The team of Časlav Brukner at IQOQI Vienna and researchers at the University of Bristol, the University of the Balearic Islands and The University of Vienna have shown how quantum systems can simultaneously evolve along two opposite time arrows — both forward and backward in time
Artistic illustration of a gondolier trapped in a quantum superposition of time flows. Credit:
© Aloop Visual & Science, University of Vienna, Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences
Microscopic physical laws are time-symmetric, hence, a priori there exists no preferential temporal direction. However, the second law of thermodynamics allows one to associate the “forward” temporal direction to a positive variation of the total entropy produced in a thermodynamic process, and a negative variation with its “time-reversal” counterpart. This definition of a temporal axis is normally considered to apply in both classical and quantum contexts. Yet, quantum physics admits also superpositions between forward and time-reversal processes, whereby the thermodynamic arrow of time becomes quantum-mechanically undefined. In this work, we demonstrate that a definite thermodynamic time’s arrow can be restored by a quantum measurement of entropy production, which effectively projects such superpositions onto the forward (time-reversal) time-direction when large positive (negative) values are measured. Finally, for small values (of the order of plus or minus one), the amplitudes of forward and time-reversal processes can interfere, giving rise to entropy-production distributions featuring a more or less reversible process than either of the two components individually, or any classical mixture thereof.
Rubino, G., Manzano, G. & Brukner, Č.
Commun Phys 4, 251 (2021). https://doi.org/10.1038/s42005-021-00759-1
Quantum Optics, Quantum Nanophysics and Quantum Information
Faculty of Physics, University of Vienna