These machines would have huge benefits, but scientists are struggling to create oneby Philip Ball / November 14, 2013 / Leave a comment
Scientists at Raytheon BBN Technologies work on building a quantum computer
When people first hear about quantum computers, a common response is “where and when can I get one?” But that’s the wrong question, and not just because you’ll be disappointed with the answer. Quantum computers are often said to promise faster, bigger, more multi-layered computation—but they are not, and might never be, an upgrade of your laptop. They’re just not that sort of machine. So what are they, and why do we want them?
You could argue that your laptop is already a quantum computer, because the laws of quantum physics govern the ways that electricity passes through it. In part that’s just pointing out that, ultimately, quantum physics governs all the properties of materials at the atomic scale. What’s more, as the scale of electronics shrinks, strange quantum effects that don’t usually manifest in the everyday world, such as the ability of electrons to leap through walls, are becoming important. This “quantum tunnelling,” for example, is the basis of flash memory.
Real quantum computers go far beyond any of that, though. In the end, all of today’s computers work using old-fashioned binary logic: by encoding information in strings of 1’s and 0’s, values known as “bits.” These are then manipulated in “logic gates,” devices built from electronic components such as transistors. A particular set of input bits prompts the gate to produce another set of output bits. That’s what computation is; the rest is a question of building software and interfaces that turn these bits into a letter to Mum glowing on the screen.
Quantum computers will also use 1’s and 0’s, but with a crucial difference. As well as having just one or other of these values, a quantum bit, or “qubit,” could have any mixture of them—it can be counter-intuitively in two states at once. While a regular bit can only be 1 or 0, a qubit can be simultaneously 1 and 0, or 1 with a tiny bit of 0, and so on. These mixtures are called superpositions, and they are a fundamental feature of objects that obey quantum rules. A photon of light, for example, can be polarised either vertically or horizontally, or can be in a superposition of both polarisations.
That gives qubits access to a vast range of states, so you can…