Article 1 - https://www.qcguy.com/blog-1-quantum-mechanics-the-basis-of-quantum-computing/
In 1947, the world’s first transistor was built. They formed the building blocks of the computing revolution that has ensued since then. Today, every year the world manufactures over 10,000,000,000,000,000,000 (that folks, in words is 10 quintillion. A Very large number.), which is one hundred times more than the sum total of all the grains of rice consumed every year by the world’s seven billion residents.
The world’s first transistor computer was built in Manchester in 1953 and had 92 of them. Today, you can buy over a hundred thousand transistors for a cost of a single grain of rice and there are about 6 billion of them in your latest iPhone. The rate at which the technology has evolved since the invention of the humble transistor in 1947 is rapid, to say the least. The Moore’s law – doubling of how many transistors can fit on a computer chip every two years – has largely been followed for decades since its inception almost half a century ago (Moore’s Law was coined on the back of an article written in 1965 by Gordon Moore, the then CEO of Intel, about this doubling). More transistors mean more computing power and thus more powerful applications to aid mankind’s various ambitions, from flying to the mars to mining bitcoins and everything in between and more.
Graph tracking the Moore’s Law from 1970 to 2018 (Source – Wikipedia of course!)
Although performance increases had previously simply followed from this Moore's law type scaling, these straightforward improvements have significantly waned over the past decade and now we are seeing this speed of computing evolution being tested to its limit. The only way to pack a higher and higher number of transistors on a single CPU chip is by reducing the size of the transistors and making the packing denser which doesn’t come cheap. We are already using cutting edge technologies that manufacture transistors that measure just 7nm (nanometres). To appreciate that size (or lack thereof) - a human hair is around 75 microns (abbreviated 75μm) or 75,000nm in diameter. The relationship between a nanometre and that hair is similar to the relationship between one mile and an inch - one mile is 63,360 inches. A human red blood cell is 6,000-8,000nm across, and the Ebola virus is about 1,500nm long and 50nm wide. I digress. To go any smaller than 7nm requires exponentially more amount of investment from the manufacturers and soon we will be tackling with laws of diminishing returns, the point where the cost of developing a technology to reduce the size of transistors would be more than the benefits it reaps from such investments.
So, in short, we are testing the limits of this rapid growth in the power and speed of “classical computers”. Don’t get me wrong though, classical computing isn’t going to die anytime soon and is going to be our companion in achieving greater things for years to come. The technology to go smaller is being tested in the labs as I write this article. Even down to 3nm! But no matter how excruciatingly small we get our transistors to be, it is safe for me to say that if we stick with classical computers the future is going to be more of the same, evolutionary rather than revolutionary.
Enter Quantum Computing (QC). QC brings with it the promise of bringing true revolution in terms of not just more power and higher speed but more importantly the capability to get answers to some of the fundamental questions that mankind has been pondering since it has achieved consciousness. How can one make such an audacious statement on QC’s capability you ask? Well, that is because it’s unlike what we have seen or done with “classical computers”. QC is a tool that uses nature’s own laws of operation, or in other words the laws of Quantum Physics and as you may have read in my previous article the quote from Richard Feynman, “Nature isn’t classical” (if you were brave enough to read through to the end of my previous article that is).
As human beings, what are we but a species that is looking for answers to some of the fundamental mysteries of the nature that finally aid in the understanding of our role in this universe. Classical computers have given us the chance to expand our understanding in that quest but I can safely say that they may not get us the answers we are looking for because, as Quantum Mechanics (QM) has proven, the building blocks of this universe, the atoms, the electrons, the neutrons, the photons, etc. don’t work in definitive zeros and ones but are more fluid and probabilistic, in that they take the form of zero and one and everything in between all at once! Indeed, Richard Feynman believed that QCs would be the only type of computers capable of accurately modelling the complexity of a universe itself built on quantum processes — from the formation of molecules and stars to the activity of the human brain.
In transistor-based classical computers, a transistor represents a binary classical bit that can store one bit of information. Classical bits are found in one of two distinct logical states: logic state 0 or logic state 1. “State 0" simply corresponds to the transistor switch being “off" (e.g., no voltage is applied to the transistor gate, and so no current flows in the transistor channel), and “state 1", as you may have guessed, corresponds to the transistor switch being “on". These discrete states are robust and can be measured with near certainty.
Now compare those to the fundamental elements of quantum computers, the “quantum bits", typically referred to as “qubits." Qubits are quantum-mechanical two-level systems. They are binary in the sense that they can be initialized in classical states 0 or 1. However, as quantum mechanical objects, qubits can also be prepared in a quantum superposition state: a single quantum state that embodies aspects of both state 0 and state 1 and is most famously illustrated by the thought-experiment, Schrödinger’s cat (I will talk about this cat in my future articles). Because of this complex nature of qubits, they have to be represented using something more suitable than just ON and OFF and Bloch sphere allows for that complex superposition state to be denoted in addition to the classical states.
Bloch sphere representing a qubit.
I will delve into the details of the Bloch sphere and how to read it in my future articles.
Superposition is the reason every logical qubit (again for my future article to explain why logical qubits and why not physical) added to the system increases its computational power exponentially. So, two qubits can process (in parallel) the same amount of information as four classical bits, three qubits the same as eight bits and so on. The figure below shows a representation of how it works. Each atom representing a qubit allowing us to play with 8 classical states thus making 3 qubits = 8 classical bits. The arrows pointing up and down represent the spin up (State 0) and spin down (State 1) of the qubits while the superposition state is represented by, you must’ve guessed it by now, arrows pointing somewhere in between the states 0 and 1.
The figure above depicts 3 qubits and the resulting 8 states that are similar to 8 classical bits.
It is calculated that once a QC has 56 logical qubits, it could outperform a quantum computer simulation running on a classical supercomputer (a point known, somewhat misleadingly, as quantum supremacy). A recent achievement by Google claims to have reached that quantum supremacy stage which is quite an exciting feat (link - https://www.ft.com/content/b9bb4e54-dbc1-11e9-8f9b-77216ebe1f17). And at 100 logical qubits, a single quantum computer processor would, theoretically, be more powerful than all the supercomputers on the planet combined. So, it isn’t a question of if but when will we achieve this stage but whenever we do the point to ponder now would be, is the world ready for the Quantum Computing revolution? Even with a QC with a relatively small number of logical qubits we have identified algorithms that can take advantage of such a QC to break the most secure form of public encryption scheme there is, with the help of which world's information is securely transmitted from one point to another, the RSA encryption. Think of the possibilities when a QC can be programmed to simulate a molecule or even a bacteria. These kinds of simulations could solve substantial problems, from fighting deadly diseases to finding more energy-efficient fertilizer manufacturing process (yes, fertilizer manufacturing! And it’s an interesting problem that I will talk about in my next article).
QC also provides the capabilities to use some of the cool features of QM. Features such as -
Quantum Parallelism and Quantum Interference – Quantum parallelism and quantum interference form the basis of how a quantum computer processes information. With just one operation, quantum parallelism and quantum interference allow us to simultaneously manipulate and change the values of the many states that comprise a superposition state. For instance, take the above diagram with 3 qubits and their equivalent 8 states. Quantum parallelism and quantum interference allow us to simultaneously operate on all 8 states with just 1 gated operation (we will learn more about the quantum gates in my next article). That’s like working on all 8 classical bits at the same time and that’s just with 3 qubits! Most modern computers have 64-bit processors and that is after 6 decades of computing evolution, we will achieve an equivalent 64-bit processor with just 6 logical qubit QC, which is already under grasp! And at a fundamental level, quantum parallelism and quantum interference are the reason why we can efficiently implement quantum algorithms on a quantum computer.
Quantum Entanglement – While the probabilistic nature of the QM irritated Einstein the entanglement feature of QM “spooked him”!
Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles are generated, interact, or share spatial proximity in ways such that the quantum state of each particle cannot be described independently of the state of the others, even when the particles are separated by a large distance.
In other words, quantum entanglement can be explained using this example nicely explained by Philip Ball in this video - https://www.youtube.com/watch?v=5_0o2fJhtSc. If you are short of time, just listen to the first 5 min. of this video.
The various governments around the globe are working on using this property of QM. China, for instance, is at the forefront of this. They have deployed a quantum communication satellite to perform various experiments on this property and what they have achieved so far is pretty extra-ordinary. They have established a photon entanglement over 1200 km. What this means is that when one of the two entangled photons is sent to the satellite and the other one kept on the station on earth, the moment the first photon is observed at precisely the same moment (yes, the same moment, not after 1 second or 1 microsecond or 1 nanosecond, the same moment) the other one settles in a state that is exactly opposite of the first photon. Now for the more learned readers, the question would arise about Einstein’s law that nothing travels faster than the speed of light and does this process then violates it? The answer is no because no real information has been passed between the two photons. This is better explained by the below example taken from a Cornell Q&A session -
Say you agree to send out two beams of light to your two friends who live on opposite sides of the galaxy (you live in the middle). Ahead of time, you tell them that if one of the beams of light is red the other will be blue. So you send the blue beam to your friend on one side and immediately she knows that your other friend is receiving a red beam at the same time. Aha! You say, my friends have now communicated at a speed faster than the speed of light and violated relativity, but no real information has been passed between them. You have told both of them at normal sub-luminal speed about what you just did and that's all. (A way of proving there's no faster than light communication is that you could lie and send them both the same coloured beam of light and they would never know!). With QM is gets a bit more complicated because theoretically no-one knows the state of the particle until it has been observed, but you still cannot affect the state of the particle so the argument is the same.
Physicists at the University of Glasgow set up a complicated experiment to capture in a single image what Einstein called "spooky action at a distance." A pair of photons were shot from a laser, split and sent on very different journeys before being captured by a special camera. The resulting image consistently showed what looks like a pair of photons mirroring each other to form a ring shape and here is an image of it that was captured for the first time ever.
Actual image of quantum entanglement on a pair of photons. Spooky to say the least!
Quantum Teleportation – This leads to our third and final property that I feel is worth mentioning about QC. Although the name is inspired by the teleportation commonly used in fiction, quantum teleportation is limited to the transfer of information rather than matter itself. Quantum teleportation is not a form of transportation, but communication: it provides a way of transporting a qubit from one location to another without having to move a physical particle along with it. But it nevertheless is a pretty cool property to talk about.
If a picture speaks a thousand words then a video speaks a million. Here is a video that will help you get your head around this property - https://www.youtube.com/watch?time_continue=78&v=DxQK1WDYI_k
If you have reached this part of the article, kudos to you. Not many would be brave to take in so much information at one go especially if a lot of it is new information. And if your brain is hurting with all this information, don’t blame me! Well yes, blame me but also blame it on the weird and wonderful world of QM and QC. QM and QC force us to expand our imaginations to a point where we start questioning what is real and what is not (Just like Elon Musk does when he says we could part of a simulation and there is no way of knowing whether we are or not! link here - https://www.vice.com/en_us/article/8q854v/elon-musk-simulated-universe-hypothesis). But I am hopeful that when a large number of us get our heads around this phenomenon we will be slightly more advanced as a species than what we are today. It is believed that we were this advanced, and more, in our thinking and had the understanding of Quantum Mechanics thousands of years ago based on what we can see written in the texts of the Upanishads, the Vedas, and the Puranas but that is what I call a fringe topic which I shall, sure enough, touch upon in my future articles.
I am cognizant that I have noted at various places about the topics I would pick up in my next articles. This is to ensure that I do not dump a lot of technical information on the readers at one go and to keep the articles digestible even for the ones who haven’t had their careers that revolve around computers and technology.
In the next articles, we will explore the practical usages of Quantum Computers and its various capabilities in detail. We will also look at the areas where QC has already made an impact so far and the possibilities that lie ahead.
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