Quantum computers made headlines last week when giant defense contractor Lockheed Martin announced it would begin using the technology on a commercial scale. What are quantum computers, and how are they more powerful than conventional computers?
Manufacturers have been able to fit twice as many transistors onto the same sized circuits roughly every 18 months or so for most of modern computing's history in an observation known as Moore's law. This phenomenon has allowed for more powerful, smaller computers to revolutionize our life, not just in smartphones and laptops, but in "The Internet of Things" such as cars and thermometers. Eventually, however, Moore's law will fail: chips can only be so small before they fall into the realm of quantum mechanics, which requires different engineering. Quantum computing research is motivated not just by the promise of more powerful computers, but by the fear of this barrier.
In a conventional computer, transistors make bits, which can be in a state of 1 or 0. These bits are used in calculations. Conventional computers' calculations are thus limited by how fast they can process individual bits. Quantum computers work with quantum bits, or qubits. Qubits operate probabilistically. A qubit inhabits a superposition of both 1, 0, both 1 and 0, and all the states in between, with probabilities assigned to each state. They are modeled after physical systems that behave the same way, such as the direction of an electron's spin, or the spin of a metal's magnetic field.
There's a simple example that illustrates why qubits are inherently superior: a group of three conventional bits can represent any number between 0 and 7, but only one number at a time. A group of three qubits can represent all the numbers between 0 and 7 at once! This allows for many more calculations to be performed at once, called parallelism. Your laptop may operate with a basic form of this process by using two or more conventional computer chips at once, Intel's Core Duo and Quad Core processors being brand-name examples.
Right now, quantum computers have not been built on a scale large enough to harness much of their potential. However, within one area the technology is developing very quickly. Canadian company D-Wave, which supplied Lockheed Martin with their computer, has scaled up from a 16 qubit computer in 2007 to a 512 qubit computer last year. D-Wave's computers use a different process than most other quantum computer hopefuls. Rather than making quantum version of logic gates, one step up from bits in the architecture of conventional computers, D-Wave makes an energy landscape. Critics say that this is not truly a quantum computer, but the computer does operate using quantum mechanics; it's just not a computer that can perform any type of calculation, which is called a universal computer. Researchers have been stymied in their quest for a quantum universal computer because connecting qubits with one another collapses them into regular bits. D-Wave's computers cannot replace your laptop, but it is also the only practical quantum technology existent, and its publicity might help fund research for a universal quantum computer.
D-Wave's adiabatic computer is a series of loops of the rare metal niobium. When cooled to just above absolute zero, the loops superconduct and behave according to quantum mechanics. The energy states of the qubits are mapped to the calculation being solved for, which can be thought of as a series of many, many hills and valleys. This approach can only solve optimization problems, but these are many of the most important problems for science and business. Optimization problems include analyzing the human genome's protein folding, finding the most efficient routes and schedules, and building image recognition software (Google has worked with D-Wave on image recognition software for their self-driving cars). The optimal solution is one that uses the least energy, which can be thought of as the lowest valley. A conventional computer must travel every hill and every valley step by step, whereas a quantum computer explores multiple regions of the energy landscape at once, making problems that would take a long time or literally forever with current computers solvable very quickly. This is called "tunneling", as if the qubits tunnel through the hills, but the qubits really are in multiple places at once.
Quantum computers' potential is truly awe-inspiring. A universal quantum computer is still far off in the future, but the pace of development is quickening, and D-Wave's success will bring more research and funding. Recently, scientists devised a way to store quantum information. Once we can crack quantum logic gates or find another way to compute using quantum mechanics, the the gains we have seen from more powerful, tinier computers would pale in comparison to those of quantum computers. Currently unbreakable RSA encryption would be easily cracked; artificial intelligence could become a reality. The most transformative innovations are unpredictable, but we can know one thing for sure: it truly would be a second computer revolution.