Scientists show how to erase information without using energy

Maxwell’s demon can extract work from a single heat reservoir at a cost of spin angular momentum. In step (a), the demon has no memory and the gas in the heat reservoir is in thermal equilibrium. In step (b), the demon divides the reservoir in two, trapping the fastest moving molecules on the right side, and uses a heat engine operating between the two partitions to extract work. In step (c), the demon's memory is erased using a spin reservoir and the two partitions are allowed to return to equilibrium

In the study, physicists Joan Vaccaro from Griffith University in Queensland, Australia, and Stephen Barnett from the University of Strathclyde in Glasgow, UK, have quantitatively described how information can be erased without any energy, and they also explain why the result is not as contentious as it first appears. Their paper is published in a recent issue of the Proceedings of the Royal Society A.

Traditionally, the process of erasing information requires a cost that is calculated in terms of energy – more specifically, heat dissipation. In 1961, Rolf Landauer argued that there was a minimum amount of energy required to erase one bit of information, i.e. to put a bit in the logical zero state. The energy required is positively related to the temperature of the system’s thermal reservoir, and can be thought of as the system’s thermodynamic entropy. As such, this entropy is considered to be a fundamental cost of erasing a bit of information.

However, Vaccaro and Barnett have shown that an energy cost can be fully avoided by using a reservoir based on something other than energy, such as spin angular momentum. Subatomic particles have spin angular momentum, a quantity that, like energy, must be conserved. Basically, instead of heat being exchanged between a qubit and thermal reservoir, discrete quanta of angular momentum are exchanged between a qubit and spin reservoir. The scientists described how repeated logic operations between the qubit’s spin and a secondary spin in the zero state eventually result in both spins reaching the logical zero state. Most importantly, the scientists showed that the cost of erasing the qubit’s memory is given in terms of the quantity defining the logic states, which in this case is spin angular momentum and not energy.

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