algorithm: A fully-specified procedure for solving a mathematical problem (such as finding the greatest common divisor of two numbers) in a finite number of steps, often involving repetition of an operation.

angular momentum: For a particle, the cross product of the vector from a specified reference point to the particle and the particle's linear momentum; for an assembly of particles, the sum of the individual angular momentums.

avalanche photodiode: A photodiode operated in the avalanche breakdown region to achieve internal photocurrent multiplication, thereby providing rapid light-controlled switching operation. A photodiode is a semiconductor diode in which the reverse current varies with illumination.

basis: A reference system for measuring a physical property, such as a horizontal/vertical frame for measuring photon polarization.

Bell's Theorem: A theorem stating that for any collection of objects with three different properties, A, B, and C, the number of objects that have property A but not property B plus the number of objects which have property B but not property C is greater than or equal to the number of objects which have property A but not property C.

breakdown voltage: The voltage measured at a specified current in the electrical breakdown region of a semiconductor diode. Electrical breakdown is a large, usually abrupt, rise in electric current in the presence of a small increase in voltage.

cipher: 1. A method of transforming texts character by character in order to conceal their meaning. 2. Such transformed messages.

CODE: A method of transforming texts by substituting words or phrases according to an agreed dictionary or codebook.

coherent light: Radiant electromagnetic energy of approximately the same wavelength with fixed phase relationships between different points in the field.

conjugate bases: Two bases oriented so that a measurement in one randomizes the measurement in the other.

cryptogram: A communication in cipher or code.

cryptotext: In cryptology, a text of visible writing which conveys no intelligible meaning, or conveys an intelligible meaning that is not the real meaning.

decrypting: The converting (as a coded message) into intelligible form by decodeing or deciphering.

electron avalanche: The cumulative process in which an electron accelerated by a strong electric field collides with and ionizes gas molecules, thereby releasing additional electrons that in turn have more collisions, so that the discharge is self-maintained.

encodeing: The conversion of a message or body of information into coded form.

encrypting: Converting a message or body of information into an obscured form using a cipher or CODE.

hole: A vacant electron energy state near the top of an energy band in a solid. It behaves as though it were a positively charged particle, and is also known as electron hole. An energy band is a restricted range in which the energies of electrons in solids lie, or from which they are excluded, as understood in quantum mechanical terms.

key: In cryptology, a set of parameters used in conjunction with an encrypting algorithm.

key distribution problem: The problem of two parties agreeing on a secure key in a way that cannot be learned by eavesdroppers.

n-type semiconductor: An extrinsic semiconductor in which the conduction electron density exceeds the hole density.

p-n junction: The interface between two regions in a semiconductor crystal which have been treated so that one is a p-type semiconductor and the other is an n-type semiconductor.

p-type semiconductor: An extrinsic semiconductor in which the hole density exceeds the conduction electron density.

plaintext: Readable information that is to be encrypted.

prime number: An integer greater than 1 with no positive integer divisors other than 1 and itself; e.g., 2, 3, 5, 7, 11, 13, 17, and 19 are prime numbers.

quantum: 1. For certain physical properties such as energy or momentum, a fundamental unit such that the property appears in integer multiples of the unit. 2. An elementary particle carrying one unit of the property. For example, the photon is the quantum of the electromagnetic field, carrying an energy equal to Planck's contant times the frequency of the light and an angular momentum equal to a multiple of Planck's constant divided by 2p.

quantum computation: Computation by exploitation of the quantum mechanical nature of particles such as electrons or atomic nuclei. This property is used to enCODE information as quantum bits, or qubits, which can take on the values of 0 or 1 or both at the same time.

quantum computer: A proposed computer that would exploit the quantum mechanical nature of particles, such as electrons or atomic nuclei, to manipulate information as quantum bits, or qubits. Whereas an ordinary bit has at any time a value of either 0 or 1, a qubit can also take on both values at once. Because a quantum computer can act on these multiple states simultaneously, it is potentially many times as powerful as a conventional computer.

quantum theory: The modern theory of discretized matter, electromagnetic radiation, and interaction between matter and radiation. It generalizes and supersedes classical physics, mainly in the realm of atomic and subatomic phenomena. Also known as quantum mechanics.

qubit: A basic unit of quantum information, representing either 0 or 1 but capable of being carried by a particle in both states until measured or resolved.

vacancy: A defect in the form of an unoccupied lattice position in a crystal.