From Bits to Qubits
A digital computer both stores and processes information using bits, which can be either 0 or 1. Physically, a bit can be anything that has two distinct configurations: one represented by “0”, and the other represented by “1”. It could be a light bulb that is on or off, a coin that is heads or tails, or any other system with two distinct and distinguishable possibilities. In modern computing and communications, bits are represented by the absence or presence of an electrical signal, encoding “0” and “1” respectively.
A quantum bit is any bit made out of a quantum system, like an electron or photon. Just like classical bits, a quantum bit must have two distinct states: one representing “0” and one representing “1”. Unlike a classical bit, a quantum bit can also exist in superposition states, be subjected to incompatible measurements, and even be entangled with other quantum bits. Having the ability to harness the powers of superposition, interference and entanglement makes qubits fundamentally different and much more powerful than classical bits.
To build quantum computers and other quantum information technologies we need quantum objects that will act as qubits. Scientists have learned to harness and control many physical systems to act as qubits. This allows us to match the demands of different quantum technologies to the advantages of each type of qubit.
Meet the Qubits
There are many kinds of qubits, some occurring naturally and others that are engineered. Some of the most common types include:
Most quantum particles behave like little magnets. We call this property spin. The spin orientation is always pointing either fully up or fully down but never in between. Using the spin states of up and down, we can build a spin qubit.
0 = pointing up, 1 = pointing down
We can use the energy levels of electrons in neutral atoms or ions as qubits. In their natural state, these electrons occupy the lowest possible energy levels. Using lasers, we can “excite” them to a higher energy level. We can assign the qubit values based on their energy state.
0 = low energy state, 1 = high energy state
We can use photons, which are individual particles of light, as qubits in several ways.
Each photon carries an electromagnetic field with a specific direction, known as its polarization. The two states used to define qubits are horizontal polarization and vertical polarization.
0 = horizontal, 1 = vertical
The path a photon takes is another way to define a qubit. We can actually put a photon in a superposition of being “here” and “there”, by using beam-splitters.
0 = top path, 1 = bottom path
It is also possible to build a photon qubit using its time of arrival. We can create a quantum superposition of a “photon arriving early” and a “photon arriving late.”
0 = photon arrives early, 1 = photon arrives later
When cooled to a low temperature, some materials allow an electrical current to flow with no resistance. We call these superconductors. We can design electrical circuits based on superconductors to behave like qubits. Unlike the other examples of qubits, these fabricated systems are made of billions of atoms, however, they still behave as a single quantum system. One way we can build a superconducting qubit is by assigning a value to the direction that the current flows around an electrical circuit.
0 = clockwise current , 1 = counterclockwise current