resources. These include prior entanglement shared between the sender and receiver and free classical bits of forward and backward communication. Inequalities and trade-off curves are derived. In particular an alternative proof is given that in the absence of feedback and shared entanglement, forward classical communication does not increase the quantum capacity of a channel.

Source: http://arxiv.org/abs/quant-ph/0406111v3

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Sep 21, 2013
09/13

by
Vittorio Giovannetti; Daniel Burgarth

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We demonstrate that the quantum communication between two parties can be significantly improved if the receiver is allowed to store the received signals in a quantum memory before decoding them. In the limit of an infinite memory, the transfer is perfect. We prove that this scheme allows the transfer of arbitrary multipartite states along Heisenberg chains of spin-1/2 particles with random coupling strengths.

Source: http://arxiv.org/abs/quant-ph/0508022v2

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Sep 18, 2013
09/13

by
Vittorio Giovannetti; Simone Severini

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By revisiting the Kirchhoff's Matrix-Tree Theorem, we give an exact formula for the number of spanning trees of a graph in terms of the quantum relative entropy between the maximally mixed state and another state specifically obtained from the graph. We use properties of the quantum relative entropy to prove tight bounds for the number of spanning trees in terms of basic parameters like degrees and number of vertices.

Source: http://arxiv.org/abs/1102.2398v1

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Sep 18, 2013
09/13

by
Vittorio Giovannetti; Rosario Fazio

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A new class of state transformations that are quantum mechanically prohibited is introduced. These can be seen as the generalization of the universal-NOT transformation which, for all pure inputs state of a given Hilbert space produces pure outputs whose projection on the original state is fixed to a value smaller than one. The case of not pure output states is also addressed. We give an application of these transformations in the context of separability criteria.

Source: http://arxiv.org/abs/quant-ph/0502149v3

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Sep 23, 2013
09/13

by
Daniel Burgarth; Vittorio Giovannetti

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In this addendum of our paper [D. Burgarth and V. Giovannetti, Phys. Rev. Lett. 99, 100501 (2007)] we prove that during the transformation that allows one to enforce control by relaxation on a quantum system, the ancillary memory can be kept at a finite size, independently from the fidelity one wants to achieve. The result is obtained by introducing the quantum analog of defragmentation algorithms which are employed for efficiently reorganizing classical information in conventional hard-disks....

Source: http://arxiv.org/abs/1006.4107v1

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Sep 18, 2013
09/13

by
Filippo Caruso; Vittorio Giovannetti

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A characterization of qubit quantum channels is introduced. In analogy to what happens in the context of Bosonic channels we exploit the possibility of representing the states of the system in terms of characteristic function. The latter are functions of non-commuting variables (Grassmann variables) and are defined in terms of generalized displacement operators. In this context we introduce the set of Gaussian channels and show that they share similar properties with the corresponding Bosonic...

Source: http://arxiv.org/abs/0707.4443v2

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Jun 27, 2018
06/18

by
Ludovico Lami; Vittorio Giovannetti

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The set of Entanglement Saving (ES) quantum channels is introduced and characterized. These are completely positive, trace preserving transformations which when acting locally on a bipartite quantum system initially prepared into a maximally entangled configuration, preserve its entanglement even when applied an arbitrary number of times. In other words, a quantum channel $\psi$ is said to be ES if its powers $\psi^n$ are not entanglement-breaking for all integers $n$. We also characterize the...

Topic: Quantum Physics

Source: http://arxiv.org/abs/1505.00461

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Sep 19, 2013
09/13

by
Daniel Burgarth; Vittorio Giovannetti

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Homogenization protocols model the quantum mechanical evolution of a system to a fixed state independently from its initial configuration by repeatedly coupling it with a collection of identical ancillas. Here we analyze these protocols within the formalism of "relaxing" channels providing an easy to check sufficient condition for homogenization. In this context we describe mediated homogenization schemes where a network of connected qudits relaxes to a fixed state by only partially...

Source: http://arxiv.org/abs/0708.2657v2

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Sep 18, 2013
09/13

by
Vittorio Giovannetti; David Vitali

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We study the dynamics of an optical mode in a cavity with a movable mirror subject to quantum Brownian motion. We study the phase noise power spectrum of the output light, and we describe the mirror Brownian motion, which is responsible for the thermal noise contribution, using the quantum Langevin approach. We show that the standard quantum Langevin equations, supplemented with the appropriate non-Markovian correlation functions, provide an adequate description of Brownian motion.

Source: http://arxiv.org/abs/quant-ph/0006084v3

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Sep 18, 2013
09/13

by
Janet Anders; Vittorio Giovannetti

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We define thermodynamic configurations and identify two primitives of discrete quantum processes between configurations for which heat and work can be defined in a natural way. This allows us to uncover a general second law for any discrete trajectory that consists of a sequence of these primitives, linking both equilibrium and non-equilibrium configurations. Moreover, in the limit of a discrete trajectory that passes through an infinite number of configurations, i.e. in the reversible limit,...

Source: http://arxiv.org/abs/1211.0183v1

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Sep 21, 2013
09/13

by
Francesco Ciccarello; Vittorio Giovannetti

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It was recently discovered that the quantum correlations of a pair of disentangled qubits, as measured by the quantum discord, can increase solely because of their interaction with a local dissipative bath. Here, we show that a similar phenomenon can occur in continuous-variable bipartite systems. To this aim, we consider a class of two-mode squeezed thermal states and study the behavior of Gaussian quantum discord under various local Markovian non-unitary channels. While these in general cause...

Source: http://arxiv.org/abs/1108.2143v2

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Sep 20, 2013
09/13

by
Filippo Caruso; Vittorio Giovannetti

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We analyze qubit channels by exploiting the possibility of representing two-level quantum systems in terms of characteristic functions. To do so, we use functions of non-commuting variables (Grassmann variables), defined in terms of generalized displacement operators, following an approach which resemble the one adopted for continuous-variable (Bosonic) systems. It allows us to introduce the notion of qubit Gaussian channels and to show that they share similar properties with the corresponding...

Source: http://arxiv.org/abs/0802.2822v1

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Sep 23, 2013
09/13

by
Francesco Ciccarello; Vittorio Giovannetti

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We have recently presented a collision-model-based framework to approach non-Markovian quantum dynamics [Ciccarello F Palma G M and Giovannetti V 2013 Phys. Rev. A 87, 040103(R)]. As a distinctive feature, memory is introduced in a dynamical way by adding extra inter-ancillary collisions to a standard (memoryless) collision model. Here, we focus on the case where such intra-bath collisions are described by incoherent partial swap operations. After briefly reviewing the model, we show how to...

Source: http://arxiv.org/abs/1305.0225v1

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Sep 18, 2013
09/13

by
Daniel Burgarth; Vittorio Giovannetti

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We demonstrate a scheme for controlling a large quantum system by acting on a small subsystem only. The local control is mediated to the larger system by some fixed coupling Hamiltonian. The scheme allows to transfer arbitrary and unknown quantum states from a memory on the large system (``upload access'') as well as the inverse (``download access''). We study sufficient conditions of the coupling Hamiltonian and give lower bounds on the fidelities for downloading and uploading.

Source: http://arxiv.org/abs/0704.3027v1

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Sep 23, 2013
09/13

by
Daniel Burgarth; Vittorio Giovannetti

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We discuss an explicit protocol which allows one to externally cool and control a composite system by operating on a small subset of it. The scheme permits to transfer arbitrary and unknown quantum states from a memory on the network ("upload access") as well as the inverse ("download access"). In particular it yields a method for cooling the system.

Source: http://arxiv.org/abs/0710.0302v4

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Jun 30, 2018
06/18

by
Ludovico Lami; Vittorio Giovannetti

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We study a set of new functionals (called entanglement--breaking indices) which characterize how many local iterations of a given (local) quantum channel are needed in order to completely destroy the entanglement between the system of interest over which the transformation is defined and an external ancilla. The possibility of contrasting the noisy effects introduced by the channel iterations via the action of intermediate ({\it filtering}) transformations is analyzed. We provide some examples...

Topics: Quantum Physics, Mathematics, Mathematical Physics

Source: http://arxiv.org/abs/1411.2517

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Sep 21, 2013
09/13

by
Vittorio Giovannetti; Kazuya Yuasa

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A setup based on the Franson optical interferometer is analyzed, which allows us to detect the coherence properties of Cooper pairs emerging via tunneling from a superconductor in contact with two one-dimensional channels. By tuning the system parameters we show that both the internal coherence of the emitted Cooper pairs, which is proportional to Pippard's length, and the de Broglie wavelength of their center-of-mass motion can be measured via current-current correlation measurements.

Source: http://arxiv.org/abs/1204.3692v2

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Sep 21, 2013
09/13

by
Alessandro Farace; Vittorio GIovannetti

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Parametrically modulated optomechanical systems have been recently proposed as a simple and efficient setting for the quantum control of a micromechanical oscillator: relevant possibilities include the generation of squeezing in the oscillator position (or momentum) and the enhancement of entanglement between mechanical and radiation modes. In this paper we further investigate this new modulation regime, considering an optomechanical system with one or more parameters being modulated over time....

Source: http://arxiv.org/abs/1204.0406v2

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Sep 22, 2013
09/13

by
Vittorio Giovannetti; Seth Lloyd

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The Amosov-Holevo-Werner conjecture implies the additivity of the minimum Re'nyi entropies at the output of a channel. The conjecture is proven true for all Re'nyi entropies of integer order greater than two in a class of Gaussian bosonic channel where the input signal is randomly displaced or where it is coupled linearly to an external environment.

Source: http://arxiv.org/abs/quant-ph/0403075v3

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39

Sep 23, 2013
09/13

by
Francesco Ciccarello; Vittorio Giovannetti

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We show that two qubits, initially in a fully classical state, can develop significant quantum correlations as measured by the quantum discord (QD) under the action of a local memoryless noise (specifically we consider the case of a Markovian amplitude-damping channel). This is analytically proven after deriving in a compact form the QD for the class of separable states involved in such a process. We provide a picture in the Bloch sphere that unambiguously highlights the physical mechanism...

Source: http://arxiv.org/abs/1105.5551v3

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Sep 18, 2013
09/13

by
Daniel Burgarth; Vittorio Giovannetti

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We give a simple and physically intuitive necessary and sufficient condition for a map acting on a compact metric space to be mixing (i.e. infinitely many applications of the map transfer any input into a fixed convergency point). This is a generalization of the "Lyapunov direct method". First we prove this theorem in topological spaces and for arbitrary continuous maps. Finally we apply our theorem to maps which are relevant in Open Quantum Systems and Quantum Information, namely...

Source: http://arxiv.org/abs/quant-ph/0605197v3

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Sep 18, 2013
09/13

by
Vittorio Giovannetti; Lorenzo Maccone

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In interferometry, sub-Heisenberg strategies claim to achieve a phase estimation error smaller than the inverse of the mean number of photons employed (Heisenberg bound). Here we show that one can achieve a comparable precision without performing any measurement, just using the large prior information that sub-Heisenberg strategies require. For uniform prior (i.e. no prior information), we prove that these strategies cannot achieve more than a fixed gain of about 1.73 over Heisenberg-limited...

Source: http://arxiv.org/abs/1201.1878v2

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Sep 20, 2013
09/13

by
Vittorio Giovannetti; Rosario Fazio

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Information capacities achievable in the multi-parallel-use scenarios are employed to characterize the quantum correlations in unmodulated spin chains. By studying the qubit amplitude damping channel, we calculate the quantum capacity $Q$, the entanglement assisted capacity $C_E$, and the classical capacity $C_1$ of a spin chain with ferromagnetic Heisenberg interactions.

Source: http://arxiv.org/abs/quant-ph/0405110v3

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Sep 22, 2013
09/13

by
Filippo Caruso; Vittorio Giovannetti

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The notion of weak-degradability of quantum channels is introduced by generalizing the degradability definition given by Devetak and Shor. Exploiting the unitary equivalence with beam-splitter/amplifier channels we then prove that a large class of one-mode Bosonic Gaussian channels are either weakly degradable or anti-degradable. In the latter case this implies that their quantum capacity Q is null. In the former case instead, this allows us to establish the additivity of the coherent...

Source: http://arxiv.org/abs/quant-ph/0603257v4

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Sep 20, 2013
09/13

by
Vittorio Giovannetti; Stefano Mancini

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We discuss a Bosonic channel model with memory effects. It relies on a multi-mode squeezed (entangled) environment's state. The case of lossy Bosonic channels is analyzed in detail. We show that in the absence of input energy constraints the memory channels are equivalent to their memoryless counterparts. In the case of input energy constraint we provide lower and upper bounds for the memory channel capacity.

Source: http://arxiv.org/abs/quant-ph/0410176v3

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Jun 28, 2018
06/18

by
Matteo Rosati; Vittorio Giovannetti

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We present a new decoding protocol to realize transmission of classical information through a quantum channel at asymptotically maximum capacity, achieving the Holevo bound and thus the optimal communication rate. At variance with previous proposals, our scheme recovers the message bit by bit, making use of a series "yes-no" measurements, organized in bisection fashion, thus determining which codeword was sent in log(N) steps, N being the number of codewords.

Topics: Quantum Physics, Information Theory, Computing Research Repository, Mathematical Physics,...

Source: http://arxiv.org/abs/1506.04999

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Sep 23, 2013
09/13

by
Vittorio Giovannetti; G. Massimo Palma

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We derive the general form of a master equation describing the interaction of an arbitrary multipartite quantum system, consisting of a set of subsystems, with an environment, consisting of a large number of sub-envirobments. Each subsystem "collides" with the same sequence of sub-environments which, in between the collisions, evolve according to a map that mimics relaxations effects. No assumption is made on the specific nature of neither the system nor the environment. In the weak...

Source: http://arxiv.org/abs/1105.4506v1

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Sep 21, 2013
09/13

by
Antonella De Pasquale; Vittorio Giovannetti

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In this paper we introduce a way to quantify the noise level associated to a given quantum transformation. The key mechanism lying at the heart of the proposal is "noise addition": in other words we compute the amount of extra noise we need to add to the system, through convex combination with a reference noisy map or by reiterative applications of the original map, before the resulting transformation becomes entanglement-breaking. We also introduce the notion of entanglement-breaking...

Source: http://arxiv.org/abs/1204.5589v3

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Sep 18, 2013
09/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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We point out a general framework that encompasses most cases in which quantum effects enable an increase in precision when estimating a parameter (quantum metrology). The typical quantum precision-enhancement is of the order of the square root of the number of times the system is sampled. We prove that this is optimal and we point out the different strategies (classical and quantum) that permit to attain this bound.

Source: http://arxiv.org/abs/quant-ph/0509179v1

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Sep 18, 2013
09/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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In classical estimation theory, the central limit theorem implies that the statistical error in a measurement outcome can be reduced by an amount proportional to n^(-1/2) by repeating the measures n times and then averaging. Using quantum effects, such as entanglement, it is often possible to do better, decreasing the error by an amount proportional to 1/n. Quantum metrology is the study of those quantum techniques that allow one to gain advantages over purely classical approaches. In this...

Source: http://arxiv.org/abs/1102.2318v1

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Sep 23, 2013
09/13

by
Seth Lloyd; Vittorio Giovannetti; Lorenzo Maccone

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We study the transmission of classical information in quantum channels. We present a decoding procedure that is very simple but still achieves the channel capacity. It is used to give an alternative straightforward proof that the classical capacity is given by the regularized Holevo bound. This procedure uses only projective measurements and is based on successive "yes"/"no" tests only.

Source: http://arxiv.org/abs/1012.0106v2

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Sep 23, 2013
09/13

by
Vittorio Giovannetti; Daniel Burgarth; Stefano Mancini

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A chain of interacting spin behaves like a quantum mediator (quantum link) which allows two distant parties that control the ends of the chain to exchange quantum messages. We show that over repeated uses without resetting the study of a quantum link can be connected to correlated quantum channels with finite dimensional environment (finite memory quantum channel). Then, using coding arguments for such kind of channels and results on mixing channels we present a protocol that allows us to...

Source: http://arxiv.org/abs/0810.0170v1

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Jul 22, 2013
07/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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A random access memory, or RAM, is a device that, when interrogated, returns the content of a memory location in a memory array. A quantum RAM, or qRAM, allows one to access superpositions of memory sites, which may contain either quantum or classical information. RAMs and qRAMs with n-bit addresses can access 2^n memory sites. Any design for a RAM or qRAM then requires O(2^n) two-bit logic gates. At first sight this requirement might seem to make large scale quantum versions of such devices...

Source: http://arxiv.org/abs/0807.4994v2

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Jul 20, 2013
07/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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A method is proposed to employ entangled and squeezed light for determining the position of a party and for synchronizing distant clocks. An accuracy gain over analogous protocols that employ classical resources is demonstrated and a quantum-cryptographic positioning application is given, which allows only trusted parties to learn the position of whatever must be localized. The presence of a lossy channel and imperfect photodetection is considered. The advantages in using partially entangled...

Source: http://arxiv.org/abs/quant-ph/0107140v3

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Sep 19, 2013
09/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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Entanglement or entanglement generating interactions permit to achieve the maximum allowed speed in the dynamical evolution of a composite system, when the energy resources are distributed among subsystems. The cases of pre-existing entanglement and of entanglement-building interactions are separately addressed. The role of classical correlations is also discussed.

Source: http://arxiv.org/abs/quant-ph/0206001v5

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Sep 19, 2013
09/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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We propose a cheat sensitive quantum protocol to perform a private search on a classical database which is efficient in terms of communication complexity. It allows a user to retrieve an item from the server in possession of the database without revealing which item she retrieved: if the server tries to obtain information on the query, the person querying the database can find it out. Furthermore our protocol ensures perfect data privacy of the database, i.e. the information that the user can...

Source: http://arxiv.org/abs/0708.2992v2

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Jun 29, 2018
06/18

by
Vasco Cavina; Andrea Mari; Vittorio Giovannetti

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According to the second law of thermodynamics, for every transformation performed on a system which is in contact with an environment of fixed temperature, the extracted work is bounded by the decrease of the free energy of the system. However, in a single realization of a generic process, the extracted work is subject to statistical fluctuations which may allow for probabilistic violations of the previous bound. We are interested in enhancing this effect, i.e. we look for thermodynamic...

Topics: Quantum Physics, Statistical Mechanics, Condensed Matter

Source: http://arxiv.org/abs/1604.08094

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3.0

Jun 29, 2018
06/18

by
Matteo Rosati; Andrea Mari; Vittorio Giovannetti

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A scheme for transferring classical information over a lossy bosonic channel is proposed by generalizing the proposal presented in Phys. Rev. Lett. 106, 240502 (2011) by Guha. It employs codewords formed by products of coherent states of fixed mean photon number with multiple phases which, through a passive unitary transformation, reduce to a Pulse-Position Modulation code with multiple pulse phases. The maximum information rate achievable with optimal, yet difficult to implement, detection...

Topic: Quantum Physics

Source: http://arxiv.org/abs/1610.05676

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Sep 18, 2013
09/13

by
Davide Rossini; Vittorio Giovannetti; Rosario Fazio

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By means of a variational calculation using Matrix Product States with periodic boundary conditions, we accurately determine the extension of the spin-supersolid phase predicted to exist in the spin-1 anisotropic Heisenberg chain. We compute both the structure factor and the superfluid stiffness, and extract the critical exponents of the supersolid-to-solid phase transition.

Source: http://arxiv.org/abs/1102.3545v2

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Sep 19, 2013
09/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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A random access memory (RAM) uses n bits to randomly address N=2^n distinct memory cells. A quantum random access memory (qRAM) uses n qubits to address any quantum superposition of N memory cells. We present an architecture that exponentially reduces the requirements for a memory call: O(log N) switches need be thrown instead of the N used in conventional (classical or quantum) RAM designs. This yields a more robust qRAM algorithm, as it in general requires entanglement among exponentially...

Source: http://arxiv.org/abs/0708.1879v2

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Sep 20, 2013
09/13

by
Daniel Burgarth; Vittorio Giovannetti; Sougato Bose

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We present a communication protocol for chains of permanently coupled qubits which achieves perfect quantum state transfer and which is efficient with respect to the number chains employed in the scheme. The system consists of $M$ uncoupled identical quantum chains. Local control (gates, measurements) is only allowed at the sending/receiving end of the chains. Under a quite general hypothesis on the interaction Hamiltonian of the qubits a theorem is proved which shows that the receiver is able...

Source: http://arxiv.org/abs/quant-ph/0410175v3

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Sep 24, 2013
09/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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Quantum mechanics, through the Heisenberg uncertainty principle, imposes limits to the precision of measurement. Conventional measurement techniques typically fail to reach these limits. Conventional bounds to the precision of measurements such as the shot noise limit or the standard quantum limit are not as fundamental as the Heisenberg limits, and can be beaten using quantum strategies that employ `quantum tricks' such as squeezing and entanglement.

Source: http://arxiv.org/abs/quant-ph/0412078v1

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Jun 30, 2018
06/18

by
Alberto Carlini; Andrea Mari; Vittorio Giovannetti

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We consider the problem of time optimal control of a continuous bosonic quantum system subject to the action of a Markovian dissipation. In particular, we consider the case of a one mode Gaussian quantum system prepared in an arbitrary initial state and which relaxes to the steady state due to the action of the dissipative channel. We assume that the unitary part of the dynamics is represented by Gaussian operations which preserve the Gaussian nature of the quantum state, i.e. arbitrary phase...

Topic: Quantum Physics

Source: http://arxiv.org/abs/1409.7342

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Jul 20, 2013
07/13

by
Davide Rossini; Vittorio Giovannetti; Rosario Fazio

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We analyze the communication efficiency of quantum information transfer along unmodulated spin chains by computing the communication rates of various protocols. The effects of temporal correlations are discussed, showing that they can be exploited to boost the transmission efficiency.

Source: http://arxiv.org/abs/quant-ph/0609022v2

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Jul 19, 2013
07/13

by
Cosmo Lupo; Vittorio Giovannetti; Stefano Mancini

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With increasing communication rates via quantum channels, memory effects become unavoidable whenever the use rate of the channel is comparable to the typical relaxation time of the channel environment. We introduce a model of a bosonic memory channel, describing correlated noise effects in quantum-optical processes via attenuating or amplifying media. To study such a channel model, we make use of a proper set of collective field variables, which allows us to unravel the memory effects, mapping...

Source: http://arxiv.org/abs/1005.2878v2

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Sep 18, 2013
09/13

by
Vittorio Giovannetti; Seth Lloyd; Lorenzo Maccone

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We establish the minimum time it takes for an initial state of mean energy E and energy spread DE to move from its initial configuration by a predetermined amount. Distances in Hilbert space are estimated by the fidelity between the initial and final state. In this context, we also show that entanglement is necessary to achieve the ultimate evolution speed when the energy resources are distributed among all subsystems.

Source: http://arxiv.org/abs/quant-ph/0210197v1

42
42

Sep 22, 2013
09/13

by
Davide Rossini; Vittorio Giovannetti; Simone Montangero

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We analyze the quality of the quantum information transmission along a correlated quantum channel by studying the average fidelity between input and output states and the average output purity, giving bounds for the entropy of the channel. Noise correlations in the channel are modeled by the coupling of each channel use with an element of a one dimensional interacting quantum spin chain. Criticality of the environment chain is seen to emerge in the changes of the fidelity and of the purity.

Source: http://arxiv.org/abs/0805.4277v1