Ultrafast Gates in Atomic Qubits

"Ultrafast Gates for Single Atomic Qubits," W. C. Campbell, J. Mizrahi, Q. Quraishi, C. Senko, D. Hayes, D. Hucul, D. N. Matsukevich, P. Maunz, and C. Monroe, Phys. Rev. Lett. 105, 090502 (2010).

An ion has internal energy levels that can serve as a qubit. The qubit begins in the down state ([0]). By absorbing and emitting photons from a single pulse (shown below) the qubit can be flipped to the up ([1]) state.

The pulse can be optically shaped to make any combination or superposition of the two qubit states: [1] and [0].

12.6 GHz

PFC experimentalists in the Trapped Ion Quantum Information group have performed a gate that flips the state of a single atomic qubit in less than 50 picoseconds. The time to perform this same operation with continuous wave (CW) lasers, a standard for these types of atomic systems, is typically over 10,000 times slower.

In conventional computers a bit can be in the state 0 or 1, but not both simultaneously. By contrast, a quantum bit, or qubit, can reside in a combination of the two states. Qubits can made from any quantum system having two energy levels. In this experiment, the qubit is a laser cooled, singly ionized Ytterbium atom having two ground state electronic levels labeled [1] and [0], or up and down. The state of this system can be controllably manipulated with lasers or microwave radiation.

The researchers drive qubit rotations using a highly energetic single pulse or by optically dividing the pulse into two counterpropagating pulses. The gate, whether performed with CW or pulsed lasers, is a process that requires two photons. Here, the key technology is an ultraviolet laser that emits a pulse of light that is 10 picosecond long every 8 nanoseconds. Within each individual pulse there are photons that have the frequency separation required (12.6 GHz) to coherently manipulate the qubit.