A qubit is a quantum bit, a unit of information that can exist in more than one state simultaneously. Unlike a classical bit, a qubit can be in multiple states at the same time and can be manipulated and measured as if it were a single entity.
Quantum bits are the basic units of quantum information. They are the quantum analogues of classical bits, which are the basic units of classical information. A classical bit can store only two values, 0 or 1, but a qubit can store an infinite number of values between 0 and 1. This makes qubits much more powerful than classical bits.
Qubits are often used in conjunction with other quantum particles, such as photons and atoms. When two or more qubits are combined, they form a register called a qubyte (pronounced kyoo-byte). A group of eight qubits is called an octet (pronounced ock-tet). There is no limit to how many qubits can be combined into a register; however, large numbers of qubits are difficult to manipulate and control.
The most common way to create a qubit is with polarized photons—photons whose electric fields vibrate in only one plane. The polarization of each photon represents one value in a set of two possible values, so two photons can represent two different classical bits. For example, if the first photon is vertically polarized and the second photon is horizontally polarized, then they represent the binary values 01 (vertical = 0; horizontal = 1). However, because photons also have spin—a property that gives them angular momentum—they can represent more than two values. In fact, any number of values between 0 and 1 can be represented by combining the spin states of multiple photons.
Another way to create a qubit is with atoms or molecules that have unpaired electrons—electrons that do not pair up with other electrons to fill orbitals around the nucleus. Unpaired electrons have spin just like photons do, so they too can represent any number of values between 0 and 1 when placed into magnetic fields pointing in different directions. For example, an atom with one unpaired electron could represent the value 0 when its electron’s spin points up (away from you) and 1 when its electron’s spin points down (toward you). By orienting these atoms in different ways within magnetic fields pointing in different directions relative to each other—a process known as nuclear magnetic resonance—it’s possible to encode any desired pattern of zeros and ones into their spins