What on earth could be so strange that Einstein himself called it “spooky?” Well, as you might have guessed, it has to do with quantum physics. The weird, the wacky, and yes, the spooky, all come together in the concept of quantum entanglement or “spooky action at a distance.” This phenomenon has puzzled physicists for generations, and this article will explore some of the qualities and applications of entangled particles.
First, it is necessary to understand quantum states. When you zoom in really far into an object, even past its atoms, you get to the level of quantum physics: the level of the fundamental building blocks of the universe. This level of reality doesn’t behave in the way that ours does; the properties of particles are not definite but are represented through probabilities and the wave function. For instance, a particle could have a 25% chance of being on the couch and a 75% chance of being on the floor. However, when the particle is observed, the wave function collapses, and the particle is now firmly in one quantum state: either on the couch or the floor, but not both, like it was previously.
Entangled particles exhibit this same property but in a strange way. When one entangled particle of the pair is observed and its quantum state is defined, the other particle’s quantum state is also changed. This works no matter how far away the two are; when one is measured the other is instantaneously impacted. Some physicists believe it happens truly instantaneously, but estimates conclude it happens at 10,000 times the speed of light. This is perplexing because, according to known laws of reality, nothing travels faster than light. Unfortunately, we still do not know how it is possible for entangled particles to “communicate” this fast.
These funky little beasts don’t just pop out of thin air, with scientists racing around hoping to get lucky. Researchers have developed methods of producing entangled particles for study. Usually, they use photons because they tend to be the easiest to entangle.
The oldest method, developed in the 1980s, is a clunky method in which a number of calcium atoms are excited. An atom in an excited state has an electron in a higher energy level than it should be. Normally, this electron drops down to the next lowest energy level and releases a photon. In this method, however, that electron is prevented from coming back down to its “ground state.” After a little while of basically holding the electron hostage, it emits two photons instead of one. These photons are entangled from birth! Pretty neat process, but the release of the photons is extremely unpredictable. The photons follow the conservation of momentum laws and are released in opposite directions. Unfortunately, that’s all researchers know for sure, so they can’t place detectors in strategic spots. This technique, dubbed “cascade” transition, is not as widely used today because of the difficulty of data collection.
A more modern approach to creating entangled particles is splitting a photon into two, which are entangled. Parametric downconversion, as it’s called, involves the firing of a laser through two very thin layers stuck together of a certain type of crystal, nonlinear optical crystals specifically, which splits the photon. This method is far more common than cascade transition due to its high success rate. Because researchers can control the laser’s angle, and conservation of momentum still applies, detectors can be placed exactly where they need to be, and large amounts of data can be collected.
These crazy properties aren’t just for show: scientists have found uses for entangled particles. The most concrete research has been into the applications of quantum entanglement in quantum computing. A quantum computer is also known as a “supercomputer,” with larger processing power than any computers currently on the planet. Quantum computing uses “qubits,” which, unlike bits, which can only be 0 or 1, exist as a combination of the two, not unlike our particle that was 25% on the couch and 75% on the floor. Qubits can be extremely unstable, but entanglement can provide a solution. By entangling the qubits, the collapse of one qubit results in the collapse of others, creating a larger problem that is actually easier to see and to fix.
Even if entanglement doesn’t provide the fix for quantum computing physicists hope for, it is still an incredible phenomenon shrouded in mystery and the unknown. Besides, isn’t that leap into the dark what makes science so fun?
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