Quantum computing involves performing calculations through the use of certain principles of quantum mechanics. Quantum mechanics is a branch of physics that describes the fundamental behavior of matter and energy, particularly that of subatomic particles (particles smaller than atoms ). A quantum computer can solve certain kinds of problems much more quickly than could a classical, non-quantum computer.
Representing information.
Subatomic particles, including electrons and photons (particles of light), exhibit certain behaviors that may seem strange or impossible. For example, an electron has a quality called spin that can take the value of “up” or “down.” But the electron can also exist in a superposition of these states, in which its spin is both “up” and “down” at the same time. However, the superposition collapses (becomes one state or the other) when the electron’s spin is measured. For this reason, scientists can never directly observe superposition.
In classical computing, information is encoded in bits. Each bit consists of a binary digit: 1 or 0. Modern computer chips simulate bits with billions of tiny electronic switches called transistors . A transistor can either switch on, for 1, or off, for 0. A quantum computer, in contrast, uses the quantum bit, or qubit, as its basic unit of information. A qubit can take on the value of 1 or 0, but it can also exist in a superposition of both values. Quantum computing takes advantage of superposition and quantum entanglement to compute in fundamentally new ways. In entanglement, two particles are held in complementary superposition. If one particle collapses into one state, the other particle collapses into the other state, even if the particles are separated by vast distances.
Qubits are simulated using subatomic particles that can be held in a superposition of values of some physical property. One method uses an electron’s spin, which can be either “up” or “down” or a superposition of these values. Another method relies on a property of photons called polarization. Polarization is the orientation of a photon’s electromagnetic field.
History.
In 1982, the American physicist Richard Feynman suggested that a quantum device would be needed to accurately simulate a quantum system. Feynman proposed a basic model for such a device’s construction. In 1985, the British physicist David Deutsch described a universal computer that could operate using quantum principles. A universal computer is an imaginary model of a machine that can store information and run programs.
In 1994, the American mathematician Peter Shor created a quantum algorithm. An algorithm is a step-by-step procedure that can be turned into a computer program. Shor’s quantum algorithm could be used to find the prime factors of a given number much more quickly than could any known classical algorithm. Algorithms like Shor’s, if run on a suitable quantum computer, could in theory break a widely used method of encryption called RSA. Encryption involves encoding computer information to keep it secure. RSA encryption is extremely hard to break because it requires factoring large numbers. This task takes classical computers a long time, but it could be done more quickly with Shor’s algorithm.
The first quantum computers were built in 1998. A group of researchers from Oxford University used a device called a nuclear magnetic resonance (NMR) machine to build a two-qubit computer. The computer succeeded in running a quantum algorithm developed by Deutsch faster than could any classical computer. At the same time, another two-qubit computer was created using an NMR machine by a group of researchers from IBM, the Massachusetts Institute of Technology (MIT), and the University of California, Berkeley. This group executed a quantum program developed by the Indian-born American mathematician Lov K. Grover. In 2009, researchers at the United States National Institute of Standards and Technology (NIST) constructed a programmable two-qubit computer.
Working with qubits.
To understand the advantage that quantum computing brings to solving certain kinds of problems, imagine trying to find a certain string of eight characters in a list of all such strings. Imagine that the string was also case-sensitive—that is, it distinguished between upper- and lowercase letters. A classical computer would accomplish this task by checking every string on the list until it found the correct one. There are over 50 trillion possible combinations of upper- and lowercase letters. Thus, if the computer could check 100 strings per second, this task could take up to 17,000 years. A quantum computer, on the other hand, could create a quantum state that is the superposition of all possible combinations. Using Grover’s quantum search algorithm, such a computer could find the correct string in just over 20 hours.
Maintaining superposition or entanglement for even a few seconds of computing time remains a difficult engineering problem. But researchers are working to build more powerful quantum computers.
