The complex mathematics behind these unsettled states of entangled 'spinning coins' can be plugged into special algorithms to make short work of problems that would take a classical computer a long time to work out Such algorithms would be useful in solving complex mathematical problems, producing hard-to-break security codes, or predicting multiple particle interactions in chemical reactions. Building a functional quantum computer requires holding an object in a superposition state long enough to carry out various processes on them.
Unfortunately, once a superposition meets with materials that are part of a measured system, it loses its in-between state in what's known as decoherence and becomes a boring old classical bit. Devices need to be able to shield quantum states from decoherence, while still making them easy to read. Different processes are tackling this challenge from different angles, whether it's to use more robust quantum processes or to find better ways to check for errors.
For the time being, classical technology can manage any task thrown at a quantum computer. One of the branches of physics is quantum mechanics , which studies the laws of nature on the scale of individual atoms and particles.
If we try to simulate quantum mechanics on a computer, we run into a fundamental problem. The full description of quantum physics has so many variables that we cannot keep track of all of them on a computer. If one particle can be described by two variables, then to describe the most general state of n particles, we need 2n variables. If we have particles, we need variables, which is roughly 1 with 30 zeros.
This number is so big that computers will never have so much memory. By itself, this problem was nothing new—many physicists already knew that. But Feynman took it one step further. He asked whether we could turn this problem into something positive: If we cannot simulate quantum physics on a computer, maybe we can build a quantum mechanical computer—which would be better than the ordinary computers?
This question was asked by the most famous physicist of the time. Yet, over the next few years, almost nothing happened. The idea of quantum computers was so new and so unusual that nobody knew how to start thinking about it. But Feynman kept telling his ideas to others, again and again. He managed to inspire a small number of people who started thinking: what would a quantum computer look like? And what would it be able to do? Quantum mechanics, the basis for quantum computers, emerged from attempts to understand the nature of matter and light.
At the end of the nineteenth century, one of the big puzzles of physics was color. The color of an object is determined by the color of the light that it absorbs and the color of the light that it reflects. On an atomic level, we have electrons rotating around the nucleus of an atom.
An electron can absorb a particle of light photon , and this causes the electron to jump to a different orbit around the nucleus. In the nineteenth century, experiments with heated gasses showed that each type of atom only absorbs and emits light of some specific frequencies.
For example, visible light emitted by hydrogen atoms only consists of four specific colors. The big question was: how can we explain that? Physicists spent decades looking for formulas that would predict the color of the light emitted by various atoms and models that would explain it.
Eventually, this puzzle was solved by Danish physicist Niels Bohr in when he postulated that atoms and particles behave according to physical laws that are quite different from what we see on a macroscopic scale. In , Bohr, who would become a frequent Member at the Institute, was awarded a Nobel Prize for this discovery.
To understand the difference, we can contrast Earth which is orbiting around the Sun and an electron which is rotating around the nucleus of an atom. Earth can be at any distance from the Sun. Physical laws do not prohibit the orbit of Earth to be a hundred meters closer to the Sun or a hundred meters further. Because of this, electrons can only absorb the light of colors that correspond to a difference between two valid orbits.
Around the same time, other puzzles about matter and light were solved by postulating that atoms and particles behave differently from macroscopic objects. Eventually, this led to the theory of quantum mechanics, which explains all of those differences, using a small number of basic principles. Quantum mechanics has been an object of much debate. And, even today, popular lectures on quantum mechanics often emphasize the strangeness of quantum mechanics as one of the main points. But I have a different opinion.
The path of how quantum mechanics was discovered was very twisted and complicated. But the end result of this path, the basic principles of quantum mechanics, is quite simple. There are a few things that are different from classical physics and one has to accept those.
But, once you accept them, quantum mechanics is simple and natural. Essentially, one can think of quantum mechanics as a generalization of probability theory in which probabilities can be negative.
In the last decades, research in quantum mechanics has been moving into a new stage. Earlier, the goal of researchers was to understand the laws of nature according to how quantum systems function. In many situations, this has been successfully achieved. Qubits are made using physical systems, such as the spin of an electron or the orientation of a photon.
These systems can be in many different arrangements all at once, a property known as quantum superposition. Qubits can also be inextricably linked together using a phenomenon called quantum entanglement. The result is that a series of qubits can represent different things simultaneously. For instance, eight bits is enough for a classical computer to represent any number between 0 and But eight qubits is enough for a quantum computer to represent every number between 0 and at the same time.
A few hundred entangled qubits would be enough to represent more numbers than there are atoms in the universe. This is where quantum computers get their edge over classical ones.
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