Researchers usually strive to perfectly shield ions - charged atoms - in quantum computing experiments from the outside world. Any "noise" or interference, including heat generated by the experiment and measurements that cause fragile quantum states to collapse, can ruin data and prevent reliable logic operations, the conventional approach to quantum information processing.
Turning bug into feature, a collaboration of physicists from NIST and the University of Copenhagen in Denmark decided to think and work outside the box. They cleverly linked the experiment to the outside world to establish and maintain the entanglement of two ions. Entanglement is a curious feature of the quantum world that will be necessary to process and transport quantum data or correct errors in future quantum computers.
The new research is described in aNaturepaper posted on-line November 24, along with similar work at Yale University using superconducting circuits. The authors of the paper are Y. Lin, J.P. Gaebler, F. Reiter, T.R. Tan, R. Bowler, A.S. Sorensen, D. Leibfried and D.J. Wineland. The paper is titled "Dissipative production of a maximally entangled steady state".
"These new methods might be used to create entangled states that would be a resource in a traditional, logic-based quantum computer", NIST postdoctoral researcher John Gaebler stated. "But there are also alternative architectures in which, for example, one couples a quantum computer to a specific noise environment and the resulting state of the computer contains the solution to the target problem."
The NIST experiments used two beryllium ions as quantum bits (qubits) to store quantum information and two partner magnesium ions, which were cooled with three ultraviolet laser beams to release heat.
The qubits were entangled by two ultraviolet laser beams and induced to "leak" any unwanted quantum states to the environment through continuous application of microwaves and one laser beam. The unwanted data were coupled to the outgoing heat in such a way that the qubits were left in only the desired entangled state - which happens to be the point of lowest motional energy, where no further heat and information is released to the environment.
Unlike a logic operation, the process can be started from any state of the ions and still yield the same final state. The scheme also can tolerate some kinds of noise that might cause a traditional logic gate to fail. For instance, the lasers and microwaves had no negative effects on the target entangled state but reshuffled any unwanted states.
All operations applied at the same time quickly drove the two qubits into a specific entangled state and kept them in that state most of the time. The qubits approached the target state within a few milliseconds and were found to be in the correct entangled state 75 percent of the time. The qubit state deteriorated slightly over longer times as the qubits were disturbed by errant laser emissions. By applying about 30 repetitions of the four steps in a particular order, scientists boosted the success rate to 89 percent in a separate experiment.
The NIST process for using lost data to generate entanglement works like this: Two ultraviolet laser beams entangle the two ion qubits' internal "spins", analogous to tiny bar magnets pointing up or down. The lasers are carefully tuned to couple the ions' synchronized, back-and-forth sideways motion to their spins, entangling this motion with the spins.
The spins have three possible correlations: Both qubits spin up, both spin down, or one is up and one is down. The desired entangled state is a superposition of spins up-down and down-up at the same time. Superposition is another special feature of the quantum world. A measurement of this state with another special-purpose laser beam causes quantum states to collapse, resulting in spins up-down, or the opposite, spins down-up. Such measurements are made by detecting light signals; spin up scatters laser light, whereas spin down does not.
If the two spins are in the desired entangled state and the lowest motional energy state, they are unaffected by all laser and microwave fields. But microwaves and one ultraviolet laser beam reshuffle all other spin states and at the same time boost the qubits to an intermediate state with higher motional energy. This energy is then removed from the qubits by three cooling laser beams applied to the magnesium ions. This continuous feedback loop alters the qubits spins until they settle into the entangled state that is no longer affected by the driving fields.
Co-authors of the paper include two collaborators at QUANTOP, The Niels Bohr Institute, University of Copenhagen. The work was supported in part by the Intelligence Advanced Research Projects Activity, Office of Naval Research, and the European Union's Seventh Framework Programme.