609 research outputs found
Stimulated Raman adiabatic passage-like protocols for amplitude transfer generalize to many bipartite graphs
Adiabatic passage techniques, used to drive a system from one quantum state
into another, find widespread application in physics and chemistry. We focus on
techniques to spatially transport a quantum amplitude over a strongly coupled
system, such as STImulated Raman Adiabatic Passage (STIRAP) and Coherent
Tunnelling by Adiabatic Passage (CTAP). Previous results were shown to work on
certain graphs, such as linear chains, square and triangular lattices, and
branched chains. We prove that similar protocols work much more generally, in a
large class of (semi-)bipartite graphs. In particular, under random couplings,
adiabatic transfer is possible on graphs that admit a perfect matching both
when the sender is removed and when the receiver is removed. Many of the
favorable stability properties of STIRAP/CTAP are inherited, and our results
readily apply to transfer between multiple potential senders and receivers. We
numerically test transfer between the leaves of a tree, and find surprisingly
accurate transfer, especially when straddling is used. Our results may find
applications in short-distance communication between multiple quantum
computers, and open up a new question in graph theory about the spectral gap
around the value 0.Comment: 17 pages, 3 figures. v2 is made more mathematical and precise than v
Magnetoresistive transducer for absolute position detection
In this paper a new method is presented for the measurement of absolute linear or angular position. The digital position information is recorded serially into one track of a suitable hard-magnetic medium. The stray field of this information layer determines the angular magnetisation distribution in a ferromagnetic (permalloy) detection strip which is positioned parallel to the track but with its plane perpendicular to the hard-magnetic layer. The bit pattern representing the position co-ordinate is regained by detection of the planar magnetoresistance effect in the sensor strip. Experiments have been performed using sensors with a resolution of 250 Âżm and 1 mm respectively and longitudinally recorded audio tape. Suitable sensor output signals could be measured without hysteresis
Investigation of the structure of recording head fields
Head fields of two recording heads (gap lengths 2.1 and 2.8 micrometer) have been measured with the help of a series of magnetoresistive transducers. The widths of these transducers are 1.9, 4.1 and 7.0 micrometer and they have been positioned in the gap field region very accurately using optical methods. The transducer outputs have been compared with computer simulated results. A systematic deviation between theoretical and experimental results may lead to the assumption that the surface of the recording head is magnetically inactive over a few tenths of a micrometer
Efficient forward propagation of time-sequences in convolutional neural networks using Deep Shifting
When a Convolutional Neural Network is used for on-the-fly evaluation of
continuously updating time-sequences, many redundant convolution operations are
performed. We propose the method of Deep Shifting, which remembers previously
calculated results of convolution operations in order to minimize the number of
calculations. The reduction in complexity is at least a constant and in the
best case quadratic. We demonstrate that this method does indeed save
significant computation time in a practical implementation, especially when the
networks receives a large number of time-frames
Measurement system for two-dimensional magnetic field distributions, applied to the investigation of recording head fields
The system described is built around a very accurate positioner into which a sensitive transducer and the object of analysis is mounted. The properties of the applied magnetoresistive transducer are described. This transducer, a very narrow permalloy strip placed at the edge of a glass substratum, can be used to measure both components of the field distribution. The analysis of the measured results can be accomplished with the help of a computer simulation of the transducer response curves. The performance of the system is demonstrated by measurements on a number of ferrite heads and conclusions about the so called 'dead layer'-structures on these heads are given
Many-body strategies for multi-qubit gates - quantum control through Krawtchouk chain dynamics
We propose a strategy for engineering multi-qubit quantum gates. As a first
step, it employs an eigengate to map states in the computational basis to
eigenstates of a suitable many-body Hamiltonian. The second step employs
resonant driving to enforce a transition between a single pair of eigenstates,
leaving all others unchanged. The procedure is completed by mapping back to the
computational basis. We demonstrate the strategy for the case of a linear array
with an even number N of qubits, with specific XX+YY couplings between nearest
neighbors. For this so-called Krawtchouk chain, a 2-body driving term leads to
the iSWAP gate, which we numerically test for N = 4 and 6.Comment: 10 pages, 3 figure
Two-dimensional coding for probe recording on magnetic patterned media
The effect of intertrack intersymbol interference in a magnetic patterned medium is studied. A two-dimensional (2-D) channel code is proposed, dedicated to perpendicularly magnetized media without soft underlayer, which exhibit read pulses showing overshoot. Read pulse shapes were investigated using a magnetic-force microscope tip scanning the patterned medium row-by-row. To test different codes, a bit-detection simulation program was developed to generate large amounts of data on which bit error rates can be measured. Application of the 2-D channel code, which implies recording of particular dot positions with fixed bits ("ones", "zeros"), resulted in the elimination of 2-D worst-case bit patterns and a subsequent reduction of detected-bit errors. The accompanying redundancy of 22% is inevitable for this type of 2-D code
Stimulated Raman adiabatic passage-like protocols for amplitude transfer generalize to many bipartite graphs
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