Qubits, or quantum bits, are the key building blocks at the heart of every quantum computer. In order to perform a computation, signals are directed to and from qubits. However, qubits are extremely sensitive to interference from their environment, and need to be shielded from outside signals, in particular from magnetic fields. It is a serious problem that the devices built to shield qubits from unwanted signals, known as nonreciprocal devices, produce magnetic fields themselves. Moreover, they are several centimeters in size, which is problematic, given that a large number of such elements is required in each quantum processor. We have decreased the size of nonreciprocal devices by two orders of magnitude. Their device, which they compare to a traffic roundabout for photons, is only about a tenth of a millimeter in size, and—more importantly—it is not magnetic.
We have implemented the first nonreciprocal mechanical on-chip microwave circulator. Directional circulation is achieved with controlled phase-sensitive interference of six distinct electro-mechanical signal conversion paths. The presented circulator is compact, its silicon-on-insulator platform is compatible with both superconducting qubits and silicon photonics, and its noise performance is close to the quantum limit. This frequency tunable and in situ reconfigurable signal processing device can be used as a filter, wavelength converter, beam splitter, isolator or circulator for microwave photons and it paves the way to implement the on-chip microwave to optical converter.
Beside that we have developed the theory of and discuss a potential realization for the controllable flow of thermal noise in quantum systems. We have demonstrated theoretically that the unidirectional flow of thermal noise is possible within quantum cascaded systems.
