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The increasing speed of fibre-optic-based telecommunications has focused attention on high-speed optical processing of digital information 1 . Complex optical processing requires a high-density, high-speed, low-power optical memory that can be integrated with planar semiconductor technology for buffering of decisions and telecommunication data 2 . Recently, ring lasers with extremely small size and low operating power have been made 3 , 4 , 5 , 6 , 7 , and we demonstrate here a memory element constructed by interconnecting these microscopic lasers. Our device occupies an area of 18 × 40 µm 2 on an InP/InGaAsP photonic integrated circuit, and switches within 20 ps with 5.5 fJ optical switching energy. Simulations show that the element has the potential for much smaller dimensions and switching times. Large numbers of such memory elements can be densely integrated and interconnected on a photonic integrated circuit: fast digital optical information processing systems employing large-scale integration should now be viable.

A fully-packaged hybrid-integrated all-optical flip-flop, where InP-based semiconductor optical amplifiers are assembled onto a planar silica waveguide board, is demonstrated. It is shown experimentally that the flip-flop can dynamically toggle between its two states by injecting 150ps optical pulses with 6dBm peak power via its set and reset port.