Albert Einstein famously disparaged quantum entanglement as “spooky action at a distance,” because the idea that two particles separated by light-years could become “entangled” and instantaneously affect one another was counter to classical physics and intuitive reasoning. All fundamental particles have a property called spin, angular momentum and orientation in space. When measuring spin, either the measurement direction is aligned with the spin of a particle -classified as spin up- or the measurement is opposite the spin of the particle -classified as spin down. If the particle spin is vertical but we measure it horizontally the result is a 50/50 chance of being measured spin up or spin down. Likewise, different angles produce different probabilities of obtaining spin up or spin down particles. Total angular momentum of the universe must stay constant, and therefore in terms of entangled particles, they must have opposite spins when measured in the same direction. Einstein’s theory of relativity was centered around the idea that nothing can move faster than the speed of light, but somehow, these particles appeared to be communicating instantaneously to ensure opposite spin. He surmised that all particles were created with a definite spin regardless of the direction they were measured in, but this theory proved to be wrong. Quantum entanglement is not science fiction; it is a real phenomenon which will fundamentally shape the future of teleportation and computing.
Quantum entanglement is the physical phenomenon that occurs when two or more particles are generated or interact so that the quantum state of each particle cannot be described independently. Rather, the quantum state of each particle must be described as a result of the system as a whole. But how does entanglement happen? The most common experimental method is called spontaneous parametric down conversion, where a laser beam is fired at a nonlinear crystal -where laser light can modify the optical properties of the material system- and is used to split photon beams into pairs of photons with combined energies and momenta equal to that of the original photon. Other experiments have used laser light to entangle two electrons trapped in synthetic diamonds. The two particles are connected in such a way that when the property of one changes the other particle changes instantaneously, regardless of the distance between them.
The superposition principle and Heisenberg’s uncertainty principle are the root of quantum entanglement. In the famous Schrodinger’s Cat thought-experiment, a cat is theoretically put in a box with a fatal bomb that has a 50% chance of going off. When we open the box and observe the cat, it can exist in only one state, dead or alive. But, until we observe it, there is a 50% chance of the cat being dead or alive, and therefore it exists in a superposition state where it is simultaneously dead and alive. The Heisenberg Uncertainty Principle -another pillar of quantum physics- reasons that taking a measurement on a quantum system fundamentally affects the system. Because quantum mechanics deal with microscopic objects that operate by their own set of rules, to “see” a particle we must bounce a photon off it for the particle to be registered by our eyes -photon receptors. The particle is so small that its interaction with the photon accelerates the particle at high speeds, meaning it no longer exists in the position we have observed. When two entangled objects are separated, taking a measurement on one must affect what happens to the other. Adhering to the superposition principle, until we observe a particle, it exists in multiple states. An example is a coin flat on a table, and only when it is observed would it “choose” to be heads or tails, while its entangled partner would be the other option. To anthropomorphize an entangled particle, it hasn’t made up it’s mind on which state to be in until it is observed. At this point, it instantaneously communicates with the entangled partner, ensuring opposite spin.
Bell’s inequality proved that particles did not carry a hidden message accounting for their coordinated behavior, adding credence to the theory that particles communicate faster than the speed of light to maintain opposite spins. Bell proposed an experiment in which two entangled particles were measured using two spin detectors, each capable of measuring spin in three directions. The directions were selected randomly and independently of one another, and the experiment was repeated, with different variables. If the particles had a predetermined spin, as Einstein proposed, then they would have different spin at least 55.55% of the time. With predetermined spin where each particle is not always spin up or spin down, a third of the time the particles would have different spin (when they are measured in the same direction). Two-thirds of the time we chose different directions, and in one-third of these instances, the particles have different spins. Adding this up, we obtain a 5/9th or 55.55% chance of the particles having different spins. Additionally, in cases where both entangled particles are either spin up or spin down regardless of the direction measured, the results would be different 100% of the time. This means that in order for Einstein’s hidden variables theory (known as the EPR experiment) to hold true, the results must be different more than 55.55% of the time. However, the results of Bell’s experiment showed that the spins are different exactly 50% of the time. In terms of classical physics, the notion that a measurement at one place can immediately influence a measurement far away, that the entangled particles “conspire” to have opposite spins, seems absurd. But Bell’s theory shows that the entangled particle spins cannot be predetermined and must communicate over a theoretically infinite distance to maintain opposite spin.
Quantum entanglement is a strange concept, and now scientists have focused on whether humanity can leverage the weird, seemingly counterintuitive behavior in the quantum world and apply it to our macroscopic world. When the subject of teleportation comes up most us think of “beam me up Scotty,” but few know that we have already achieved teleportation in a lab. Quantum teleportation does not involve the movement of atoms, but instead is the transfer of the quantum state of an atom or particle from one place to another (without information moving in the area between the two objects). Using entangled particles, researchers can remotely reconstruct data that is in a quantum state at a place arbitrarily far away. The particles themselves do not move, but rather their state changes. For example, some experiments involve trapping matter (a nitrogen atom) and one entangled electron in a nonlinear crystal or synthetic diamond. A piece of information is then encoded onto the matter and information regarding the measurements of the electron and atom are sent to a distant location. Here, the second entangled electron can be manipulated to recreate the encoded information. The information does not travel the distance between them, and it is therefore teleported.
The no-cloning theorem of quantum physics dictates that at the quantum state, it is impossible to create an identical copy without destroying the original. Although the ability to teleport human beings remains wishful thinking, it presents an interesting dilemma. The philosophical paradox, Theseus’ ship, asks if every component in a ship is eventually replaced, is it the same ship? Because the matter of a teleported object must be destroyed and recreated in another location, to teleport a human they must first die. The resulting person would appear identical, but is actually constructed of different atoms which have adopted the quantum state of the original human, begging the question of whether the teleported human is the same person.
Quantum computing is another application of entanglement with serious implications for the future. Quantum computing promises to solve problems that we can’t currently solve or even think to solve with traditional computers. Instead of using binary digits that exist in either an off or on (0 or 1) state, quantum computers use superposition particles known as qubits. Because entangled qubits can simultaneously be in 2 states, they output 2^(n # of qubits) possible qubits of information. For example, with an algorithm on a classical computer, the machine tests and compares the function at every possible variable value, while a quantum computer can test every possible solution simultaneously and will output the most successful one. A traditional computer is like reading a book. You can read each page and understand the contents individually. With quantum entanglement, the information is stored in correlations spread out across the entire book, and therefore to understand the information a computer must be able to read everything at once and understand the system as a whole. Theoretically while a classical computer takes longer than the age of the universe to factor a 500 digit number, a quantum computer could do it in two seconds because it is testing all possible solutions simultaneously.
Despite the promise of quantum computers, strange restrictions within the quantum world make engineering a quantum computer problematic. Because quantum information is in a superposition state, if a quantum computer is observed during processing then decoherence, the destruction of quantum information when exposed to the environment, occurs. There is a need for secret computation and therefore, in line with the no-cloning theorem, the computer should not remember what it was doing because no record exists of the intermediate stages of computation. Theoretical quantum computers also pose a huge security risk in our increasingly digital world. Because Quantum computers can theoretically test all possible solutions at once, using the brute force decryption method, a quantum computer could decrypt any data or password in a fraction of a second. But quantum computing is still in its early stages, and the idea of applying quantum physics, which only exist at an atomic scale, to create systems that are big enough to see, remains far-fetched.
Quantum physics remains a new, relatively unknown field. Quantum entanglement is a strange, logic-defying phenomenon, indicative of potential future technology. The superposition principle and idea that particles exist in a combined spin-up and spin-down state until they are observed and “decide” which condition they assume, seem counter to everything we know about the physical world. But the quantum world is different, and the behavior of quantum information is alien to us. Einstein’s skepticism was not unwarranted: we still cannot explain how the particles communicate faster than light, but rather, leading quantum physicists use this theory of instantaneous communication to explain the odd behavior of entangled particles. Quantum teleportation and quantum computing are both theoretically possible examples of quantum entanglement, posing interesting philosophical questions and engineering challenges.