measurement) is then often implied by the operands, for example as the required state space for solving a problem. In Grover's algorithm, Lov named this generic basis vector set "the database". The selection of basis vectors against to measure a quantum state will influence the outcome of the measurement. See change of basis and Von Neumann entropy for details. In this article, we always use the computational basis, which means that we have labeled the basis vectors of an -qubit register or use the binary representation In quantum mechanics, the basis vectors constitute an orthonormal basis. An example of usage of an alternative measurement basis is in the BB84 cipher. The effect of measurement on entangled states If two quantum states (i.e. qubits, or registers) are entangled (meaning that their combined state cannot be expressed as a tensor product), measurement of one register affects or reveals the state of the other register by partially or entirely collapsing its state too. This effect can be used for computation, and is used in many algorithms. The Hadamard-CNOT combination acts on the zero-state as follows: This resulting state is the Bell state It cannot be described as a tensor product of two qubits. There is no solution for because for example needs to be both non-zero and zero in the case of and . The quantum state spans the two qubits. This is called entanglement. Measuring one of the two qubits that make up this Bell state will result in that the other qubit logically must have the same value, both must be the same: Either it will be found in the state or in the state If we measure one of the qubits to be for example then the other qubit must also be because their combined state became Measurement of one of the qubits collapses the entire quantum state, that span the two qubits. The GHZ state is a similar entangled quantum state that spans three or more qubits. This type of value-assignment occurs instantaneously over any distance and this has as of 2018 been experimentally verified by QUESS for distances of up to 1200 kilometers. That the phenomena appears to happen instantaneously as opposed to the time it would take to traverse the distance separating the qubits at the speed of light is called the EPR paradox, and it is an open question in physics how to resolve this. Originally