A Beginner’s Guide to Lukewarm Entanglement

Entanglement, the ability of one quantum particle to influence another particle instantly, has been both a bane and boon to quantum mechanics ever since Einstein wrote about this "spooky action at a distance" in 1935.

In Einstein's day, entanglement was a much more controversial notion than it is today. For the most part, physicists have come to accept it as a fundamental part of nature, albeit a very bizarre one. It remains difficult, however, to experimentally produce an entangled system, which often involves complex processes and stringent environmental conditions. It makes entanglement difficult to apply to anything practical outside of a state-of-the-art laboratory.

Recently, a team of researchers at the University of Chicago made a significant stride toward making entanglement an accessible phenomenon for practical technologies by demonstrating that particle entanglement can be achieved at room temperatures and without the use of massive magnetic fields.

Their findings, which were published Friday in Science Advances, could have a wide variety of applications for quantum technologies, ranging from quantum computers to perfectly synchronized GPS satellites to ultrasensitive biosensors implanted in the human body.

Producing entangled systems is in part a complicated process because entanglement requires that particles start out in a highly ordered state. This is contrary to the principles of thermodynamics (the relationship between heat and energy), which probabilistically favor disorder. This means that producing entanglement is extremely difficult on macroscopic scales, where a system has a massive number of particles. Yet if entanglement is ever to become a useful component of practical technologies (computers, satellites, etc.), then figuring out how to manipulate entangled particles on a macroscopic scale is crucial.

"The macroscopic world that we are used to seems very tidy, but it is completely disordered at the atomic scale," Paul Klimov, a graduate student in the University of Chicago's Institute for Molecular Engineering and lead author of the study, said in a statement. "The laws of thermodynamics generally prevent us from observing quantum phenomena in macroscopic objects."

Prior to Klimov's discovery, in order to achieve the degree of order necessary to entangle the spin states of particles on macroscopic scales in solids and liquids, scientists resorted to bringing systems to super low temperatures (about -270 C) and applying huge magnetic fields to the system (fields approximately 1,000 times larger than the field of a refrigerator magnet). The end result is that the spin states of the particles or particle collection become entangled.

Yet Klimov and his team successfully demonstrated that neither of these very limiting environmental factors (low temperatures and massive magnetic fields) is necessary for entangling the spin of large collections of particles. Rather, in an unprecedented experiment, they were able to produce a macroscopic system of particles with entangled spin states at room temperature and with a relatively small magnetic field.

"Spin is an incredibly versatile property in the sense that it can interact with many other degrees of freedom: optical fields, magnetic fields, electric fields—essentially anything," Klimov told Motherboard in an email. "The fabrication techniques developed in the high-power electronics and optoelectronics industries around silicon carbide (our substrate) could be used for the development of sophisticated entanglement-harnessing devices that interact with these other degrees of freedom to do something useful, such as sensing."

The team accomplished this by using infrared lasers to order the magnetic states of thousands of electrons and nuclei, which it then entangled by way of electromagnetic pulses similar to those used in magnetic resonance imaging (MRI). This created a macroscopic entangled system of silicon carbide (SiC) on a semiconductor about 40 cubic micrometers in volume—approximately the size of a red blood cell.

According to the team, the choice of a SiC semiconductor wafer was instrumental in achieving these results.

"We know that the spin states of atomic nuclei associated with semiconductor defects have excellent quantum properties at room temperature," David Awschalom, a senior scientist at Argonne National Laboratory who worked with Klimov on the experiment, said in a statement. "They are coherent, long-lived and controllable with photonics and electronics. Given these quantum 'pieces,' creating entangled quantum states seemed like an attainable goal."

While the team's results are likely to remain esoteric for a while, they have the potential for some pretty wild applications. Initially, the techniques demonstrated by Klimov and his colleagues could be put to use to surpass the sensitivity threshold of non-quantum sensors. What this means is that, since these entangled states can be realized in ambient conditions, they could be used in biosensors implanted in living organisms for more accurate results.

"Entangled states can be used, in general, for achieving a higher signal-to-noise ratio (a figure of merit that tells you the precision of the sensor) than is possible in traditional non-quantum sensors," said Klimov. "The exact way that the sensing would work is of course application dependent but very superficially, it would require one to bring the sensor in close proximity to the to-be-sensed 'thing,' and then apply a specific quantum algorithm (sequence of optical, microwave, and radiofrequency pulses—similar to those used in conventional magnetic resonance imaging (MRI) ) to extract the to-be-sensed signal (perhaps a magnetic field, electric field, temperature, or pressure, which are all very important variables in a biological system)."

In the long term, the team speculates that it might be possible to go beyond an entangled system on a single SiC semiconductor to create an entangled system between disparate SiC chips. Long-distance, macroscopic entanglement could have some pretty awesome applications, such as perfectly synchronized GPS satellites or communication systems impervious to eavesdroppers, since interception of the message would irreversibly corrupt it. "The primary (and extremely difficult-to-overcome) challenge in realizing such a technology, however, is distributing the entanglement," said Klimov. "This is one reason why it could pay to use many spins (an ensemble of spins, which we use in our study) to encode quantum information as opposed to single spins (which are actively being explored for such technologies as well)."

Although these are highly speculative applications, the work being done at the Institute for Molecular Engineering is a big step in the direction of mastering macroscopic entangled systems.