Electromagnetically induced transparency

Extreme dispersion is also created within this transparency "window" which leads to "slow light", described below.

It is in essence a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium.

[1] Observation of EIT involves two optical fields (highly coherent light sources, such as lasers) which are tuned to interact with three quantum states of a material.

The "probe" field is tuned near resonance between two of the states and measures the absorption spectrum of the transition.

If the states are selected properly, the presence of the coupling field will create a spectral "window" of transparency which will be detected by the probe.

The coupling laser is sometimes referred to as the "control" or "pump", the latter in analogy to incoherent optical nonlinearities such as spectral hole burning or saturation.

EIT is based on the destructive interference of the transition probability amplitude between atomic states.

Any real material system may contain many triplets of states which could theoretically support EIT, but there are several practical limitations on which levels can actually be used.

In any real system at non-zero temperature there are processes which cause a scrambling of the phase of the quantum states.

EIT research uses atomic systems in dilute gases, solid solutions, or more exotic states such as Bose–Einstein condensate.

[7][8] EIT was first proposed theoretically by professor Jakob Khanin and graduate student Olga Kocharovskaya at Gorky State University (renamed to Nizhny Novgorod in 1990), Russia;[9] there are now several different approaches to a theoretical treatment of EIT.

One approach is to extend the density matrix treatment used to drive Rabi oscillation of a two-state, single field system.

In this picture the probability amplitude for the system to transfer between states can interfere destructively, preventing absorption.

Another approach is the "dressed state" picture, wherein the system + coupling field Hamiltonian is diagonalized and the effect on the probe is calculated in the new basis.

In this picture EIT resembles a combination of Autler-Townes splitting and Fano interference between the dressed states.

Between the doublet peaks, in the center of the transparency window, the quantum probability amplitudes for the probe to cause a transition to either state cancel.

Here, the photons of the probe are coherently "transformed" into "dark state polaritons" which are excitations of the medium.

This rapid and positive change in refractive index produces an extremely low group velocity.

[10] The first experimental observation of the low group velocity produced by EIT was by Boller, İmamoğlu, and Harris at Stanford University in 1991 in strontium.

In 1999 Lene Hau reported slowing light in a medium of ultracold sodium atoms,[11] achieving this by using quantum interference effects responsible for electromagnetically induced transparency (EIT).

"Using detailed numerical simulations, and analytical theory, we study properties of micro-cavities which incorporate materials that exhibit Electro-magnetically Induced Transparency (EIT) or Ultra Slow Light (USL).

We find that such systems, while being miniature in size (order wavelength), and integrable, can have some outstanding properties.

In particular, they could have lifetimes orders of magnitude longer than other existing systems, and could exhibit non-linear all-optical switching at single photon power levels.

Potential applications include miniature atomic clocks, and all-optical quantum information processing.

"[13] The current record for slow light in an EIT medium is held by Budker, Kimball, Rochester, and Yashchuk at U.C.

[14] Stopped light, in the context of an EIT medium, refers to the coherent transfer of photons to the quantum system and back again.

In principle, this involves switching off the coupling beam in an adiabatic fashion while the probe pulse is still inside of the EIT medium.

[17] EIT has been used to laser cool long strings of atoms to their motional ground state in an ion trap.

Due to the quantum interference of transition amplitudes, a weaker "cooling" laser driving the

"blue" sideband lies in a region of low excitation probability, as shown in the figure below.

The effect of EIT on a typical absorption line. A weak probe normally experiences absorption shown in blue. A second coupling beam induces EIT and creates a "window" in the absorption region (red). This plot is a computer simulation of EIT in an InAs/GaAs quantum dot
EIT level schemes can be sorted into three categories; vee, ladder, and lambda.
Rapid change of index of refraction (blue) in a region of rapidly changing absorption (gray) associated with EIT. The steep and positive linear region of the refractive index in the center of the transparency window gives rise to slow light
Three level lambda structure that is used for EIT cooling, with the Rabi frequencies and detunings of the cooling and coupling laser, respectively.
Absorption profile seen by the cooling laser as a function of detuning . The Rabi frequency is chosen so that the red sideband (red dashed line) lies on the narrow peak of the Fano-like feature and the blue sideband (blue dashed line) lies in a region of low probability. The carrier (black dashed line) lies on the dark resonance where the detunings are equal, i.e. , such that absorption is zero.