Simon C. Benjamin

Cellular automata models of traffic flow along a highway containing a junction

We examine various realistic generalizations of the basic cellular automaton model describing traffic flow along a highway. In particular, we introduce a slow-to-start rule which simulates a possible delay before a car pulls away from being stationary. Having discussed the case of a bare highway, we then consider the presence of a junction. We study the effects of acceleration, disorder, and slow-to-start behaviour on the queue length at the entrance to the highway. Interestingly, the junctions efficiency is improved by introducing disorder along the highway, and by imposing a speed limit.

Comment on "Quantum Games and Quantum Strategies''

A Comment on the Letter by Jens Eisert, Martin Wilkens, and Maciej Lewenstein, Phys.Rev.Lett. 83, 3077 (1999). The authors of the Letter offer a Reply.

Magnetic Field Sensing Beyond the Standard Quantum Limit Using 10-Spin NOON States

Quantum-Enhanced Measurement The single electron spin in a molecule, atom, or quantum dot precesses in a magnetic field and so can be used as a magnetic field sensor. As the number of spins in a sensor increases, so too does the sensitivity. Quantum mechanical entanglement of the spin ensemble should then allow the sensitivity to increase much more than would be expected from a simple increase in the number of individual spins in the ensemble. Using the highly symmetric molecule, trimethyl phosphite, a molecule containing a central P atom surrounded by nine hydrogen atoms, Jones et al. (p. 1166, published online 23 April) quantum mechanically entangled the 10 spins (or qubits) to generate a nearly 10-fold enhancement in the magnetic field sensitivity. The results pave the way for the further development of quantum sensors. Quantum mechanical entanglement of nuclei in a single molecule results in an enhancement of the magnetic field sensitivity. Quantum entangled states can be very delicate and easily perturbed by their external environment. This sensitivity can be harnessed in measurement technology to create a quantum sensor with a capability of outperforming conventional devices at a fundamental level. We compared the magnetic field sensitivity of a classical (unentangled) system with that of a 10-qubit entangled state, realized by nuclei in a highly symmetric molecule. We observed a 9.4-fold quantum enhancement in the sensitivity to an applied field for the entangled system and show that this spin-based approach can scale favorably as compared with approaches in which qubit loss is prevalent. This result demonstrates a method for practical quantum field sensing technology.

Coherence of spin qubits in silicon

Given the effectiveness of semiconductor devices for classical computation one is naturally led to consider semiconductor systems for solid state quantum information processing. Semiconductors are particularly suitable where local control of electric fields and charge transport are required. Conventional semiconductor electronics is built upon these capabilities and has demonstrated scaling t ol argecomplicated arrays of interconnected devices. However, the requirements for a quantum computer are very different from those for classical computation, and it is not immediately obvious how best to build one in a semiconductor. One possible approach is to use spins as qubits: of nuclei, of electrons, or both in combination. Long qubit coherence times are a prerequisite for quantum computing, and in this paper we will discuss measurements of spin coherence in silicon. The results are encouraging—both electrons bound to donors and the donor nuclei exhibit low decoherence under the right circumstances. Doped silicon thus appears to pass the first test on the road to a quantum computer. (Some figures in this article are in colour only in the electronic version)

Topological quantum computing with a very noisy network and local error rates approaching one percent

A scalable quantum computer could be built by networking together many simple processor cells, thus avoiding the need to create a single complex structure. The difficulty is that realistic quantum links are very error prone. A solution is for cells to repeatedly communicate with each other and so purify any imperfections; however prior studies suggest that the cells themselves must then have prohibitively low internal error rates. Here we describe a method by which even error-prone cells can perform purification: groups of cells generate shared resource states, which then enable stabilization of topologically encoded data. Given a realistically noisy network (≥10% error rate) we find that our protocol can succeed provided that intra-cell error rates for initialisation, state manipulation and measurement are below 0.82%. This level of fidelity is already achievable in several laboratory systems.

Prospects for measurement-based quantum computing with solid state spins

This article aims to review the developments, both theoretical and experimental, that have in the past decade laid the ground for a new approach to solid state quantum com- puting. Measurement-based quantum computing (MBQC) re- quires neither direct interaction between qubits nor even what would be considered controlled generation of entanglement. Rather it can be achieved using entanglement that is generated probabilistically by the collapse of quantum states upon mea- surement. Single electronic spins in solids make suitable qubits for such an approach, offering long coherence times and well defined routes to optical measurement. We will review the the- oretical basis of MBQC and experimental data for two fron- trunner candidate qubits - nitrogen-vacancy (NV) centres in diamond and semiconductor quantum dots - and discuss the prospects and challenges that lie ahead in realising MBQC in the solid state. fast switched optical multiplexer

Freely Scalable Quantum Technologies using Cells of 5-to-50 Qubits with Very Lossy and Noisy Photonic Links

Exquisite quantum control has now been achieved in small ion traps, in nitrogen-vacancy centres and in superconducting qubit clusters. We can regard such a system as a universal cell with diverse technological uses from communication to large-scale computing, provided that the cell is able to network with others and overcome any noise in the interlinks. Here we show that loss-tolerant entanglement purification makes quantum computing feasible with the noisy and lossy links that are realistic today: With a modestly complex cell design, and using a surface code protocol with a network noise threshold of 13.3%, we find that interlinks which attempt entanglement at a rate of 2MHz but suffer 98% photon loss can result in kilohertz computer clock speeds (i.e. rate of high fidelity stabilizer measurements). Improved links would dramatically increase the clock speed. Our simulations employed local gates of a fidelity already achieved in ion trap devices.

A possible nanometer-scale computing device based on an adding cellular automaton

We present a simple one-dimensional cellular automaton (CA) which has the property that an initial state composed of two binary numbers evolves quickly into a final state which is their sum. We call this CA the adding cellular automaton (ACA). The ACA requires only 2N two-state cells in order to add any two N−1 bit binary numbers. The ACA could be directly realized as a wireless nanometer-scale computing device. A possible implementation using coupled quantum dots is outlined.

Nanoscale solid-state quantum computing

Most experts agree that it is too early to say how quantum computers will eventually be built, and several nanoscale solid–state schemes are being implemented in a range of materials. Nanofabricated quantum dots can be made in designer configurations, with established technology for controlling interactions and for reading out results. Epitaxial quantum dots can be grown in vertical arrays in semiconductors, and ultrafast optical techniques are available for controlling and measuring their excitations. Single–walled carbon nanotubes can be used for molecular self–assembly of endohedral fullerenes, which can embody quantum information in the electron spin. The challenges of individual addressing in such tiny structures could rapidly become intractable with increasing numbers of qubits, but these schemes are amenable to global addressing methods for computation.

A New Type of Radical-Pair-Based Model for Magnetoreception

Certain migratory birds can sense the Earths magnetic field. The nature of this process is not yet properly understood. Here we offer a simple explanation according to which birds literally see the local magnetic field through the impact of a physical rather than a chemical signature of the radical pair: a transient, long-lived electric dipole moment. Based on this premise, our picture can explain recent surprising experimental data indicating long lifetimes for the radical pair. Moreover, there is a clear evolutionary path toward this field-sensing mechanism: it is an enhancement of a weak effect that may be present in many species.