Decoherence Eﬀects Break Reciprocity in Matter Transport
Propagation of an electron wave packet in a nanodevice consisting of an asymmetric quantum ring with two contacts. The ring is biased with a magnetic field. The video shows the propagation of the electron as given by the Schrödinger equation. This propagation is interrupted by three quantum collapses highlighted in purple. These collapse processes correspond to quantum measurements or decoherence events. They occur at finite temperatures and are induced by small interactions of the electron with the environment.
The transition between the quantum and classical worlds is intriguing. This transition harbors fundamental questions concerning the appropriate description of quantum decoherence or, analogously, quantum mechanical collapse events and quantum measurements.
We have discovered that this transition regime enables a novel type of matter transport. Applying this discovery, we present nanoscale devices in which decoherence or random quantum jumps produce fundamentally novel phenomena by modifying the propagation of electron wave packets.
Non-centrosymmetric conductors with mesoscopic length scales act as two-terminal rectiﬁers with unique and completely novel properties (see Fig.1). These devices transport single electrons preferably in one direction and not in the other, much as the famous demon considered by James Maxwell in 1867 was supposed to do.
In these devices, the well-known thermal equilibrium occupancy of quantum states of an electron becomes unstable, giving rise to states characterized by charge separation between the two leads, or, in closed circuits, to the generation of persistent currents (see Fig. 2). Amazingly, these persistent currents are supposed to flow even in the presence of processes that push the system back towards equilibrium and thereby model dissipation.
This behavior is a strict prediction resulting from the laws of quantum physics, yet it is not consistent with the laws of thermodynamics. The new dynamics provides an exemption to the reciprocity relation, for which Lars Onsager has been awarded the Nobel prize in 1968, as it forms the foundation of nonequilibrium thermodynamics.
According to the current state of our work, it cannot even be excluded that the described effects violate the second law of thermodynamics, which effectively says that it is not possible to invent a Maxwell demon which continuously generates usable work from a device in thermal equilibrium.