A Clear Signal Emerging from Quantum Noise

Marine Le Bouar
1 day ago
4 min read

Surprising signals can arise from the coupling of light particles. © Oliver Diekmann

Researchers at TU Wien and the Okinawa Institute of Science and Technology (OIST) have demonstrated an unexpected effect: in a quantum system that is highly disordered, coherent microwave radiation can suddenly emerge.

Two candles emit twice as much light as one. And ten candles have ten times the intensity. This rule seems completely trivial—but in the quantum world it can be broken. When quantum particles are excited to a higher-energy state, they can emit light as they relax back to a lower-energy state. However, when many such quantum particles are coupled together, they can collectively generate a light pulse that is far stronger than the sum of the individual contributions. The pulse intensity scales with the square of the number of particles—this phenomenon is known as superradiance. It is a form of collective emission in which all quantum particles in the system release energy almost instantaneously and, so to speak, “in lockstep.”

TU Wien and the Okinawa Institute of Science and Technology (Japan) have now discovered a different, completely unexpected manifestation of this phenomenon. They observed superradiance in irregular diamonds and found that after the initial superradiant pulse, a series of additional pulses follows, emitting further radiation in a coherent and perfectly regular manner. This is about as surprising as if the uncoordinated chirping of many crickets were suddenly to merge into a single, synchronized bang.

Black Diamonds

“We study diamonds—but not the clear, transparent kind people are familiar with. Our diamonds contain trillions of defects where nitrogen atoms are embedded. As a result, they are not transparent, but black,” explains Wenzel Kersten from the Atominstitut at TU Wien.

The defects in the diamond possess a spin—a quantum-mechanical angular momentum that can point either up or down. Using magnetic fields and microwaves in a microwave resonator, it is possible to prepare a large number of these defects in a spin state with elevated energy. This state is unstable, much like a snow-covered slope where a tiny trigger can suddenly cause an avalanche. At some point the state collapses, the energy is released, and a “superradiant avalanche” occurs.

“This is already very interesting from a physics perspective, but for us it was really only the beginning of the story,” says Wenzel Kersten. The team discovered that after the expected superradiant pulse, there is a pause of a few microseconds—followed by a sequence of additional microwave pulses. “This was highly unexpected and did not fit any existing theory of superradiance,” says Oliver Diekmann (TU Wien).

Disorder Enables Energy Exchange

What is particularly remarkable about this phenomenon is that the system is actually extremely disordered. The defects in the diamond are not identical; they are not arranged in a perfectly regular pattern; they all have slightly different energies; and they are coupled to one another in complex ways. This is essentially the opposite of a clean, well-ordered system in which one would normally expect quantum effects to emerge.

“But as it turns out, it is precisely this disorder that is responsible for the highly ordered microwave pulses,” says Elena Redchenko (TU Wien). During the superradiant pulse, not all defect sites in the diamond release their energy—only those with exactly the right energy do. This energy range is then depleted: there are no spins left that match this energy value. According to conventional thinking, that should be the end of the process.

However, precisely because the diamond is so disordered, the defect sites can exchange energy with one another. In this way, the depleted energy range is refilled: other defect sites gain or lose energy—and in doing so, some of them again reach the energy level that had been swept clean during the superradiant pulse.

“When this energy state eventually becomes sufficiently populated again, another discharge occurs and a new microwave pulse is generated,” says Nikolaus de Zordo (TU Wien). “And this pulse, too, is coherent—like a laser. The emitted microwave photons therefore oscillate in perfect synchrony.” In this way, bursts of radiation are produced without any external trigger.

A New Approach for Emerging Quantum Technologies

“Essentially, the system drives itself,” says Prof. William Munro (OIST). “These spin–spin interactions trigger a sequence of microwave transitions and thereby reveal a fundamentally new form of collective quantum behavior.”

This new type of microwave radiation could be used as a timing reference. The phenomenon is also of interest for sensor applications: “It could potentially be used to detect extremely small changes in electric or magnetic fields,” says Prof. Jörg Schmiedmayer (VCQ / TU Wien). “Such advances could improve medical imaging, contribute to materials research, or be applied in environmental analysis. Fundamentally, new insights from quantum theory repeatedly lead to highly promising technologies.”