Synopsis: No Free Lunch, or Measurements

Quantum measurements are often treated as if they cost zero energy, but in a Physical Review E Rapid Communication, Kurt Jacobs, of the University of Massachusetts, Boston, quantifies the intrinsic cost of such measurements. He considers a measuring device that cyclically assesses the quantum states of a system, and stores this “information” in a quantum memory. Using a thought experiment to illustrate thermodynamic calculations, Jacobs shows that this measurement is like a heat engine extracting work from the system: Under certain conditions, the energy cost paid by the measurement is equal to the extracted work, which in turn is equal to the product of the initial temperature of the device and the average information obtained by the measurement.

In previous studies, the cost of a measurement was associated with the operation of erasing the measuring device’s memory. In his new work, Jacobs treats the measuring device as a sort of Maxwell’s demon and demonstrates that the measurement cost can be interpreted as intrinsic to the measurement process because it raises the entropy of the memory. In addition to addressing a fundamental issue, the result may have important implications for technologies such as quantum computing. – Ron Dickman