arXiv subjects

Sougato Bose

Publications and source records attributed to Sougato Bose.

At least 163 records · Page 9Linked to original sources

arXiv2015

A Toolbox for Linear Optics in a 1D Lattice via Minimal Control

Tight binding lattices offer a unique platform in which particles may be either static or mobile depending on the potential barrier between the sites. How to harness this mobility in a many-site lattice for useful operations is still an open question. We show how effective linear optics-like operations between arbitrary lattice sites can be implemented by a minimal local control which introduces a local impurity in the middle of the lattice. In particular we show how striking is the difference of the two possible correlations with and without the impurity. Our scheme enables the observation of the Hong-Ou-Mandel effect between distant wells without moving them next to each other with, e.g., tweezers. Moreover, we show that a tunable Mach-Zehnder interferometer is implemented adding a step-like potential, and we prove the robustness of our linear optics scheme to inter-particle interactions.

quant-ph↗

arXiv2015

Adiabatic many-body state preparation and information transfer in quantum dot arrays

Quantum simulation of many-body systems are one of the most interesting tasks of quantum technology. Among them is the preparation of a many-body system in its ground state when the vanishing energy gap makes the cooling mechanisms ineffective. Adiabatic theorem, as an alternative to cooling, can be exploited for driving the many-body system to its ground state. In this paper, we study two most common disorders in quantum dot arrays, namely exchange coupling fluctuations and hyperfine interaction, in adiabatically preparation of ground state in such systems. We show that the adiabatic ground state preparation is highly robust against those disorder effects making it good analog simulator. Moreover, we also study the adiabatic classical information transfer, using singlet-triplet states, across a spin chain. In contrast to ground state preparation the transfer mechanism is highly affected by disorder and in particular, the hyperfine interaction is very destructive for the performance. This suggests that for communication tasks across such arrays adiabatic evolution is not as effective and quantum quenches could be preferable.

quant-ph↗

arXiv2015

Macroscopic quantum resonators (MAQRO): 2015 Update

Do the laws of quantum physics still hold for macroscopic objects - this is at the heart of Schrödinger's cat paradox - or do gravitation or yet unknown effects set a limit for massive particles? What is the fundamental relation between quantum physics and gravity? Ground-based experiments addressing these questions may soon face limitations due to limited free-fall times and the quality of vacuum and microgravity. The proposed mission MAQRO may overcome these limitations and allow addressing those fundamental questions. MAQRO harnesses recent developments in quantum optomechanics, high-mass matter-wave interferometry as well as state-of-the-art space technology to push macroscopic quantum experiments towards their ultimate performance limits and to open new horizons for applying quantum technology in space. The main scientific goal of MAQRO is to probe the vastly unexplored "quantum-classical" transition for increasingly massive objects, testing the predictions of quantum theory for truly macroscopic objects in a size and mass regime unachievable in ground-based experiments. The hardware for the mission will largely be based on available space technology. Here, we present the MAQRO proposal submitted in response to the (M4) Cosmic Vision call of the European Space Agency for a medium-size mission opportunity with a possible launch in 2025.

quant-ph↗

arXiv2015

Perfect wave-packet splitting and reconstruction in a one-dimensional lattice

Particle delocalization is a common feature of quantum random walks in arbitrary lattices. However, in the typical scenario a particle spreads over multiple sites and its evolution is not directly useful for controlled quantum interferometry, as may be required for technological applications. In this paper we devise a strategy to perfectly split the wave-packet of an incoming particle into two components, each propagating in opposite directions, which reconstruct the shape of the initial wavefunction after a particular time $t^*$. Therefore, a particle in a delta-like initial state becomes exactly delocalized between two distant sites after $t^*$. We find the mathematical conditions to achieve the perfect splitting which are satisfied by viable example Hamiltonians with static site-dependent interaction strengths. Our results pave the way for the generation of peculiar many-body interference patterns in a many-site atomic chain (like the Hanbury Brown and Twiss and quantum Talbot effects) as well as for the distribution of entanglement between remote sites. Thus, as for the case of perfect state transfer, the perfect wave-packet splitting can be a new tool for varied applications.

quant-ph↗

arXiv2014

Optimal Quench for Distance-Independent Entanglement and Maximal Block Entropy

We optimize a quantum walk of multiple fermions following a quench in a spin chain to generate near ideal resources for quantum networking. We first prove an useful theorem mapping the correlations evolved from specific quenches to the apparently unrelated problem of quantum state transfer between distinct spins. This mapping is then exploited to optimize the dynamics and produce large amounts of entanglement distributed in very special ways. Two applications are considered: the simultaneous generation of many Bell states between pairs of distant spins (maximal block entropy), or high entanglement between the ends of an arbitrarily long chain (distance-independent entanglement). Thanks to the generality of the result, we study its implementation in different experimental setups using present technology: NMR, ion traps and ultracold atoms in optical lattices.

quant-ph↗

arXiv2014

Nonlinearity-Induced Entanglement Stability in a Qubit-Oscillator System

We propose a system composed of a qubit interacting with a quartic (undriven) nonlinear oscillator (NLO) through a conditional displacement Hamiltonian. We show that even a modest nonlinear perturbation in the NLO potential can enhance and stabilize the quantum entanglement dynamically generated between the qubit and the NLO. In absence of the nonlinearity the entanglement between the qubit and the oscillator is known to periodically oscillate between 0 and 1, whereas the nonlinearity suppresses the dynamical decay of entanglement once it is generated. While the generation of entanglement is due to the superposition principle combined with conditional displacements in the NLO, as noted in several works before, the improvements in this entanglement is because of the squeezing and other complex processes induced by two- and four-phonon interactions. Finally, we have solved the Markovian master equation, and even in this open case the system preserves the previous features and remain robust.

quant-ph↗

arXiv2014

Cavity-assisted energy relaxation for quantum many-body simulations

We propose an energy relaxation mechanism whereby strongly correlated spin systems decay into their ground states. The relaxation is driven by cavity quantum electrodynamics interaction and the decay of cavity photons. It is shown that by applying broadband driving fields, strongly correlated systems can be cooled regardless of the specific details of their energy level profile. The scheme would also have significant implications in other contexts, such as adiabatic quantum computation and steady-state entanglement in dissipative systems.

quant-ph↗

arXiv2014

An order parameter for impurity systems at quantum criticality

A quantum phase transition may occur in the ground state of a system at zero temperature when a controlling field or interaction is varied. The resulting quantum fluctuations which trigger the transition produce scaling behavior of various observables, governed by universal critical exponents. A particularly interesting class of such transitions appear in systems with quantum impurities where a non-extensive term in the free energy becomes singular at the critical point. Curiously, the notion of a conventional order parameter which exhibits scaling at the critical point is generically missing in these systems. We here explore the possibility to use the Schmidt gap, which is an observable obtained from the entanglement spectrum, as an order parameter. A case study of the two-impurity Kondo model confirms that the Schmidt gap faithfully captures the scaling behavior by correctly predicting the critical exponent of the dynamically generated length scale at the critical point.

cond-mat.str-el↗

arXiv2014

Coherently opening a high-Q cavity

We propose a general framework to effectively `open' a high-Q resonator, that is, to release the quantum state initially prepared in it in the form of a traveling electromagnetic wave. This is achieved by employing a mediating mode that scatters coherently the radiation from the resonator into a one-dimensional continuum of modes such as a waveguide. The same mechanism may be used to `feed' a desired quantum field to an initially empty cavity. Switching between an `open' and `closed' resonator may then be obtained by controlling either the detuning of the scatterer or the amount of time it spends in the resonator. First, we introduce the model in its general form, identifying (i) the traveling mode that optimally retains the full quantum information of the resonator field and (ii) a suitable figure of merit that we study analytically in terms of the system parameters. Then, we discuss two feasible implementations based on ensembles of two-level atoms interacting with cavity fields. In addition, we discuss how to integrate traditional cavity QED in our proposal using three-level atoms.

quant-ph↗

arXiv2014

Quench Induced Growth of Distant Entanglement from Product and Locally Entangled States in Spin Chains

We study the problem of entangling two spins at the distant ends of a spin chain by exploiting the nonequilibrium dynamics of the system after a sudden global quench. As initial states we consider a canted/spiral order product state of the spins, and singlets of neighboring pairs of spins. We find that within the class of canted order initial states, no entanglement is generated at any time except for the special case of the Neel state. While an earlier work had shown that the Neel state is indeed an excellent starting resource for the dynamical generation of long distance entanglement, the curious fact that this is the sole point within a large class of initial product states of the spins was not noted. On the other hand, we find that an initial state which is a series of nearest neighbor Bell states, and well motivated by some physical realizations, is also a good starting resource for end to end entanglement in a similar way to the Neel state.

quant-ph↗

arXiv2013

Two-qubit gates for decoherence-free qubits using a ring exchange interaction

It is known that it is possible to encode a logical qubit over many physical qubits such that it is immune to the effects of collective decoherence, and it is possible to perform universal quantum computation using these `decoherence-free' qubits. However, current proposed methods of performing gates on these encoded qubits could be difficult to implement, or could take too much time to perform. Here we investigate whether exploiting ring-exchange interactions, which may be naturally present, can simplify the implementation of these gates in any way. Using a ring exchange interaction, we have found a way to create a controlled-Z gate on the 4-qubit decoherence-free subspace and the 3-qubit decoherence-free subsystem using a sequence with 5 pulses. This could be useful in situations where simplicity is important or where ring exchange interactions are prominent. We also investigate how timing errors and magnetic field fluctuations affect the fidelity of this gate.

quant-ph↗

arXiv2012

Classical Vs Quantum correlations in composite systems

Here we provide the contributions' abstracts published in a volume we edited as a special issue in International Journal of Modern Physics B. The volume deals with the recent progress in quantifying quantum correlations beyond the generic notion of 'correlations in a quantum system'. The main goal of the special issue is to provide authoritative reviews on selected topics discussed in the field in the last few years. Nevertheless many articles contain significant original research. The opening section of the issue is dedicated to foundational aspects of quantum mechanics. The second section deals with diverse quantum information insights into the analysis of quantum resources (like entanglement and quantum discord). Many of the concepts presented in the first part of the issue are applied to spin systems in the third section. The last section of the issue is focused on the dynamics of quantum discord in open systems and in quantum communication protocols.

quant-ph↗

arXiv2012

Quantum resources for hybrid communication via qubit-oscillator states

We investigate a family of qubit-oscillator states as resources for hybrid quantum communication. They result from a mechanism of qubit-controlled displacement on the oscillator. For large displacements, we obtain analytical formulas for entanglement and other nonclassical correlations, such as entropic and geometric discord, in those states. We design two protocols for quantum communication using the considered resource states, a hybrid teleportation and a hybrid remote state preparation. The latter, in its standard formulation, is shown to have a performance limited by the initial mixedness of the oscillator, echoing the behaviour of the geometric discord. If one includes a further optimization over non-unitary correcting operations performed by the receiver, the performance is improved to match that of teleportation, which is directly linked to the amount of entanglement. Both protocols can then approach perfect efficiency even if the oscillator is originally highly thermal. We discuss the critical implications of these findings for the interpretation of general quantum correlations.

quant-ph↗

arXiv2012

Entanglement probe of two-impurity Kondo physics in a spin chain

We propose that real-space properties of the two-impurity Kondo model can be obtained from an effective spin model where two single-impurity Kondo spin chains are joined via an RKKY interaction between the two impurity spins. We then use a DMRG approach, valid in all ranges of parameters, to study its features using two complementary quantum-entanglement measures, the negativity and the von Neumann entropy. This non-perturbative approach enables us to uncover the precise dependence of the spatial extent $ξ_K$ of the Kondo screening cloud with the Kondo and RKKY couplings. Our results reveal an exponential suppression of the Kondo temperature $T_K \sim 1/ξ_K$ with the size of the effective impurity spin in the limit of large ferromagnetic RKKY coupling, a striking display of "Kondo resonance narrowing" in the two-impurity Kondo model. We also show how the antiferromagnetic RKKY interaction produces an effective decoupling of the impurities from the bulk already for intermediate strengths of this interaction, and, furthermore, exhibit how the non-Fermi liquid quantum critical point is signaled in the quantum entanglement between various parts of the system.

cond-mat.str-el↗

arXiv2012

Reconstructing the quantum state of oscillator networks with a single qubit

We introduce a scheme to reconstruct arbitrary states of networks composed of quantum oscillators--e.g., the motional state of trapped ions or the radiation state of coupled cavities. The scheme uses minimal resources, in the sense that it i) requires only the interaction between one-qubit probe and one constituent of the network; ii) provides the reconstructed state directly from the data, avoiding any tomographic transformation; iii) involves the tuning of only one coupling parameter. In addition, we show that a number of quantum properties can be extracted without full reconstruction of the state. The scheme can be used for probing quantum simulations of anharmonic many-body systems and quantum computations with continuous variables. Experimental implementation with trapped ions is also discussed and shown to be within reach of current technology.

quant-ph↗

arXiv2011

Negativity for two blocks in the one dimensional Spin 1 AKLT model

In this paper we compute the entanglement, as quantified by negativity, between two blocks of length $L_A$ and $L_B$, separated by $L$ sites in the one dimensional spin-1 AKLT model. We took the model with two different boundary conditions. We consider the case of $N$ spins 1 in the bulk and one spin 1/2 at each boundary which constitute an unique ground state, and the case of just spins 1, even at the end of the chain, where the degeneracy of the ground state is four. In both scenarios we made a partition consisting of two blocks $A$ and $B$, containing $L_A$ and $L_B$ sites respectively. The separation of these two blocks is $L$. In both cases we explicitly obtain the reduced density matrix of the blocks $A$ and $B$. We prove that the negativity in the first case vanishes identically for $L\geq 1$ while in the second scenario it may approach a constant value $N=1/2$ for each degenerate eigenstate depending on the way one constructs these eigenstates. However, as there is some freedom in constructing these eigenstates, vanishing entanglement is also possible in the latter case. Additionally, we also compute the entanglement between non-complementary blocks in the case of periodic boundary conditions for the spin-1 AKLT model for which there is a unique ground state. Even in this case, we find that the negativity of separated blocks of spins is zero.

quant-ph↗

arXiv2011

Entanglement in bipartite pure states of an interacting boson gas obtained by local projective measurements

We quantify the extractable entanglement of excited states of a Lieb-Liniger gas that are obtained from coarse-grained measurements on the ground state in which the boson number in one of two complementary contiguous partitions of the gas is determined. Numerically exact results obtained from the coordinate Bethe ansatz show that the von Neumann entropy of the resulting bipartite pure state increases monotonically with the strength of repulsive interactions and saturates to the impenetrable boson limiting value. We also present evidence indicating that the largest amount of entanglement can be extracted from the most probable projected state having half the number of bosons in a given partition. Our study points to a fundamental difference between the nature of the entanglement in free-bosonic and free-fermionic systems, with the entanglement in the former being zero after projection, while that in the latter (corresponding to the impenetrable boson limit) being non-zero.

quant-ph↗

arXiv2011

Oscillator state reconstruction via tunable qubit coupling in Markovian environments

We show that a parametrically coupled qubit can be used to fully reconstruct the quantum state of a harmonic oscillator, even when both systems are subject to decoherence. By controlling the coupling strength of the qubit over time, the characteristic function of the oscillator at any phase space point can be directly measured by combining the expectation values of two Pauli operators. The effect of decoherence can be filtered out from the measured data, provided a sufficient number of experimental runs is performed. In situations where full state reconstruction is not practical or not necessary, the method can still be used to estimate low order moments of the mechanical quadratures. We also show that in the same framework it is possible to prepare superposition states of the oscillator. The model is very general but particularly appropriate for nanomechanical systems.

quant-ph↗