April 5, 2011 | 6
Quantum information science is a bit like classroom management—the larger the group, the harder it is to keep everything together.
But to build a practical quantum computer physicists will need many particles working in synchrony as quantum bits, or quibits. Each qubit can be a 0 and a 1 simultaneously, vaulting the number-crunching power of a hypothetical quantum computer well past that of ordinary computers. With each qubit in a superposition, a quantum computer can manipulate an exponentially large quantity of numbers at once—2^n numbers for a system of n qubits. So each step toward generating large sets of qubits pushes practical quantum computing closer to reality.
Thomas Monz of the Institute for Experimental Physics at the University of Innsbruck in Austria and his colleagues mark just such a step forward in the April 1 issue of Physical Review Letters. Monz and his co-authors report creating entangled states with a record 14 qubits. Other researchers had previously demonstrated entangled states with 10 qubits.
Entanglement is a key quantum phenomenon by which particles share correlated properties, even though they are spatially separated. (A common analogy for entangled particles is a pair of dice that always land on matching numbers; if one die comes up 5, for instance, the other will, too.) But entangled particles are a bit like rambunctious children—keeping them on-task is difficult work, and the situation only gets harder to control as more particles are added.
Monz and his colleagues encoded information onto a string of trapped calcium atoms, using two energy levels of the atom to represent 0 and 1. Using lasers to manipulate the atomic qubits, the group set the entire ensemble into superpositions of 0 and 1—in other words, each qubit is in a sense both a 0 and a 1 until it is measured, at which point it is forced to settle on being one or the other. With a set of qubits entangled in an certain way, measuring one qubit forces the rest of the set to follow suit, resulting in all 0s or all 1s.
Monz and his colleagues ran their experiment with qubit sets of different sizes, and managed to demonstrate multiparticle entanglement with up to 14 qubits. But the data leave some room for doubt—whereas sets of 2, 4, or even 8 qubits showed strong signs of entanglement, the set of 14 just barely cleared the benchmark for fidelity. (Entanglement does not work every time, so it is usually verified by certain statistical tests.) The researchers say that the data support 14-qubit entanglement with a confidence interval of 76 percent, so there is certainly room for improvement.
Photo credit: University of Innsbruck