Entangling Atoms with Optical Frequency Combs For the first time, scientists have employed a powerful technique of laser physics – the “optical frequency comb” – to entangle two trapped atoms. This form of control is a promising candidate for use as a logic gate for quantum computing and information-processing, and offers substantial operational advantages over other methods of laser-generated entanglement. The team began by preparing two ytterbium ions, spaced about five micrometers apart in an electrical trap, in identical minimal-energy ground states. The goal of the experiment was to entangle these two ions – that is, to place them in a condition in which the quantum state of one is inextricably connected to the state of the other – using light from a single high-speed pulsed laser. Each pulse consists of multiple frequencies, which over time build up into a pattern of sharply defined frequency spikes that are uniformly spaced like the teeth in a comb. (See red and blue curves at right.) If the energy difference between any two comb frequencies corresponds exactly with an atomic quantum transition, it will produce it. But many desired effects – including those sought by the PFC team – do not precisely match any spacing in the teeth of a single comb. So the scientists split the main pulsed laser beam into separate beams and applied slight frequency offsets to them with devices called acousto-optic modulators (AOMs). By tuning the system with the AOMs, the team can create any frequency difference they want to produce both the target effects: generating qubit states in the ions, and entangling the ions by momentum kicks. Lab setup: The dots (QD1 and QD2) are excited and then their output photons are routed simultaneously to a beamsplitter in which coalescence can be observed. “Entanglement of Atomic Qubits Using an Optical Frequency Comb,” D. Hayes et al, Phys. Rev. Lett. 104, 140501 (2010).