Diamonds Are A Quantum Memory’s Best Friend
A Canadian team report the successful implementation of a bulk diamond for quantum processing/memory purposes. Utilising THz-bandwidth single photons, researchers were able to reliably store and retrieve quantum information at room temperatures that could see the technique used for future research: ‘The device requires no cooling or optical preparation before storage and is a few millimetres in size’, say the team, ‘diamond is therefore a robust, convenient and high-speed test-bed system in which to evaluate operational memory parameters, study the effects of noise, and develop quantum protocols’.
The system works by pumping the diamond with pairs of photons (herald and signal), using spontaneous parametric down-conversion (SPDC) techniques. The absorbed photons are then stored via an off-resonant Raman transition within the crystal’s optical phonon modes (lattice vibrations). The team state that because the transitions were far from any optical resonances the memory can store photons of various wavelengths (visible and near-infrared), making the technique highly flexible. Also, memory bandwidth is broad and has a large tuning range, making storage compatible with high speed SPDC sources.
This attribute is especially helpful as decay from optical to acoustic phonons occurs after a mere 3.5ps, thus setting the maximum time limit for memory storage: ‘While this lifetime is prohibitively short for some applications, the advantage of the rapid acoustic decay is that it returns the crystal lattice to the ground state, resetting the memory such that it is ready to store the next photon. This sub-nanosecond reset time permits GHz repetition rates in the diamond phonon system’. This fast memory/reset characteristic, although undesirable for certain applications (i.e. quantum repeaters/networks), will be essential for quantum computing, where fast processing speeds are critical.
The team report, for a storage time of 0.5ps: ‘The second-order correlation of the memory output is g (2) (0) = 0.65 ± 0.07 (5σ below the classical limit), demonstrating preservation of non-classical photon statistics throughout storage and retrieval’. However, it was noted that the g (2) (0) function increases with storage time due to background noise, although non-classical statistics for the memory output were maintained for >2.5ps—thus confirming the retention of quantum properties post memory interaction.
Further uses are cited to include: quantum frequency conversion, memory-enhanced optical nonlinearities and/or programmable linear-optical components.
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