Quantum memory takes a leap forward

Light is a useful carrier of quantum information, but no method yet exists for storing and retrieving it without significantly corrupting its quantum state. Now, Matthew Sellars (Australian National University, Canberra) and colleagues have taken a step closer to a useful quantum memory by mapping a light pulse’s state onto an ensemble of praseodymium ions doped into a transparent crystal. Capitalizing on the Stark effect, the researchers used an electric-field gradient, as shown in the figure, to shift the ions’ transition energies. Absorption of an input light pulse created a coherent superposition of ground and excited states, whose phase in each ion evolved at a rate proportional to the transition energy. Flipping the electric field’s sign reversed the transition-energy distribution and brought the ions back into phase, at which point the pulse was re-emitted. The innovation was in how the researchers countered the spectral line’s natural broadening due to inhomogeneities in the crystal lattice: Applying a frequency-stabilized laser beam to the side of the long, thin crystal, they pumped atoms whose transition energies lay outside a narrow range into an out-of-the-way hyperfine state. Using the sharpened spectral line, which Sellars says may be “the sharpest optical filter ever demonstrated,” the researchers stored pulses of up to 500 photons more faithfully than could be done classically. When pulses were stored for 1.3 µs, the retrieved ones were 69% as intense—the previous record was 15%. (M. P. Hedges et al., Nature 465, 1052, 2010 .)—Johanna Miller