Just one century ago, entanglement was at the center of intense theoretical debate, leaving scientists like Albert Einstein baffled. Today, however, entanglement is accepted as a fact of nature and is actively being explored as a resource for future technologies including quantum computers, quantum communication networks, and high-precision quantum sensors.

Entanglement is also one of nature’s most elusive phenomena. Producing entanglement between particles requires that they start out in a highly ordered state, which is disfavored by thermodynamics, the process that governs the interactions between heat and other forms of energy. This poses a particularly formidable challenge when trying to realize entanglement at the macroscopic scale, among huge numbers of particles.

“The macroscopic world that we are used to seems very tidy, but it is completely disordered at the atomic scale. The laws of thermodynamics generally prevent us from observing quantum phenomena in macroscopic objects,” said Paul Klimov, a graduate student in the University of Chicago’s Institute for Molecular Engineering and lead author of new research on quantum entanglement. The institute is a partnership between UChicago and Argonne National Laboratory.

Previously, scientists have overcome the thermodynamic barrier and achieved macroscopic entanglement in solids and liquids by going to ultra-low temperatures (-270 degrees Celsius) and applying huge magnetic fields (1,000 times larger than that of a typical refrigerator magnet) or using chemical reactions. In the Nov. 20 issue of Science Advances, Klimov and other researchers in David Awschalom’s group at the Institute for Molecular Engineering have demonstrated that macroscopic entanglement can be generated at room temperature and in a small magnetic field.

The researchers used infrared laser light to order (preferentially align) the magnetic states of thousands of electrons and nuclei and then electromagnetic pulses, similar to those used for conventional magnetic resonance imaging (MRI), to entangle them. This procedure caused pairs of electrons and nuclei in a macroscopic 40 micrometer-cubed volume (the volume of a red blood cell) of the semiconductor SiC to become entangled.

“We know that the spin states of atomic nuclei associated with semiconductor defects have excellent quantum properties at room temperature,” said Awschalom, Liew Family Professor in Molecular Engineering and a senior scientist at 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.”

In addition to being of fundamental physical interest, “the ability to produce robust entangled states in an electronic-grade semiconductor at ambient conditions has important implications on future quantum devices,” Awschalom said.

In the short-term, the techniques used here in combination with sophisticated devices enabled by advanced SiC device-fabrication protocols could enable quantum sensors that use entanglement as a resource for beating the sensitivity limit of traditional (non-quantum) sensors. Given that the entanglement works at ambient conditions and the fact that SiC is bio-friendly, one particularly exciting application is biological sensing inside a living organism.

“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.

In the long term, it might even be possible to go from entangled states on the same SiC chip to entangled states across distant SiC chips. Such efforts could be facilitated by physical phenomena that allow macroscopic quantum states, as opposed to single quantum states (in single atoms), to interact very strongly with one another, which is important for producing entanglement with a high success rate. Such long-distance entangled states have been proposed for synchronizing global positioning satellites and for communicating information in a manner that is fundamentally secured from eavesdroppers by the laws of physics.

In certain nanomaterials, electrons are able to race through custom-built roadways just one atom wide. To achieve excellent efficiency, these one-dimensional paths must be paved with absolute perfection–a single errant atom can stop racing electrons in their tracks or even launch it backwards. Unfortunately, such imperfections are inevitable.

Now, a pair of scientists from the U.S. Department of Energy’s Brookhaven National Laboratory and Ludwig Maximilian University in Munich have proposed the first solution to such subatomic stoppage: a novel way to create a more robust electron wave by binding together the electron’s direction of movement and its spin. The trick, as described in a paper published November 16 in Physical Review Letters and featured as an Editor’s Selection, is to exploit magnetic ions lacing the electron racetrack. The theory could drive advances in nanoscale engineering for data- and energy-storage technologies.

“One-dimensional materials can only be very good conductors if they are defect-free, but nothing in this world is perfect,” said Brookhaven physicist Alexei Tsvelik, one of two authors on the paper. “Our theory, the first of its kind, lays out a way to protect electron waves and optimize these materials.”

The work relies on a model system called a Kondo chain, where flowing electrons interact with local magnetic moments within a material. Properly harnessed, this powerful interaction could allow materials to behave like perfect conductors and offer high efficiency.

Protecting the transport

Atom-wide channels only allow motion in one of two opposing directions: right or left. Electrons traveling through such a narrow path–racing along in what are called charge-density waves–can be easily reversed by virtually any obstacle.

“The wave rises like an electronic tsunami that is expected to carry electrons smoothly in one direction,” Tsvelik said. “But it turns out that this tsunami can be very easily pinned by disorder, by impurities in the material.”

This “tsunami” shifts direction through a conductivity-smothering phenomenon called backscattering–like a wave breaking against sheer cliffs. But while direction is easily reversed, another feature of the electron is much more resilient: spin. The spin of an electron–like a perpetually spinning quantum top–can only be described as either up or down, and it is impervious to simple imperfections in the material. The trick, then, is to teach the directional wave to lean on spin for support.

“As the electrons flow, they interact with magnetic moments embedded in the material–these pockets of intrinsic magnetism are the key to producing the bound state,” said Ludwig Maximilian University physicist Oleg Yevtushenko, the other collaborator on the paper. “The magnetic moments bind spin and direction tightly together, so any disturbance would need to flip the electron’s spin in order to change its direction.”

These rolling electron waves could then be described as right-moving with spin up, left-moving with spin down, and so on. In each instance, the direction is bolstered by spin.

Building an electron bicycle

Imagine walking along a narrow path barely wide enough for both feet. In such a simple system, turning around is easy–one can pivot around at the slightest provocation.

“But what if we give our pedestrian a bicycle?” Tsvelik said. “It suddenly becomes very difficult to break that angular momentum and change directions–especially on such a narrow path. This bound spin-direction state is like our electron’s bicycle, keeping it rolling along powerfully enough to overcome bumps in the one-dimensional road.”

To verify the efficacy of this theoretical electron bicycle, scientists will need to apply this theory to stringent tests.

“The magnetic ions in materials such as cesium, iron, and manganese all make excellent candidates for generating and exploring this promising bound state,” Yevtushenko said.

The process of synthesizing functional one-dimensional materials–as thin metallic wires or paths conjured by chemistry–continues to evolve and push both theory and industry forward. Scientists in Brookhaven Lab’s Condensed Matter Physics and Materials Science Department and Center for Functional Nanomaterials specialize in similar one-of-a-kind atomic architectures.

“We hope our colleagues will leap at this challenge, especially as it’s the only method proposed to enhance flow at this 1D scale,” Tsvelik said. “Who knows where these fundamental concepts might lead? The wonder of science is that it brings surprise.”