The dynamical evolution of a quantum register of arbitrary length coupled to an environment of arbitrary coherence length is predicted within a relevant model of decoherence. The results are reported for quantum bits (qubits) coupling individually to different environments (`independent decoherence') and qubits interacting collectively with the same reservoir (`collective decoherence'). In both cases, explicit decoherence functions are derived for any number of qubits. The decay of the coherences of the register is shown to strongly depend on the input states: we show that this sensitivity is a characteristic of $both$ types of coupling (collective and independent) and not only of the collective coupling, as has been reported previously. A non-trivial behaviour ("recoherence") is found in the decay of the off-diagonal elements of the reduced density matrix in the specific situation of independent decoherence. Our results lead to the identification of decoherence-free states in the collective decoherence limit. These states belong to subspaces of the system's Hilbert space that do not get entangled with the environment, making them ideal elements for the engineering of ``noiseless'' quantum codes. We also discuss the relations between decoherence of the quantum register and computational complexity based on the new dynamical results obtained for the register density matrix.Comment: Typos corrected. Discussion and references added. 1 figure + 3 tables added. This updated version contains 13 (double column) pages + 8 figures. PRA in pres

We show that excitons in coupled quantum dots are ideal candidates for reliable preparation of entangled states in solidstate systems. An optically controlled exciton transfer process is shown to lead to the generation of Bell and GHZ states in systems comprising two and three coupled dots, respectively. The strength and duration of selective light-pulses for producing maximally entangled states are identified by both analytic, and full numerical, solution of the quantum dynamical equations. Experimental requirements to build such entangled states are discussed.PACS numbers: 03.67, 71.10.Li, 73.20.D Quantum information, quantum computation, quantum cryptography and quantum teleportation represent exciting new arenas which exploit intrinsic quantum mechanical correlations.1 A fundamental requirement for the experimental realization of such proposals is the successful generation of highly entangled quantum states. In particular, coherent evolution of two quantum bits (qbits) in an entangled state of the Bell type is fundamental to both quantum cryptography and quantum teleportation. Maximally entangled states of three qbits, such as the so-called Greenberger-Horne-Zeilinger (GHZ) states 2 , are not only of intrinsic interest but are also of great practical importance in such proposals. New systems and methods for the preparation and measurement of such maximally entangled states are therefore being sought intensively. Most of the theoretical and experimental activity to date has been associated with atomic and quantum-optic systems 3-5 . Solid-state realizations of such quantum-based phenomena have received little attention despite the fact that semiconductor nanostructures such as quantum dots (QDs), with quantum-mechanical electron confinement in all three directions, have been fabricated and studied by many groups.6 In addition, recent experimental work by Bonadeo et al. 7,8 suggests that optically-generated electron-hole pairs (excitons) in semiconductor QDs represent ideal candidates for achieving coherent wavefunction control on the nanometer and femtosecond scales.In this paper we give a detailed prescription for producing such entangled states in semiconductor quantum dot systems. We show that the resonant transfer interaction between spatially separated excitons can be exploited to produce such entanglement, starting from suitably initialized states. The system requirements are realizable in current experiments employing ultrafast optical spectroscopy of quantum dots.When two quantum dots are sufficiently close, there is a resonant energy-transfer process originating from the Coulomb interaction whereby an exciton can hop between dots 9 . Experimental evidence of such energytransfers between quantum dots was reported recently 7 ; the resonant process also plays a fundamental role in biological and organic systems, and is commonly called the Forster process 10 . Unlike usual single-particle transport measurements, the Forster process does not require the physical transfer of the electron and hole, just...

PACS. 03.65.Ud -Entanglement and quantum nonlocality. PACS. 73.43.Nq -Quantum phase transitions. PACS. 75.10.-b -General theory and models of magnetic ordering.Abstract. -We show that the quantum phase transition arising in a standard radiationmatter model (Dicke model) belongs to the same universality class as the infinitely-coordinated, transverse field XY model. The effective qubit-qubit exchange interaction is shown to be proportional to the square of the qubit-radiation coupling. A universal finite-size scaling is derived for the corresponding two-qubit entanglement (concurrence) and a size-consistent effective Hamiltonian is proposed for the qubit subsystem.Quantum phase transitions (QPTs) are associated with a dramatic change in the physical properties of a system at zero temperature when a parameter varies around its critical value. It is well-known that very different systems can exhibit similar behavior in this critical regime, giving rise to the concept of universality. Enlarging a given universality class by the addition of systems from very different areas of physics, is a very important step toward unifying our understanding of the basic physics underlying apparently disconnected complex phenomena. Recently there have been studies of light-controlled condensed matter systems displaying QPTs with atoms in extreme one-dimensional confinements [1], ions driven by properly tuned and pulsed light [2] and fermionic atoms in optical superlattices [3]. Fully quantum mechanical models of radiation-matter systems are also being considered, and are important for several reasons: Scalable and distributed quantum information processing (QIP) devices will demand the integration of matter quantum bits (qubits) such as atoms, trapped ions, semiconductor quantum dots or SQUIDs with photons. In addition, the capability of photons to control and modify the coupling between physically distant qubits makes them appropriate for manipulating and transferring quantum information.

Results on heat current, entropy production rate and entanglement are reported for a quantum system coupled to two different temperature heat reservoirs. By applying a temperature gradient, different quantum states can be found with exactly the same amount of entanglement but different purity degrees and heat currents. Furthermore, a nonequilibrium enhancement-suppression transition behavior of the entanglement is identified.Comment: 5 pages and 5 figures(eps). Minor changes. Accepted version to be published in Phys. Rev.

We reveal universal dynamical scaling behavior across adiabatic quantum phase transitions in networks ranging from traditional spatial systems (Ising model) to fully connected ones (Dicke and Lipkin-Meshkov-Glick models). Our findings, which lie beyond traditional critical exponent analysis and adiabatic perturbation approximations, are applicable even where excitations have not yet stabilized and, hence, provide a time-resolved understanding of quantum phase transitions encompassing a wide range of adiabatic regimes. We show explicitly that even though two systems may traditionally belong to the same universality class, they can have very different adiabatic evolutions. This implies that more stringent conditions need to be imposed than at present, both for quantum simulations where one system is used to simulate the other and for adiabatic quantum computing schemes.