140 research outputs found
Quantum correlations and violation of Bell inequality induced by External Field in a two photon radiative cascade
We study the polarization dependent second order correlation of a pair of
photons emitted in a four level radiative cascade driven by an external field.
It is found that the quantum correlations of the emitted photons, degraded by
the energy splitting of the intermediate levels in the radiative cascade can be
efficiently revived by a far detuned external field. The physics of this
revival is linked to an induced stark shift and the formation of dressed states
in the system by the non-resonant external field. Further, we investigated the
competition between the effect of the coherent external field and incoherent
dephasing of the intermediate levels. We found that the degradation of quantum
correlations due to the incoherent dephasing can be content for small dephasing
with the external field. We also studied the non-locality of the correlations
by evaluating the Bell's inequality in the linear polarization basis for the
radiative cascade. We find that the Bell parameter decreases rapidly with
increase in the intermediate level energy splitting or incoherent dephasing
rate to the extent that there is no violation. However, the presence of an
external field leads to control over the degrading mechanisms and preservation
of nonlocal correlation among the photons. This in turn can induce, violation
of Bell's inequality in the radiative cascade for arbitrary intermediate level
splitting and small incoherent dephasing
Synthetic gauge potential and effective magnetic field in a Raman medium undergoing molecular modulation
We theoretically demonstrate non-trivial topological effects for a probe
field in a Raman medium undergoing molecular modulation processes. The medium
is driven by two non-collinear pump beams. We show that the angle between the
pumps is related to an effective gauge potential and an effective magnetic
field for the probe field in the synthetic space consisting of a synthetic
frequency dimension and a spatial dimension. As a result of such effective
magnetic field, the probe field can exhibit topologically-protected one-way
edge state in the synthetic space, as well as Landau levels which manifests as
suppression of both diffraction and sideband generation. Our work identifies a
previously unexplored route towards creating topological photonics effects, and
highlights an important connection between topological photonics and nonlinear
optics
Quantum Coherence and Superradiant Emission: From Lasing Without Inversion to Sky Laser and the QASER
This work is focused on quantum coherence and superradiant emission in a dense pencil-like multi-level medium where many novel effects appear such as transient lasing without inversion, coherence-brightened sky laser, and quantum amplification by superradiant emission of radiation.
We start from an interesting cascade model where quantum coherence effects can lead to surprising phenomena, gain without population inversion and gain suppression, under different parameters. We further show superradiant emission inside helium plasma. The population evolution shows the decay is significantly faster than collisional decoherence and spontaneous decay rates. This indicates superradiant coherent behavior of the atomic system inside the plasma.
Based on these results, we demonstrate lasing without inversion on a time scale shorter than the decoherence time. The possibility of transient lasing without inversion holds promise for lasing in the extreme-ultraviolet/x-ray regime. We propose experiments to demonstrate this in helium or helium-like ions plasma.
We also study coherent emission from ambient air and demonstrate efficient generation of laser-like beams directed both forward and backward with respect to a nanosecond ultraviolet pumping laser beam. The emission process exhibits nonadiabatic quantum coherence, which is similar in nature to Dicke superradiance. This coherence-brightened backward light source in air provides possibility for atmospheric remote sensing through a phase-matched coherent Raman scattering process.
Finally, we present a new kind of quantum amplifier, the QASER (quantum amplification by superradiant emission of radiation), based on collective superradiant emission which does not require initial population in the excited state. We show that parametric resonance between the driving field and collective superradiant oscillations of the atomic polarization can yield light amplification at high frequencies. The resulting superradiant amplifier is many orders of magnitude more efficient than nonlinear multiphoton excitation and holds promise as a new way to generate high-frequency coherent radiation
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