We experimentally investigate surface-plasmon assisted photoemission to enhance the efficiency of metallic photocathodes for high-brightness electron sources. A nanohole array-based copper surface was designed to exhibit a plasmonic response at 800 nm, fabricated using the focused ion beam milling technique, optically characterized and tested as a photocathode in a high power radio frequency photoinjector. Because of the larger absorption and localization of the optical field intensity, the charge yield observed under ultrashort laser pulse illumination is increased by more than 100 times compared to a flat surface. We also present the first beam characterization results (intrinsic emittance and bunch length) from a nanostructured photocathode.

In this Letter, we report on the efficient generation of electrons from metals using multiphoton photoemission by use of nanostructured plasmonic surfaces to trap, localize, and enhance optical fields. The plasmonic surface increases absorption over normal metals by more than an order of magnitude, and due to the localization of fields, this results in over 6 orders of magnitude increase in effective nonlinear quantum yield. We demonstrate that the achieved quantum yield is high enough for use in rf photoinjectors operating as electron sources for MHz repetition rate x-ray free electron lasers.

We present a scheme for universal quantum computing using XY Heisenberg spin chains. Information is encoded into packets propagating down these chains, and they interact with each other to perform universal quantum computation. A circuit using g gate blocks on m qubits can be encoded into chains of lengthfor all d > 0 with vanishingly small error.

Nanocavities fabricated in a metallic surface have important and technologically useful properties of complete light absorption and strong field enhancement. Here, we demonstrate how a nanometerthick alumina deposition inside such a cavity can be used to gain an exquisite control over the resonance wavelength. This process allows achieving a precise control over the spectral response and is completely reversible allowing many tuning attempts to be made on a single structure until the optimum performance is achieved.

We investigate graphs that can be disconnected into small components by removing a vanishingly small fraction of their vertices. We show that, when a controllable quantum network is described by such a graph and the gaps in eigenfrequencies and in transition frequencies are bounded exponentially in the number of vertices, the network is efficiently controllable, in the sense that universal quantum computation can be performed using a control sequence polynomial in the size of the network while controlling a vanishingly small fraction of subsystems. We show that networks corresponding to finite-dimensional lattices are efficiently controllable and explore generalizations to percolation clusters and random graphs. We show that the classical computational complexity of estimating the ground state of Hamiltonians described by controllable graphs is polynomial in the number of subsystems or qubits. DOI: 10.1103/PhysRevLett.118.260501 Controlling large quantum networks and performing universal quantum computation are two important and related problems in quantum information processing. A common goal is to perform control and computation efficiently, by accessing a minimum number of directly controlled parts. Quantum networks were introduced in Ref. [1]. In Ref.[2], it was shown that almost any quantum network with a single probe is controllable. If controls can be applied to quantum degrees of freedom in a pairwise fashion, then the control is computationally universal [3]. The connectivity of the graph of interactions plays an important role in controllability and computation [4]. Under mild assumptions about network topology and the algebra of controls, Ref.[5] gave sufficient conditions for a network to be controlled using a small number of control qubits, without regard for the efficiency of the control sequence. See [6] for a similar study of classical linear systems. In suitable systems, quantum computation is possible with only a few control qubits [7,8]. As suggested in these papers, spin chains with specific Hamiltonians can give controllability as well as the ability to enact efficient universal quantum computation on the chain. These results raise the question of when it is possible to perform universal quantum control and quantum computation efficiently on a general quantum network. This Letter shows in a general setting that it is possible to perform universal quantum control and computation in time polynomial in network size on a wide variety of controllable quantum networks with Hamiltonians whose gaps in eigenfrequencies and in transition frequencies are bounded exponentially in the number of vertices while acting on only a vanishingly small fraction of their nodes. This naturally leads one to define an interesting class of graphs, which we call efficiently controllable graphs, which admit efficient control by acting on a vanishingly small fraction of controlled nodes. The existence, construction, and analysis of this new class of graphs pose an intriguing problem in the graph theory. In this work, ...