1,720,984 research outputs found
Full control by locally induced relaxation
No full textWe demonstrate a scheme for controlling a large quantum system by acting on a small subsystem only. The local control is mediated to the larger system by some fixed coupling Hamiltonian. The scheme allows us to transfer arbitrary and unknown quantum states from a memory to the large system (“upload access”) as well as the inverse (“download access”). We study the sufficient conditions of the coupling Hamiltonian and give lower bounds on the fidelities for downloading and uploading
Mediated Homogenization
No full textHomogenization protocols model the quantum mechanical evolution of a system to a fixed state independently from its initial configuration by repeatedly coupling it with a collection of identical ancillas. Here we analyze these protocols within the formalism of ldquorelaxingrdquo channels providing an easy-to-check sufficient condition for homogenization. In this context we describe mediated homogenization schemes where a network of connected qudits relaxes to a fixed state by only partially interacting with a bath. We also study configurations which allow us to introduce entanglement among the elements of the network. Finally we analyze the effect of having competitive configurations with two different baths and we prove the convergence to dynamical equilibrium for Heisenberg chains
Improved transfer of quantum information using a local memory
No full textWe demonstrate that the quantum communication between two parties can be significantly improved if the receiver is allowed to store the received signals in a quantum memory before decoding them. In the limit of an infinite memory, the transfer is perfect. We prove that this scheme allows the transfer of arbitrary multipartite states along Heisenberg chains of spin-1/2 particles with random coupling strengths
A protocol For Cooling and Controlling Composite Systems by Local Interactions
No full textWe discuss an explicit protocol which allows one to externally cool and control a composite system by operating on a small subset of it The scheme permits to transfer arbitrary and unknown quantum states from a memory on the network ("upload access") as well as the inverse ("download access") In particular it yields a method for cooling the syste
Quantum defragmentation algorithm
No full textIn this addendum to our paper [D. Burgarth and V. Giovannetti, Phys. Rev. Lett. 99, 100501 (2007)] we prove that during the transformation that allows one to enforce control by relaxation on a quantum system, the ancillary memory can be kept at a finite size, independently from the fidelity one wants to achieve. The result is obtained by introducing the quantum analog of defragmentation algorithms which are employed for efficiently reorganizing classical information in conventional hard disks
The generalized Lyapunov Theorem and its Application to Quantum Channels
Full text linkWe give a simple and physically intuitive necessary and sufficient condition for a map acting on a compact metric space to be mixing (i.e. infinitely many applications of the map transfer any input into a fixed convergency point). This is a generalization of the 'Lyapunov direct method'. First we prove this theorem in topological spaces and for arbitrary continuous maps. Finally we apply our theorem to maps which are relevant in open quantum systems and quantum information, namely quantum channels. In this context, we also discuss the relations between mixing and ergodicity (i.e. the property that there exists only a single input state which is left invariant by a single application of the map) showing that the two are equivalent when the invariant point of the ergodic map is pure.We give a simple and physically intuitive necessary and sufficient condition for a map acting on a compact
metric space to be mixing (i.e. infinitely many applications of the map transfer any input into a fixed
convergency point). This is a generalization of the 'Lyapunov direct method'. First we prove this theorem in
topological spaces and for arbitrary continuous maps. Finally we apply our theorem to maps which are
relevant in open quantum systems and quantum information, namely quantum channels. In this context, we
also discuss the relations between mixing and ergodicity (i.e. the property that there exists only a single input
state which is left invariant by a single application of the map) showing that the two are equivalent when the
invariant point of the ergodic map is pure
Communication Through a Quantum Link
No full textA chain of interacting spin behaves like a quantum mediator (quantum link), which allows two distant parties that control the ends of the chain to exchange quantum messages. We show that over repeated uses without resetting the study of a quantum link can be connected to correlated quantum channels with a finite dimensional environment (finite memory quantum channel). Then, using coding arguments for such kinds of channels and results on mixing channels we present a protocol that allows us to achieve perfect information transmission through a quantum link
Efficient and perfect state transfer in quantum chains
No full textWe 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 of 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 can be proved which shows that the receiver is able to asymptotically recover the messages by repetitive monitoring of his qubits. 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 time scale of the order of 0.33h /J N1.7 ln P
Optimal quantum chain communication by end gates
No full textThe scalability of solid-state quantum computation relies on the ability of connecting the qubits to the macroscopic world. Quantum chains can be used as quantum wires to keep regions of external control at a distance. However, even in the absence of external noise their transfer fidelity is too low to assure reliable connections. We propose a method of optimizing the fidelity by minimal usage of the available resources, consisting of applying a suitable sequence of two-qubit gates at the end of the chain. Our scheme also allows the preparation of states in the first excitation sector as well as cooling
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