Quantum Entanglement
I am not going to deny that the complexity of this particular title might be a deterrent for some of you, but allow me to offer a consolation prize summed up in one word. And that is teleportation. It is, after all, the concept of quantum entanglement that laid out one of the first fundamental frameworks for making teleportation a possibility. And while it has not been observed on the macro level as yet, it is undoubtedly one of the most bewildering sightings of late to have occurred in the quantum world — better known as quantum teleportation. And while it had initially sealed a stamp of disapproval from the majority of physicists, it is only now that we begin to appreciate just how much of a far-reaching impact quantum entanglement has in the real world.
Let us first get our head around the more intuitive aspect of the topic — entanglement. In layman’s terms, entanglement is an act of getting twisted with or being caught up in something. So imagine two loose barbed wires getting caught up with each other when you’ve plugged in far too many devices in the same socket. On the other hand, the word Quantum comes from the Latin word Quantus — meaning how big. And so a quantum is related to the amount of physical entity (matter, energy, radiation, etc) involved in an interaction. Some of the more common theories to have incorporated a quantum in a similar manner include Max Planck’s Theory of Quantization, and its application as Einstein’s Theory of the Quantization of Light.
It is paramount here to understand that a quantum system is a portion of the world around us that is testament to the existence of wave-particle duality. Hence, it has an associative wave function and a probability distribution for that. And while a wave function describes the position of a wave in the fabric of space, its corresponding probability distribution entails a range of possible values for its position in a plot against their likelihood/probability. This would be suggestive that at any given point, the position does not have a singular discrete value — as is our notion in Classical Physics — but instead encompasses a range of possibilities.
Furthermore, a quantum state defines the condition in which a quantum system exists. Its state of existence — a compilation of the probability distributions of each of its properties (position, momentum, wavelength, etc). The reason we attribute each of these properties to a probability distribution, instead of a singular discrete number, is because we can never know their exact state — owing to the Uncertainty Principle — unless we disturb said system by making a measurement on it.
Finally taking the plunge, let us now discuss how quantum entanglement is an occurrence whereby two or more particles interact in a way that makes them so deeply intertwined with each other that they are thought to share a mutual existence. They then behave as one entity. Any fluctuations in the quantum state of one are reflected immediately in all others: they share and overlap their quantum states. They are created at the same point and instant in time, and can either be clumped together, or millions of miles apart. Upon identifying the quantum state of one of these particles, we automatically find out the quantum states of all others. And following on from what we have discussed about probability distributions, an entangled particle will always exist in multiple quantum states simultaneously, till we make a measurement or an observation on it — at which point it then collapses to a singular value.
What was the real intrigue for physicists was that this collapse seemingly occurred instantaneously. So suppose we make a measurement of the spin of Particle A in an entangled system. We would see to our surprise, that at the same instant, all other particles in the system collapse to singular and discrete spin values of their own: an inevitable consequence of us disturbing the system by making a measurement on it in the first place (Particle A’s spin). If this was not bewildering enough for you, this next bit certainly is. In the same situation, these entangled particles in fact collapse to the singular values they are supposed to take, such that the properties of the entangled system as a whole are preserved. Let us first imagine that upon measurement the overall spin of a two-particle entangled system was 0. We have also measured for Particle A in this system to have a spin of 1/2. You have guessed correctly if you deliberated that, at the same instant, Particle B would collapse to take a spin value of -1/2, such that the overall spin remains 0 and corroborates our first measurement.
“Spooky action at a distance.”
Because the only explanation for this seemingly instantaneous communication within an entangled system hinged on a transfer of information possibly faster than the speed of light, it comes as little surprise that Albert Einstein vehemently dismissed its existence, as encapsulated in the quote. This was because it was in direct violation of his Theory of Special Relativity, and that was simply not acceptable! And so together with Boris Podolsky and Nathan Rosen, he coined the EPR Paradox paper. This paper claimed that there was no communication or transfer of information within the entangled system to begin with — each particle was entirely independent of the measurements made on its neighbors. And the only reason Particle B could have ever taken the value it did was because it was pre-determined from before any measurement was made on the system in the first place. And so was the case for every other particle in that system. They had little patience for the particle properties taking up a range of values at any given point, and instead believed that they could record discrete values in this case, and those too with a 100% precision. And so, the EPR Paradox was also in violation of the Heisenberg Uncertainty Principle. The trio had further claimed that the oddities of the entangled system might be attributed to the presence of what we refer to as hidden variables today. These hidden variables were supposedly responsible for conserving the definitive nature of the particle properties, and would also rule out the likelihood of there existing any communication between the entangled particles.
John Stewart Bell attempted to create a harmony between the two clashing claims that had arisen thereof. He aimed to abridge the seemingly contradictory notions put forth by quantum entanglement and its rebuttal in the EPR papers. He conceived the Bell Inequality Theorem as a quantitative criterion to delineate how far Particle B would emulate the changes made on its entangled neighbour Particle A if indeed there were hidden variables in play, as suggested by Einstein, Podolosky, and Rosen. And the fact that there even existed this limit attests to how said quantum theory might as well be rendered incomplete! In any case, Bell further demonstrated, through experimental verification, that if indeed hidden variables were responsible for quantum entanglement, they would have to be non-local. This meant that the hidden variables were no longer a finite distance from the entangled system as had been pointed out in the EPR papers, but could even trigger internal communication from infinity.
In the hopes of adding to our persuasion powers, we are going to wrap up with a discussion on some of the applications of quantum entanglement around us. And while we require ultra-high resolution microscopes to even observe these occurrences, we can be assured that they are there. And that it is only a matter of time that we catch the same enigma before our own eyes.
While it has been nothing short of a blessing in our lives, we have to acknowledge also the limitations of the classical computer. There is no denying that the world could do with quicker medical diagnoses, more efficient equipment, and fast-paced technology, and so it becomes imperative we avail what we can from quantum computing. While classical computing employs binary data in the form of 0s and 1s as its language of communication, quantum computing harnesses the power of qubits. In a qubit, all physical properties are accompanied by their corresponding dual states (for example, spin up and down for the spin of a particle). What is in fact fascinating is that these dual states are always in constant superposition with each other. They overlap — the particle is spin up and down at the same time. On this note, it becomes almost intuitive to appreciate that a quantum computer has greater storage and processing capability than its classical counterpart, as the former contains all the data required to work through any and all permutations and combinations for the outcome in one place. This opens up a whole new realm of possibility of making faster statistical predictions and logistical deductions that may be especially significant in matters of say national security and industrial production. In fact, industrial giants as Hitachi, Pasqal, and Toshiba have beaten their competitors in the race to employ quantum technology.
A second application is the marvel that is quantum key distribution. This phenomenon relies on the two fundamental characteristics of quantum entanglement. First, it is contingent upon the interdependence between the two ports at either end of the communication channel, as one would expect of an entangled pair — for a port to reflect immediately any variation made to the other port. And second, just as any external disturbance forces the quantum system to collapse to a single value, any third-person intrusion would similarly disrupt the communication stream and raise a warning. Executing quantum properties in this manner falls in the ambit of quantum cryptography. As for the application, quantum key distribution has been shown to be a secure channel whereby one can create and distribute an encryption key. The information requisite of conceiving such a key is available to only the two entangled ports that are always in touch with each other.
For the final application under our scrutiny, let us jog our memories and reiterate that teleportation is the transfer of mass or energy over some distance without physically traversing that space. American physicists Charles Bennett and Giles Brassard led a group of physicists in an instrumental attempt to help establish the viability of transferring qubits across a communication channel without disturbing any physical entity along the way. Confined to only the transmission of data for now, qubits are thereby transferred along a communication stream towards the termination port, where they are then unscrambled back into a digital (classical) format, without actually coming into contact with any physical entity that may lie along its way.
And because teleportation and most of all other entanglement applications are not yet generalized to the transmission of tangible physical matter, we can not immediately deem it fallacious, but should patiently understand that it is indeed the gateway to getting there. And we are closer than ever before. And it is the same optimism that we are prompted to conclude that it won’t be long before quantum technology takes over our lives! Good thing or bad, is for you to decide.
