One difficulty of making quantum systems is because the qubits have to be maintained coherent during the whole process. Thus, due to the current technology, the qubits must be very close to each other, about 10 to 20 nm apart, in order to communicate. This leaves little room to place the electronics needed to make a quantum computer work. And one of these essential part to make a functional circuit is the circulator.

The circulator, like the insular, is crucial to communication systems for the manipulation of signals. For example, in the case of a RF signal, the isolator can be used to protect other RF components from excessive signal reflection. On the other hand, the RF circulator is usually used to control the direction of the signal flow in a circuit. These devices are essential to give a strict direction to processing signals and avoid any parasitic backward movement. The control of such devices is usually done by controlling the magnetic field. Being able to build such circuit for qubit will help to go closer to a functional quantum computer.

“Even if we had millions of qubits today, it is not clear that we have the classical technology to control them. […] Realizing a scaled-up quantum computer will require the invention of new devices and techniques at the quantum-classical interface.”

David Reilly, physicist at the University of Sydney and Director of Microsoft Station Q.

Example of circuit with an optical circulator.

Recently, a team had achieved a key stone toward the realization of a zerofield microwave circulator.  They incorporated ferromagnetic dopants into a three-dimensional topological insulator thin films leading to the realization of a quantum anomalous Hall effect. This result provides a measure without contacting the sample, and pave the way for a circulator on-chip circuit.

The quantum anomalous Hall effect (QAHE), the last member of Hall family, was predicted to exhibit quantized Hall conductivity without any external magnetic feld. The QAHE shares a similar physical phenomenon with the integer quantum Hall effect (QHE), whereas its physical origin relies on the intrinsic topological inverted band structure and ferromagnetism. Since the QAHE does not require external energy input in the form of magnetic feld, this effect has unique potential for applications in future electronic devices with low-power consumption.


Over a long period of exploration, the successful observation of quantized version of anomalous Hall effect (AHE) in thin film of magnetically-doped topological insulator completed a quantum Hall trio—quantum Hall effect (QHE), quantum spin Hall effect (QSHE), and quantum anomalous Hall effect (QAHE). On the theoretical front, it was understood that intrinsic AHE is related to Berry curvature and U(1) gauge field in momentum space.

The signature of this phase is the quantum anomalous Hall effect (QAHE), in which the transverse conductance of a magnetized Hall bar remains quantized in units of the conductance quantum, even in the absence of an external magnetic field. A major step was overcome when researchers managed to create a room-temperature QAHE, in which edge states propagate without dissipation.

illustration of the quantum anomalous Hall effect in a three-dimensional topological insulator thin film with ferromagnetic dopants (left). Cartoon of a three-port circulator device with a magnetic topological insulator (middle). Illustration of chiral edge transport in a circulator setup for different magnetisation and port configurations (right).

The team of Alice Mahoney showed that the response of the system exhibits resonances that can be explained by accounting for the slow velocity of edge plasmons as they traverse an arc-length of the TI disk’s edge rather than the bulk. Taken together, their microwave measurements provide strong evidence that this material system indeed supports robust, chiral edge states at zero magnetic field, opening the prospect of compact microwave components based on magnetic topological insulators

Such compact circulators could be implemented in a variety of quantum hardware platforms, irrespective of the particular quantum system used.

Alice Mahoney, University of Sydney

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