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, our channel allows distillable entanglement to be shared over arbitrary distances.

Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. However, the lack of empirical evidence has lead to a debate on whether gravity is a quantum entity. Despite varied proposed probes for quantum gravity, it is fair to say that there are no feasible ideas yet to test its quantum coherent behavior directly in a laboratory experiment. Here, we introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple spin correlation measurements.

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 single two level system (atom/quantum dot/Cooper pair) and can be individually addressed from outside. In the Mott phase each atomcavity 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.

We investigate the entanglement between any two spins in a one dimensional Heisenberg chain as a function of temperature and the external magnetic field. We find that the entanglement in an antiferromagnetic chain can be increased by increasing the temperature or the external field. Increasing the field can also create entanglement between otherwise disentangled spins. This entanglement can be confirmed by testing Bell's inequalities involving any two spins in the solid.It is well known that distinct quantum systems can be correlated in a "stronger than classical" manner [1][2][3]. This "excess correlation", called entanglement, has recently become an important resource in quantum information processing [4]. Like energy, it is quantifiable [5][6][7]. This motivates us to ask how much entanglement exists in a realistic system such as a solid (the likely final arena for quantum computing [8]) at a finite temperature. The 1D Heisenberg model [9,10] is a simple but realistic [11] and extensively studied [12][13][14][15] solid state system. We analyze the dependence of entanglement in this system on temperature and external field. We find that the entanglement between two spins in an antiferromagnetic solid can be increased by increasing the temperature or the external field. Increasing the field to a certain value can also create entanglement between otherwise disentangled spins. We show that the presence entanglement can be confirmed by observing the violation of Bell's inequalities. However, on exceeding a critical value of the field, the entanglement vanishes at zero temperature and decays off at a finite temperature. In the ferromagnetic solid, on the other hand, entanglement is always absent. We compare the entanglement in these systems to the total correlations.The entanglement of formation [5] is a computable entanglement measure for two spin-1 2 systems (qubits) [16]. We will use this measure to compute the entanglement between different spins in the 1D isotropic spin-1 2 Heisenberg model. This model describes a system of an arbitrary number of linearly arranged spins, each interacting only with its nearest neighbors. Recently, entanglement in linear arrays of qubits have attracted interest [18][19][20] and in Ref.[19] the entanglement in the ground state of a Heisenberg antiferromagnet has been computed. But entanglement in the natural state of a system as a function of its temperature remains to be studied and the possibilities of increasing this entanglement by an external magnetic field remains to be explored. The Hamiltonian for the 1D Heisenberg chain in a constant external magnetic field B, is given bywhere σ i = (σ i x , σ i y , σ i z ) in which σ i x/y/z are the Pauli matrices for the ith spin (we assume cyclic boundary conditions 1 + N = 1). J > 0 and J < 0 correspond to the antiferromagnetic and the ferromagnetic cases respectively. The state of the above system at thermal equilibrium (temperature T ) is ρ(T ) = e −H/kT /Z where Z is the partition function and k is Boltzmann's constant. To find th...

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.

We describe how a quantum system composed of a cavity field interacting with a movable mirror can be utilized to generate a large variety of nonclassical states of both the cavity field and the mirror. First we consider state preparation of the cavity field. The system dynamics will prepare a single mode of the cavity field in a multicomponent Schrödinger-cat state, in a similar manner to that in a Kerr medium. In addition, when two or more cavity modes interact with the mirror, they can be prepared in an entangled state, which may be regarded as a multimode generalization of the even and odd coherent states. We show also that near-number states of a single mode may be prepared by performing a measurement of the position of the mirror. Second we consider state preparation of the mirror and show that this macroscopic object may be placed in a Schrödinger-cat-like state by a quadrature measurement of the light field. In addition, we examine the effect of the damping of the motion of the mirror on the field states inside the cavity and compare this with the effect of cavity field damping.

We generalize the procedure of entanglement swapping to obtain a scheme for manipulating entanglement in multiparticle systems. We describe how this scheme allows one to establish multiparticle entanglement between particles belonging to distant users in a communication network through a prior distribution of singlets followed by only local measurements. We show that this scheme can be regarded as a method of generating entangled states of many particles and compare it with existing schemes using simple quantum computational networks. We highlight the practical advantages of using a series of entanglement swappings during the distribution of entangled particles between two parties. Applications of multiparticle entangled states in cryptographic conferencing and in reading messages from more than one source through a single measurement are also described.

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.