6,367 research outputs found
Use of composite rotations to correct systematic errors in NMR quantum computation
We implement an ensemble quantum counting algorithm on three NMR
spectrometers with 1H resonance frequencies of 500, 600 and 750 MHz. At higher
frequencies, the results deviate markedly from naive theoretical predictions.
These systematic errors can be attributed almost entirely to off-resonance
effects, which can be substantially corrected for using fully-compensating
composite rotation pulse sequences originally developed by Tycko. We also
derive an analytic expression for generating such sequences with arbitrary
rotation angles.Comment: 8 pages RevTex including 7 PostScript figures (18 subfigures
Robust Logic Gates and Realistic Quantum Computation
The composite rotation approach has been used to develop a range of robust
quantum logic gates, including single qubit gates and two qubit gates, which
are resistant to systematic errors in their implementation. Single qubit gates
based on the BB1 family of composite rotations have been experimentally
demonstrated in a variety of systems, but little study has been made of their
application in extended computations, and there has been no experimental study
of the corresponding robust two qubit gates to date. Here we describe an
application of robust gates to Nuclear Magnetic Resonance (NMR) studies of
approximate quantum counting. We find that the BB1 family of robust gates is
indeed useful, but that the related NB1, PB1, B4 and P4 families of tailored
logic gates are less useful than initially expected.Comment: 6 pages RevTex4 including 5 figures (3 low quality to save space).
Revised at request of referee and incorporting minor corrections and updates.
Now in press at Phys Rev
Experimental investigation of non-uniform heating on flow boiling instabilities in a microchannels based heat sink
Two-phase flow boiling in microchannels is one of the most promising cooling technologies able to cope with high heat fluxes generated by the next generation of central processor units (CPU). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as a cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated, with hotspots located in different positions along the heat sink. It was observed that boiling inside microchannels with non-uniform heating led to high temperature non-uniformity in transverse direction
Ohio's Nursing Homes and Residential Care Facilities in the Economic Downturn: How have they adjusted?
Long-term care facilities have been impacted by the economic downturn and other changes in the long-term care system. This research brief describes strategies most often implemented by Ohio's nursing homes and residential care facilities to enhance economic prospects
Equivalent qubit dynamics under classical and quantum noise
We study the dynamics of quantum systems under classical and quantum noise,
focusing on decoherence in qubit systems. Classical noise is described by a
random process leading to a stochastic temporal evolution of a closed quantum
system, whereas quantum noise originates from the coupling of the microscopic
quantum system to its macroscopic environment. We derive deterministic master
equations describing the average evolution of the quantum system under
classical continuous-time Markovian noise and two sets of master equations
under quantum noise. Strikingly, these three equations of motion are shown to
be equivalent in the case of classical random telegraph noise and proper
quantum environments. Hence fully quantum-mechanical models within the Born
approximation can be mapped to a quantum system under classical noise.
Furthermore, we apply the derived equations together with pulse optimization
techniques to achieve high-fidelity one-qubit operations under random telegraph
noise, and hence fight decoherence in these systems of great practical
interest.Comment: 5 pages, 2 figures; converted to PRA format, added Fig. 2, corrected
typo
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Experimental pool boiling investigations of vertical coalescence for FC-72 on silicon from an isolated artificial cavity
In this study bubble growth from an isolated artificial cavity micro-fabricated on a horizontal 380 µm thick silicon wafer was investigated. The horizontally oriented boiling surface was heated by a thin resistance heater integrated on the rear of the silicon test section. The temperature was measured using an integrated micro-sensor situated on the boiling surface with the artificial cavity located in its geometrical centre. A resistive track was used as the sensor, which when calibrated, exhibited a near-linear behaviour with increasing temperature. To conduct pool boiling experiments the test section was immersed in degassed fluorinert FC-72. Bubble nucleation, growth and detachment at different pressures were observed using high-speed imaging. Coalescence was observed at the boundary between the isolated bubble and interference regimes. The occurrence of vertical coalescence was found to be more frequent, with increasing wall superheat and decreasing pressure.
The equivalent sphere volumes of two bubbles before and after coalescence were evaluated from area measurements. It was observed that the second nucleated bubble is always smaller than its predecessor. The vapour generation appears not to stop during coalescence as the volume of the merged bubble was typically 5-18% larger than the sum of the bubble volumes just before coalescence
Tackling Systematic Errors in Quantum Logic Gates with Composite Rotations
We describe the use of composite rotations to combat systematic errors in
single qubit quantum logic gates and discuss three families of composite
rotations which can be used to correct off-resonance and pulse length errors.
Although developed and described within the context of NMR quantum computing
these sequences should be applicable to any implementation of quantum
computation.Comment: 6 pages RevTex4 including 4 figures. Will submit to Phys. Rev.
A precise CNOT gate in the presence of large fabrication induced variations of the exchange interaction strength
We demonstrate how using two-qubit composite rotations a high fidelity
controlled-NOT (CNOT) gate can be constructed, even when the strength of the
interaction between qubits is not accurately known. We focus on the exchange
interaction oscillation in silicon based solid-state architectures with a
Heisenberg Hamiltonian. This method easily applies to a general two-qubit
Hamiltonian. We show how the robust CNOT gate can achieve a very high fidelity
when a single application of the composite rotations is combined with a modest
level of Hamiltonian characterisation. Operating the robust CNOT gate in a
suitably characterised system means concatenation of the composite pulse is
unnecessary, hence reducing operation time, and ensuring the gate operates
below the threshold required for fault-tolerant quantum computation.Comment: 9 pages, 8 figure
High fidelity one-qubit operations under random telegraph noise
We address the problem of implementing high fidelity one-qubit operations
subject to time dependent noise in the qubit energy splitting. We show with
explicit numerical results that high fidelity bit flips and one-qubit NOT gates
may be generated by imposing bounded control fields. For noise correlation
times shorter than the time for a pi-pulse, the time optimal pi-pulse yields
the highest fidelity. For very long correlation times, fidelity loss is
approximately due to systematic error, which is efficiently tackled by
compensation for off-resonance with a pulse sequence (CORPSE). For intermediate
ranges of the noise correlation time we find that short CORPSE, which is less
accurate than CORPSE in correcting systematic errors, yields higher fidelities.
Numerical optimization of the pulse sequences using gradient ascent pulse
engineering results in noticeable improvement of the fidelities for the bit
flip and marginal improvement for the NOT gate.Comment: 7 pages, 6 figure
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