A stepping stone to measuring quantity

A group of theoretical physicists, including two physicists from the University of Groningen, have proposed a ‘table’ device that could measure gravitational waves. However, their real purpose is to answer one of the biggest questions in physics: is gravity a quantum phenomenon? The most important element for the device is the quantum superposition of large objects. Their design was published in New Journal of Physics on August 6th.

Already in the preprint stage was the paper written by Ryan J. Marshman, Peter F. Barker and Sougato Bose (University College London, UK), Gavin W. Morley (University of Warwick, UK) and Anupam Mazumdar and Steven Hoekstra (University of Groningen, the Netherlands) was considered a new method for measuring gravitational waves. Instead of the current mile-sized LIGO and VIRGO detectors, the physicists working in the United Kingdom and the Netherlands proposed a table detector. This device would be sensitive to lower frequencies than the current detectors and it would be easy to point them to specific parts of the air – in contrast, the current detectors only see a fixed part.

Diamond

The most important part of the device is a thin diamond, but a few nanometers in size. “In this diamond, one of the carbohydrates is replaced by a nitrogen atom,” explains Assistant Professor Anupam Mazumdar. This atom introduces a free space in the valence band, which can be filled with an extra electron. Quantum theory states that when the electron is irradiated with laser light, it can absorb or not absorb the photon energy. The energy absorbed would change the spin of the electron, a magnetic moment that can be up or down.

“Like Schrödinger’s cat, which is both dead and alive, this electron spin does not absorb and absorb the photon energy, so its spin is both up and down.” This phenomenon is called quantum superposition. Since the electron is part of the diamond, it is the whole object – with a mass of about 10^-17 kilograms, which is enormous for quantum phenomena – is in quantum superposition.

“We have a diamond that has spin and down at the same time,” Mazumdar explains. By applying a magnetic field, it is possible to separate the two quantum states. If these quantum states are brought together again by switching off the magnetic field, they will create an interference pattern. “The nature of this interference depends on the distance traveled by the two separate quantum states. And this can be used to measure gravitational waves.” These waves are contractions of space, so their passage affects the distance between the two separate states and thus the interference pattern.

Link is missing

The paper shows that this setup could indeed detect gravitational waves. But that is not what Mazumdar and his colleagues are really interested in. “A system in which we can obtain quantum superposition from a mesoscopic object such as the diamond, and for a reasonable amount of time, would be a real breakthrough,” says Mazumdar. “It would allow all kinds of measurements to be taken, and one of these could be used to determine whether gravity itself is a quantum phenomenon.” Quantum gravity has been ‘the missing link’ in physics for almost a century.

In a paper published in 2017, Mazumdar and his long-time collaborator Sougato Bose, along with several colleagues, suggested that pollution between two mesoscopic objects could be used to determine whether gravity itself is a quantum phenomenon. Put simply, entanglement is a quantum phenomenon, so if two objects that interact only show gravity entanglement, this proves that gravity is a quantum phenomenon.

Technology

“In our latest paper, we describe how you can create mesoscopic quantum superposition. With two of these systems, we were able to see pollution.” As they noted during their work, the only system would be sensitive to gravitational waves and this became the focus of the New Journal of Physics paper.

“The technology to build these systems could take several decades to develop,” Mazumdar admits. A vacuum of 10^-15 Pascal is required, although the working temperature should be as low as possible, close to absolute zero (-273 ° C). “Technology to achieve both high vacuum and low temperature is available, but we need the technology to achieve both at the same time.” Furthermore, the magnetic field must be constant. “Any fluctuation would cause the quantum superposition to collapse.”

Free fall

The reward for making this kind of system would be great. “It could be used for all kinds of measurements in fields such as low-energy physics or quantum processing, for example.” And it could of course be used to determine whether gravity is a quantum phenomenon. Mazumdar, Bose and colleagues have just uploaded another preprint in which they describe how this experiment could be performed. “To ensure that the only interaction between the two scattered objects is gravity between them, the experiment should be done in free fall,” Mazumdar explains. With visible enthusiasm, he describes a one-kilometer-long dropshaft in a deep mine, to reduce interference. Two entangled mesoscopic quantum systems must be repeatedly lowered to obtain a reliable measurement. “I think this can be done in my life. And the result would finally solve one of the biggest questions in physics.”