Showing papers by "Mark Hillery published in 1997"

Quantum copying: A network

Abstract: We present a network consisting of quantum gates that produces two imperfect copies of an arbitrary qubit. The quality of the copies does not depend on the input qubit. We also show that for a restricted class of inputs it is possible to use a very similar network to produce three copies instead of two. For qubits in this class, the copy quality is again independent of the input and is the same as the quality of the copies produced by the two-copy network.

Broadcasting of entanglement via local copying

Abstract: We show that inseparability of quantum states can be partially broadcasted (copied or cloned) with the help of local operations, i.e., distant parties sharing an entangled pair of spin-1/2 states can generate two pairs of partially nonlocally entangled states using only local operations. This procedure can be viewed as an inversion of quantum purification procedures.

Quantum copying: Fundamental inequalities

Abstract: How well can one copy an arbitrary qubit? To answer this question we consider two arbitrary vectors in a two-dimensional state space and an abstract copying transformation which will copy these two vectors If the vectors are orthogonal, then perfect copies can be made If they are not, then errors will be introduced The size of the error depends on the inner product of the two original vectors We derive a lower bound for the amount of noise induced by quantum copying We examine both copying transformations which produce one copy and transformations which produce many, and show that the quality of each copy decreases as the number of copies increases

Quantized fields in a nonlinear dielectric medium : a microscopic approach

Abstract: Theories which have been used to describe the quantized electromagnetic field interacting with a nonlinear dielectric medium are either phenomenological or derived by quantizing the macroscopic Maxwell equations. Here we take a different approach and derive a Hamiltonian describing interacting fields from one which contains both field and matter degrees of freedom. The medium is modelled as a collection of two-level atoms, and these interact with the electromagnetic field. The atoms are grouped into effective spins and the Holstein- Primakoff representation of the spin operators is used to expand them in one over the total spin. When the lowest-order term is combined with the free atomic and field Hamiltonians, a theory of noninteracting polaritons results. When higher-order terms are expressed in terms of polariton operators, standard nonlinear optical interactions emerge.

Direct and indirect strategies for phase measurement

Abstract: Recently, Torgerson and Mandel [Phys Rev Lett 76, 3939 (1996)] have reported a disagreement between two schemes for measuring the phase difference of a pair of optical fields We analyze these schemes and derive their associated phase-difference probability distributions, including both their strong and weak field limits Our calculation confirms the main point of Torgerson and Mandel of the non-uniqueness of an operational definition of the phase distribution We further discuss the role of postselection of data and argue that it cannot meaningfully improve the sensitivity