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In physics
Physics

Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
, the Rabi cycle is the cyclic behaviour of a two-state quantum system in the presence of an oscillatory driving field. A two-state system has two possible states, and if they are not degenerate energy level
Degenerate energy level

In physics two or more different physical states are said to be degenerate if they are all at the same energy level. Physical states differ if and only if they are linearly independent....
s the system can become "excited" when it absorbs a quantum
Quantum

In physics, a quantum is an indivisible entity of a quantity that has the same units as the Planck constant and is related to both energy and momentum of elementary particles of matter and of photons and other bosons....
 of energy.

The effect is important in quantum optics
Quantum optics

Quantum optics is a field of research in physics, dealing with the application of quantum mechanics to phenomena involving light and its interactions with matter....
, nuclear magnetic resonance and quantum computing. The term is named in honour of Isidor Isaac Rabi
Isidor Isaac Rabi

Isidor Isaac Rabi was a Galicia -born American physicist and Nobel laureate recognised in 1944 for his discovery of nuclear magnetic resonance....
.

When an atom
Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
 (or some other two-level system
Two-level system

In quantum mechanics, a two-state system is a system which has two possible states. More formally, the Hilbert space of a two state system has two degrees of freedom , so a complete basis spanning the space must consist of two independent states....
) is illuminated by a coherent beam of photon
Photon

In physics, the photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation....
s, it will cyclically absorb
Absorption (electromagnetic radiation)

In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom....
 photons and re-emit them by stimulated emission
Stimulated emission

In optics, stimulated emission is the process by which an electron, perturbed by a photon having the correct energy, may drop to a lower energy level resulting in the creation of another photon....
.






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In physics
Physics

Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
, the Rabi cycle is the cyclic behaviour of a two-state quantum system in the presence of an oscillatory driving field. A two-state system has two possible states, and if they are not degenerate energy level
Degenerate energy level

In physics two or more different physical states are said to be degenerate if they are all at the same energy level. Physical states differ if and only if they are linearly independent....
s the system can become "excited" when it absorbs a quantum
Quantum

In physics, a quantum is an indivisible entity of a quantity that has the same units as the Planck constant and is related to both energy and momentum of elementary particles of matter and of photons and other bosons....
 of energy.

The effect is important in quantum optics
Quantum optics

Quantum optics is a field of research in physics, dealing with the application of quantum mechanics to phenomena involving light and its interactions with matter....
, nuclear magnetic resonance and quantum computing. The term is named in honour of Isidor Isaac Rabi
Isidor Isaac Rabi

Isidor Isaac Rabi was a Galicia -born American physicist and Nobel laureate recognised in 1944 for his discovery of nuclear magnetic resonance....
.

When an atom
Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
 (or some other two-level system
Two-level system

In quantum mechanics, a two-state system is a system which has two possible states. More formally, the Hilbert space of a two state system has two degrees of freedom , so a complete basis spanning the space must consist of two independent states....
) is illuminated by a coherent beam of photon
Photon

In physics, the photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation....
s, it will cyclically absorb
Absorption (electromagnetic radiation)

In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom....
 photons and re-emit them by stimulated emission
Stimulated emission

In optics, stimulated emission is the process by which an electron, perturbed by a photon having the correct energy, may drop to a lower energy level resulting in the creation of another photon....
. One such cycle is called a Rabi cycle and the inverse of its duration the Rabi frequency
Rabi frequency

The Rabi frequency for a given atomic transition in a given light field gives the strength of the coupling between the light and the transition....
 of the photon beam.

This mechanism is fundamental to quantum optics. It can be modelled using the Jaynes-Cummings model
Jaynes-Cummings model

The Jaynes-Cummings model is a theoretical model in quantum optics. It describes the system of a Two-level system interacting with a quantized mode of an optical cavity, with or without the presence of light....
 and the Bloch vector formalism.

For example, for a two-state atom (an atom in which an electron can either be in the excited or ground state) in an electromagnetic field with frequency tuned to the excitation energy, the probability of finding the atom in the excited state is found from the Bloch equations to be: where is the Rabi frequency.

More generally, one can consider a system where the two levels under consideration are not energy eigenstates. Therefore if the system is initialized in one of these levels, time evolution will make the population of each of the levels oscillate with some characteristic frequency, whose angular frequency
Angular frequency

In physics , angular frequency ? is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity....
 is also known as the Rabi frequency.

See also

  • Atomic coherence
    Atomic coherence

    In physics, atomic coherence is the induced Coherence between levels of a multi-level atomic system sometimes observed when it interacts with a coherent electromagnetic field....
  • Bloch sphere
    Bloch sphere

    In quantum mechanics, the Bloch sphere is a geometrical representation of the pure state space of a two-level system named after the physicist Felix Bloch....
  • Laser pumping
    Laser pumping

    Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms....
  • Optical pumping
    Optical pumping

    Optical pumping is a process in which light is used to raise electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction, to laser pumping the active laser medium so as to achieve population inversion....
  • Rabi frequency
    Rabi frequency

    The Rabi frequency for a given atomic transition in a given light field gives the strength of the coupling between the light and the transition....
  • Rabi problem
    Rabi problem

    The Rabi problem concerns the response of an atom to an applied normal mode electric field, with an applied frequency very close to the atom's natural frequency....
  • Vacuum Rabi oscillation
    Vacuum Rabi oscillation

    A vacuum Rabi oscillation is a damped oscillation of an initially excited atom coupled to an electromagnetic resonator or cavity in which the atom alternately emits photon into a single-mode electromagnetic cavity and reabsorbs them....


External links

A Java applet that visualizes Rabi Cycles of two-state systems (laser driven).
extended version of the applet. Includes electron phonon interaction.