Any input into a computer is translated to strings of bits, which are sequences of zeros and ones. These sequences are then manipulated by a set of simple operations. All operations that take in two bits and output one, lead to an increase in entropy. The entropy of a system is measured by the number of possible states the system can be in. Two bits can be in one of four states: 00,01,10 or 11, while one bit can only be in one of two states: 0 or 1.

According to the Landauer’s principle, as dictated by the second law of thermodynamics, a computation that reduces the number of states from four to only two, has a fundamental energy cost. As a computer consists of millions of such operations, Landauer’s principles sets a fundamental limit to the energy efficiency of computers. A barrier that we yet have to encounter as our computers are by far less efficient. Once we get there however, quantum computers might offer a solution as they are reversible and do not reduce the number of states.

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