If a source emits a wave that scatters off an object and is then measured by a detector, the principle of reciprocity states that the measured signal will be unchanged if the source and detector switch places. This symmetry is a prevalent feature in all physical systems, but for certain applications it constitutes an obstacle. For example, to create an isolator—a gadget that allows signals to pass in one direction but not in the other—reciprocity must be broken. Such nonreciprocal devices—ones defined by preferential direction or “chirality” in their emission or absorption—are valuable in many fields. Recently, nonreciprocal devices have been implemented in the superconducting electrical circuits used in quantum computing, but all have had drawbacks. Now Chaitali Joshi and her colleagues from the California Institute of Technology have constructed a simpler nonreciprocal device: an “artificial atom” made from a superconducting circuit, which can be coupled exclusively to either left- or right-moving signals in a microwave waveguide [1]. This chiral design could be used in quantum networks to enable control over information flow between multiple artificial atoms coupled to a waveguide.
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