Constructive Quantum Interference Research Articles

Dynamical effects of the four-wave mixing enhancement induced by constructive quantum interference**Project supported by National Natural Science Foundation of China (Grant Nos. 10774059 and 10904047), the National Basic Research Program of China (Grant No. 2006CB921103), the doctoral program foundation of institution of High Education of China (Grant No. 20060183046) and the basic research foundation of Jilin University of China (Grant

In a four-level system of ultracold 87Rb atoms, through analytical and numerical calculations we propose an efficient scheme to achieve the enhanced four-wave mixing process and demonstrate its dynamical control by various parameters such as the travel distance z, probe detuning δ and the probe pulse width τ. In particular, we find that the maximal intensity of the nonlinearly generated signal pulse can be about 80% of the initial input probe under the optimal condition. This greatly enhanced conversion efficiency occurs due to the constructive quantum interference between two different components of the generated signal pulse.

Enhanced selectivity and efficiency of coherent population transfer via a train of pulse pairs

We propose a way to significantly enhance selectivity and efficiency of coherent population transfer in a {Lambda}-type four-level system with a closely spaced doublet in the final state by using a train of counterintuitively ordered pump-Stokes pulse pairs. Due to temporal constructive and destructive quantum interference between the sequential transitions and subsequent coherent accumulation and annihilation in the time domain, the spectral resolution of selective and perfect population transfer to either of the final states can be improved by more than one order of magnitude as compared to the conventionally widely used stimulated Raman adiabatic passage and chirped adiabatic passage techniques, which can also be well understood as a result of the formation of a comblike structure of the pulse train spectrum in the frequency domain. Moreover, an arbitrary coherent superposition between the closely separated doublet can be achieved by suitably tuning the repetition period and detuning of the laser fields. This method has potential applications in control of chemical reactions and quantum information processing.

Coherence control for photonic logic gates via Y-configuration double-control quantum interferences

Destructive and constructive quantum interferences exhibited in a four-level Y-configuration double-control atomic system are suggested. It is shown that the probe transition (driven by the probe field) can be manipulated by the quantum interferences between two control transitions (driven by the control fields) of the four-level system. The atomic vapor is opaque (or transparent) to the probe field if the destructive (or constructive) quantum interference between the control transitions emerges. The optically sensitive responses due to double-control quantum interferences can be utilized to realize some quantum optical and photonic devices such as the logic-gate devices, e.g., the NOT, OR, NOR and EXNOR gates.

Anderson transition in 1D systems with spatial disorder

A simple Kronig–Penney model for 1D mesoscopic systems with δ peak potentials is used to study numerically the influence of spatial disorder on conductance fluctuations and distribution at different regimes. The Lévy laws are used to investigate the statistical properties of the eigenstates. It is found that an Anderson transition occurs even in 1D meaning that the disorder can also provide constructive quantum interferences. The critical disorder W c for this transition is estimated. In these 1D systems, the metallic phase is well characterized by a Gaussian conductance distribution. Indeed, the results relative to conductance distribution are in good agreement with the previous works in 2D and 3D systems for other models. At this transition, the conductance probability distribution has a system size independent shape with large fluctuations in good agreement with previous works.

Ultrafast coherent population transfer with a train of weak femtosecond pulse pairs

We investigate ultrafast coherent population transfer in a Λ-type three-level system driven by a train of weak phase-locked pump-Stokes femtosecond pulse pairs, where the pump and Stokes pulses in each pair are applied in counterintuitive order, similar to that in the stimulated Raman adiabatic passage technique. It is shown that due to temporal constructive quantum interference and the subsequent coherent accumulation, complete population transfer can be achieved by a few pulse pairs with suitable interpair and intrapair time delays even if a single pair of pulses in the train is so weak that nearly no population transfer can take place. The method is efficient and robust to moderate variations in the interpair and intrapair time delays, number of pulse pairs, and laser intensity, which is particularly favorable to coherent population transfer in quantum systems with weak oscillator strengths.

Control of coherent population transfer via spontaneous decay-induced coherence

We investigate the effect of the spontaneous decay-induced coherence on the control of coherent population transfer in a closed double $\ensuremath{\Lambda}$-type four-level system with the energy separation of the excited doublet comparable to their decay rates by using the stimulated Raman adiabatic passage technique. It is shown that when the two pulse laser fields are tuned to the particular frequency where the condition for quantum interference is satisfied, remarkable enhancement or suppression of population transfer can be realized through the spontaneous emission constructive or destructive quantum interference even with a finite pulse area, which is very favorable to coherent population transfer in quantum systems with weak oscillator strengths.

Characterizing temporal distinguishability of anN-photon state by a generalized photon bunching effect with multiphoton interference

The complementarity principle of quantum mechanics relates qualitatively the visibility of quantum interference with path indistinguishability. A quantitative study was recently presented [Z. Y. Ou, Phys. Rev. A 74, 063808 (2006)]. Following the formalism of this study, we investigate another scheme for characterizing quantitatively the degree of temporal distinguishability of an $N$-photon state, based on constructive quantum interference between an $N$-photon and a single-photon state. This scheme is related to a generalized photon bunching effect in the form of a ``bump,'' in contrast to the ``dip'' for the destructive interference effect in the previous study. Generalization to other more complicated cases is straightforward and is much simpler than for the scheme of destructive interference in the previous study. A degree of (in)distinguishability is defined and can be determined experimentally by the measurement of the size of the constructive multiphoton interference effect.

Transient evolutional behaviours of double-control electromagnetically induced transparency

The evolutional optical behaviours (turn-on dynamics) of a four-level double-control tripod-configuration (electromagnetically induced transparency) system are considered based on the transient solution to the equation of motion of the probability amplitudes of the atomic levels. As the most remarkable property (quantum interference between the two control transitions) will arise in the present tripod-configuration system, the transient evolution of the permittivity in cases of both destructive and constructive quantum interferences is presented. It can be shown that the four-level double-control vapour can become a destructive-interference medium, (exhibiting a two-level resonant absorption) and a constructive-interference medium, (exhibiting transparency to the probe field), respectively, under certain conditions (related to the ratio of the two control field intensities). The present double-control scenario can be applicable to designs of some new photonic and quantum optical devices such as logic and functional devices (logic gate circuits) and the key component of the technology of quantum coherent information storage.

Model of collision-induced constructive quantum interference

Yang and collaborators reported recently observation of unexpected resonaces in so-called hybrid excitation spectra of sodium vapour. Yang and Xie presented a theoretical model in which the resonances were interpreted as in terms of collision constructive quantum interference effects. This work reports results of testing of the applicability range of the model and demonstrates its serious limitations.

Nonlinear optical effects in a doubly driven four-level atom

We have studied nonlinear quantum optical effects in a doubly driven four-level atomic system in a homogeneously broadened regime. On application of two strong fields the system can be made to display double electromagnetically induced transparency (EIT) because of constructive quantum interference giving rise to nonlinear absorption at three-photon resonance condition. This fact may be used to obtain other phenomena like the effect of photon switching in a pulsed regime and large cross phase modulation (XPM) effect. We also show that the behaviour of spatial modulation of transparency of the weak probe beam in a three-level system interacted with a strong standing wave pump field can be dramatically changed in our four-level ladder type system. Here, strong coupling field acting on the uppermost transition is represented by a standing wave whereas strong pump and weak probe fields acting successively on the lower transitions are represented by travelling waves. The effect of double EIT on nonlinear light generation based on the four-wave-mixing (FWM) technique is also investigated. We have examined that the four wave mixing efficiency can be effectively controlled by varying the strength of the pump field.

Double-control quantum interferences in a four-level atomic system

A new scheme is suggested to manipulate the probe transitions (and hence the optical properties of atomic vapors) via double-control destructive and constructive quantum interferences. The influence of phase coherence between the two control transitions on the probe transition is also studied. The most remarkable feature of the present scheme is that the optical properties (absorption, transparency and dispersion) of an atomic system can be manipulated using this double-control multi-pathway interferences (multiple routes to excitation). It is also shown that a four-level system will exhibit a two-level resonant absorption because the two control levels (driven by the two control fields) form a dark state (and hence a destructive quantum interference occurs between the two control transitions). However, the present four-level system will exhibit electromagnetically induced transparency to the probe field when the three lower levels (including the probe level and the two control levels) form a three-level dark state. The present scenario has potential applications in new devices (e.g. logic gates and sensitive optical switches) and new techniques (e.g. quantum coherent information storage).

Laboratory observation of quantum control level sets

In controlled quantum dynamics, a level set is defined as the collection of control fields that produce a specific value for a particular observable. This paper explores the relationship between individual solutions to a control problem, and presents the first experimentally observed quantum control level sets, which are found to be continuous submanifolds. Level sets are observed for two photon transitions where the control is the spectral phase function, which is expressed as a fourth-order polynomial. For the systems studied here, the level sets are shown to be closed surfaces in the spectral phase control space. A perturbation analysis provides insight into the observed topology of the level set, which is shown to be preserved by the low-order polynomial phase representation. Each of the multiple control fields forming a level set preserves the observable value by its own distinct manipulation of constructive and destructive quantum interferences. Thus, the richness of quantum control fields meeting a particular observable value is accompanied by an equally diverse family of control mechanisms.

Effect of quantum interference on tunneling photoionization rates of N2 and O2 molecules

In this work, we reexamine the photoionization rates of N(2) and O(2) molecules using the previously published photoionization rate theory which is based on the original atomic Keldysh theory [K. Mishima et al., Phys. Rev. A 66, 033401 (2002); ibid.66, 053408 (2002)]. We have found that the constructive quantum interference takes place for N(2) molecule while the destructive quantum interference plays an important role for O(2) molecule. This is consistent with the experimental and theoretical results reported in the literature. The formulas derived in this paper clearly show that this is due to the different symmetries of the valence orbitals of N(2) and O(2) molecules.

Electrical conductance of carbon nanotori in contact with single-wall carbon nanotubes

The realization of the potential of carbon nanotori as elements of nanoscale devices based on their recently predicted unusual properties requires a thorough understanding of contacting these nanostructures. We carried out a series of calculations of the electric conductance of carbon nanotori contacted by single-wall carbon nanotubes to shed light on the effects of the geometry as well as the chemistry of the contacts. The relaxed structures of the contacted nanotori were determined by an order-N nonorthogonal tight-binding molecular dynamics scheme. The conductance was calculated using the Landauer-Büttiker formula based on a π-orbital Hamiltonian. We found that the conductance of the contacted carbon nanotorus is very sensitive to the transparency (chemistry) of the contacts. We also found that the equivalence (chemistry as well as geometry) of the contacts plays an important role in the transport properties. For example, a difference in the right contact and left contact will diminish the constructive quantum interference of the transmission as compared to the situation when the two contacts are equivalent. This conclusion is general and is expected to be applicable for any metallic contacts to the contacted carbon nanotori.

Large enhancement of fully resonant sum-frequency generation through quantum control via continuum states

A theory of quantum control of short-wavelength sum-frequency generation, which employs the continuum states, is developed. The proposed scheme employs all-resonant coupling and trade-off optimization of the accompanying constructive and destructive quantum interference effects in the lower-order and higher-order polarizations controlled by the overlap of two autoionizinglike laser-induced continuum structures. The scheme does not rely on adiabatic passage, coherent population trapping or maximum atomic coherence as a means to facilitate maximum output. The opportunities for manipulating transparency of the medium and refractive index for the fundamental and generated radiations, as well as nonlinear polarization in the multiple-resonant medium, are shown. This opens the feasibility of creating frequency-tunable narrowband filters, polarization rotators, and dispersive elements for vacuum ultraviolet radiation. The features specific for quantum interference in Doppler-broadened media are investigated. The feasibility of almost complete conversion of long-wavelength fundamental radiation into generated short-wavelength radiation, and of a dramatic decrease in the intensity of required fundamental radiations, is shown.

Quantum controllable optical selective resonant excitation in a dense medium

Selective resonant excitations can be realized with a shaped ultrashort pulse in an optically dense collection of multilevel individual atoms, where the near dipole-dipole interactions induce intrinsic frequency shifts of the atomic resonance. A proper transform-limited pulse and modulation of excitation pulses can be controlled to establish constructive or destructive quantum interference between different excitation pathways towards a desired selective resonant excitation, even driven by pulses with broadband spectral components.

Collision-induced constructive quantum interference

We theoretically study the collision-induced constructive quantum interference in an open four-level system with the density-matrix approach based on the experimental observation of constructive quantum interference between two transition pathways ${3P}_{1/2}\ensuremath{-}5S$ (or $4D)$ and ${3P}_{3/2}\ensuremath{-}5S$ (or $4D)$ via equal-frequency hybrid excitation in the ${\mathrm{Na}}_{2}\ensuremath{-}\mathrm{Na}$ system. The effects of the collision-induced coherent and incoherent decay rates and the ratio of the two transition dipole moments on the interference are analyzed. It is shown that through the incoherent process (collision), the coherence between a widely separated doublet and subsequent constructive quantum interference can be realized. The physical origin of the constructive interference can be seen clearly in the dressed-atom picture. The theoretical results can also be used to qualitatively explain the dependence of quantum interference on the experimental buffer gas pressure and sample temperature.

Theory of excess noise in unstable resonator lasers

We theoretically investigate the quantum dynamics of an unstable resonator laser. Compared to a stable cavity laser of equal gain and loss it exhibits a K-fold enhanced linewidth. This excess noise factor K is a measure of the nonorthogonality of the resonator eigenmodes and amounts to an enlargement of the quantum vacuum fluctuations. Using a quantum treatment starting from first principles based on the nonorthogonal eigenmodes, we put previous theoretical predictions onto a more firm ground. While we find a position-dependent enhancement of the spontaneous emission rate into an empty mode of only $\sqrt{K},$ the constructive quantum interference of the spontaneous emission with a single oscillating mode lets the Petermann excess noise factor K reappear in the phase diffusion (laser linewidth). Hence locally enhanced spontaneous emission as well as noise enhanced by interference (amplified spontaneous emission) play an equal role in the origin of excess noise.

Spontaneous emission spectrum from a V-type three-level atom in a double-band photonic crystal

The spontaneous emission spectrum from a V-type three-level atom embedded in a double-band photonic band gap (PBG) material has been investigated for the first time. Most interestingly it is shown that there is not only a black dark line, but also a narrow spontaneous line near the edges of the double photonic band. The positions of the dark line and narrow spontaneous line are near the transition from an empty upper level to a lower level. The lines stem from destructive and constructive quantum interferences, which induce population transfer between the two upper levels, in the PBG reservoirs. The effects of system parameters on the interference have been discussed in detail.

The effect of two-photon quantum interference on amplitude-squared squeezing of a quantum field

In this paper, we consider a superposition of a weak coherent field and a two-photon source and examine the amplitude-squared squeezing (ASS) properties of the combined field. We demonstrate that the efficient and lasting ASS effect of the field could be observed due to constructive two-photon quantum interference. The relationship between ASS and photon antibunching effects is discussed. It is found that photon antibunching may accompany weak ASS, but in most cases ASS effects appear accompanying photon bunching.