Quantum Computing with Globally Controlled Exchange-type Interactions

If the interaction between qubits in a quantum computer has a non-diagonal form (e.g. the Heisenberg interaction), then one must be able to "switch it off" in order to prevent uncontrolled propagation of states. Therefore, such QC schemes typically demand local control of the interaction strength between each pair of neighboring qubits. Here we demonstrate that this degree of control is not necessary: it suffices to switch the interaction collectively - something that can in principle be achieved by global fields rather than with local manipulations. This observation may offer a significant simplification for various solid state, optical lattice and NMR implementations.Comment: 3 pages inc. 3 figure

Quantum Computing in Arrays Coupled by 'Always On' Interactions

It has recently been shown that one can perform quantum computation in a Heisenberg chain in which the interactions are 'always on', provided that one can abruptly tune the Zeeman energies of the individual (pseudo-)spins. Here we provide a more complete analysis of this scheme, including several generalizations. We generalize the interaction to an anisotropic form (incorporating the XY, or Forster, interaction as a limit), providing a proof that a chain coupled in this fashion tends to an effective Ising chain in the limit of far off-resonant spins. We derive the primitive two-qubit gate that results from exploiting abrupt Zeeman tuning with such an interaction. We also demonstrate, via numerical simulation, that the same basic scheme functions in the case of smoothly shifted Zeeman energies. We conclude with some remarks regarding generalisations to two- and three-dimensional arrays.Comment: 16 pages (preprint format) inc. 3 figure

Efficient Graph State Construction Under the Barrett and Kok Scheme

Recently Barrett and Kok (BK) proposed an elegant method for entangling separated matter qubits. They outlined a strategy for using their entangling operation (EO) to build graph states, the resource for one-way quantum computing. However by viewing their EO as a graph fusion event, one perceives that each successful event introduces an ideal redundant graph edge, which growth strategies should exploit. For example, if each EO succeeds with probability p=0.4 then a highly connected graph can be formed with an overhead of only about ten EO attempts per graph edge. The BK scheme then becomes competitive with the more elaborate entanglement procedures designed to permit p to approach unity.Comment: 3 pages, 3 figures. Small refinement

\u3ci\u3eZiglar v. Abbasi\u3c/i\u3e and the Decline of the Right to Redress

Part I briefly describes the facts of Ziglar, its journey through the federal courts, and the Court’s treatment of it. Part II offers a commentary on Justice Kennedy’s opinion in Ziglar, focusing especially on his analysis of the reasons for and against recognizing a Bivens action and his choice to disposeof the case through a Bivens framework. I argue that his reasoning in Ziglar reflects an untenably narrow conception of the place of private rights of action in our legal system. In this respect, Part III suggests that the atrophy of Bivens in the Supreme Court exemplifies a wide range of changes in the Court’s outlook on many aspects of litigation. The Court’s decisions on standing, class actions, punitive damages, federal preemption, pleading, summary judgment, and immunities have all been deeply affected by a failure to take the basis of private rights of action seriously. This skewed mindset largely came into place in the Rehnquist era and has thrived in the Roberts Court. Part IV suggests that some aspects of this hostility to private rights of action have been absorbed by the bench and bar as a kind of centrist, pragmatic wisdom about what our court system can tolerate