arXiv subjects

Vittorio Giovannetti

Publications and source records attributed to Vittorio Giovannetti.

At least 253 records · Page 14Linked to original sources

arXiv2010

Achieving the Holevo bound via sequential measurements

We present a new decoding procedure to transmit classical information in a quantum channel which, saturating asymptotically the Holevo bound, achieves the optimal rate of the communication line. Differently from previous proposals, it is based on performing a sequence of (projective) YES/NO measurements which in N steps determines which codeword was sent by the sender (N being the number of the codewords). Our analysis shows that as long as N is below the limit imposed by the Holevo bound the error probability can be sent to zero asymptotically in the length of the codewords.

quant-ph↗

arXiv2010

Memory effects in attenuation and amplification quantum processes

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 the n-fold concatenation of the memory channel to a unitarily equivalent, direct product of n single-mode bosonic channels. We hence estimate the channel capacities by relying on known results for the memoryless setting. Our findings show that the model is characterized by two different regimes, in which the cross correlations induced by the noise among different channel uses are either exponentially enhanced or exponentially reduced.

quant-ph↗

arXiv2010

The optimal unitary dilation for bosonic Gaussian channels

A generic quantum channel can be represented in terms of a unitary interaction between the information-carrying system and a noisy environment. Here, the minimal number of quantum Gaussian environmental modes required to provide a unitary dilation of a multi-mode bosonic Gaussian channel is analyzed both for mixed and pure environment corresponding to the Stinespring representation. In particular, for the case of pure environment we compute this quantity and present an explicit unitary dilation for arbitrary bosonic Gaussian channel. These results considerably simplify the characterization of these continuous-variable maps and can be applied to address some open issues concerning the transmission of information encoded in bosonic systems.

quant-ph↗

arXiv2010

The quantum mechanics of time travel through post-selected teleportation

This paper discusses the quantum mechanics of closed timelike curves (CTC) and of other potential methods for time travel. We analyze a specific proposal for such quantum time travel, the quantum description of CTCs based on post-selected teleportation (P-CTCs). We compare the theory of P-CTCs to previously proposed quantum theories of time travel: the theory is physically inequivalent to Deutsch's theory of CTCs, but it is consistent with path-integral approaches (which are the best suited for analyzing quantum field theory in curved spacetime). We derive the dynamical equations that a chronology-respecting system interacting with a CTC will experience. We discuss the possibility of time travel in the absence of general relativistic closed timelike curves, and investigate the implications of P-CTCs for enhancing the power of computation.

quant-ph↗

arXiv2010

Quantum Defragmentation Algorithm

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. Our result also implies that the reduced dynamics in any noisy system can be simulated with finitely many resources.

quant-ph↗

arXiv2010

Closed timelike curves via post-selection: theory and experimental demonstration

Closed timelike curves (CTCs) are trajectories in spacetime that effectively travel backwards in time: a test particle following a CTC can in principle interact with its former self in the past. CTCs appear in many solutions of Einstein's field equations and any future quantum version of general relativity will have to reconcile them with the requirements of quantum mechanics and of quantum field theory. A widely accepted quantum theory of CTCs was proposed by Deutsch. Here we explore an alternative quantum formulation of CTCs and show that it is physically inequivalent to Deutsch's. Because it is based on combining quantum teleportation with post-selection, the predictions/retrodictions of our theory are experimentally testable: we report the results of an experiment demonstrating our theory's resolution of the well-known `grandfather paradox.

quant-ph↗

arXiv2010

Communication at the quantum speed limit along a spin chain

Spin chains have long been considered as candidates for quantum channels to facilitate quantum communication. We consider the transfer of a single excitation along a spin-1/2 chain governed by Heisenberg-type interactions. We build on the work of Balachandran and Gong [1], and show that by applying optimal control to an external parabolic magnetic field, one can drastically increase the propagation rate by two orders of magnitude. In particular, we show that the theoretical maximum propagation rate can be reached, where the propagation of the excitation takes the form of a dispersed wave. We conclude that optimal control is not only a useful tool for experimental application, but also for theoretical enquiry into the physical limits and dynamics of many-body quantum systems.

quant-ph↗

arXiv2010

Efficient Generation of a Maximally Entangled State by Repeated On- and Off-Resonant Scattering of Ancilla Qubits

A scheme for preparing two fixed non-interacting qubits in a maximally entangled state is presented. By repeating on- and off-resonant scattering of ancilla qubits, the state of the target qubits is driven from an arbitrary initial state into the singlet state with probability 1 (perfect efficiency). Neither the preparation nor the post-selection of the ancilla spin state is required. The convergence from an arbitrary input state to the unique fixed point (mixing property) is proved rigorously, and its robustness is investigated, by scrutinizing the effects of imperfections in the incident wave of the ancilla, such as mistuning to a resonant momentum, imperfect monochromatization, and fluctuation of the incident momentum, as well as detector efficiency.

quant-ph↗

arXiv2009

Optimal Control at the Quantum Speed Limit

Optimal control theory is a promising candidate for a drastic improvement of the performance of quantum information tasks. We explore its ultimate limit in paradigmatic cases, and demonstrate that it coincides with the maximum speed limit allowed by quantum evolution (the quantum speed limit).

quant-ph↗

arXiv2009

Local controllability of quantum networks

We give a sufficient criterion that guarantees that a many-body quantum system can be controlled by properly manipulating the (local) Hamiltonian of one of its subsystems. The method can be applied to a wide range of systems: it does not depend on the details of the couplings but only on their associated topology. As a special case, we prove that Heisenberg and Affleck-Kennedy-Lieb-Tasaki chains can be controlled by operating on one of the spins at their ends. In principle, arbitrary quantum algorithms can be performed on such chains by acting on a single qubit.

quant-ph↗

arXiv2009

Teleportation-Induced Correlated Quantum Channels

Quantum teleportation of a n-qubit state performed using as entangled resource a general bipartite state of 2n qubits instead of n Bell states is equivalent to a correlated Pauli channel. This provides a new characterization of such channels in terms of many-body correlation functions of the teleporting media. Our model is then generalized to the Continuous Variable case. We show that this new representation provides a relatively simple method for determining whether a correlated quantum channel is able to reliably convey quantum messages by studying the entanglement properties of the teleportation mediating system.

quant-ph↗

arXiv2009

Capacities of lossy bosonic memory channels

We introduce a general model for a lossy bosonic memory channel and calculate the classical and the quantum capacity, proving that coherent state encoding is optimal. The use of a proper set of collective field variables allows to unravel the memory, showing that the n-fold concatenation of the memory channel is unitarily equivalent to the direct product of n single-mode lossy bosonic channels.

quant-ph↗

arXiv2008

The quantum optical Josephson interferometer

The interplay between coherent tunnel coupling and on-site interactions in dissipation-free bosonic systems has lead to many spectacular observations, ranging from the demonstration of number-phase uncertainty relation to quantum phase transitions. To explore the effect of dissipation and coherent drive on tunnel coupled interacting bosonic systems, we propose a device that is the quantum optical analog of a Josephson interferometer. It consists of two coherently driven linear optical cavities connected via a central cavity with a single-photon nonlinearity. The Josephson-like oscillations in the light emitted from the central cavity as a function of the phase difference between two pumping fields can be suppressed by increasing the strength of the nonlinear coupling. Remarkably, we find that in the limit of ultra-strong interactions in the center-cavity, the coupled system maps on to an effective Jaynes-Cummings system with a nonlinearity determined by the tunnel coupling strength. In the limit of a single nonlinear cavity coupled to two linear waveguides, the degree of photon antibunching from the nonlinear cavity provides an excellent measure of the transition to the nonlinear regime where Josephson oscillations are suppressed.

cond-mat.mes-hall↗

arXiv2008

A Protocol For Cooling and Controlling Composite Systems by Local Interactions

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.

quant-ph↗

arXiv2008

Quantum MERA Channels

Tensor networks representations of many-body quantum systems can be described in terms of quantum channels. We focus on channels associated with the Multi-scale Entanglement Renormalization Ansatz (MERA) tensor network that has been recently introduced to efficiently describe critical systems. Our approach allows us to compute the MERA correspondent to the thermodynamic limit of a critical system introducing a transfer matrix formalism, and to relate the system critical exponents to the convergence rates of the associated channels.

quant-ph↗

arXiv2008

Architectures for a quantum random access memory

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 impractical, due to the difficulty of constructing and operating coherent devices with large numbers of quantum logic gates. Here we analyze two different RAM architectures (the conventional fanout and the "bucket brigade") and propose some proof-of-principle implementations which show that in principle only O(n) two-qubit physical interactions need take place during each qRAM call. That is, although a qRAM needs O(2^n) quantum logic gates, only O(n) need to be activated during a memory call. The resulting decrease in resources could give rise to the construction of large qRAMs that could operate without the need for extensive quantum error correction.

quant-ph↗

arXiv2008

Quantum Illumination with Gaussian States

An optical transmitter irradiates a target region containing a bright thermal-noise bath in which a low-reflectivity object might be embedded. The light received from this region is used to decide whether the object is present or absent. The performance achieved using a coherent-state transmitter is compared with that of a quantum illumination transmitter, i.e., one that employs the signal beam obtained from spontaneous parametric downconversion (SPDC). By making the optimum joint measurement on the light received from the target region together with the retained SPDC idler beam, the quantum illumination system realizes a 6 dB advantage in error probability exponent over the optimum reception coherent-state system. This advantage accrues despite there being no entanglement between the light collected from the target region and the retained idler beam.

quant-ph↗

arXiv2008

Communication Through a Quantum Link

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 achieve perfect information transmission through a quantum link.

quant-ph↗