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Nilanjana Datta

Publications and source records attributed to Nilanjana Datta.

108 records · Page 6Linked to original sources
arXiv2006

Beyond i.i.d. in Quantum Information Theory

The information spectrum approach gives general formulae for optimal rates of codes in many areas of information theory. In this paper the quantum spectral divergence rates are defined and properties of the rates are derived. The entropic rates, conditional entropic rates, and spectral mutual information rates are then defined in terms of the spectral divergence rates. Properties including subadditivity, chain rules, Araki-Lieb inequalities, and monotonicity are then explored.

quant-ph↗
arXiv2006

Coding Theorem for a Class of Quantum Channels with Long-Term Memory

In this paper we consider the transmission of classical information through a class of quantum channels with long-term memory, which are given by convex combinations of product channels. Hence, the memory of such channels is given by a Markov chain which is aperiodic but not irreducible. We prove the coding theorem and weak converse for this class of channels. The main techniques that we employ, are a quantum version of Feinstein's Fundamental Lemma and a generalization of Helstrom's Theorem.

quant-ph↗
arXiv2006

Quantum Coding Theorems for Arbitrary Sources, Channels and Entanglement Resources

The information spectrum approach gives general formulae for optimal rates of various information theoretic protocols, under minimal assumptions on the nature of the sources, channels and entanglement resources involved. This paper culminates in the derivation of the dense coding capacity for a noiseless quantum channel, assisted by arbitrary shared entanglement, using this approach. We also review the currently known coding theorems, and their converses, for protocols such as data compression for arbitrary quantum sources and transmission of classical information through arbitrary quantum channels. In addition, we derive the optimal rate of data compression for a mixed source

quant-ph↗
arXiv2005

Complementarity and additivity for depolarizing channels

In this paper we use the method of a recent paper (quant-ph/0509101) to compute complementary channels for certain important cases, such as depolarizing and transpose-depolarizing channels. This method allows us to easily obtain the minimal Kraus representations from non--minimal ones. We also study the properties of the output purity of the tensor product of a channel and its complement.

quant-ph↗
arXiv2005

Maximal output purity and capacity for asymmetric unital qudit channels

We consider generalizations of depolarizing channels to maps in which the identity channel is replaced by a convex combinations of unitary conjugations. We show that one can construct unital channels of this type for which the input which achieves maximal output purity is unique. We give conditions under which multiplicativity of the maximal p-norm and additivity of the minimal output entropy. We also show that the Holevo capacity need not equal log d - the minimal entropy as one might expect for a convex combination of unitary conjugations. Conversely, we give examples for which this condition holds, but the channel has no evident covariance properties.

quant-ph↗
arXiv2004

Mirror Inversion of Quantum States in Linear Registers

Transfer of data in linear quantum registers can be significantly simplified with pre-engineered but not dynamically controlled inter-qubit couplings. We show how to implement a mirror inversion of the state of the register in each excitation subspace with respect to the centre of the register. Our construction is especially appealing as it requires no dynamical control over individual inter-qubit interactions. If, however, individual control of the interactions is available then the mirror inversion operation can be performed on any substring of qubits in the register. In this case a sequence of mirror inversions can generate any permutation of a quantum state of the involved qubits.

quant-ph↗
arXiv2004

Additivity for transpose depolarizing channels

Additivity of the minimal output entropy for the family of transpose depolarizing channels introduced by Fannes et al. [quant-ph/0410195] is considered. It is shown that using the method of our previous paper [quant-ph/0403072] allows us to prove the additivity for the range of the parameter values for which the problem was left open in [quant-ph/0410195]. Together with the result of [quant-ph/0410195], this covers the whole family of transpose depolarizing channels.

quant-ph↗
arXiv2004

Perfect Transfer of Arbitrary States in Quantum Spin Networks

We propose a class of qubit networks that admit perfect state transfer of any two-dimensional quantum state in a fixed period of time. We further show that such networks can distribute arbitrary entangled states between two distant parties, and can, by using such systems in parallel, transmit the higher dimensional systems states across the network. Unlike many other schemes for quantum computation and communication, these networks do not require qubit couplings to be switched on and off. When restricted to $N$-qubit spin networks of identical qubit couplings, we show that $2\log_3 N$ is the maximal perfect communication distance for hypercube geometries. Moreover, if one allows fixed but different couplings between the qubits then perfect state transfer can be achieved over arbitrarily long distances in a linear chain. This paper expands and extends the work done in PRL 92, 187902.

quant-ph↗
arXiv2004

Additivity in Isotropic Quantum Spin Channels

We prove additivity of the minimum output entropy and the Holevo capacity for rotationally invariant quantum channels acting on spin-1/2 and spin-1 systems. The physical significance of these channels and their relations to other known channels is also discussed.

quant-ph↗
arXiv2004

Perfect state transfer in quantum spin networks

We propose a class of qubit networks that admit perfect transfer of any quantum state in a fixed period of time. Unlike many other schemes for quantum computation and communication, these networks do not require qubit couplings to be switched on and off. When restricted to N-qubit spin networks of identical qubit couplings, we show that 2 log_3 N is the maximal perfect communication distance for hypercube geometries. Moreover, if one allows fixed but different couplings between the qubits then perfect state transfer can be achieved over arbitrarily long distances in a linear chain.

quant-ph↗
arXiv2004

A quantum channel with additive minimum output entropy

We give a direct proof of the additivity of the minimum output entropy of a particular quantum channel which breaks the multiplicativity conjecture. This yields additivity of the classical capacity of this channel, a result obtained by a different method by Matsumoto and Yura [quant-ph/0306009]. Our proof relies heavily upon certain concavity properties of the output entropy which are of independent interest.

quant-ph↗
arXiv2003

Expansions for one quasiparticle states in spin 1/2 systems

Convergent expansions of the wavefunctions for the ground state and low-lying excited states of quantum transverse Ising systems are obtained. These expansions are employed to prove that the dispersion relation for a single quasi-particle has a convergent expansion.

cond-mat.stat-mech↗
arXiv2003

Instability of interfaces in the antiferromagnetic XXZ chain at zero temperature

For the antiferromagnetic, highly anisotropic XZ and XXZ quantum spin chains, we impose periodic boundary conditions on chains with an odd number of sites to force an interface (or kink) into the chain. We prove that the energy of the interface depends on the momentum of the state. This shows that at zero temperature the interface in such chains is not stable. This is in contrast to the ferromagnetic XXZ chain for which the existence of localized interface ground states has been proven for any amount of anisotropy in the Ising-like regime.

math-ph↗
arXiv2002

Data compression limit for an information source of interacting qubits

A system of interacting qubits can be viewed as a non-i.i.d quantum information source. A possible model of such a source is provided by a quantum spin system, in which spin-1/2 particles located at sites of a lattice interact with each other. We establish the limit for the compression of information from such a source and show that asymptotically it is given by the von Neumann entropy rate. Our result can be viewed as a quantum analog of Shannon's noiseless coding theorem for a class of non - i.i.d. quantum information sources.

quant-ph↗
arXiv2000

Random Matrix approach to the crossover from Wigner to Poisson statistics of energy levels

We analyze a class of parametrized Random Matrix models, introduced by Rosenzweig and Porter, which is expected to describe the energy level statistics of quantum systems whose classical dynamics varies from regular to chaotic as a function of a parameter. We compute the generating function for the correlations of energy levels, in the limit of infinite matrix size. Our computations show that for a certain range of values of the parameter, the energy-level statistics is given by that of the Wigner-Dyson ensemble. For another range of parameter values, one obtains the Poisson statistics of uncorrelated energy levels. However, between these two ranges, new statistics emerge, which is neither Poissonnian nor Wigner. The crossover is measured by a renormalized coupling constant. The model is exactly solved in the sense that, in the limit of infinite matrix size, the energy-level correlation functions and their generating function are given in terms of a finite set of integrals.

cond-mat↗
arXiv1999

Rigidity of interfaces in the Falicov-Kimball model

We analyze the thermodynamic properties of interfaces in the three-dimensional Falicov Kimball model, which can be viewed as a primitive quantum lattice model of crystalline matter. In the strong coupling limit, the ionic subsystem of this model is governed by the Hamiltonian of an effective classical spin model whose leading part is the Ising Hamiltonian. We prove that the 100 interface in this model, at half-filling, is rigid, as in the three-dimensional Ising model. However, despite the above similarities with the Ising model, the thermodynamic properties of its 111 interface are very different. We prove that even though this interface is expected to be unstable for the Ising model, it is stable for the Falicov Kimball model at sufficiently low temperatures. This rigidity results from a phenomenon of "ground state selection" and is a consequence of the Fermi statistics of the electrons in the model.

math-ph↗
arXiv1998

Effective Hamiltonians and Phase Diagrams for Tight-Binding Models

We present rigorous results for several variants of the Hubbard model in the strong-coupling regime. We establish a mathematically controlled perturbation expansion which shows how previously proposed effective interactions are, in fact, leading-order terms of well defined (volume-independent) unitarily equivalent interactions. In addition, in the very asymmetric (Falicov-Kimball) regime, we are able to apply recently developed phase-diagram technology (quantum Pirogov-Sinai theory) to conclude that the zero-temperature phase diagrams obtained for the leading classical part remain valid, except for thin excluded regions and small deformations, for the full-fledged quantum interaction at zero or small temperature. Moreover, the phase diagram is stable upon the addition of arbitrary, but sufficiently small, further quantum terms that do not break the ground-states symmetries. This generalizes and unifies a number of previous results on the subject; in particular published results on the zero-temperature phase diagram of the Falikov-Kimball model (with and without magnetic flux) are extended to small temperatures and/or small ionic hopping. We give explicit expressions for the first few orders, in the hopping amplitude, of these equivalent interactions, and we describe the resulting phase diagram. Our approach, however, yields algorithms to compute arbitrary high orders.

math-ph↗