1,857 research outputs found
The Geometry of Single-Qubit Maps
The physically allowed quantum evolutions on a single qubit can be described
in terms of their geometry. From a simple parameterisation of unital
single-qubit channels, the canonical form of all such channels can be given.
The related geometry can be used to understand how to approximate positive maps
by completely-positive maps, such as in the case of optimal eavesdropping
strategies. These quantum channels can be generated by the appropriate network
or through dynamical means. The Str{\o}mer-Woronowisc result can also be
understood in terms of this geometry.Comment: 10 pages, 8 figures, REVTeX
Controlled phase gate for solid-state charge qubits
We describe a mechanism for realizing a controlled phase gate for solid-state
charge qubits. By augmenting the positionally defined qubit with an auxiliary
state, and changing the charge distribution in the three-dot system, we are
able to effectively switch the Coulombic interaction, effecting an entangling
gate. We consider two architectures, and numerically investigate their
robustness to gate noise.Comment: 14 pages, 11 figures, 2 tables, RevTeX
Experimental Hamiltonian identification for controlled two-level systems
We present a strategy to empirically determine the internal and control Hamiltonians for an unknown two-level system (black box) subject to various (piecewise constant) control fields when direct readout by measurement is limited to a single, fixed observable
Direct estimation of functionals of density operators by local operations and classical communication
We present a method of direct estimation of important properties of a shared bipartite quantum state, within the "distant laboratories" paradigm, using only local operations and classical communication. We apply this procedure to spectrum estimation of shared states, and locally implementable structural physical approximations to incompletely positive maps. This procedure can also be applied to the estimation of channel capacity and measures of entanglement
Quantum cryptography based on qutrit Bell inequalities
We present a cryptographic protocol based upon entangled qutrit pairs. We analyze the scheme under a symmetric incoherent attack and plot the region for which the protocol is secure and compare this with the region of violations of certain Bell inequalities
Reference frames for Bell inequality violation in the presence of superselection rules
Superselection rules (SSRs) constrain the allowed states and operations in quantum theory. They limit preparations and measurements and hence impact our ability to observe non-locality, in particular the violation of Bell inequalities. We show that a reference frame compatible with a particle number SSR does not allow observers to violate a Bell inequality if and only if it is prepared using only local operations and classical communication. In particular, jointly prepared separable reference frames are sufficient for obtaining violations of a Bell inequality. We study the size and non-local properties of such reference frames using superselection-induced variance. These results suggest the need for experimental Bell tests in the presence of superselection
Robust Charge-based Qubit Encoding
We propose a simple encoding of charge-based quantum dot qubits which
protects against fluctuating electric fields by charge symmetry. We analyse the
reduction of coupling to noise due to nearby charge traps and present single
qubit gates. The relative advantage of the encoding increases with lower charge
trap density.Comment: 6 Pages, 7 Figures. Published Versio
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