arXiv subjects

Sougato Bose

Publications and source records attributed to Sougato Bose.

At least 199 records · Page 11Linked to original sources

arXiv2009

Information Transferring Ability of the Different Phases of a finite XXZ Spin Chain

We study the transmission of both classical or quantum information through all the phases of a finite XXZ spin chain. This characterizes the merit of the different phases in terms of their ability to act as a quantum wire. As far as quantum information is concerned, we need only consider the transmission of entanglement as the direct transmission of a quantum state is equivalent. The isotropic anti-ferromagnetic spin chain is found to be the optimal point of the phase diagram for the transmission of quantum entanglement when one considers both the amount of transmitted entanglement, as well as the velocity with which it is transmitted. But this optimal point in the phase diagram moves to the Neel phase when decoherence or thermal fluctuations are taken to account. This chain may also be able to transfer classical information even when, due to a large magnitude of the noise, quantum information is not transmitted at all. For a certain range of anisotropies of the model, a curious feature is found in the flow of quantum information inside the chain, namely, a hopping mode of entanglement transfer which skips the odd numbered sites. Our predictions will potentially be testable in several physical systems.

quant-ph↗

arXiv2009

Long-lived spin entanglement induced by a spatially correlated thermal bath

We investigate how two spatially separated qubits coupled to a common heat bath can be entangled by purely dissipative dynamics. We identify a dynamical time scale associated with the lifetime of the dissipatively generated entanglement and show that it can be much longer than either the typical single-qubit decoherence time or the time scale on which a direct exchange interaction can entangle the qubits. We give an approximate analytical expression for the long-time evolution of the qubit concurrence and propose an ion trap scheme in which such dynamics should be observable.

quant-ph↗

arXiv2009

Long Range Free Bosonic Models in Block Decimation Notation: Applications and Entanglement

We study the effect of long range particle exchange in bosonic arrangements. We show that by combining the solution of the Heisenberg equations of motion with matrix product state representation it is possible to investigate the dynamics as well as the ground state while including particle exchange beyond next-neighbours sites. These ideas are then applied to study the emergence of entanglement as a result of scattering in boson chains. We propose a scheme to generate highly entangled multi-particle states that exploits collision as a powerful entangling mechanism.

quant-ph↗

arXiv2009

Exploiting Quench Dynamics in Spin Chains for Distant Entanglement and Quantum Communication

We suggest a method of entangling significantly the distant ends of a spin chain using minimal control. This entanglement between distant individual spins is brought about solely by exploiting the dynamics of an initial mixed state with Neel order if the lattice features nearest-neighbor XXZ interaction. There is no need to control single spins or to have engineered couplings or to pulse globally. The method only requires an initial nonadiabatic switch (a quench) between two Hamiltonians followed by an evolution under the second Hamiltonian. The scheme is robust to randomness of the couplings as well as the finiteness of an appropriate quench and could potentially be implemented in various experimental setups, ranging from atoms in optical lattices to Josephson-junction arrays.

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

End-to-end entanglement in Bose-Hubbard chains

We study the ground state as well as the dynamics of chains of bosons with local repulsive interactions and nearest-neighbour exchange using numerical techniques based on density matrix decimation. We explore the development of entanglement between the terminal sites of such chains as mechanisms are invoked to concentrate population in these sites. We find that long-range entanglement in the ground state emerges as a result of hopping taking place at the whole chain length in systems with appropriate hopping coefficients. Additionally, we find appropriate perturbations to increase the entanglement between the end sites above their ground state values.

quant-ph↗

arXiv2009

Quantum Walk-based Generation of Entanglement Between Two Walkers

Quantum walks can be used either as tools for quantum algorithm development or as entanglement generators, potentially useful to test quantum hardware. We present a novel algorithm based on a discrete Hadamard quantum walk on a line with one coin and two walkers whose purpose is to generate entanglement between walkers. We provide several classical computer simulations of our quantum algorithm in which we show that, although the asymptotical amount of entanglement generated between walkers does not reach the highest degree of entanglement possible at each step for either coin measurement outcome, the entanglement ratio (entanglement generated/highest value of entanglement possible, for each step) tends to converge, and the actual convergence value depends on the coin initial state and on the coin measurement outcome. Furthermore, our numerical simulations show that, for the quantum walks used in our algorithm, the value towards which entanglement ratio converges also depends on the position probability distribution symmetry of a quantum walk computed with one single walker and the same coin initial state employed in the corresponding quantum walk with two walkers.

quant-ph↗

arXiv2009

Simulation of high-spin Heisenberg models in coupled cavities

We propose a scheme to realize the Heisenberg model of any spin in an arbitrary array of coupled cavities. Our scheme is based on a fixed number of atoms confined in each cavity and collectively applied constant laser fields, and is in a regime where both atomic and cavity excitations are suppressed. It is shown that as well as optically controlling the effective spin Hamiltonian, it is also possible to engineer the magnitude of the spin. Our scheme would open up an unprecedented way to simulate otherwise intractable high-spin problems in many-body physics.

quant-ph↗

arXiv2008

Fractional Quantum Hall State in Coupled Cavities

We propose a scheme to realize the fractional quantum Hall system with atoms confined in a two-dimensional array of coupled cavities. Our scheme is based on simple optical manipulation of atomic internal states and inter-cavity hopping of virtually excited photons. It is shown that as well as the fractional quantum Hall system, any system of hard-core bosons on a lattice in the presence of an arbitrary Abelian vector potential can be simulated solely by controlling the phases of constantly applied lasers. The scheme, for the first time, exploits the core advantage of coupled cavity simulations, namely the individual addressability of the components and also brings the gauge potential into such simulations as well as the simple optical creation of particles.

quant-ph↗

arXiv2008

Large Spin Entangled Current from a Passive Device

We show that a large entangled current can be produced from a very simple passive device: a cluster of three resonant quantum dots, tunnel coupled to one input lead and two output leads. The device can function in a `clean' mode, when almost all emitted electrons are paired in Bell states, or a `dirty' mode with a far higher emission rate but a significant portion of non-entangled electrons. Subsequent charge detection can enhance performance by identifying the pairs that are most likely to be entangled. The device is robust to specific choice of system parameters and therefore lends itself to immediate experimental demonstration. Applications include quantum repeaters and unconditionally secure interfaces.

quant-ph↗

arXiv2008

Heralded generation of entanglement with coupled cavities

We propose a scheme to generate two-photon, two-atom, or atom-photon entangledstates with a coupled system of two cavities. In our scheme, two cavity photonsare exchanged by the direct inter-cavity coupling, while atoms in the cavitiessimply play the role of generating and probing them. By virtue of the highefficiency of atomic state measurement, this method enables the realization ofefficient heralded entanglement generation robust against photon loss, whichgreatly facilitates applications in quantum information processing.

quant-ph↗

arXiv2008

Communication in XYZ All-to-All Quantum Networks with a Missing Link

We explicitate the relation between Hamiltonians for networks of interacting qubits in the XYZ model and graph Laplacians. We then study evolution in networks in which all sites can communicate with each other. These are modeled by the complete graph K_{n} and called all-to-all networks. It turns out that K_{n} does not exhibit perfect state transfer (PST). However, we prove that deleting an edge in K_{n} allows PST between the two non-adjacent sites, when n is a multiple of four. An application is routing a qubit over n different sites, by switching off the link between the sites that we wish to put in communication. Additionally, we observe that, in certain cases, the unitary inducing evolution in K_{n} is equivalent to the Grover operator.

quant-ph↗

arXiv2008

Multi-level, multi-party singlets as ground states and their role in entanglement distribution

We show that a singlet of many multi-level quantum systems arises naturally as the ground state of a physically-motivated Hamiltonian. The Hamiltonian simply exchanges the states of nearest-neighbours in some network of qudits (d-level systems); the results are independent of the strength of the couplings or the network's topology. We show that local measurements on some of these qudits project the unmeasured qudits onto a smaller singlet, regardless of the choice of measurement basis at each measurement. It follows that the entanglement is highly persistent, and that through local measurements, a large amount of entanglement may be established between spatially-separated parties for subsequent use in distributed quantum computation.

quant-ph↗

arXiv2008

Steady state entanglement between hybrid light-matter qubits

We study the case of two polaritonic qubits localized in two separate cavities coupled by a fiber/additional cavity. We show that surprisingly enough, even a coherent classical pump in the intermediate cavity/fiber can lead to the creation of entanglement between the two ends in the steady state. The stationary nature of this entanglement and its survival under dissipation opens possibilities for its production under realistic laboratory conditions. To facilitate the verification of the entanglement in an experiment we also construct the relevant entanglement witness measurable by accessing only a few local variables of each polaritonic qubit.

quant-ph↗

arXiv2008

Many body effects and cluster state quantum computation in strongly interacting systems of photons

We discuss the basic properties of a recently proposed hybrid light-matter system of strongly interacting photons in an array of coupled cavities each doped with a single two level system. Using the non-linearity generated from the photon blockade effect, we predict strong correlations between the hopping photons in the array, and show the possibility of observing a phase transition from a polaritonic insulator to a superfluid of photons. In the Mott phase, this interaction can be mapped to an array of spins. We show how the remaining Hamiltonian, in conjunction with individual spin manipulation, can thus be used for simulating spin chains (useful for state transfer protocols) and cluster state quantum computation.

quant-ph↗

arXiv2008

Quantum Communication through Spin Chain Dynamics: an Introductory Overview

We present an introductory overview of the use of spin chains as quantum wires, which has recently developed into a topic of lively interest. The principal motivation is in connecting quantum registers without resorting to optics. A spin chain is a permanently coupled 1D system of spins. When one places a quantum state on one end of it, the state will be dynamically transmitted to the other end with some efficiency if the spins are coupled by an exchange interaction. No external modulations or measurements on the body of the chain, except perhaps at the very ends, is required for this purpose. For the simplest (uniformly coupled) chain and the simplest encoding (single qubit encoding), however, dispersion reduces the quality of transfer. We present a variety of alternatives proposed by various groups to achieve perfect quantum state transfer through spin chains. We conclude with a brief discussion of the various directions in which the topic is developing.

cond-mat.other↗

arXiv2007

Boundary effects to the entanglement entropy and two-site entanglement of the spin-1 valence-bond solid

We investigate the von Neumann entropy of a block of subsystem for the valence-bond solid (VBS) state with general open boundary conditions. We show that the effect of the boundary on the von Neumann entropy decays exponentially fast in the distance between the subsystem considered and the boundary sites. Further, we show that as the size of the subsystem increases, its von Neumann entropy exponentially approaches the summation of the von Neumann entropies of the two ends, the exponent being related to the size. In contrast to critical systems, where boundary effects to the von Neumann entropy decay slowly, the boundary effects in a VBS, a non-critical system, decay very quickly. We also study the entanglement between two spins. Curiously, while the boundary operators decrease the von Neumann entropy of L spins, they increase the entanglement between two spins.

quant-ph↗

arXiv2007

Photon blockade induced Mott transitions and XY spin models in coupled cavity arrays

As photons do not interact with each other, it is interesting to ask whether photonic systems can be modified to exhibit the phases characteristic of strongly coupled many-body systems. We demonstrate how a Mott insulator type of phase of excitations can arise in an array of coupled electromagnetic cavities, each of which is coupled resonantly to a {\em single} two level system (atom/quantum dot/Cooper pair) and can be individually addressed from outside. In the Mott phase each atom-cavity system has the same integral number of net polaritonic (atomic plus photonic) excitations with photon blockade providing the required repulsion between the excitations in each site. Detuning the atomic and photonic frequencies suppresses this effect and induces a transition to a photonic superfluid. We also show that for zero detuning, the system can simulate the dynamics of many body spin systems.

quant-ph↗