Vacuum-induced Coherence Research Articles

Enhancement of magnon bistability and magnon–phonon entanglement using vacuum induced coherence in cavity magnomechanics

We propose a novel approach to enhance magnon bistability and magnon–phonon entanglement in a microwave cavity containing a driving atomic ensemble and a ferrimagnetic crystal of yttrium iron garnet (YIG), where the cavity is simultaneously coupled with the magnon and atoms while the magnon couples to phonon through magnetostrictive interaction. When the magnon-atom coherent coupling is established via the exchange of virtual photon, it is of interest that the magnon bistability is significantly enhanced by the vacuum induced coherence (VIC) from the atomic spontaneous. Moreover, the magnon–phonon entanglement that arises from parametric and beam-splitter interactions are also enhanced, wherein the atomic ensemble acts as a reservoir to cooling the magnon and phonon further. The present scheme provides a feasible way to control the nonlinear magnon–phonon coupling strength by quantum interference, which may be useful for macroscopic quantum information processing.

Production and protection of entanglement via vacuum induced coherences

The entanglement dynamics of a pair of three-level V-type atoms decaying spontaneously in a common vacuum reservoir is investigated. Under the condition that the decaying transitions in the atoms have parallel dipole moments, the effect of coherences induced by spontaneous emission is considered in the atomic dynamics. We show that vacuum-induced coherence (VIC) and collective effects in atomic decay play a significant role in the creation of entanglement. By using negativity as a measure of entanglement, we study the time evolution of entanglement for initial separable and generalized Dicke states as well as maximally entangled qutrit states. We show that the effects of VIC enhance the production of entanglement from initial separable states of the atoms. We also show that the entanglement can be protected in steady-state for atoms evolving from initial entangled states. The amount of entanglement that can be preserved is more in the presence of VIC than in its absence.

Postponing entanglement sudden death by quantum interferences

We investigate the entanglement dynamics in a 2 ⨂ 3 quantum system consisting of a two-level atom and a three-level V-type atom interacting independently with their environments. Pure and mixed initial states, including one-parameter and two-parameter states of the atoms are analyzed. It is observed that the entanglement in the qubit-qutrit system disappears in a finite time, commonly referred to as ESD (entanglement sudden death). We analytically calculate the ESD time using the negativity as an entanglement measure. Remarkably, the presence of vacuum-induced coherence in the three-level atom (qutrit) delays the occurrence of ESD and effectively protects the entanglement from decoherence.

Simulating quantum interference effects in the fluorescence of a V-system with perpendicular dipole moments

We present a scheme that enables the observation of interference effects in the resonance fluorescence of a V-type atom with orthogonal dipole moments. Specifically, we consider the atomic configuration of a J g = 0 to J e = 1 transition driven by a single laser field. By employing polarization-sensitive detection in such a way that the light emitted on the two transitions become indistinguishable, we show that one can simulate the effect of vacuum-induced coherence on the resonance fluorescence of this system. In addition, we demonstrate the possibility of realizing atomic transitions with both parallel and antiparallel dipole moments and their effects on the fluorescence spectrum. The interference induced leads to interesting features in the fluorescence spectrum such as asymmetric spectral peaks, enhancement and suppression of the sidebands, and disappearance of fluorescence in the particular direction of detection. The numerical results are understood in the context of the dressed states of the combined atom–field system.

Monitoring and manipulation of vacuum-induced coherences in frequency-resolved correlation

The frequency-resolved correlations of the fluorescence emitted by a four-level system in the $J=1/2$ to $J=1/2$ transition driven by a linearly polarized laser field are theoretically investigated. In the cross two-photon correlation of the $\ensuremath{\pi}$ and ${\ensuremath{\sigma}}^{+}$ transitions, we show that VICs can induce the interference of time orderings between the two-photon paths with different dipole moments, which can serve as a good scheme to exhibit the effects of vacuum-induced coherences (VICs). An external magnetic field can cause frequency differences between two-photon paths with different time orderings. When the differences can be distinguished by detectors, the effects of VICs are destroyed and time asymmetry of the two-photon correlations appears due to the reveal of ``which path'' information. However, the which path information can be erased by tuning the filtering frequencies so the perfect interference effects of time orderings and time symmetry are restored.

Squeezing in resonance fluorescence via vacuum induced coherences

The squeezing spectrum of the fluorescence field emitted from a four-level atom in J = 1/2 to J = 1/2 configuration driven by two coherent fields is studied. We find that the squeezing properties of the fluorescence radiation are significantly influenced by the presence of vacuum-induced coherence in the atomic system. It is shown that such coherence induces spectral squeezing in phase quadratures of the fluorescence light for both weak and strong driving fields. The dependence of the squeezing spectrum on the relative phase of the driving fields is also investigated. Effects such as enhancement or suppression of the squeezing peaks are shown in the spectrum as the relative phase is varied. An analytical explanation of the numerical findings is presented using dressed-states of the atom-field system.

Suppressing deleterious effects of spontaneous emission in creating bound states in cold atom continuum

In a previous paper [Deb and Agarwal 2014 Phys. Rev. A 90 063417], it was theoretically shown that, magneto-optical manipulation of low energy scattering resonances and atom-molecule transitions could lead to the formation of a bound state in continuum (BIC), provided there is no spontaneous emission. We find that even an exceedingly small spontaneous decay from exited molecular states can spoil the BIC. In this paper, we show how to circumvent the detrimental effect of spontaneous emission by making use of vacuum-induced coherence (VIC) which results in the cancellation or suppression of spontaneous emission. VIC occurs due to the destructive interference between two spontaneous decay pathways. An essential condition for VIC is the non-orthogonality of two transition dipole moments associated with the radiative decays form two excited states. Furthermore, the interference between two decay pathways requires that the spacing between the two decaying states must be comparable to or smaller than the square root of the product of the two spontaneous linewidths. We demonstrate that these conditions can be fulfilled by optically manipulating appropriately chosen molecular excited states, opening a promising prospect for the experimental realization of BIC of cold atoms.

Effects of multiple vacuum-induced coherence on absorption properties in a four-level inverted-tripod scheme atomic system

When the three transitions from the three nearly degenerate excited levels to single ground level in a four-level inverted-tripod scheme atomic system interact with the same vacuum, there appears multiple vacuum induced coherence (VIC) induced by the interference among these transitions. We investigate the effects of the VIC on the probe absorption properties, and find that the VIC from symmetric transitions enhances the central absorption, while suppresses the sideband absorptions within the double transparency spectrum. On the other hand, the VIC from nonsymmetric transitions increases the minimum value of the double transparency.

Fluorescence control through vacuum induced coherences

The resonance fluorescence of a four-level atom in J = 1/2 to J = 1/2 transition driven by two coherent fields is studied. We find that the incoherent fluorescence spectrum shows a direct indication of vacuum-induced coherence in the atomic system. We show that such coherence manifests itself via an enhancement or suppression of the spectral peaks in the π-polarized fluorescence. The effect of the relative phase of the driving fields on the spectral features is also investigated. We show that phase-dependent enhancement or suppression of the fluorescence peaks appears in the incoherent spectrum emitted along the σ transitions. It is found that this phase dependence occurs because of the polarization-detection scheme employed for the observation of the fluorescence light. We present an analytical explanation, based on dressed-states of the atom-field system, to interpret the numerical results.

Comment on “Protecting bipartite entanglement by quantum interferences”

In an interesting article [Phys. Rev. A 81, 052341 (2010)], Das and Agarwal have discussed the preservation of bipartite entanglement in three-level atoms employing the coherences induced by spontaneous emission. The authors considered various initially entangled qubits prepared from two $\mathsf{V}$-type three-level atoms and showed that more than $50%$ of the initial (bipartite) entanglement can be preserved in steady state due to vacuum-induced coherence. In this Comment, we point out that their analytical formulas for the entanglement measure contain errors affecting all the numerical results of that article. We substantiate our claim by giving correct analytical results for the time evolution of the two-atom system.

Population trapping in the excited states using vacuum-induced coherence and adiabatic process

We theoretically investigate how population can be trapped in the closely spaced excited levels in presence of vacuum-induced coherence (VIC). We employ delayed pulses to transfer population from a meta-stable state to the excited states. Subsequently, spontaneous emission from these excited states builds coherence between them. This coherence can be probed by using chirping, which leads to the decoupling of the excited states from the ground state thereby ensuring population transfer via delayed pulses. Our results indicate that the existence of VIC leads to the generation of a mixed state in the excited state manifold, where trapping of the population occurs even in the presence of large decay. This trapping may be realized in molecular systems and can be interpreted as a sensitive probe of VIC. We present suitable numerical analysis to support our results.

Coherence and multimode correlations from vacuum fluctuations in a microwave superconducting cavity.

The existence of vacuum fluctuations is one of the most important predictions of modern quantum field theory. In the vacuum state, fluctuations occurring at different frequencies are uncorrelated. However, if a parameter in the Lagrangian of the field is modulated by an external pump, vacuum fluctuations stimulate spontaneous downconversion processes, creating squeezing between modes symmetric with respect to half of the frequency of the pump. Here we show that by double parametric pumping of a superconducting microwave cavity, it is possible to generate another type of correlation, namely coherence between photons in separate frequency modes. The coherence correlations are tunable by the phases of the pumps and are established by a quantum fluctuation that stimulates the simultaneous creation of two photon pairs. Our analysis indicates that the origin of this vacuum-induced coherence is the absence of which-way information in the frequency space.

Probing vacuum-induced coherence via magneto-optical rotation in molecular systems

We investigate theoretically the effects of vacuum-induced coherence (VIC) on magneto-optical rotation (MOR). We carry out a model study to show that VIC in the presence of a control laser and a magnetic field can lead to large enhancement in the rotation of the plane of polarization of a linearly polarized weak laser with vanishing circular dichroism. This effect can be realized in cold molecular gases and may be used as a sensitive probe for VIC. Such a large MOR angle can also be used to detect weak magnetic field with large measurement sensitivity.

High Refractive Index without Absorption in a Ladder-Type System

A three-level ladder-type system with vacuum-induced coherence and incoherent pumping is studied to obtain a high refractive index without absorption. It is shown that the enhancement of the refractive index without absorption can be accomplished by choosing the proper values of the incoherent pumping, the angle between the two dipole moments, the probe detuning and the relative phase of the two coherent fields.

Transient properties in a resonant ladder-type atomic system with vacuum-induced coherence and incoherent pumping

The transient absorption and population in a resonant ladder-type atomic system with or without an incoherent pumping field are theoretically investigated. We find that the vacuum-induced coherence and the relative phase between probe and coupling fields can clearly affect the transient absorption (gain) properties, but the steady-state value of the probe absorption (gain) is remarkably dependent on the incoherent pumping. And the steady population distribution is sensitive to the incoherent pumping and insensitive to the relative phase. Additionally, it is shown that the lasing with inversion can be reached with the incoherent pumping.

Vacuum-induced coherence in quantum dot systems

We present a theoretical study of vacuum-induced coherence in a pair of vertically stacked semiconductor quantum dots. The process consists in a coherent excitation transfer from a single-exciton state localized in one dot to a delocalized state in which the exciton occupation gets trapped. We study the influence of the factors characteristic of quantum dot systems (as opposed to natural atoms): energy mismatch, coupling between the single-exciton states localized in different dots, and different and nonparallel dipoles due to sub-band mixing, as well as coupling to phonons. We show that the destructive effect of the energy mismatch can be overcome by an appropriate interplay of the dipole moments and coupling between the dots which allows one to observe the trapping effect even in a structure with technologically realistic energy splitting of the order of milli--electron volts. We also analyze the impact of phonon dynamics on the occupation trapping and show that phonon effects are suppressed in a certain range of system parameters. This analysis shows that the vacuum-induced coherence effect and the associated long-living trapped excitonic population can be achieved in quantum dots.

Transient response in a three-level system with the squeezed vacuum

In this paper, the effects of the squeezed vacuum (SV) on the transient response in a ∨-type three-level atomic medium are investigated. We find that the properties of transient process can be greatly affected by the SV whether the vacuum-induced coherence (VIC) exists or not. The combined influence of the SV and the VIC can dramatically modify the transient absorption and gain. Especially, the transient gain can be almost eliminated in the absence of VIC. And the response time from the transient to the steady situation is dependent on the SV. When including the VIC, the transient and steady optical properties display a reverse variation with the SV. Besides, it is shown that the considerable steady state population without inversion can be always induced by the SV.

Vacuum-induced coherence in ultracold photoassociative rovibrational excitations

We show that coherence between two excited ro-vibrational states belonging to the same molecular electronic configuration arises quite naturally due to their interaction with electromagnetic vacuum. For initial preparation of a molecule in the desired ro-vibrational states, we propose to employ the method of ultracold photoassociation. Spontaneous decay of the excited molecule then gives rise to vacuum induced coherence between the excited ro-vibrational states. We demonstrate theoretically an interesting interplay of effects due to vacuum induced coherence and photoassociation. We apply our theory to photoassociation of bosonic Ytterbium (^{174}Yb) atoms which appear to be a promising system for exploring such interplay. The effects discussed here can be important for controlling decoherence and dissipation in molecular systems.

Effect of Doppler broadening on the vacuum induced coherence-dependent probe field absorption in Y-type four-level system

Using the numerical result, the influence of the Doppler broadening on the vacuum induced coherence (VIC) related probe field absorption is discussed. It is shown that if there exists no Doppler broadening, only when VIC is absent can the electromagnetically induced transparency (EIT) phenomenon occur; VIC leads the probe field absorption to vary obviously and the gain to appear; in both cases with and without VIC, the absorption curve has a double-peak structure which is symmetrical about the probe detuning. If there exists the Doppler broadening, in both cases with and without VIC, EIT phenomenon can always occur; VIC still leads the probe field absorption to vary obviously and the gain to appear; no matter whether the VIC is present. The probe field absorption has the following characteristics: the absorption curve no longer has symmetry about the probe field detuning and changes gradually from the double-peak structure to the single-peak structure with Doppler broadening width (D) increasing; the probe field absorption does not monotonically increase or decrease with D increasing; the probe field absorption, when the probe and driving fields propagate along the opposite directions, is smaller than that when the probe and driving fields propagate along the same directions.

Phase-dependence of inversionless gain in a Doppler broadened open four-level system

The effect of the relative phase (ϕ) between the probe and driving fields on the gain without inversion (GWI) in a Doppler broadened open quasi Λ-type four level atomic system with vacuum induced coherence (VIC) for both co- and counter-propagating probe and driving fields cases is studied. It is shown that: (1) GWI and the probe detuning region in which GWI exists are very sensitive to variation of the relative phase; when values of the other parameters keep unvarying, by adjusting value of ϕ, the largest GWI can be obtained. (2) The Doppler width (D) also has dramatically modulation role on the phase-dependent GWI. When value of D is smaller, the value of ϕ which corresponds to the largest value of GWI is about π, when value of D is large enough, it is about π/2. (3) GWI varies periodically with ϕ varying, the period is 2π, but the concrete variation rule is closely related to value of D. (4) In the co-propagating case we can obtain much larger GWI than that in the counter-propagating case.