quantum physics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, q ...

, coincidence counting is used in experiments testing particle non-locality and

quantum entanglement Quantum entanglement is the phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state o ...

. In these experiments two or more particles are created from the same initial packet of energy, inexorably linking/ entangling their physical properties. Separate

particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing particles, such as those produced by ...

s measure the

quantum state In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution in ...

s of each particle and send the resulting signal to a coincidence counter. In any experiment studying entanglement, the entangled particles are vastly outnumbered by non-entangled particles which are also detected; patternless noise that drowns out the entangled signal. In a two detector system, a coincidence counter alleviates this problem by only recording detection signals that strike both detectors simultaneously (or more accurately, recording only signals that arrive at both detectors and correlate to the same emission time). This ensures that the data represents only entangled particles. However, since no detector/counter circuit has infinitely precise temporal resolution (due both to limitations in the electronics and the laws of the Universe itself), detections must be sorted into time bins (detection windows equivalent to the temporal resolution of the system). Detections in the same bin ''appear'' to occur at the same time because their individual detection times cannot be resolved any further. Thus in a two detector system, two unrelated, non-entangled particles may randomly strike both detectors, get sorted into the same time bin, and create a false-coincidence that adds noise to the signal. This limits coincidence counters to improving the

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in d ...

to the extent that the quantum behavior can be studied, without removing the noise completely. Every experiment to date that has been used to calculate Bell's inequalities, perform a

quantum eraser In quantum mechanics, the quantum eraser experiment is an interferometer experiment that demonstrates several fundamental aspects of quantum mechanics, including quantum entanglement and complementarity. The quantum eraser experiment is a variat ...

, or conduct any experiment utilizing quantum entanglement as an information channel has only been possible through the use of coincidence counters. This unavoidably prevents

superluminal communication Superluminal communication is a hypothetical process in which information is sent at faster-than-light (FTL) speeds. The current scientific consensus is that faster-than-light communication is not possible, and to date it has not been achieved in ...

since, even if a random or purposeful decision ''appears'' to be affecting events that have already transpired (as in the

delayed choice quantum eraser A delayed-choice quantum eraser experiment, first performed by Yoon-Ho Kim, R. Yu, S. P. Kulik, Y. H. Shih and Marlan O. Scully, and reported in early 1998, is an elaboration on the quantum eraser experiment that incorporates concepts considered ...

), the signal from the past cannot be seen/decoded until the coincidence circuit has correlated both the past and future behavior. Thus the "signal" in the past is only visible ''after'' it is "sent" from the future, precluding quantum entanglement from being exploited for the purposes of faster-than-light communication or data time travel.

See also