5 research outputs found
Discrete Cosine Transforms on Quantum Computers
A classical computer does not allow to calculate a discrete cosine transform
on N points in less than linear time. This trivial lower bound is no longer
valid for a computer that takes advantage of quantum mechanical superposition,
entanglement, and interference principles. In fact, we show that it is possible
to realize the discrete cosine transforms and the discrete sine transforms of
size NxN and types I,II,III, and IV with as little as O(log^2 N) operations on
a quantum computer, whereas the known fast algorithms on a classical computer
need O(N log N) operations.Comment: 5 pages, LaTeX 2e, IEEE ISPA01, Pula, Croatia, 200
Engineering superpositions of displaced number states of a trapped ion
We present a protocol that permits the generation of a subtle with
superposition with 2^(l+1) displaced number states on a circle in phase space
as target state for the center-of-mass motion of a trapped ion. Through a
sequence of 'l' cycles involving the application of laser pulses and
no-fluorescence measurements, explicit expressions for the total duration of
laser pulses employed in the sequence and probability of getting the ion in the
upper electronic state during the 'l' cycles are obtained and analyzed in
detail. Furthermore, assuming that the effective relaxation process of a
trapped ion can be described in the framework of the standard master equation
for the damped harmonic oscillator, we investigate the degradation of the
quantum interference effects inherent to superpositions via Wigner function.Comment: 14 pages, 10 figure
Crosstalk Suppression for Fault-tolerant Quantum Error Correction with Trapped Ions
Physical qubits in experimental quantum information processors are inevitably
exposed to different sources of noise and imperfections, which lead to errors
that typically accumulate hindering our ability to perform long computations
reliably. Progress towards scalable and robust quantum computation relies on
exploiting quantum error correction (QEC) to actively battle these undesired
effects. In this work, we present a comprehensive study of crosstalk errors in
a quantum-computing architecture based on a single string of ions confined by a
radio-frequency trap, and manipulated by individually-addressed laser beams.
This type of errors affects spectator qubits that, ideally, should remain
unaltered during the application of single- and two-qubit quantum gates
addressed at a different set of active qubits. We microscopically model
crosstalk errors from first principles and present a detailed study showing the
importance of using a coherent vs incoherent error modelling and, moreover,
discuss strategies to actively suppress this crosstalk at the gate level.
Finally, we study the impact of residual crosstalk errors on the performance of
fault-tolerant QEC numerically, identifying the experimental target values that
need to be achieved in near-term trapped-ion experiments to reach the
break-even point for beneficial QEC with low-distance topological codes.Comment: 30 pages, 13 figures, 1 tabl
Synthesis and evaluation of fault-tolerant quantum computer architectures
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.Includes bibliographical references (p. 241-247).Fault-tolerance is the cornerstone of practical, large-scale quantum computing, pushed into its prominent position with heroic theoretical efforts. The fault-tolerance threshold, which is the component failure probability below which arbitrarily reliable quantum computation becomes possible, is one standard quality measure of fault-tolerant designs based on recursive simulation. However, there is a gulf between theoretical achievements and the physical reality and complexity of envisioned quantum computing systems. This thesis takes a step toward bridging that gap. We develop a new experimental method for estimating fault-tolerance thresholds that applies to realistic models of quantum computer architectures, and demonstrate this technique numerically. We clarify a central problem for experimental approaches to fault-tolerance evaluation--namely, distinguishing between potentially optimistic pseudo-thresholds and actual thresholds that determine scalability. Next, we create a system architecture model for the trapped-ion quantum computer, discuss potential layouts, and numerically estimate the fault-tolerance threshold for this system when it is constrained to a local layout. Finally, we place the problem of evaluation and synthesis of fault-tolerant quantum computers into a broader framework by considering a software architecture for quantum computer design.by Andrew W. Cross.S.M