37 research outputs found
Fast Quantum Computing with Buckyballs
We have found that encapsulated atoms in fullerene molecules, which carry a
spin, can be used for fast quantum computing. We describe the scheme for
performing quantum computations, going through the preparation of the qubit
state and the realization of a two-qubit quantum gate. When we apply a static
magnetic field to each encased spin, we find out the ideal design for the
preparation of the quantum state. Therefore, adding to our system a time
dependent magnetic field, we can perform a phase-gate. The operational time
related to a phase gate is of the order of . This finding shows that,
during the decoherence time, which is proportional to , we can perform
many thousands of gate operations. In addition, the two-qubit state which
arises after a gate is characterized by a high degree of entanglement.
This opens a new avenue for the implementation of fast quantum computation.Comment: 9 pages, 4 figure
Switching and symmetry breaking behaviour of discrete breathers in Josephson ladders
We investigate the roto-breathers recently observed in experiments on
Josephson ladders subjected to a uniform transverse bias current. We describe
the switching mechanism in which the number of rotating junctions increases. In
the region close to switching we find that frequency locking, period doubling,
quasi-periodic behaviour and symmetry breaking all occur. This suggests that a
chaotic dynamic occurs in the switching process. Close to switching the induced
flux increases sharply and clearly plays an important role in the switching
mechanism. We also find three critical frequencies which are independent of the
dissipation constant , provided that is not too large
Direct current generation due to harmonic mixing: From bulk semiconductors to semiconductor superlattices
We discuss an effect of dc current and dc voltage (stopping bias) generation
in a semiconductor superlattice subjected by an ac electric field and its
phase-shifted n-th harmonic. In the low field limit, we find a simple
dependence of dc voltage on a strength, frequency, and relative phase of mixing
harmonics for an arbitrary even value of n.
We show that the generated dc voltage has a maximum when a frequency of ac
field is of the order of a scattering constant of electrons in a superlattice.
This means that for typical semiconductor superlattices at room temperature
operating in the THz frequency domain the effect is really observable.
We also made a comparison of a recent paper describing an effect of a
directed current generation in a semiconductor superlattice subjected by ac
field and its second harmonic (n=2) [K.Seeger, Appl.Phys.Lett. 76(2000)82] with
our earlier findings describing the same effect [K.Alekseev et al., Europhys.
Lett. 47(1999)595; cond-mat/9903092 ].
For the mixing of an ac field and its n-th harmonic with n>=4, we found that
additionally to the phase-shift controlling of the dc current, there is a
frequency control. This frequency controlling of the dc current direction is
absent in the case of n=2. The found effect is that, both the dc current
suppression and the dc current reversals exist for some particular values of ac
field frequency. For typical semiconductor superlattices such an interesting
behavior of the dc current should be observable also in the THz domain.
Finally, we briefly review the history of the problem of the dc current
generation at mixing of harmonics in semiconductors and semiconductor
microstructures.Comment: 9 pages, 1 figure, RevTEX, EPS
Exotic structures on magnetic multilayers
To characterize the possible magnetic structures created on magnetic
multilayers a model has been formulated and studied. The interlayer
inhomogeneous structures found indicate either (i) a regular periodic, (ii) a
quasiperiodic change in the magnetization or (iii) spatially chaotic glass
states. The magnetic structures created depend mainly on the ratio of the
magnetic anisotropy constant to the exchange constant.
With the increase of this ratio the periodic structures first transform into
the quasiperiodic and then into the chaotic glass states. The same tendency
arises with the depolarization of the magnetic moments of the first layer
deposited on the substrate.Comment: 4 pages, 1 figur
Gravitational vortices and clump formation In Saturn's F ring during an encounter with Prometheus
Saturn rings are most beautiful and dynamic places in the solar system, consisting of ice particles in a constant battle between the gravitational forces of Saturn and its many moons. Fan, spiral, propellers, moonlets and streamer-channels observed by CASSINI in the F-ring have been attributed to encounters by Prometheus on the F ring, with investigations of optical thickness revealing large populations of transient moonlets. Taking into account gravitational interaction between particles and a multi-stranded F-ring structure we show that Prometheus' encounters create rotational flows, like atmospheric vortices and the self-gravity enhances the accelerated growth and size of moonlets. Vortex patches form caustics, which is a primary cause of the transient particle density clumps of 20 km width and 100 km length, and they are elongated to cover an area of 1600 km by 150 km, which may eventually combine into a vortex sheet
Chaos and rectification of electromagnetic wave in a lateral semiconductor superlattice
We find the conditions for a rectification of electromagnetic wave in a
lateral semiconductor superlattice with a high mobility of electrons. The
rectification is assisted by a transition to a dissipative chaos at a very high
mobility. We show that mechanism responsible for the rectification is a
creation of warm electrons in the superlattice miniband caused by an interplay
of the effects of nonlinearity and finite band width.Comment: 4 pages, 4 figures (2 color figs). Sufficient revision in comparison
with version1: More explanations on physics of the effect are added. Removed
from version1 material will be published elsewher
Two-Dimensional Ising Model with Competing Interactions as a Model for Interacting pi-Rings
We investigate the Ising model on a square lattice with antiferromagnetic exchange between nearest and next-nearest neighbors and show that at low temperatures stripe-like and droplet-like superstructures appear. We show that the competing interactions introduce a strong frustration that could plausibly describe the systems with dipole{dipole type interactions and pay particular attention to arrays of interacting Josephson ¼-rings