In a lab on the western edge of Paris, where the River Seine flows wide and trams slide past glass-fronted buildings and blossoming cherry trees, a technician called Rémi makes some adjustments with a spanner. The machine, a cascade of gold and silver-coloured cylinders descending through a cloud of wires, is a cryostat, a device that cools so much it slows activity even at the molecular level. Minus 273 degrees Celsius in the cylinder at the bottom. A temperature at which even the tiniest