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Researchers have created a quantum circuit which allows them to listen to the weakest possible radio signal allowed by quantum mechanics. They hope that this innovation will eventually help reveal the connection between quantum theory and gravity.

The scientists at Delft University of Technology — led by Professor Gary Steele — hope that the quantum circuit will have significant applications in radio astronomy, medicine and allow for the creation of experiments that can finally shed some light on the connection between quantum mechanics and gravity — perhaps even leading to a long-sought quantum theory of gravity.

If you have ever been annoyed by a weak radio signal — whether that be your car radio tuning out during your favourite song or your Wi-Fi signal dropping out at an inopportune time — your usual solution has probably been to try and boost that signal. This could be by re-tuning your radio or by moving closer to your router — seeking a stronger signal.

This quantum chip (1×1 cm big) allows the researchers to listen to the smallest radio signal allowed by quantum mechanics (TU Delft)

Weak radio signals can be a more serious problem for scientists attempting to use Magnetic Resonance Imaging (MRI) scanners at hospitals and for astronomers using radio telescopes to peer into space. In these situations, it may not be possible to seek to boost a signal — one can hardly move closer to a distant galaxy at a whim.

But what if, instead of boosting the signal, we could just design equipment that could ‘listen’ to these signals more carefully?

This is what Steele and his team have attempted to do. To create a circuit capable of detecting single photons — or quanta of energy — the weakest possible signal allowed by quantum mechanics.

Photons as Quantum ‘packets’ of energy

The breakthrough idea o quantum mechanics was that energy was delivered not in a steady, continuous stream, but in distinct chunks called ‘quanta’.

Marcus Gely, the lead researcher on the project, explains the significance of this discovery: Say I am pushing a kid on a swing. In the classical theory of physics, if I want the kid to go a little bit faster I can give them a small push, giving them more speed and more energy.

“Quantum mechanics says something different: I can only increase the kid’s energy one ‘quantum step’ at a time. Pushing by half of that amount is not possible.”

Electron microscope picture of the quantum circuit built by the researchers. The width of the picture corresponds to only a third of a millimetre (UT Delft)

In quantum mechanics, an example of this is electrons circling an atomic nucleus. The electrons can exist in orbits that are defined by their energy levels. To move up a level, an electron must absorb a photon that matches the energy difference between those to states. To jump back down, it must emit a photon of the same energy.

So, consider an electron orbiting a simple hydrogen nucleus — the most abundant atom in the universe — the difference between the lowest ‘step’, what physicists call the ground state and the next ‘step’ up — the first excited state — is 11.2eV. To make this jump, an electron needs to ‘accept’ a photon of 11.2eV. Two photons of energies 5.6eV won’t facilitate this jump. The electron needs the right quanta of energy.

For a kid on a swing, these ‘quantum steps’ are so tiny that they are too small to notice — and until recently, the same was true for radio waves. However, the research team in Delft developed a circuit that can actually detect these chunks of energy in radio frequency signals, opening up the potential for sensing radio waves at the quantum level.

But how does this development potentially open a door to a theory of quantum gravity?

Tuning into quantum gravity

Beyond applications in quantum sensing, the group in Delft is interested in taking quantum mechanics to the next level — although the theory of quantum electromagnetism was developed nearly 100 years ago, physicists are still puzzled today on how to fit gravity into quantum mechanics.

Grely says: “Using our quantum radio, we want to try to listen to and control the quantum vibrations of heavy objects, and explore experimentally what happens when you mix quantum mechanics and gravity.

“Such experiments are hard, but if successful we would be able to test if we can make a quantum superposition of space-time itself, a new concept that would test our understanding of both quantum mechanics and general relativity.”

Original research available at http://science.sciencemag.org/content/363/6431/1072





















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