Strong Probe Field Research Articles

Quantum computation using weak nonlinearities: Robustness against decoherence

We investigate decoherence effects in the recently suggested quantum-computation scheme using weak nonlinearities, strong probe coherent fields, detection, and feedforward methods. It is shown that in the weak-nonlinearity-based quantum gates, decoherence in nonlinear media can be made arbitrarily small simply by using arbitrarily strong probe fields, if photon-number-resolving detection is used. On the contrary, we find that homodyne detection with feedforward is not appropriate for this scheme because in this case decoherence rapidly increases as the probe field gets larger.

Trichromatic phase manipulation of the response of a two-level medium to an arbitrarily intense probe field

The phase dependence and independence of the response of a trichromatically driven two-level medium to an arbitrarily intense probe field have been studied. The sum of the relative phases of the sideband components of the trichromatic field compared to the central component plays a crucial role in the response of the medium. For a weak probe field, as the sum of the relative phases changes from 0 to π, multiple switching can be achieved, in which switching from normal to anomalous dispersion occurs in multiple separate frequency regimes. The remarkable dependence on the sum phase is also shown for a strong probe field. On the other hand, when the sum of the two relative phases is fixed, the changes in the respective phases have no influence on the response of the medium.

Nonlinear response of a three-level system driven by a polychromatic field and probed by an arbitrarily intense field

A numerical method is developed in which we study the nonlinear absorption and dispersion of a three-level system driven by a polychromatic field and probed by an arbitrarily intense monochromatic field. This method is based on a harmonic expansion of all eight density matrix elements and a transformation of an infinite set of equations for the slowly varying amplitudes into a vector equation. When we substitute a strong probe field for a weak probe field, the absorption and dispersion spectra evolve from the well resolved multiple narrow peaks into powered broadened broad structures. The present method has an advantage over the previous studies in that it does no longer require a single population recurrence relation and it is applicable for the systems with more atomic levels and more field components.

Laser-induced birefringence in a wavelength-mismatched cascade system of inhomogeneously broadened Yb atoms

We report the observation of laser induced birefringence (LIB) in a wavelength-mismatched cascade system (J=0 \leftrightarrow J=1 \leftrightarrow J=0 transitions) of inhomogeneously broadened ytterbium atoms with strong pump and probe fields. We investigate the transmission spectrum of two circular polarization (\sigma_p^+ and \sigma_p^-) components of strong probe field at fixed frequency, depending on the detuning of circularly polarized (\sigma_c^-) coupling field from two-photon resonance. We find that \sigma_p^+ (\sigma_p^-) polarized component exhibits a narrow electromagnetically induced absorption (transparency) spectrum. Numerical solutions of density matrix equations show qualitative agreement with experimental results. A Doppler-free dispersive LIB signal is obtained by detecting the Stokes parameter of the probe field, enabling us to stabilize the frequency of coupling laser without frequency modulation.

Strong-field index enhancement via incoherent excitation

We show that it is possible, in principle, to employ an incoherent field as a control fieldto achieve maximal atomic coherence for a probe field, which characterizes ultralargeindex of refraction and vanishing absorption. For this purpose we consider a three-levelVsystem in which there are two dipole transitions, of which one is weak and theother is strong. The strong probe field is applied to the weak transition and theincoherent field drives the strong transition. For such an arrangement, selectivedressed-population trapping, which corresponds to maximal atomic coherence, isestablished by relying on the incoherent field and the spontaneous relaxation totransfer population from one dressed state to the other. This scheme as anextreme example explains that strong-field index enhancement based on selectivedressed-population trapping is resistant to the control field fluctuations.

Maximal atomic coherence via selective trapping of dressed states

We consider strong-field index enhancement in three-level systems in which there is a strong spontaneous decay from an auxiliary level into either of two states that the probe field couples. A control field is detuned resonant with the transition between the auxiliary level and one of the dressed states produced by the strong probe field. It is shown that it is possible to achieve maximal atomic coherence, which characterizes an ultralarge index of refraction and vanishing absorption. This scheme is based on selective dressed population trapping, which is established by relying on the control field and the strong decay to transfer population from one dressed state to another through the auxiliary level. An advantage of the present scheme is that the conditions for its realization are accessible experimentally.

Pulse propagation in a Raman pumped, four-level medium that exhibits inversionless gain

We numerically study the effect of two, coherent, copropagating pulses incident on an atomic beam of identical, homogeneously broadened, four-level atoms. The injected pulses have different frequencies that are nearly resonant with the allowed dipole transitions. The complex polarization that gives rise to gain and dispersion is examined at various field strengths; including strong probe field strengths where standard linear treatments are not valid. We conclude that in contrast to the steady state case, the maximum gain decreases at the sidebands with increasing pump strengths. In the nonlinear regime, where pump and probe are of comparable magnitude, competition between the upper and lower coherences may lead to loss of gain.