Referred to by Albert Einstein as “spooky action at a distance,” quantum entanglement involves two or more particles that are connected in such a manner that an action on one particle affects the other(s), even if they are separated by long distance(s).
Particles have physical properties such as position, momentum, polarization and spin. When these properties are measured on entangled particles, they are found to be connected. For instance, if one photon in an entangled pair has an up-spin, the other photon will be found to have the corollary down-spin. Hence, the quantum state of an entangled system must be described as a whole, and not as states of individual particles.
According to the tenets of quantum physics, an unobserved particle exists in all possible states at the same time but, when observed or measured, the particle has just one state. Furthermore, any observation of a particle's property leads to an irreversible collapse that alters original quantum state. With regards to entangled particles, such a measurement would be of the entire system, and therefore the entire system would collapse if observed.
Given that the statistics of these measurements cannot be replicated by models where each particle has its own independent state, it appears one particle of an entangled pair “knows” if measurement has been conducted on the other, and what the outcome was of that measurement, even though there is no recognized method for such information to be passed between particles, which may be separated by large distances at the moment of measurement.
While Einstein and others regarded such a phenomenon to be impossible, these counterintuitive theories of quantum mechanics were verified experimentally in trials involving the spin of entangled particles measured at separate locations.
How Quantum Entanglement Occurs
Capable of happening in a number of different ways, entanglement occurs when particles, such as photons, physically interact. For example, a laser beam that passes through a particular kind of crystal can trigger the entanglement of photons pairs.
Or, subatomic particles can break down into entangled pairs of other particles. The decay of these particles obeys various conservation laws of quantum physics and consequently, the measurements of one daughter particle are highly correlated with the measurements of the other daughter particle. Hence, total angular momenta, energy and other qualities remain about the same before and after the decay process. For example, a spin-zero particle could break down into a couple of spin-½ particles. Because the total spin before and after this must be zero, whenever the first particle is determined to be spin-up on some axis, the other, when observed on the same axis, is always discovered to be spin-down.
An entangled state can be broken when the particles decohere through interaction with their surroundings; for instance, when a measurement is conducted.
Quantum Entanglement and Cryptography
Due to its highly unique and “spooky” properties, quantum entanglement could have revolutionary applications.
One of the most oft-cited applications of quantum entanglement is quantum cryptography. Traditional cryptography uses ‘keys’: A sender uses one key to encode data, and a recipient has a different key to decipher the message. The two major problems with traditional cryptography are the risk of an eavesdropper, and a key being compromised.
Quantum entanglement can be used to fix these issues through a technique called quantum key distribution (QKD). In QKD, the key is transmitted via entangled photons that have been arbitrarily polarized. This limits the photon so that it vibrates in just one plane-for instance, up-and-down or left-and-right. A quantum key recipient can use polarized filters to decode the key and then use a specific algorithm to safely encrypt a message. Sensitive information still gets sent over normal communication channels, but deciphering the message isn't possible without the exact quantum key. Trying to read” the entangled photons changes their states, which means any attempt to hack the communication will alert the communicators.
Experimental quantum cryptography systems have been successfully tested, but the rollout of such a system appears to be several years away.