On-Chip Microwave Quantum Hall Circulator
  Circulators are non-reciprocal circuit elements integral to technologies including radar systems, microwave communication transceivers, and the readout of quantum information devices. Their non-reciprocity arises from the interference of microwaves over the centimetre-scale of the signal wavelength in the presence of bulky magnetic media that break time-reversal symmetry. Here we realize a completely passive on-chip microwave circulator with size one-thousandth the wavelength by exploiting the chiral, slow-light response of a 2-dimensional electron gas (2DEG) in the quantum Hall regime. For an integrated GaAs device with 330 um diameter and 1 GHz centre frequency, a non-reciprocity of 25 dB is observed over a 50 MHz bandwidth. Furthermore, the direction of circulation can be selected dynamically by varying the magnetic field, an aspect that may enable reconfigurable passive routing of microwave signals on-chip. 
A. C. Mahoney, J. I. Colless, S. J. Pauka, J. M. Hornibrook, J. D.   Watson, G. C. Gardner, M. J. Manfra, A. C. Doherty, and...
cond-mat.mes-hall quant-ph