20 research outputs found
Property lattices for independent quantum systems
We consider the description of two independent quantum systems by a complete
atomistic ortho-lattice (cao-lattice) L. It is known that since the two systems
are independent, no Hilbert space description is possible, i.e. ,
the lattice of closed subspaces of a Hilbert space (theorem 1). We impose five
conditions on L. Four of them are shown to be physically necessary. The last
one relates the orthogonality between states in each system to the
ortho-complementation of L. It can be justified if one assumes that the
orthogonality between states in the total system induces the
ortho-complementation of L. We prove that if L satisfies these five conditions,
then L is the separated product proposed by Aerts in 1982 to describe
independent quantum systems (theorem 2). Finally, we give strong arguments to
exclude the separated product and therefore our last condition. As a
consequence, we ask whether among the ca-lattices that satisfy our first four
basic necessary conditions, there exists an ortho-complemented one different
from the separated product.Comment: Reports on Mathematical Physics, Vol. 50 no. 2 (2002), p. 155-16
Weak tensor products of complete atomistic lattices
Abstract.: Given two complete atomistic lattices and , we define a set of complete atomistic lattices by means of three axioms (natural regarding the description of separated quantum compound systems), or in terms of a universal property with respect to a given class of bimorphisms. We call the elements of weak tensor products of and . We prove that is a complete lattice. We compare the bottom element with the separated product of Aerts and with the box product of Grätzer and Wehrung. Similarly, we compare the top element with the tensor products of Fraser, Chu and Shmuely. With some additional hypotheses on and (true for instance if and are moreover irreducible, orthocomplemented and with the covering property), we characterize the automorphisms of weak tensor products in terms of those of and {\mathcal{L}}_{2}$
Decoherence in a N-qubit solid-state quantum computer
We investigate the decoherence process for a quantum register composed of N
qubits coupled to an environment. We consider an environment composed of one
common phonon bath and several electronic baths. This environment is relevant
to the implementation of a charge based solid-state quantum computer. We
explicitly compute the time evolution of all off-diagonal terms of the
register's reduced density matrix. We find that in realistic configurations,
"superdecoherence" and "decoherence free subspaces" do not exist for an N-qubit
system. This means that all off-diagonal terms decay like exp(-q(t)N), where
q(t) is of the same order as the decay function of a single qubit.Comment: 11 page
Orthocomplementation and compound systems
In their 1936 founding paper on quantum logic, Birkhoff and von Neumann
postulated that the lattice describing the experimental propositions concerning
a quantum system is orthocomplemented. We prove that this postulate fails for
the lattice L_sep describing a compound system consisting of so called
separated quantum systems. By separated we mean two systems prepared in
different ``rooms'' of the lab, and before any interaction takes place. In that
case the state of the compound system is necessarily a product state. As a
consequence, Dirac's superposition principle fails, and therefore L_sep cannot
satisfy all Piron's axioms. In previous works, assuming that L_sep is
orthocomplemented, it was argued that L_sep is not orthomodular and fails to
have the covering property. Here we prove that L_sep cannot admit and
orthocomplementation. Moreover, we propose a natural model for L_sep which has
the covering property.Comment: Submitted for the proceedings of the 2004 IQSA's conference in
Denver. Revised versio