October 31, 2016

Nanodrums near the quantum limit can be used for data encryption

Researchers at Aalto University and the University of Jyväskylä have developed a new method of measuring microwave signals extremely accurately. This method can be used for processing quantum information, for example by efficiently transforming signals from microwave circuits to the optical regime.

Important quantum limit

If you are trying to tune in a radio station but the tower is too far away, the signal gets distorted by noise. The noise results mostly from having to amplify the information carried by the signal in order to transfer it into an audible form. According to the laws of quantum mechanics, all amplifiers add noise. In the early 1980s, US physicist Carlton Caves proved theoretically that the Heisenberg uncertainty principle for such signals requires that at least half an energy quantum of noise must be added to the signal. In everyday life, this kind of noise does not matter, but researchers around the world have aimed to create amplifiers that would come close to Caves' limit.

Micro drums enable a nearly noiseless measurement of radio signals. The drum is made of thin superconducting aluminum film on top of a quartz chip (blue background). CREDIT Mika Sillanpää

Physical Review X - Low-Noise Amplification and Frequency Conversion with a Multiport Microwave Optomechanical Device

So far, the solution for getting closest to the limit is an amplifier based on superconducting tunnel junctions developed in the 1980s, but this technology has its problems. Led by Sillanpää, the researchers from Aalto and the University of Jyväskylä combined a nanomechanical resonator - a vibrating nanodrum - with two superconducting circuits, i.e. cavities.

'As a result, we have made the most accurate microwave measurement with nanodrums so far', explains Caspar Ockeloen-Korppi from Aalto University, who conducted the actual measurement.

In addition to the microwave measurement, this device enables transforming quantum information from one frequency to another while simultaneously amplifying it.

'This would for example allow transferring information from superconducting quantum bits to the "flying qubits" in the visible light range and back', envision the creators of the theory for the device, Tero Heikkilä, Professor at the University of Jyväskylä, and Academy Research Fellow Francesco Massel. Therefore, the method has potential for data encryption based on quantum mechanics, i.e. quantum cryptography, as well as other applications.


High-gain amplifiers of electromagnetic signals operating near the quantum limit are crucial for quantum information systems and ultrasensitive quantum measurements. However, the existing techniques have a limited gain-bandwidth product and only operate with weak input signals. Here, we demonstrate a two-port optomechanical scheme for amplification and routing of microwave signals, a system that simultaneously performs high-gain amplification and frequency conversion in the quantum regime. Our amplifier, implemented in a two-cavity microwave optomechanical device, shows 41 dB of gain and has a high dynamic range, handling input signals up to 10^13 photons per second, 3 orders of magnitude more than corresponding Josephson parametric amplifiers. We show that although the active medium, the mechanical resonator, is at a high temperature far from the quantum limit, only 4.6 quanta of noise is added to the input signal. Our method can be readily applied to a wide variety of optomechanical systems, including hybrid optical-microwave systems, creating a universal hub for signals at the quantum level.

SOURCES - Eurekalert

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