microwave signals quantum data encryption

Nanodrum signal measurement nears quantum limit

A highly accurate method of measuring microwave signals using ‘nanodrums’ could be used for data encryption based on quantum cryptography.

Developed in Finland by researchers at Aalto University and the University of Jyväskylä, the method could theoretically be used for processing quantum information, for example by efficiently transforming signals from microwave circuits to the optical regime.

The system works in a similar fashion to radio station towers. When a receiver is too far away from a tower, the signal is weak and affected by distortion, which typically manifests itself as a crackling sound on FM radios.

This noise results mostly from having to amplify the information carried by the signal in order to convert 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.

“The quantum limit of amplifiers is essential for measuring delicate quantum signals, such as those generated in quantum computing or quantum mechanical measuring, because the added noise limits the size of signals that can be measured,” explains Professor Mika Sillanpää.

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, or vibrating nanodrum (pictured above), with two superconducting circuits, i.e. cavities.

Nanodrums enable a nearly noiseless measurement of radio signals. The drum is made of thin superconducting aluminium film on top of a quartz chip.

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

 “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.

“This would for example allow transferring information from superconducting quantum bits to the ‘flying qubits’ in the visible light range and back,” said Tero Heikkilä, Professor at the University of Jyväskylä, and Academy Research Fellow Francesco Massel.

According to the team, this phenomenon allows for data encryption based on quantum mechanics, i.e. quantum cryptography, as well as other applications.

In September, a team demonstrated a quantum encryption technology that for the first time allows users to secure messages with keys shorter than the messages themselves. 

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