Preserving qubit coherence

To build a quantum computer, you need to pull off a delicate balancing act. Whatever objects embody the qubits must preserve their quantum coherence—which requires isolation. But the qubits must also be initialized, entangled, manipulated, and read out—which requires violating their isolation. Michael Biercuk, Hermann Uys, and John Bollinger of NIST in Boulder, Colorado, and their collaborators have just demonstrated a method for preserving qubit coherence using trapped ions, one of the leading qubit contenders. The NIST method is based on spin-echo, a venerable technique borrowed from nuclear magnetic resonance. Environmental noise causes the information encoded in spins, electronic or nuclear, to decay and would induce fatal errors in a spin-based quantum computer. If the noise fluctuates slowly, administering a single short electromagnetic pulse of the right frequency and duration—a spin echo pulse—will send the spins back to their original state; in principle, a sequence of such pulses should boost coherence times. In 2007, Götz Uhrig proposed that modifications to a standard sequence of spin-echo pulses could prolong coherence by orders of magnitude, even if the noise fluctuates rapidly. To test Uhrig’s scheme, the NIST team loaded about 1000 beryllium ions into a Penning trap (see image; the spacing is 10 μm). Optical and microwave pulses initialize the ions in a superposition of spin-up and spin-down states. Without intervention, information embodied by the relative phase between spin-up and spin-down ions would be lost. The NIST experiment not only validated Uhrig's prediction, it also extended his work. Using measurement feedback, the NIST team found sequences that could forestall dephasing for arbitrary and unknown noise spectra. (M. J. Biercuk et al., Nature 458, 996, 2009.) — Charles Day