163 research outputs found
Effective Hamiltonians in quantum physics: Resonances and geometric phase
Effective Hamiltonians are often used in quantum physics, both in time-dependent and time-independent contexts. Analogies are drawn between the two usages, the discussion framed particularly for the geometric phase of a time-dependent Hamiltonian and for resonances as stationary states of a time-independent Hamiltonian. © 2006 The Royal Swedish Academy of Sciences
Generic Two-Qubit Photonic Gates Implemented by Number-Resolving Photodetection
We combine numerical optimization techniques [Uskov et al., Phys. Rev. A 79,
042326 (2009)] with symmetries of the Weyl chamber to obtain optimal
implementations of generic linear-optical KLM-type two-qubit entangling gates.
We find that while any two-qubit controlled-U gate, including CNOT and CS, can
be implemented using only two ancilla resources with success probability S >
0.05, a generic SU(4) operation requires three unentangled ancilla photons,
with success S > 0.0063. Specifically, we obtain a maximal success probability
close to 0.0072 for the B gate. We show that single-shot implementation of a
generic SU(4) gate offers more than an order of magnitude increase in the
success probability and two-fold reduction in overhead ancilla resources
compared to standard triple-CNOT and double-B gate decompositions.Comment: 5 pages, 3 figure
General linear-optical quantum state generation scheme: Applications to maximally path-entangled states
We introduce schemes for linear-optical quantum state generation. A quantum
state generator is a device that prepares a desired quantum state using product
inputs from photon sources, linear-optical networks, and postselection using
photon counters. We show that this device can be concisely described in terms
of polynomial equations and unitary constraints. We illustrate the power of
this language by applying the Grobner-basis technique along with the notion of
vacuum extensions to solve the problem of how to construct a quantum state
generator analytically for any desired state, and use methods of convex
optimization to identify bounds to success probabilities. In particular, we
disprove a conjecture concerning the preparation of the maximally
path-entangled |n,0)+|0,n) (NOON) state by providing a counterexample using
these methods, and we derive a new upper bound on the resources required for
NOON-state generation.Comment: 5 pages, 2 figure
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