Microwaves in the pedestrian zone.
© Chuyang Zheng
The propagation of pulses of electromagnetic radiation is usually governed by the properties - such as dispersion - of the medium or waveguide that hosts the field. Now researchers at the Ecole Polytechnique Fédérale de Lausanne (EPFL), the Walther-Meissner-Institute (WMI), the Technische Universität München (TUM) and the Max Planck Institute of Quantum Optics (MPQ) have introduced a radically different approach to control the propagation of microwave signals by using the electromechanical interaction of a vibrating silicon nitride nanostring. They are thus able to dynamically switch, slow down, and accelerate the propagation of microwave pulses. Signals can be retrieved after milliseconds of tunable delay, with negligible loss and distortion.
A coaxial cable realizing the same delay would need to be 600 km long, thus very lossy, hardly tunable and expensive.
The teams of Prof. Tobias Kippenberg at EPFL (LPQM) and MPQ and Prof. Rudolf Gross at WMI and TUM are collaborating closely on the investigation of superconducting microwave circuits coupled to nanomechanical oscillators.
These hybrid systems combine two platforms that have traditionally been studied in separate contexts:
Superconducting microwave circuits are essentially printed circuit boards that operate at very high frequencies (6 GHz) and low temperatures (0.05 K above absolute zero). In recent years, these circuits have been established as one of the most promising candidates for future quantum information processors, due to their excellent coherence, operation speed and scalability. Nanomechanical oscillators, on the other hand, have been under physicists’ scrutiny for the yet unexplored quantum aspects of their vibrational motion, probing the laws of quantum mechanics in an unprecedentedly literal sense.
In their recent study, the Swiss and German teams combined these two research endeavors in a quest for new mechanical functionalities for superconducting quantum circuits, but also potentially new handles on the nanomechanical oscillator’s properties provided by the microwave probe.
As they report in the forthcoming issue of Nature Physics, they could realize a coupling between the microwave field and the vibration of a nanomechanical string with a diameter of only 150 nm (a human hair is about 100-times larger) in an on-chip platform of finger nail size. The coupling proceeds by a tiny capacitance change in the circuit induced by the motion of the nanostring.
By analogy with phenomena known in atomic and optical physics, they found that the nanomechanical oscillator can mediate interference between microwaves, and exploited to control the propagation of microwave signals. In particular, they could create extremely sharp spectral transmission windows (linewidth 10 Hz) for microwave radiation, which are accompanied by very long delays of the order of milliseconds. Thus, a microwave pulse takes the same time to travel across the microchip as if it was sent the 600 km from Garching (site of the WMI...) to Lausanne at the speed of light. Moreover, the width, delay, but also amplitude of the transmission window, i.e., the fraction of radiation power that is transmitted, can be dynamically controlled by a second microwave field.
The device under study could be used for the processing of quantum information as it can be operated in a regime in which decoherence, a potentially destroying effect for quantum states, is not a critical issue.
‘We built a platform to manipulate wave propagation in a fully integrated architecture without the need of photon detection and regeneration. Its implications extend to the field of quantum computing and quantum information processing’’ said Xiaoqing Zhou, the PhD student who developed the device in EPFL’s Center of MicroNanoTechnology (CMi).
The device shows properties relevant to research fields, like atomic physics or quantum state storage and transfer or to more technological domains, like electromechanics, nanomechanics and superconducting circuits. In particular, the authors are exploring follow-up experiments involving more sophisticated superconducting circuits, which may allow preparing and manipulating the quantum state of the nanomechanical oscillator.