Quantum Information Seminar Series

Viv  Kendon Abstract:

Efficient generation of cluster states is crucial for engineering large-scale measurement-based quantum computers. Hybrid matter-optical systems offer a robust, scalable path to this goal. Such systems have an ancilla which acts as a bus connecting the qubits. We show that by generating the cluster in smaller sections of interlocking bricks, reusing one ancilla per brick, the cluster can be produced with maximal efficiency, requiring fewer than half the operations compared with no bus reuse. By reducing the time required to prepare sections of the cluster, bus reuse more than doubles the size of the computational workspace that can be used before decoherence effects dominate. A row of buses in parallel provides fully scalable cluster-state generation requiring only 20 controlled-PHASE gates per bus use. Similar efficiencies are obtained for other quantum computations, including a linear QFT, implying that qubus quantum computing is equivalent to the circuit model with unbounded fan-out.

Robert Scholten Abstract:

“Quantum technology” normally conjures thoughts of computation and communication, but also has exciting potential applications in new ways of measuring things at the nanometre, particularly for biological systems. The nitrogen-vacancy (NV) defect centre in diamond is an especially promising single spin system for quantum measurements in biology. Our first experiments have demonstrated optically detected magnetic resonance (ODMR) of individual fluorescent nanodiamond nitrogen-vacancy centres inside living human HeLa cells, and measured their spin levels and spin coherence times while tracking their location and orientation with nanoscale precision. We have measured quantum coherence through Rabi and spin-echo sequences, and orientation with 1° angular precision, over long (>10 h) periods. Variations in the decoherence rates are linked to changes in the local environment inside the cells, representing a new non-destructive imaging modality for intracellular biology.
While quantum-based imaging is an exciting prospect, even classical imaging is a vexing problem at the atomic scale, where there is enormous potential for example to determine the structure of bio-molecules. We have recently demonstrated a new source of high-coherence electron bunches based on photoionisation of laser-cooled atoms. With laser control of the cold atom cloud, we can shape the electron bunches, and because the electrons are so cold, they retain their shape during propagation. We have created “the world’s most expensive TV”, with a new bunch shape every frame – allowing the control needed to generate ultra-high-brightness bunches for coherent diffractive imaging at nanometer and femtosecond resolution.