Quantum computers hold the promise of massive increases in computational speed for certain classes of problems. However, what quantum computers are and how they work can be a bit of a mystery, mostly because quantum physics is a mystery. We are going to dive in and find out how they work.
First, a little bit about quantum physics. At the turn of the century, physicists were a pretty confident lot. Maxwell had described electromagnetism with Maxwell’s equations and most problems appeared to be solve. However, a couple of items persisted that could not be described by “classical” physics, such as blackbody radiation or the photoelectric effect. It is safe to say, understanding these phenomena and the discovery of Quantum Mechanics was the accomplishment of the 20th century.
Quantum Mechanics describes the physical world at the size of the atom, at which point, the classical laws of Newton and Maxwell fall apart. One of the main takeaways, especially for Quantum Computing, is the idea that NOTHING is specific at the quantum level. Particles, such as electrons, at the quantum level exists in states; aka their position, speed and energy. They may be low energy, they may be high energy, but knowing these states is not always doable unless we measure them. You may know a probability for the energy of an electron, but not the exact value. Schrödinger’s Equation, which describes the probability of an electron being in any state, not exactness. So while the laws of Newton allowed us to put a person on the moon with great accuracy, the laws at the quantum level just tell us probabilities. It is more feelings than certainty!
Schrödinger’s Equations lead us to one of the key components of Quantum Computing, and that is superposition. One of the outcomes of Schrodinger’s Equation, is that any valid quantum states can be added together to create another valid quantum state. Another way of saying this, there are many valid solutions at any given time, and ALL solutions are valid. In the physical world, observing what the answer is, chooses one of those valid solutions. It is a lot to get one’s head around.
To illustrate the madness of what we are talking about, consider the famous example of Schrödinger’s Cat, a thought experiment in physics. The version I have heard, is you put a cat in a box with a radioactive isotope that has a 50% chance of decaying in a minute. If the particle decays within a minute, the cat dies, if it doesn’t the cat lives. With quantum physics, the craziness comes is that you have no way of knowing if the cat is alive or dead, it exists in both states. This is superposition. There are two valid solutions to the equation, the possible states for the cat are alive or dead. However, superposition states that any solution can be added together and that is also a valid solution, so the cat is also alive and dead. It isn’t until we observe, that one of the states is chosen. If you really want to have your mind blown, check out this YouTube video describing Schrödinger’s Cat.
So if we think of our current computers and transistors, they are based on the idea of a bit; either on or off or 0 and 1. In quantum computing, bits are replaced by quantum bits, or qubits. A qubit can be a 0 or a 1, it can be both a 0 and 1, or any value in between. In the current world with bits, only two options exist at any single time, and steps are performed linearly, in the quantum world, with the concept of qubits you can store multiple values at all at once and all the solutions are processes in parallel. Once you determine the state by measuring or observing, you get a single answer.
In the real world, what does this mean. So far, not a whole lot. You aren’t going to get a tremendous increase in performance playing Minecraft. Quantum Computing has so far been proven to be much faster than traditional computing for a small subset of problems. One of these is Shor’s Algorithm, which is very helpful in figuring out prime numbers, which has very important implications for cryptography.
As more and more research happens, more types of problems are discovered that can be solved with quantum computing. This should continue to grow over time.
So your next PC will not be a quantum computer, but the types and classes of problems that can be solved by Quantum Computing will continue to grow and provide real world benefits in the years ahead!