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An international team of physicists from the Universities of Illinois in the United States, Pavia, Italy, and Toronto, Canada has recently published a paper in Physical Review Letters that demonstrates a new method for generating quantum entanglement in an optical fiber.
This cutting-edge research will not only help experimental physicists gather rich and useful data on quantum states, but could also be used in the search for so-called “quantum supremacy” – the theoretical future state in which quantum computers can be shown to perform any task faster or better than the conventional classical computers we use today are capable of.
What is Quantum Entanglement?
Quantum entanglement is one of the peculiar phenomena observed in matter at the quantum scale, which is the smallest possible interacting scale of size. Here in the realm of atoms, neutrinos and electrons, the conventional laws of classical mechanics are turned on their head. Particles can exist in two opposite states at once in a quantum superposition, as illustrated by the famous Schrödinger’s cat example, or can pass through material like a ghost going through a wall in quantum tunneling.
Quantum entanglement is at the heart of these peculiarities. This is the phenomenon in which two or more particles are generated, interact with one another, or are spatially close to each other in such a way that individual particles cannot be described independently of the pair or group.
When quantum entanglement occurs, measuring, for example, the movement of one particle in the group reveals perfect correlation with other entangled particles. In addition to this, causing one entangled particle to spin results in other entangled particles spinning as well. This effect is observable, despite the particles’ separation by vast distances.
Albert Einstein and Erwin Schrödinger could not believe the phenomenon was possible when they first discussed it in 1935. To this end, Einstein referred to entanglement as “spooky action at a distance,” whereas Schrödinger soon realized it was “not … one but rather the characteristic trait of quantum mechanics.” Several recent experiments have emerged as a result of these hypotheses.
How is Quantum Entanglement Generated?
Quantum entanglement is observed in nature as atoms with multiple electrons that always have entangled electrons in their electron shells. Some physicists have also suggested that the energy transfer from kinetic energy to chemical energy in photosynthesis involves quantum entanglement.
In the laboratory, entanglement can be generated by several induced interactions between particles. For example, converting one high-energy photon into two low-energy photons results in a pair of particles that are entangled in polarization.
Another method involves the mixing of photons in a fiber coupler, which is a joint connecting optical fibers. The latest research further enhances this method by generating entanglement between three particles, which is significant and not just a trivial extension of methods for generating bipartite entanglement.
The team emitted light from modelocked titanium: sapphire laser and then split it using a series of colored filters. This technique is known as spontaneous four-wave mixing (SFWM). However, the new method involved simultaneously performing this kind of mixing with different color filters. Ultimately, multiple photons were separated by distance yet remained entangled in one system that could only be described as a whole.
Applications for Quantum Entanglement
The researchers anticipate that their method for low-noise and low-loss quantum entanglement generation “will find useful applications in quantum information protocols.” Quantum information science is the field of research that works with the hypothesis that all information is bound by and operates according to the laws of quantum mechanics.
Within quantum information science, quantum computing applies the findings of physicists like those who found the new method for generating quantum entanglement to create exponentially more powerful quantum computers. This aspect of quantum information science requires methods for generating quantum entanglement that can be replicated at scale with minimal defects. At present, quantum computers can only be formed from arrays of particles in strictly controlled laboratory environments and with expensive machines.
However, with research like this, new methods for generating quantum entanglement will be discovered and eventually applied to practical quantum computers, thereby bringing quantum supremacy to new light. When that happens, researchers anticipate that a new information revolution comparable to the drastic change in society that resulted from the discovery and application of transistor chips in modern computers will occur.
References and Further Reading
Fang, B., Menotti, M., Liscidini, M., Sipe, J.E. and Lorenz, V.O. (2019). Three-Photon Discrete-Energy-Entangled W State in an Optical Fiber. Physical Review Letters, 123(7). DOI: 10.1103/PhysRevLett.123.070508