Geometrical-phase quantum-logic gate demonstrated.

The recent demonstration of a novel, two-qubit, geometrical-phase quantum-logic gate significantly advances prospects for quantum computation. This new gate appears to have features that overcome a number of difficulties identified earlier.

In this device, rather than manipulating the direction of the spin states, NIST researchers control the phases of the spin states using optical dipole forces. An important advantage of this change is that the requirements on control of the laser phase are signficantly reduced. For gates of the sort the researchers demonstrated and reported 2 years ago in Nature, the laser phase had to be kept constant relative to the qubit phase during the gate operation. For the new gate, only the phase of the dipole force relative to the motion needs to be maintained constant. In general, this is much easier to do.

The geometrical-phase gate also shares many of the positive attributes of the earlier gate. It is a one-step gate. Also, individual-ion addressing is not required. In fact, we want to illuminate the ions simultaneously. This means that careful laser focusing is not needed. Finally, motional eigenstates are not required as long as the ions are tightly contained and within the Lamb-Dicke limit.

The new gate is particularly well-suited to application in multiplexed trap systems, a design approach that will provide for scaling to larger logic systems. For example, there is no requirement for maintaining equal laser couplings to each ion as in the previous, two qubit gate. This means that sympathetic cooling can be used in more complex systems without introducing difficulties in maintaining specific laser-beam couplings to the ions. The additional phases accumulated on individual qubits from unequal coupling can be removed in software by changing the phase of subsequent applied operations rather than applying correction pulses to the ions themselves.

CONTACT: David Wineland, [phone]; [e-mail].