Macroscopic quantum entanglement achieved at room temperature

The researchers believe that the advance could lead to entanglement-enhanced magnetic resonance imaging probes(Credit: Awschalom Group/University of Chicago)

In quantum physics, the creation of a state of entanglement in particles any larger and more complex than photons usually requires temperatures close to absolute zero and the application of enormously powerful magnetic fields to achieve. Now scientists working at the University of Chicago (UChicago) and the Argonne National Laboratory claim to have created this entangled state at room temperature on a semiconductor chip, using atomic nuclei and the application of relatively small magnetic fields.

When two particles, such as photons, are entangled – that is, when they interact physically and are then forcibly separated – the spin direction imparted to each is directly opposite to the other. However, when one of the entangled particles has its spin direction measured, the other particle will immediately display the reverse spin direction, no matter how great a distance they are apart. This is the "spooky action at a distance" phenomenon (as Albert Einstein put it) that has already seen the rise of applications once considered science fiction, such as ultra-safe cryptography and a new realm of quantum computing.

Ordinarily, quantum entanglement is a rarely observed occurence in the natural world, as particles coupled in this way first need to be in a highly ordered state before they can be entangled. In essence, this is because thermodynamic entropy dictates that a general chaos of particles is the standard state of things at the atomic level and makes such alignments exceedingly rare. Going up a scale to the macro level, and the sheer number of particles involved makes entanglement an exceptionally difficult state to achieve.

"The macroscopic world that we are used to seems very tidy, but it is completely disordered at the atomic scale," said Paul Klimov, a graduate student in the Institute for Molecular Engineering (a facility formed as a cooperation between UChicago and the Argonne National Laboratory). "The laws of thermodynamics generally prevent us from observing quantum phenomena in macroscopic objects."

In standard sub-atomic quantum entanglement experiments using photons, for example, very high energy value photons are generated using a laser and then directed through a nonlinear crystal. The majority of the crystals will pass straight through unimpeded, however some will undergo a process known as spontaneous parametric down-conversion (SPDC) where, simply stated, a single high-energy photon will be split into two lower-energy photons. As a result of this SPDC, the two photons will have been created entangled, with opposing spin polarizations, because they both were spawned from a single particle.

At a macroscopic level, however, things aren't quite as simple, and particles such as atoms in solids and liquids are particularly difficult to wrangle into a quantum state. This is because the difficulties of overcoming quantum decoherence (put simply, where interfering wave functions from surrounding atoms cause the collapse of quantum states) in entangling particles normally means that ultra-low temperatures (around -270° C (-454° F)) and enormous magnetic fields (about 1,000 times greater than that of an average refrigerator magnet) are required. This is to keep atomic movement close to zero and contain the entangled particles, both of which reduce the likelihood of decoherence.

Given that a practical application of entanglement to macroscopic particles is to enhance quantum electronic devices in real world situations and at ambient temperatures, the researchers sought a different approach to this problem. Using an infrared laser, they coaxed into order (known in scientific circles as "preferentially aligned") the magnetic states of many thousands of electrons and nuclei and then proceeded to entangle them by bombarding them with short electromagnetic pulses, just like those used in standard magnetic resonance imaging (MRI). As a result, many entangled pairs of electrons and nuclei were created in an area equal to the size and volume of a red blood cell on a Silicon Carbide (SiC) semiconductor.

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

With the techniques demonstrated used in concert with other SiC-derived devices, quantum sensors may be constructed in the near future that use entanglement to improve the sensitivity limit over and above that found in current, non-quantum sensors. As the entanglement operates at ordinary temperatures and the SiC device is biologically inert, sensing within a living being is also a potential application.

"We are excited about entanglement-enhanced magnetic resonance imaging probes, which could have important biomedical applications," said Abram Falk of IBM's Thomas J. Watson Research Center and a co-author of the research findings.

Aside from the usual applications in secure communication and information processing, and high-capacity, minimal error data transfer, the research team believes that other technologies, such as synchronizing global positioning satellites could also benefit from this breakthrough.

The results of this research were published in the journal Science Advances.

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