1. Physics Department, Lancaster University, Lancaster LA1 4YB, United Kingdom
2. Amethyst Research Ltd., Kelvin Campus, West of Scotland Science Park, Glasgow G20 0SP, United Kingdom
3. Amethyst Research, Inc., 123 Case Circle, Ardmore, Oklahoma 73401, USA
4. Defence Science and Technology Laboratory, Porton Down, Salisbury, Wiltshire SP4 0JQ, United Kingdom