54,341 research outputs found
Bipartite all-versus-nothing proofs of Bell's theorem with single-qubit measurements
If we distribute n qubits between two parties, which quantum pure states and
distributions of qubits would allow all-versus-nothing (or
Greenberger-Horne-Zeilinger-like) proofs of Bell's theorem using only
single-qubit measurements? We show a necessary and sufficient condition for the
existence of these proofs for any number of qubits, and provide all distinct
proofs up to n=7 qubits. Remarkably, there is only one distribution of a state
of n=4 qubits, and six distributions, each for a different state of n=6 qubits,
which allow these proofs.Comment: REVTeX4, 4 pages, 2 figure
Bell inequality, Bell states and maximally entangled states for n qubits
First, we present a Bell type inequality for n qubits, assuming that m out of
the n qubits are independent. Quantum mechanics violates this inequality by a
ratio that increases exponentially with m. Hence an experiment on n qubits
violating of this inequality sets a lower bound on the number m of entangled
qubits. Next, we propose a definition of maximally entangled states of n
qubits. For this purpose we study 5 different criteria. Four of these criteria
are found compatible. For any number n of qubits, they determine an orthogonal
basis consisting of maximally entangled states generalizing the Bell states.Comment: 8 pages, no figur
Quantum Computation by Communication
We present a new approach to scalable quantum computing--a ``qubus
computer''--which realises qubit measurement and quantum gates through
interacting qubits with a quantum communication bus mode. The qubits could be
``static'' matter qubits or ``flying'' optical qubits, but the scheme we focus
on here is particularly suited to matter qubits. There is no requirement for
direct interaction between the qubits. Universal two-qubit quantum gates may be
effected by schemes which involve measurement of the bus mode, or by schemes
where the bus disentangles automatically and no measurement is needed. In
effect, the approach integrates together qubit degrees of freedom for
computation with quantum continuous variables for communication and
interaction.Comment: final published versio
- …