The problem of giving a theoretical description of sequential decays is reconsidered. It is shown that the usual quantum description of such processes cannot be reconciled with the classical one. This fact suggests a more critical analysis of the actual physical situation occurring in such processes. By taking into consideration the remarkable successes which have been obtained in the description of nonsequential decays by considering the unavoidable interactions of the decaying system with the measuring apparatuses, an analogous formalism is developed for the problem under consideration. The description of the process is given in the language of the density operator. The evolution equation for such an operator has the standard form of the dynamical equations in the theory of quantum dynamical semigroups. Also in the present case, as for the nonsequential decay, the more realistic description which takes into account the actual set-up of a decay experiment turns out to yield results which agree completely with the classical ones. Some interesting problems arising within the present formalism are discussed.
Quantum Computation and Quantum Information are continuously growing research areas which are based on nature and resources of quantum mechanics, as superposition and entanglement. In its gate array version, the use of convenient and appropriate gates is essential. But while those proposed gates adopt convenient forms for computational algorithms, in the practice, their design depends on specific quantum systems and stuff being used. Gates design is restricted to properties and limitations of interactions and physical elements being involved, where Quantum Control plays a deep role. Quantum complexity of multipartite systems and their interactions requires a tight control to manipulate their quantum states, either local and non-local ones, but still a reducibility procedure should be addressed. This work shows how a general
2d-partite two level spin system in
SU(2d) could be decomposed in
2n−1 subsystems on
SU(2), letting establish control operations. In particular, it is shown that Bell gems basis is a set of natural states on which decomposition happen naturally under some interaction restrictions. Thus, alternating the direction of local interaction terms in the Hamiltonian, this procedure states a universal exchange semantics on those basis. The structure developed could be understood as a splitting of the
2d information channels into
22d−1 pairs of
2 level information subsystems.
Many experimental results and theoretical reasonings on gedanken experiments suggest that quantum entities behave in a non-local . In order to make the discussion more precise, we propose an operational definition of non-locality . We discuss two examples of spin superposition experiments which illustrate the use of this definition. They show that quantum entities like a neutron or an atom are non-local. We make an analysis of the difficulty to imagine such non-local entities and to conciliate this non-locality with the locality of classical entities
Quantum non-locality is normally defined via violations of Bell's inequalities that exclude certain classical hidden variable theories from explaining quantum correlations. Another definition of non-locality refers to the wave function collapse thereby one can prepare a quantum state from arbitrary far away. In both cases one can debate on whether non-locality is a real physical phenomenon, e.g. , one can employ formulations of quantum mechanics that do not use collapse, or one can simply refrain from explaining quantum correlations via classical hidden variables. Here we point out that there is a non-local effect within quantum mechanics, i.e. , without involving hidden variables or collapse. This effect is seen via imprecise ( i.e. , interval-valued) joint probability of two observables, which replaces the ill-defined notion of the precise joint probability for non-commuting observables. It is consistent with all requirements for the joint probability, e.g. , those for commuting observales. The non-locality amounts to a fact that (in a two-particle system) the joint imprecise probability of non-commuting two-particle observables ( i.e. , tensor product of single-particle observables) does not factorize into single-particle contributions, even for uncorrelated states of the two-particle system. The factorization is recovered for a less precise ( i.e. , the one involving a wider interval) joint probability. This approach to non-locality reconciles it with locality, since the latter emerges as a less precise description.
Understanding how local information propagates through many-body quantum systems is a central problem in nonequilibrium dynamics, with important implications for quantum communication, state transfer, and remote sensing. In this work, we investigate information transfer along a one-dimensional open chain of qudits, focusing on the task of recovering information initially encoded at one end via measurements performed at the opposite end. By restricting the dynamics to brickwork quantum circuits, and considering small
M-qudit subsystems on the causal ``lightcone'' of the circuit, we obtain several results valid even for large system sizes
N or for nonintegrable global dynamics. Within this framework, lossless information transfer is linked to the existence of peripheral eigenvalues of a quantum channel
ΦM, which governs the evolution of the
M-qudit local subsystem along the lightcone. We investigate conditions under which brickwork circuits admit such peripheral eigenvalues. For qubit chains and
M=1, we show that the dual-unitary property is necessary, whereas for larger local subsystems (
M≥2) or higher-dimensional qudits, this requirement can be relaxed. Perhaps surprisingly, we can use the peripheral eigenvalue condition to construct examples exhibiting lossless information transfer through chains of arbitrary system size
N, even when the underlying circuit dynamics is nonintegrable and thermalising at long times.