How Things Work: Quantum computing
When you combine a technology as familiar as computers with an idea as complex as quantum mechanics, the result is a computer that interprets information significantly faster than the fastest computers in existence today.
The working of quantum computers is similar to that of regular computers since they both rely on the concept of the “Turing machine.”
The Turing machine is an infinitely long strip of tape divided into squares that are read by a machine.
In each square there is a 0 or 1, or a blank space. This is the familiar mechanism of modern computers, where the basic unit of information, the bit, is represented as a 0 or 1, and is interpreted by a read-write head.
However, for quantum computers, the basic unit of information is a subatomic particle like an electron or proton called a quantum binary digit (qubit), and is represented by 0, 1, or any superposition of 0 and 1.
Relating to the Turing machine, this means that in any square there can be a 0, 1, or both 0 and 1, and every point in between.
The superposition of the numbers in a quantum computer gives rise to the computer’s parallelism; for example, a classical computer can represent the number seven with three bits. Because of superposition, three qubits would be able to represent all the numbers from zero to seven simultaneously. Therefore, with enough qubits, information in the computer’s memory and processor can be represented simultaneously.
Parallelism is what makes quantum computing vastly faster than today’s PCs.
Qubits can be manipulated by a variety of devices called “control devices.” Traps are used to catch ions and other particles.
Quantum dots are superconductors that control the flow of electrons and can accurately measure their properties, like spin.
Another factor that makes the quantum computers extremely fast is that the circuits of quantum computers are superconducting, allowing electrons to flow with extremely low resistance. Instead of manipulating data by Boolean logic gates like an ordinary computer, quantum computers manipulate data using quantum gates, which perform operations on qubits.
Some might question the possibility of such a quantum computer by referring to the Heisenberg uncertainty principle: The idea where making observations on subatomic particles is inaccurate because the very act of observing the particles inadvertently changes their state. Scientists use an aspect of quantum mechanics called entanglement to avoid this problem. They indirectly measure the value of one qubit by relating its motion to a second atom, and detecting the values of the second atom. Modern computers can simulate quantum computing, but such simulations are very inefficient.
If there were a simulation of a 500 qubit system, for example, the computer would have to simulate 2500 quantum superposition states.
Given today’s technology, this is an almost impossible feat. As of now, quantum computing is still only a theoretical possibility.
Nevertheless, basic computers, capable of performing certain calculations, have been built. In 1994, Peter Shor, a current MIT professor, created a quantum algorithm for factoring large integers on the order of 10200, called Shor’s algorithm.
In 2001, a seven-qubit quantum computer was built by IBM and Stanford University that successfully found the prime factors of 15 using Shor’s algorithm. The number of qubits then increased to eight, and then 12. In 2007, a 16-qubit computer built by D-Wave Systems solved a Sudoku puzzle.
D-Wave claims that it will make practical computers this year, but many have pointed out inconsistencies in their claims in reference to technology available today.
Making a quantum computer involves overcoming obstacles, such as keeping its components from interacting with anything external.
They must be coherent, which means that the particles must be in phase and have the same frequency in order for quantum states to be measured. Interacting with any external force causes decoherence, as the atoms inevitably entangle with the state of the environment.
Also, the number of qubits used in computers today is not at a practical level, nor is the size and functionality of the device used to read the qubits.
Even if practical computers are years away, the idea of the unimaginable speed and capabilities of quantum computing make it an exciting prospect.
It will be valuable in many operations, from solving equations for extremely large numbers to encryption and decryption.
The applications of quantum computing also include communication and artificial intelligence. Some have theorized that every object is a quantum computer in some way; every physical process could be modeled by a quantum computer, including rational thought.
The many fields of study relating to quantum computing are still in their early stages, but are promising.
Maybe one day, just as the transistor replaced the vacuum tube, the quantum computer will make today’s modern computers obsolete.