physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

, atomic coherence is the induced coherence between levels of a multi-level

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

ic system and an electromagnetic field. The internal state of an atom is characterized by a superposition of

excited state In quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...

s and their associated

energy level A quantum mechanics, quantum mechanical system or particle that is bound state, bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical mechanics, classical pa ...

s. In the presence of external

electromagnetic field An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...

s, the atom's energy levels acquire perturbations to the excited states that describe the atom's internal state. When the acquired phase is the same over the range of internal states, the atom is coherent. Atomic coherence is characterized by the length of time over which the internal state of the atom can be reliably manipulated.

Measuring coherence

Atomic coherence can be characterized by the

coherence time For an electromagnetic wave, the coherence time is the time over which a propagating wave (especially a laser or maser beam) may be considered coherent, meaning that its phase is, on average, predictable. In long-distance transmission systems ...

. For example, the contrast in Ramsey fringes has been used to measure the relaxation time,

T_2

, in a trapped ion and in neutral atoms. Similarly, the coherence time can be characterized by measuring the population transfer over time of an atom undergoing Rabi oscillations.

Examples

Atomic interferometry

An atom interferometer creates coherent atomic beams, where the coherence is with respect to the phase of the atom's de Broglie wave.

Rabi flopping

If an electron in a two level atomic system is excited by narrow line width coherent electro-magnetic radiation, like a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

, that is on resonance with the two level transition, the electron will Rabi flop. During Rabi flopping the electron oscillates between the ground and excited states and can be described by a continuous rotation around the

Bloch sphere In quantum mechanics and computing, the Bloch sphere is a geometrical representation of the pure state space of a two-level quantum mechanical system ( qubit), named after the physicist Felix Bloch. Mathematically each quantum mechanical syst ...

. For a perfectly isolated system, a particle undergoing Rabi oscillation between two levels will remain in phase. In practice, interactions between the system and the environment introduce a phase offset in the Rabi oscillation between the two levels, causing " decoherence". If instead of a single two-level system, an ensemble of identical two level systems (such as a chain of identical atoms in an ion trap) is prepared and continuously addressed with a laser, all the atoms will coherently Rabi flop. All two level systems will initially have a defined relative phase and the system will be coherent. As atoms undergo random

spontaneous emission Spontaneous emission is the process in which a Quantum mechanics, quantum mechanical system (such as a molecule, an atom or a subatomic particle) transits from an excited state, excited energy state to a lower energy state (e.g., its ground state ...

their Rabi oscillations will accumulate a random relative phase with respect to each other and become decoherent. In actual experiments ambient magnetic field noise and thermal heating from collisions between atoms cause decoherence faster than random spontaneous emission and are the dominant uncertainties when running

atomic clocks An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

or trapped ion quantum computers. Atomic coherence can also apply to multi-level systems which require more than a single laser. Atomic coherence is essential in research on several effects, such as electromagnetically induced transparency (EIT), lasing without inversion (LWI), stimulated raman adiabatic passage (STIRAP) and nonlinear optical interaction with enhanced efficiency. Atomic systems demonstrating continuous superradiance exhibit long

, a property shared with lasers.

References

{{reflist

Measuring coherence

Examples

Atomic interferometry

Rabi flopping

See also

References