Quantum computers. I’m sure you’ve heard about them - about how they will revolutionize computing and about how blazing fast they will be. The term quantum computer has been tossed about enough that most people have some kind of vision of these lightning fast machines in their head. But do you really know what a quantum computer is or what its impact will be?
A classic computer, like the one you are using right now, uses a bit as its basic building block. A bit can have two values, 0 or 1. Internally to a computer, it is a switch that is either flipped on or off to allow electrical current to flow (or not flow). Classic computers combine multiple bits to represent useful pieces of information.
Quantum computers are built on qubits which can have a value of 0 or 1 just like a classic computer. However, a qubit can also be both a 1 and a 0 at the same time ... crazy. Rather than switches, a qubit is stored in something much smaller, such as the spin of an electron. This works because at this very very small level, quantum physics takes over and, as strange as it may sound, something like an electron can exist in two different states at the same time.
Ummm… so? What's the big deal about a qubit?
So a quantum computer uses qubits … why does that make it so great? All too often, descriptions of quantum computers stop here and we are left with this tremendous leap from, “it uses qubits” to “it's super fast”. But that is quite a leap and is certainly not intuitive.
A qubit can be both a 0 and 1 at the same time, which means that two qubits can be a 00, 01,10, and 11 at the same time. Three qubits can be 000,001,010,011,100,101,110,111 at the same time, and so on. In a three bit classical computer, you would need 8 three bit storage slots, or 24 bits to hold this information whereas in a quantum computer, you just need your three qubits.
To make things even more interesting, because we can store all of these values in the same three qubits, we can operate on them all at the same time as well. So in a classical computer, if we want to manipulate our three bit words, we need to do it one word at a time. In our quantum computer, all this information is held simultaneously within our three qubits, we can operate on them all at the same time as well!
So, 3 qubits can potentially represent 24 bits of data. 4 qubits can represent 128 bits of data, 5 qubits can represent 1024 bits (1 KB) of data, 6 qubits can can represent 4KB of data. If we keep going, it gets pretty scary. 50 qubits can represent 1,267,650,600,228,229,376 TB of information.
That isn't really true - it is a (qu)bit deceiving
When we put it this way it may seem as though we are dealing with some magical, huge, powerful hard drive. That is not true. While 50 qubits have the potential representing1,267,650,600,228,229,376 TB of data simultaneously, You cannot save and retrieve that much data. Because I am not fond of writing out 19 digit numbers, let’s go back to our three qubit example.
Our 3 qubits have the potential of being any and all of 8 numbers at the same time. When we try to read the value of our qubits it isn't like reading three bits from a 24 bit hard drive. The data we read may not even be what we expected. If I try to store the value 101 in my three qubits and then try to read the values back out, I may not get back what I entered. I may read 111 or 110, or any of the eight possible combinations. When we aren't looking, our qubits can certainly be in both states but as soon as we look, the qubit immediately settles into one of its two readable states - a one or a zero.
At this point any excitement you may have felt about the amazing powers of the qubit may be fading, instead feeling that it is about as useless as it gets. Stay with me.
When our qubits are talking to each other - one tiny little quantum particle chatting with another tiny little quantum particle - they can exchange information without losing their superposition (being in both states at once). So we can construct quantum circuits and quantum logic gates so that this vast volume of information can be processed. We just can't observe what is happening inside. We have to be content with only reading the result of whatever calculation is happening.
Not a replacement for your computer
Because quantum computers operate in such a fundamentally different way, they really aren’t candidates for replacing the computer you are using right now. Everything you currently use your computer for - reading email, writing the next pulitzer prize winning novel, doing your taxes, watching cat videos - won’t benefit from quantum computing. In fact, a quantum computer would be much slower.
There are, however, certain problems that classical computers are very bad at. For example - prime factorization. Now … set you’re wayback machine to your early education. Remember having to find factors of numbers? Say … given the number 12, the factors are 2 & 6 and 3 & 4. Prime factors are factors of a number that happen to be prime (can only be divided by 1 and itself). For example, the prime factors of 15 are 3 & 5. For really big numbers, it is almost impossible to find the prime factors in any reasonable amount of time. For example, the prime factors of 1,050,809,297,549,059,047,257 are 32,416,187,567 & 32,416,190,071.
First, stop pretending that you knew that. Second, any algorithm for finding prime factors takes a ridiculous amount of time to complete. For this reason, prime factorization is used heavily in cryptography - keeping your data safe when it moves across the internet.
A quantum computer because it can represent so many numbers in such a small space and because it can operate on those numbers all at the same time, can solve these types of problems reasonably quickly.
As you might guess, our friends in the intelligence agencies are keenly interested in quantum computers. However, there are many similarly complex problems in fields such as chemistry and biology. So beyond just cracking codes, the potential for advancements in medicine and other sciences is tremendous.
Not ready for prime time
As you can imagine, the complexity involved in using something like the spin of an electron to store data makes building a reliable quantum computer rather challenging. Over the last few years, the giants in the computing industry such as Google and IBM have been making huge leaps forward but quantum computing is still in its infancy.
The quantum computers that exist now … and yes, they do most definitely exist, cannot outperform classic computers. When you run these really hard problems through our best and fastest classic computers, the quantum computers are still slower - they have not delivered on their promises … yet. We don’t know when they will begin to perform - if you read the press releases it may be in the next few months. We’ve heard that before, though.
A very different kind of machine
In general quantum computers are viewed as replacements for our current computers. Our heads are filled with visions of ultra fast computers on or desks, in our bags, and in our pockets.
It isn't a box sitting on a desk. The engineering involved in manipulating quantum states, keeping control of individual atoms, keeping superconductors cold, and working with lasers makes for an interesting looking machine. This is not a MacBook. In fact, the computers look more like the spaceship from Close Encounters of the 3rd Kind then they do a classic computer.
The truth is, quantum computers, in theory, will be really good at a set of problems that classical computers are really bad at solving. Conversely, quantum computers will be really bad at doing those activities that our classical computers are good at.
These machines are complimentary, so don’t go selling your Intel stock yet.
The future of computing?
In the 1950’s when a computer filled an entire room it would have been hard to imagine that within 60 years computers that are thousands of times more powerful than those original behemoths would fit into our pockets. Quantum computers are barely even in their infancy so it can be hard to predict where they will be in 50 years. Maybe in 50 years you will have a quantum computer in your pocket, or maybe embedded under your skink. Or not. A lot can change in 50 years.
At least now you have a better idea of what quantum computers are and what they might offer. It isn't magic. It's physics.... quantum physics.
Superposition is a principle of quantum physics that allows any two quantum states to be combined to form a new state. The reverse is also true - any quantum state can be broken down into two other quantum states. This is what allows our qubits to be both a 1 and a 0 at the same time.
Another $10 Word
Quantum entanglement occurs when two particles become linked (entangled) such that a change to one particle's quantum state is instantaneously reflected in the other particle - no matter how far away they are. Quantum computers can use this to allow qubits to interact and to communicate at great distances without delay. Einstein described this behavior as "Spooky."
Quantum computers are cold... really cold. Like absolute zero cold. The particles we're dealing with are so small, that vibrations & interference caused by heat - any heat at all - messes with the system.
Thanks Dr. Feynman
Though quantum computers are a new and yet to be proven technology, they were originally proposed in in 1959 by Dr. Richard Feynman in a lecture where he proposed using quantum particles for computing. 22 years later, in 1981, he published a paper in which he proposed the basic model for a quantum computer.