2016/07/15 by Mischa P. Woods, Ralph Silva, Jonathan Oppenheim · 2 citations
Computer Science · Physics and Astronomy · #Advanced Thermodynamics and Statistical Mechanics #Computer science #Observer (physics) #Open quantum system #Physics #Quantum #Quantum Computing Algorithms and Architecture #Quantum Information and Cryptography #Quantum algorithm #Quantum computer #Quantum dynamics #Quantum many-body systems #Quantum mechanics #Quantum operation #Quantum process #Unitary state #quant-ph
paper · pdf · doi:10.1007/s00023-018-0736-9
Main text: 9 + 53 pages. V4: Close to the published version, J. Annales Henri Poincaré (2018) [Communicated by David Pérez-García]
openalex publication_date 2016/07/15 · arxiv created 2018/11/05 · arxiv updated 2018/11/06 · openalex created_date 2019/07/30 · openalex updated_date 2026/08/05
Processes such as quantum computation, or the evolution of quantum cellular automata are typically described by a unitary operation implemented by an external observer. In particular, an interaction is generally turned on for a precise amount of time, using a classical clock. A fully quantum mechanical description of such a device would include a quantum description of the clock whose state is generally disturbed because of the back-reaction on it. Such a description is needed if we wish to consider finite sized autonomous quantum machines requiring no external control. The extent of the back-reaction has implications on how small the device can be, on the length of time the device can run, and is required if we want to understand what a fully quantum mechanical treatment of an observer would look like. Here, we consider the implementation of a unitary by a finite sized device which we call the "Quasi-Ideal clock", and show that the back-reaction on it can be made exponentially small in the device's dimension with only a linear increase in energy. As a result, an autonomous quantum machine need only be of modest size and or energy. We are also able to solve a long-standing open problem by using a finite sized quantum clock to approximate the continuous evolution of an Idealised clock. The result has implications on the equivalence of different paradigms of quantum thermodynamics, some which allow external control and some which only allow autonomous thermal machines.