For thousands of years, students and tradespeople have used the abacus to make quick calculations. Now scientists at two European universities have devised a “photonic abacus” that uses picosecond optical pulses to do arithmetic (Nat. Commun., doi:10.1038/s41467-017-01506-3).

## Reuniting processing and storage

The abacus—in its most common form, sets of beads strung on parallel wires, which represent different place values—incorporates both data processing and data storage. Electronic computers, however, separate the computation and storage functions, based on a concept developed by mathematician John von Neumann more than seven decades ago. In practice, the separation of data processing and memory often leads to a bottleneck, which slows computational speed and wastes energy.

The device built by scientists at Münster University, Germany, and the University of Exeter, U.K., reunites the two basic tasks on a single microchip. The chip contains a 30-μm-square grid of nanoscale photonic waveguides. Sitting at the crossroads of this square waveguide grid are tiny cells that contain phase-change materials—the key ingredient in rewritable optical discs.

## Moving optical beads

Optical pulses through the waveguides control the transition of the phase-change materials from amorphous to crystalline phases. Each step in the transition represents a digit from 0 to 9, and each cell represents a different digit in front of the decimal point (tens, hundreds and so forth). A series of picosecond, 12-pJ pulses incrementally changes the refractive index of each cell to a preset level representing one of the 10 decimal digits.

When the number of pulses hitting the first cell in the grid pushes its value above 9, the cell flips back to the 0 state—with high transmission and low absorption of light. The pulses carry over to the next cell and begin to alter its phase. In that way, two single-digit numbers – say, 7 and 5 – add correctly to a two-digit number such as 12. The authors also developed a two-phase optical switching scheme to speed up the arithmetic operations.

According to the research team, the photonic abacus can be fabricated with standard silicon-photonics processes. The only limit to the speed of this type of computer is the time it takes to flip the phase-change material between crystalline and amorphous states, and the authors state that the flip rate “approaches the GHz regime” with the types of chalcogenide glasses used in their experiments.