1,329 research outputs found
Quantum Circuits for General Multiqubit Gates
We consider a generic elementary gate sequence which is needed to implement a
general quantum gate acting on n qubits -- a unitary transformation with 4^n
degrees of freedom. For synthesizing the gate sequence, a method based on the
so-called cosine-sine matrix decomposition is presented. The result is optimal
in the number of elementary one-qubit gates, 4^n, and scales more favorably
than the previously reported decompositions requiring 4^n-2^n+1 controlled NOT
gates.Comment: 4 pages, 3 figure
Experimental determination of the Berry phase in a superconducting charge pump
We present the first measurements of the Berry phase in a superconducting
Cooper pair pump. A fixed amount of Berry phase is accumulated to the
quantum-mechanical ground state in each adiabatic pumping cycle, which is
determined by measuring the charge passing through the device. The dynamic and
geometric phases are identified and measured quantitatively from their
different response when pumping in opposite directions. Our observations, in
particular, the dependencies of the dynamic and geometric effects on the
superconducting phase bias across the pump, agree with the basic theoretical
model of coherent Cooper pair pumping.Comment: 4 pages, 3 figure
Spatiotemporal Convergence of Semantic Processing in Reading and Speech Perception
Retrieval of word meaning from the semantic system and its integration with context are often assumed to be shared by spoken and written words. How is modality-independent semantic processing manifested in the brain, spatially and temporally? Time-sensitive neuroimaging allows tracking of neural activation sequences. Use of semantically related versus unrelated word pairs or sentences ending with a semantically highly or less plausible word, in separate studies of the auditory and visual modality, has associated lexical-semantic analysis with sustained activation at ∼200–800 ms. Magnetoencephalography (MEG) studies have further identified the superior temporal cortex as a main locus of the semantic effect. Nevertheless, a direct comparison of the spatiotemporal neural correlates of visual and auditory word comprehension in the same brain is lacking. We used MEG to compare lexical-semantic analysis in the visual and auditory domain in the same individuals, and contrasted it with phonological analysis that, according to models of language perception, should occur at a different time with respect to semantic analysis in reading and speech perception. The stimuli were lists of four words that were either semantically or phonologically related, or with the final word unrelated to the preceding context. Superior temporal activation reflecting semantic processing occurred similarly in the two modalities, left-lateralized at 300–450 ms and thereafter bilaterally, generated in close-by areas. Effect of phonology preceded the semantic effect in speech perception but not in reading. The present data indicate involvement of the middle superior temporal cortex in semantic processing from ∼300 ms onwards, regardless of input modality.Peer reviewe
Measurement scheme of the Berry phase in superconducting circuits
We present a measurement scheme for observing the Berry phase in a flux
assisted Cooper pair pump - the Cooper pair sluice. In contrast to the recent
experiments, in which the sluice was employed to generate accurate current
through a resistance, we consider a device in a superconducting loop. This
arrangement introduces a connection between the pumped current and the Berry
phase accumulated during the adiabatic pumping cycles. From the adiabaticity
criterion, we derive equations for the maximum pumped current and optimize the
sluice accordingly. These results apply also to the high accuracy pumping which
results in a potential candidate for a metrological current standard. For
measuring the pumped current, an additional Josephson junction is installed
into the superconducting loop. We show in detail that the switching of this
system from superconducting state into normal state as a consequence of an
external current pulse through it may be employed to probe the pumped current.
The experimental realization of our scheme would be the first observation of
the Berry phase in superconducting circuits.Comment: 19 pages, 5 figure
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