What is Quantum Entanglement? – love on subatomic scale.
Can particles be connected as if they’re joined together, even if they’re millions of miles apart? Quantum mechanics is one of the most amazing intellectual achievements in human history. For the first time, scientists were able to probe a world that was, until then, quite invisible to us. Looking at the world at the scale of atoms – a million times smaller than the width of a human hair!
One of Quantum Mechanics’ most puzzling phenomena is Quantum Entanglement. The phenomenon so riled Albert Einstein he called it “spooky action at a distance”. It is thought to be one of the trickiest concepts in science. To help answer the question of “What is Quantum Entanglement”,first, let’s look at the definition.
“Quantum entanglement is a phenomenon that occurs when a pair or group of particles is created, interacts, or shares spatial proximity so that the quantum state of each particle of the pair or group cannot be described independently of the state of the others, even if that Particles are separated by a large distance.”
In order to completely understand the concept entanglement, we must first understand the concept of ‘Spin’. All fundamental particles in physics have a property called “Spin”. Spin can be described as an intrinsic form of angular momentum with a particular orientation in space. The Spin of a particle can be measured, but we have to choose a direction in which it has to be measured. This measurement can have only two outcomes. Either the particle’s Spin is aligned with the direction of the measurement (Spin up) or it is in the opposite direction than that of the measurement (Spin down).
So what does it mean when we say two particles are entangled?
Let’s understand it with the help of an example. Think of a pair of socks. If you find the left one alone in your drawer, then you can conclude that the missing sock must be right. The two socks could be described as entangled because knowing something about one thing tells us something important about the other.
Similarly, in the case of entangled particles, when measured in the same direction they must have opposite spins assuring that the total angular momentum of the universe remains constant.
Now imagine two friends A and B with friend A having a pair of entangled particles. Friend A separates the pair and gives one of the particles to B. Let’s say A measured the state of his particle and found out that it is “Spin Up”. Now using the same logic as above Friend A can conclude that Friend B must have “Spin Down”. As soon as he knows the state of his particle, he immediately also knows the state of the other particle.
Simple, right? But wait…
Unlike socks, in quantum mechanics a particle has every spin allowed by its wave function i.e. it is both “Spin Up” and “Spin Down” at the same time and is in a state of superposition. Put Simply, the particle hasn’t made up it’s mind on which state to be in, until the moment we try to observe it.
Going back to the pair of entangled particles, we can now say that both the particles are in a state of superposition. Now as soon as Friend A observes the state of his particle, the particle immediately chooses between one of the possible states and it instantaneously communicates with the other particle, ensuring opposite spin. And Friend A would know this even if B was off on the other side of the galaxy.
To put simply, when two particles, such as atoms, photons, or electrons are entangled, they experience an inexplicable link that is maintained even if they are separated by an astronomical distance in space. The word entangled simply implies that the two particles are interdependent on each other. It is not until the Spin of the particle is measured, and the wave function collapses, and the outcome realises.
What is truly mind-bending about Quantum Entanglement is that the communication of quantum information between two entangled particles seems to be faster than the speed of light! Einstein himself thought it was absurd as this directly opposed his Theory of Relativity. Einstein and his colleagues also published a paper suggesting that the quantum description surrounding the particles must be incomplete as it opposes his Theory of Special Relativity.
They theorized a possible resolution to this paradox: Hidden Variables. The theory suggested that the results of the measurement depend on some hidden variables, whose values effectively determine, right from the point of separation, what the outcomes of the measurements on the particles will be.
Later, a physicist named John Bell devised a mathematical inequality to help distinguish whether the EPR (Einstein, Podolsky, and Rosen) representation or the Copenhagen Representation of quantum mechanics was correct, known as Bell’s inequality. It was then proven that the hidden variables theory was incorrect. This seemed to imply that quantum information can travel at a speed faster than that of light. The correlation between the particles can only be seen once the measurements are brought together and compared below or at the speed of light. So the Theory of Relativity is safe.
To understand how Bell disapproved of Einstein’s explanation, imagine there are two spin detectors, each capable of measuring the Spin of a particle in three different directions, these measurements are selected randomly and independent of each other.
Now pairs of entangled particles will be sent to these detectors and record whether the measured spins are the same or different. This will be repeated over and over, while randomly varying the measurement directions, to find the probability that the detectors will give different results i.e., one particle will be Spin up and the other Spin down. This value of probability depends on whether the particles contain some hidden variable or they do not.
It can be mathematically calculated that if the particles do contain some hidden information, the probability of getting different results is 5 out of 9 times. So what can be actually seen in an experiment? It is found that the probability of getting different results is 50%. The quantum mechanic explanation mathematically shows that the detectors get different results 50% of the time and the same results 50% of the time. Therefore the Hidden Variable theory failed to explain the paradox.
Now we know that quantum mechanics works. So does this mean that we can use entangled particles to enable communication faster than the speed of light? No, we cannot! This is because only when the observers meet up and compare notes, would they realize when their results differed and when they didn’t. This can only be done below or at the speed of light. So it does not violate the theory of Relativity.
With the aid of entanglement, otherwise impossible tasks may be achieved. Quantum Entanglement has many real-world applications in our “ordinary” world.
Quantum computing is the use of phenomena such as superposition and entanglement to perform computational tasks. These types of computers are believed to be able to solve certain computational problems, such as advanced simulations, better online security and even computation involved in space travel, substantially faster than classical computers.
These computers use quantum gates and qubits instead of logic gates and bits, it manipulates an input of superpositions, rotates probabilities and produces another superposition as its output. This allows the quantum computers to measure all possible outcomes of any problem at once rather than on a classical computer on which you would have to double-check and try again. This makes quantum computers exponentially more efficient than ever possible.
Quantum Cryptography also uses entangled photons to send a decryption key. The photons have been randomly polarised, this allows the photon to only vibrate in one plain -up and down or right and left. The receiver can then use a polarised filter to decipher the key. The data still gets sent over normal channels of communication. Still, no one can decode the message unless they have the quantum key. This is called Quantum Key Distribution.
The world of small particles is definitely weird, luckily for physicists, this weirdness is starting to spread its way into the everyday world of large things. Developing a deeper understanding of quantum entanglement can help solve problems both practical and fundamental. The quantum revolution has just begun!
