arXiv subjects

Sougato Bose

Publications and source records attributed to Sougato Bose.

216 records · Page 12Linked to original sources

arXiv2007

Transfer of a Polaritonic Qubit through a Coupled Cavity Array

We demonstrate a scheme for quantum communication between the ends of an array of coupled cavities. Each cavity is doped with a single two level system (atoms or quantum dots) and the detuning of the atomic level spacing and photonic frequency is appropriately tuned to achieve photon blockade in the array. We show that in such a regime, the array can simulate a dual rail quantum state transfer protocol where the arrival of quantum information at the receiving cavity is heralded through a fluorescence measurement. Communication is also possible between any pair of cavities of a network of connected cavities.

quant-ph↗

arXiv2007

Quantum communication via a continuously monitored dual spin chain

We analyze a recent protocol for the transmission of quantum states via a dual spin chain [Burgarth and Bose, Phys. Rev. A 71, 052315 (2005)] under the constraint that the receiver's measurement strength is finite. That is, we consider the channel where the ideal, instantaneous and complete von Neumann measurements are replaced with a more realistic continuous measurement. We show that for optimal performance the measurement strength must be "tuned" to the channel spin-spin coupling, and once this is done, one is able to achieve a similar transmission rate to that obtained with ideal measurements. The spin chain protocol thus remains effective under measurement constraints.

quant-ph↗

arXiv2006

Entanglement from the Dynamics of an Ideal Bose Gas in a Lattice

We show how the remotest sites of a finite lattice can be entangled, with the amount of entanglement exceeding that of a singlet, solely through the dynamics of an ideal Bose gas in a special initial state in the lattice. When additional occupation number measurements are made on the intermediate lattice sites, then the amount of entanglement and the length of the lattice separating the entangled sites can be significantly enhanced. The entanglement generated by this dynamical procedure is found to be higher than that for the ground state of an ideal Bose gas in the same lattice. A second dynamical evolution is shown to verify the existence of these entangled states, as well entangle qubits belonging to well separated quantum registers.

cond-mat.other↗

arXiv2006

Universal destabilization and slowing of spin transfer functions by a bath of spins

We investigate the effect of a spin bath on the spin transfer functions of a permanently coupled spin system. When each spin is coupled to a seperate environment, the effect on the transfer functions in the first excitation sector is amazingly simple: the group velocity is slowed down by a factor of two, and the fidelity is destabilized by a modulation of |cos Gt|, where G is the mean square coupling to the environment.

quant-ph↗

arXiv2006

A Processor Core Model for Quantum Computing

We describe an architecture based on a processing 'core' where multiple qubits interact perpetually, and a separate 'store' where qubits exist in isolation. Computation consists of single qubit operations, swaps between the store and the core, and free evolution of the core. This enables computation using physical systems where the entangling interactions are 'always on'. Alternatively, for switchable systems our model constitutes a prescription for optimizing many-qubit gates. We discuss implementations of the quantum Fourier transform, Hamiltonian simulation, and quantum error correction.

quant-ph↗

arXiv2006

Distributed quantum computation via optical fibres

We investigate the possibility of realising effective quantum gates between two atoms in distant cavities coupled by an optical fibre. We show that highly reliable swap and entangling gates are achievable. We exactly study the stability of these gates in presence of imperfections in coupling strengths and interaction times and prove them to be robust. Moreover, we analyse the effect of spontaneous emission and losses and show that such gates are very promising in view of the high level of coherent control currently achievable in optical cavities.

quant-ph↗

arXiv2005

Entanglement creation and distribution on a graph of exchange-coupled qutrits

We propose a protocol that allows both the creation and distribution of entanglement, resulting in two distant parties (Alice and Bob) conclusively sharing a bipartite Bell State. The system considered is a graph of three-level objects ("qutrits") coupled by SU(3) exchange operators. The protocol begins with a third party (Charlie) encoding two lattice sites in unentangled states, and allowing unitary evolution under time. Alice and Bob perform a projective measurement on their respective qutrits at a given time, and obtain a maximally-entangled Bell state with a certain probablility. We also consider two further protocols, one based on simple repetition and the other based on successive measurements and conditional resetting, and show that the cumulative probability of creating a Bell state between Alice and Bob tends to unity.

quant-ph↗

arXiv2005

Efficient and perfect state transfer in quantum chains

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 to asymptotically recover the messages by repetitive monitoring of his qubits.

quant-ph↗

arXiv2005

Perfect State Transfer, Effective Gates and Entanglement Generation in Engineered Bosonic and Fermionic Networks

We show how to achieve perfect quantum state transfer and construct effective two-qubit gates between distant sites in engineered bosonic and fermionic networks. The Hamiltonian for the system can be determined by choosing an eigenvalue spectrum satisfying a certain condition, which is shown to be both sufficient and necessary in mirror-symmetrical networks. The natures of the effective two-qubit gates depend on the exchange symmetry for fermions and bosons. For fermionic networks, the gates are entangling (and thus universal for quantum computation). For bosonic networks, though the gates are not entangling, they allow two-way simultaneous communications. Protocols of entanglement generation in both bosonic and fermionic engineered networks are discussed.

quant-ph↗

arXiv2005

Perfect quantum state transfer with randomly coupled quantum chains

We suggest a scheme that allows arbitrarily perfect state transfer even in the presence of random fluctuations in the couplings of a quantum chain. The scheme performs well for both spatially correlated and uncorrelated fluctuations if they are relatively weak (say 5%). Furthermore, we show that given a quite arbitrary pair of quantum chains, one can check whether it is capable of perfect transfer by only local operations at the ends of the chains, and the system in the middle being a "black box". We argue that unless some specific symmetries are present in the system, it will be capable of perfect transfer when used with dual-rail encoding. Therefore our scheme puts minimal demand not only on the control of the chains when using them, but also on the design when building them.

quant-ph↗

arXiv2005

Conclusive and arbitrarily perfect quantum state transfer using parallel spin chain channels

We suggest a protocol for perfect quantum communication through spin chain channels. By combining a dual-rail encoding with measurements only at the receiving end, we can get conclusively perfect state transfer, whose probability of success can be made arbitrarily close to unity. As an example of such an amplitude delaying channel, we show how two parallel Heisenberg spin chains can be used as quantum wires. Perfect state transfer with a probability of failure lower than P in a Heisenberg chain of N spin-1/2 particles can be achieved in a timescale of the order of N^1.7|ln(P)|. We demonstrate that our scheme is more robust to decoherence and non-optimal timing than any scheme using single spin chains.

quant-ph↗

arXiv2004

Engineering an interaction and entanglement between distant atoms

We propose a scheme to generate an effective interaction of arbitrary strength between the internal degrees of freedom of two atoms placed in distant cavities connected by an optical fiber. The strength depends on the field intensity in the cavities. As an application of this interaction, we calculate the amount of entanglement it generates between the internal states of the distant atoms. The scheme effectively converts entanglement distribution networks to networks of interacting spins.

quant-ph↗

arXiv2003

An exact effective two-qubit gate in a chain of three spins

We show that an effective two-qubit gate can be obtained from the free evolution of three spins in a chain with nearest neighbor XY coupling, without local manipulations. This gate acts on the two remote spins and leaves the mediating spin unchanged. It can be used to perfectly transfer an arbitrary quantum state from the first spin to the last spin or to simultaneously communicate one classical bit in each direction. One ebit can be generated in half of the time for state transfer. For longer spin chains, we present methods to create or transfer entanglement between the two end spins in half of the time required for quantum state transfer, given tunable coupling strength and local magnetic field. We also examine imperfect state transfer through a homogeneous XY chain.

quant-ph↗

arXiv2003

Qubit Assisted Probing of Coherence Between States of a Macroscopic Apparatus

I present a general scheme through which the evidence of a superposition involving distinct classical-like states of a macroscopic system can be probed. The scheme relies on a qubit being coupled to a macroscopic harmonic oscillator in such a way that it can be used to both prepare and probe a macroscopic superposition. Two potentially realizable implementations, one with a flux qubit coupled to a LC circuit, and the other with an ion-trap qubit coupled to the collective motion of several ions, are proposed.

quant-ph↗

arXiv2003

Quantum Communication Through an Unmodulated Spin Chain

We propose a scheme for using an unmodulated and unmeasured spin-chain as a channel for short distance quantum communications. The state to be transmitted is placed on one spin of the chain and received later on a distant spin with some fidelity. We first obtain simple expressions for the fidelity of quantum state transfer and the amount of entanglement sharable between any two sites of an arbitrary Heisenberg ferromagnet using our scheme. We then apply this to the realizable case of an open ended chain with nearest neighbor interactions. The fidelity of quantum state transfer is obtained as an inverse discrete cosine transform and as a Bessel function series. We find that in a reasonable time, a qubit can be directly transmitted with better than classical fidelity across the full length of chains of up to 80 spins. Moreover, the spin-chain channel allows distillable entanglement to be shared over arbitrarily large distances.

quant-ph↗

arXiv2003

Quantum Computing with an 'Always On' Heisenberg Interaction

Many promising ideas for quantum computing demand the experimental ability to directly switch 'on' and 'off' a physical coupling between the component qubits. This is typically the key difficulty in implementation, and precludes quantum computation in generic solid state systems, where interactions between the constituents are 'always on'. Here we show that quantum computation is possible in strongly coupled (Heisenberg) systems even when the interaction cannot be controlled. The modest ability of 'tuning' the transition energies of individual qubits proves to be sufficient, with a suitable encoding of the logical qubits, to generate universal quantum gates. Furthermore, by tuning the qubits collectively we provide a scheme with exceptional experimental simplicity: computations are controlled via a single 'switch' of only six settings. Our schemes are applicable to a wide range of physical implementations, from excitons and spins in quantum dots through to bulk magnets.

quant-ph↗

arXiv2001

Ponderomotive entangling of atomic motions

We propose the use of ponderomotive forces to entangle the motions of different atoms. Two situations are analyzed: one where the atoms belong to the same optical cavity and interact with the same radiation field mode; the other where each atom is placed in own optical cavity and the output field of one cavity enters the other.

quant-ph↗

arXiv1999

A quantum holographic principle from decoherence

We present a fully quantum version of the holographic principle in terms of quantum systems, subsystems, and their interactions. We use the concept of environment induced decoherence to prove this principle. We discuss the conditions under which the standard (semi-classical) holographic principle is obtained from this quantum mechanical version.

gr-qc↗