WATTech – Quantum Computing
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In my school days, I was quite fascinated with physics as a whole. What started off with an explanation of heat, light, apples falling from trees expanded to an understanding of atoms, electrons, and subatomic particles. Even for a lot of “geeky” school going students, the term “quantum” was not unknown. From Michael Crichton mentioning it in Timeline to millions of sci-fi novels, comics and series throwing it in your face – quantum was here to stay.
Now, Quantum Computing as a concept is relatively newer. Also, it’s picking up interest quite recently. I shall attempt an overview of the subject. Let’s begin with a very mild introduction of the physics involved, then move on to how normal computers work, to how quantum computers work. As we go along, the potential of these machines will be self apparent.
Ok, I shall assume that everyone understands the whole deal with atoms and electrons. Matter –> Molecules –> Atoms –> Protons, Neutrons and Electrons. Also, school would have taught you that light travels in a straight line. Light is a wave with wavelength, frequency and speed. And travelling in a straight line very well explains reflection, refraction and all other phenomena.
Now, the tricky bit came later. Planck postulated that light was comprised on tiny particles (quanta). So now, from being a wave, light was understood as a stream of particles called photons. Then, came the understanding that ALL particles displayed wave-like properties. Eventually, they concluded that all matter displayed both wave and particle properties.
Now in the early 1900s, these revelations shocked physicists. The knowledge that everything was not as simple as they figured was scary. Heisenberg came up with his uncertainty formula. For an electron in an atom, you can never know the exact position AND the exact velocity. Quatum Physics deals with uncertainty. The theory evolved. Everything was present in a variety of states with all these states in continuous flux.
Let’s try understanding this. An electron is rotating around an atom. However, it’s not there as one particle around another particle. It is in fact a wave. One entire sphere around the nucleus is the electron. And there is a finite probability you will encounter it if you try catching it. Quantum physics/mechanics deals with quantum states.
- How you were taught it looks
- How it really looks!
Schrodinger re-invented wave mechanics. But he did not like the uncertainty inherent in quantum mechanics. He states his famous “cat problem”. For a radioactive element, the number of atoms which have not disintegrated decreases by half after a specific time called the half life. So if there are 100 atoms, it comes down to 50, then 25… so on and so forth. So if there was only one atom in a box, after the half life had elapsed, what would happen to the atom. Probability of it having disintegrated = o.5. However, quantum mechanics will tell you that the particle has both disintegrated and NOT disintegrated. And until you open the box, it exists is both states. He tied this to his cat. If the life of the cat was dependent on the atom, was the cat both alive and dead simultaneously.
Quantum mechanics will tell you that the observation of an act changes it. And that until something is measured, it can exist in a wide variety of states.
So much for the physics.
Normal day computers work with 0s and 1s. Your fancy operating system and softwares are translated into more basic code that can interface with your system hardware. Your hardware further breaks it down to 0s and 1s. At the basic of your computer hardware, there are diodes, MOSFETs and other semiconductor devices. All of these work on the On-Off principle. Hence Off = 0, On = 1. Now imagine millions of these devices integrated to form higher level abstractions. Hence, all computing runs on BITS. 0s and 1s.
Quantum computing runs on Qbits. A qbit is the quantum analogue of a bit. So, if a bit exists either as a 0 or a 1, a qbit exists as a linear superposition of 0 and 1. In general, thus, the physical state of a qubit is the superposition |ψ> = |0> + β|1> (where α and β are complex numbers). If we try and measure the value of this qbit, we will get 0 with probability α^2 and 1 with the probability β^2. Also, (α^2)+(β^2) = 1, so we will definitely get a value.
What quantum computing does is that it allows the bit to occupy all possible states. Then, when asked for the value, collapses it down to the most probable state and gives the answer. For example, Feyman in QED said that light does NOT travel in straight lines. It travels in a number of paths, and it’s just that the straight line is the most probable path for you to find it.
So, what so cool about being confused about a lot of states? Because a quantum computer can contain these multiple states simultaneously, it has the potential to be millions of times more powerful than today’s most powerful supercomputers.
Quantum computers also utilize another aspect of quantum mechanics known as entanglement. If you look at a qbit in superposition to determine its value, the qbit will assume the value of either 0 or 1, but not both (effectively turning your spiffy quantum computer into a mundane digital computer).
So, in theory, a qbit may occupy infinite states. But practically? To make a practical quantum computer, scientists have to devise ways of making measurements indirectly to preserve the system’s integrity. Entanglement provides a potential answer. In quantum physics, if you apply an outside force to two atoms, it can cause them to become entangled, and the second atom can take on the properties of the first atom. So if left alone, an atom will spin in all directions. The instant it is disturbed it chooses one spin, or one value; and at the same time, the second entangled atom will choose an opposite spin, or value. This allows scientists to know the value of the qubits without actually looking at them.
Now, that’s the concept behind quantum computing. We’ve understood that it provides more computational potential. But is it achievable. Apparently, yes, using quantum algorithms.
Now, all data uses encryption. The most widely used public-key cryptography scheme is the RSA. It works on the funda that computers can’t really factor large numbers really fast. In fact, no classical algorithm is known that can factor in polynomial time. However, Shor’s algorithm shows that factoring is efficient on a quantum computer, so an appropriately large quantum computer can “break” RSA. It was also a powerful motivator for the design and construction of quantum computers and for the study of new quantum computer algorithms. In 2001, Shor’s algorithm was demonstrated by a group at IBM, who factored 15 into 3 × 5, using an NMR implementation of a quantum computer with 7 qbits.
Also, if it could break it, it could also make it. Imagine data encoded in qbits with each bit occupying hundreds of different values. If you try to observe the value, the data changes. The ultimate cypher – it changes if you try to break it.
Now, the difficulty is physically simulating these devices. The developments in quantum computing are provided here.
Now, let’s look at the criticism offered. Since the criticism is not mine, and I do not claim to understand it extremely well, here’s the link (Check the What’s Quantum in Quantum Computing). The argument offered is that there is barely anything “quantum” in quantum computing. Also, only one algorithm has actually proven that it is faster than normal algorithms. And it mentions the impracticality of building a large scale quantum computer.
There’s a lot of recent news and hype about quantum computers in general. For example, Ars Technica comments on Time Travel and Quantum Computing. There was an article recently about a breakthrough in solving large equations using quantum theory.
There was an interesting article in the Mint recently on how the amount of data produced in scientific research is impossible to keep in check. In fact, a lot of experiments require way more storage space than currently possible. We have reached a juncture where further scientific progress needs more computational power and more storage space.
As days progress, more attempts are being made to redifine computing. There’s Quantum Computing, DNA Computing, Photonics, Plasmonics…And in 1947, American computer engineer Howard Aiken said that just six electronic digital computers would satisfy the computing needs of the United State. Funny isn’t it?
For those interested in reading and knowing more : There’s a lot of material available online.
Quantum Mechanics – Physics Bit
Introduction to Quantum Mechanics
Quantum Electrodynamics
Interpretation of Quantum Mechanics
Quantum Computing
The Wikipedia Article
Qbits
Quantum Algorithms
How Stuff Works : Quantum Computers – extremely useful
Quantum Computing – an Understanding
Video : Youtube Video – Quantum Computing – Google TechTalks
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excellent article…loved reading it. got back into old days of physics..