arXiv subjects

Raymond Laflamme

Publications and source records attributed to Raymond Laflamme.

118 records · Page 7Linked to original sources

arXiv1996

A Theory of Quantum Error-Correcting Codes

Quantum Error Correction will be necessary for preserving coherent states against noise and other unwanted interactions in quantum computation and communication. We develop a general theory of quantum error correction based on encoding states into larger Hilbert spaces subject to known interactions. We obtain necessary and sufficient conditions for the perfect recovery of an encoded state after its degradation by an interaction. The conditions depend only on the behavior of the logical states. We use them to give a recovery operator independent definition of error-correcting codes. We relate this definition to four others: The existence of a left inverse of the interaction, an explicit representation of the error syndrome using tensor products, perfect recovery of the completely entangled state, and an information theoretic identity. Two notions of fidelity and error for imperfect recovery are introduced, one for pure and the other for entangled states. The latter is more appropriate when using codes in a quantum memory or in applications of quantum teleportation to communication. We show that the error for entangled states is bounded linearly by the error for pure states. A formal definition of independent interactions for qubits is given. This leads to lower bounds on the number of qubits required to correct $e$ errors and a formal proof that the classical bounds on the probability of error of $e$-error-correcting codes applies to $e$-error-correcting quantum codes, provided that the interaction is dominated by an identity component.

quant-ph↗

arXiv1996

Decoherence Bounds on Quantum Computation with Trapped Ions

Using simple physical arguments we investigate the capabilities of a quantum computer based on cold trapped ions. From the limitations imposed on such a device by spontaneous decay, laser phase coherence, ion heating and other sources of error, we derive a bound between the number of laser interactions and the number of ions that may be used. The largest number which may be factored using a variety of species of ion is determined.

quant-ph↗

arXiv1996

Effects of Loss and Decoherence on a Simple Quantum Computer

We investigate the impact of loss (amplitude damping) and decoherence (phase damping) on the performance of a simple quantum computer which solves the one-bit Deutsch problem. The components of this machine are beamsplitters and nonlinear optical Kerr cells, but errors primarily originate from the latter. We develop models to describe the effect of these errors on a quantum optical Fredkin gate. The results are used to analyze possible error correction strategies in a complete quantum computer. We find that errors due to loss can be avoided perfectly by appropriate design techniques, while decoherence can be partially dealt with using projective error correction.

quant-ph↗

arXiv1996

Perfect Quantum Error Correction Code

We present a quantum error correction code which protects a qubit of information against general one qubit errors which maybe caused by the interaction with the environment. To accomplish this, we encode the original state by distributing quantum information over five qubits, the minimal number required for this task. We give a simple circuit which takes the initial state with four extra qubits in the state |0> to the encoded state. The circuit can be converted into a decoding one by simply running it backward. Reading the extra four qubits at the decoder's output we learn which one of the sixteen alternatives (no error plus all fifteen possible 1-bit errors) was realized. The original state of the encoded qubit can then be restored by a simple unitary transformation.

quant-ph↗

arXiv1995

Quantum Computers, Factoring, and Decoherence

In a quantum computer any superposition of inputs evolves unitarily into the corresponding superposition of outputs. It has been recently demonstrated that such computers can dramatically speed up the task of finding factors of large numbers -- a problem of great practical significance because of its cryptographic applications. Instead of the nearly exponential ($\sim \exp L^{1/3}$, for a number with $L$ digits) time required by the fastest classical algorithm, the quantum algorithm gives factors in a time polynomial in $L$ ($\sim L^2$). This enormous speed-up is possible in principle because quantum computation can simultaneously follow all of the paths corresponding to the distinct classical inputs, obtaining the solution as a result of coherent quantum interference between the alternatives. Hence, a quantum computer is sophisticated interference device, and it is essential for its quantum state to remain coherent in the course of the operation. In this report we investigate the effect of decoherence on the quantum factorization algorithm and establish an upper bound on a ``quantum factorizable'' $L$ based on the decoherence suffered per operational step.

quant-ph↗

arXiv1994

Evidence for the Stability of Extremal Black p-Branes

We investigate the stability of the extremal black p-brane which contains a n-form and a dilaton. We show that the instability due to the s-mode, which was present in the uncharged and non-extremal p-brane, disappears in the extreme case. This is shown to be consistent with an entropy argument which shows that the zero entropy of the extremal black hole is approached more rapidly than the zero entropy of the black p-brane, which would mean an instability would violate the second law of thermodynamics.

hep-th↗

arXiv1994

The Least Action Method, CDM and $Ω$

Peebles has suggested an interesting method to trace back in time positions of galaxies called the least action method. This method applied on the Local Group galaxies seems to indicate that we live in an $Ω\approx 0.1$ Universe. We have studied a CDM N-body simulation with $Ω=0.2$ and $H=50 kms^{-1}/Mpc$ and compare trajectories traced back from the Least Action Principle and the center of mass of the particle forming CDM halos. We show that the agreement between these set of trajectories is at best qualitative. We also show that the line of sight peculiar velocities are underestimated. This discrepancy is due to orphans, CDM particles which do not end up in halos. By varying the density parameter $Ω$ in the least action principle we show that using this method we would underestimate the density of the Universe by a factor of 4-5.

astro-ph↗

arXiv1994

Conservation Laws in the Quantum Mechanics of Closed Systems

We investigate conservation laws in the quantum mechanics of closed systems. We review an argument showing that exact decoherence implies the exact conservation of quantities that commute with the Hamiltonian including the total energy and total electric charge. However, we also show that decoherence severely limits the alternatives which can be included in sets of histories which assess the conservation of these quantities when they are not coupled to a long-range field arising from a fundamental symmetry principle. We then examine the realistic cases of electric charge coupled to the electromagnetic field and mass coupled to spacetime curvature and show that when alternative values of charge and mass decohere, they always decohere exactly and are exactly conserved as a consequence of their couplings to long-range fields. Further, while decohering histories that describe fluctuations in total charge and mass are also subject to the limitations mentioned above, we show that these do not, in fact, restrict {\it physical} alternatives and are therefore not really limitations at all.

gr-qc↗

arXiv1994

The Instability of Charged Black Strings and p-Branes

We investigate the evolution of small perturbations around charged black strings and branes which are solutions of low energy string theory. We give the details of the analysis for the uncharged case which was summarized in a previous paper. We extend the analysis to the small charge case and give also an analysis for the generic case, following the behavior of unstable modes as the charge is modified. We study specifically a magnetically charged black 6-brane, but show how the instability is generic, and that charge does not in general stabilise black strings and p-branes.

hep-th↗

arXiv1993

Time Symmetric Quantum Cosmology and Our Universe

We investigate the time neutral formulation of quantum cosmology of Gell-Mann and Hartle. In particular we study the proposal discussed by them that our Universe corresponds to the time symmetric decoherence functional with initial and final density matrix of low entropy. We show that our Universe does not correspond to this proposal by investigating the behaviour of small inhomogeneous perturbations around a Friedman-Robertson-Walker model. These perturbations cannot be time symmetric if they were small at the Big Bang.

gr-qc↗