Quantum superposition is a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two (or more) quantum states can be added together (“superposed”) and the result will be another valid quantum state; and conversely, that every quantum state can be represented as a sum of two or more other distinct states.

Mathematically, it refers to a property of solutions to the Schrödinger equation; since the Schrödinger equation is linear, any linear combination of solutions will also be a solution.

An example of a physically observable manifestation of the wave nature of quantum systems is the interference peaks from an electron beam in a double-slit experiment. The pattern is very similar to the one obtained by diffraction of classical waves.

Another example is a quantum logical qubit state, as used in quantum information processing, which is a quantum superposition of the “basis states” |0>and |1>. Here |0> is the Dirac notation for the quantum state that will always give the result 0 when converted to classical logic by a measurement. Likewise |1> is the state that will always convert to 1.

Contrary to a classical bit that can only be in the state corresponding to 0 or the state corresponding to 1, a qubit may be in a superposition of both states. This means that the probabilities of measuring 0 or 1 for a qubit are in general neither 0.0 nor 1.0, and multiple measurements made on qubits in identical states will not always give the same result.

Successful experiments involving superpositions of relatively large (by the standards of quantum physics) objects have been performed.

- A “cat state” has been achieved with photons.
- A beryllium ion has been trapped in a superposed state.
- A double slit experiment has been performed with molecules as large as buckyballs.
- A 2013 experiment superposed molecules containing 15,000 each of protons, neutrons and electrons. The molecules were of compounds selected for their good thermal stability, and were evaporated into a beam at a temperature of 600 K.
- An experiment involving a superconducting quantum interference device (“SQUID”) has been linked to the theme of the “cat state” thought experiment.
- By use of very low temperatures, very fine experimental arrangements were made to protect in near isolation and preserve the coherence of intermediate states, for a duration of time, between preparation and detection, of SQUID currents.
- An experiment involving a flu virus has been proposed.
- A piezoelectric “tuning fork” has been constructed, which can be placed into a superposition of vibrating and non-vibrating states. The resonator comprises about 10 trillion atoms.
- Recent research indicates that chlorophyll within plants appears to exploit the feature of quantum superposition to achieve greater efficiency in transporting energy, allowing pigment proteins to be spaced further apart than would otherwise be possible.
- An experiment has been proposed, with a bacterial cell cooled to 10 mK, using an electromechanical oscillator. At that temperature, all metabolism would be stopped, and the cell might behave virtually as a definite chemical species.