We study the efficiency of different two-photon states of light to induce the simultaneous excitation of two atoms of different kinds when the sum of the energies of the two photons matches the sum of the energies of the two atomic transitions, while no photons are resonant with each individual transition. We find that entangled two-photon states produced by an atomic cascade are indeed capable of enhancing by a large factor the simultaneous excitation probability as compared to uncorrelated photons, as predicted some years ago by Muthukrishnan et al., but that several unentangled, separable, correlated states, produced either by an atomic cascade or parametric down-conversion, or even appropriate combinations of coherent states, have comparable efficiencies. We show that the key ingredient for the increase of simultaneous excitation probability is the presence of strong frequency anticorrelation and neither time correlation nor time-frequency entanglement.

Parallel double-plate capacitive proximity sensor modelling based on effective theory AIP Advances 4, 027119 (2014) Abstract. Interdigital capacitive sensors are applied in nondestructive testing and material property characterization of lowconductivity materials. The sensor performance is typically described based on the penetration depth of the electric field into the sample material, the sensor signal strength and its sensitivity. These factors all depend on the geometry and material properties of the sensor and sample. In this paper, a detailed analysis is provided, through finite element simulations, of the ways in which the sensor's geometrical parameters affect its performance. The geometrical parameters include the number of digits forming the interdigital electrodes and the ratio of digit width to their separation. In addition, the influence of the presence or absence of a metal backplane on the sample is analyzed. Further, the effects of sensor substrate thickness and material on signal strength are studied. The results of the analysis show that it is necessary to take into account a trade-off between the desired sensitivity and penetration depth when designing the sensor. Parametric equations are presented to assist the sensor designer or nondestructive evaluation specialist in optimizing the design of a capacitive sensor.

We theoretically demonstrate through numerical methods that the triple-photon state generated by threephoton spontaneous parametric down-conversion is a non-Gaussian Greenberger-Horne-Zeilinger state, showing super-Gaussian statistics. Interestingly, the degree of entanglement between the modes of the triple-photon state is stronger than that corresponding to the two-mode squeezed vacuum state produced by a quadratic Hamiltonian with the same parameters. Furthermore, we propose a model to prepare two-mode sub-Gaussian entangled states with a tunable negative Wigner function based on quadrature projection measurements. We find that these Gaussian projection measurements with outcomes X 1 not only improve the entanglement of the residual two modes but also introduce a Gaussian component, resulting in the coexistence of Gaussian and non-Gaussian entanglement.

We propose a Raman quantum memory scheme that uses several atomic ensembles to store and retrieve the multimode highly entangled state of an optical quantum frequency comb, such as the one produced by parametric down-conversion of a pump frequency comb. We analyse the efficiency and the fidelity of such a quantum memory. Results show that our proposal may be helpful to multimode information processing using the different frequency bands of an optical frequency comb.PACS numbers: 42.50. Gy, 42.50.Ct, 03.67.Bg, 42.50.Ex Quantum information is a fascinating subject, as it makes use of the deepest aspects of quantum theory, which is inherently an information theory. When the information is carried by quantum states belonging to a high-dimensional Hilbert space, for example by quantum states of highly multimode light, one expects a potential significant increase in the information capacity. In the toolbox of quantum information processing, one of the most important tools is the quantum memory, without which all the advantages of quantum information cannot be fully exploited. Quantum memories are actively studied, both at the theoretical and experimental level [1], because they constitute an important resource in longdistance quantum communication.Up to now these studies concerned mainly quantum states contained in a single pulse of single transverse mode light, i.e., single-mode configurations. They also have been extended to some kinds of multimode fields, either in the spatial or temporal domain. In the spatial domain, the quantum aspects of highly multimode light have been actively studied in the past years [2]. In particular, spatially multimode quantum memories have been designed and built [3]. In the temporal domain, multimode quantum memories have been developed to store long pulses of different mean frequencies [4][5][6][7]. Another possibility is to store different modes consisting of different time slots [8] or different temporal shapes of short light pulses [9], for which efficient pulse-shaping techniques exist. This is the reason why we have chosen to explore the problem of storing quantum states of "optical frequency combs": Highly multimode quantum frequency combs have indeed been recently experimentally implemented [10], and have been shown to exhibit genuine multipartite [11,12] and full entanglement [13]. Such highly multimode quantum states are a promising resource for quantum information processing and measurement-based quantum computing [12,[14][15][16][17].

We investigate the possibility of an experimentally feasible cascaded four-wave mixing (FWM) system [Phys. Rev. Lett. 113, 023602 (2014)] to generate tripartite entanglement. We verify that genuine tripartite entanglement is present in this system by calculating the covariances of three output beams and then considering the violations of the inequalities of the three-mode entanglement criteria, such as two-condition criterion, single-condition criterion, optimal single-condition criterion and the positivity under partial transposition (PPT) criterion. We also consider the possibilities of the bipartite entanglement of any pair of the three output beams using the Duan-Giedke-Cirac-Zoller criterion and PPT criterion. We find that the tripartite entanglement and the bipartite entanglement for the two pairs are present in the whole gain region. The entanglement characteristics under different entanglement criteria are also considered. Our results pave the way for the realization and application of multipartite entanglement based on the cascaded FWM processes.

We investigate different kinds of entanglement in a four-wave mixing process with a degenerate pump. After analyses on means and quantum fluctuations of the three output beams (Stokes, anti-Stokes, and pump), we verify the existence of genuine tripartite entanglement, and quantify bipartite, two-mode, as well as tripartite entanglement with the covariance matrix. We find out that the input pump power and the nonlinear coupling strength are the physical origins to enhance entanglement at a given photon loss.

Laser light with spectral purity and frequency stability is pursued in precision spectroscopy and precision measurements. We propose a scheme to generate millihertz-linewidth laser light with a frequency instability of 10−18 via optical four-wave mixing in alkaline-earth atoms. We show that the linewidth of the mixing laser light is ultimately limited by the natural linewidth of the atomic transition rather than by the linewidth of the input lasers. The frequency stability of the mixing laser light depends largely on the intensity stability of the input lasers. It is possible to generate a millihertz-linewidth laser light with a frequency instability of 10−18 and a power of 10−12 W when the input lasers with a relative intensity instability of 10−4 and a spectral width of 1 Hz interact with strontium (Sr) atoms with a density of 1 × 1011 cm−3.

Multimode entanglement is essential for the generation of quantum networks, which plays a central role in quantum information processing and quantum metrology. Here, we study the spatial multimode entanglement characteristics of the large scale quantum states via a dual-pumped four-wave-mixing (FWM) process of Rubidium atomics vapors. A linear mode transform approach is applied to solve the four- and six-mode Gaussian states and the analytical input-output relations are presented. Moreover, via reconstructing the full covariance matrix of the produced states, versatile entanglement with from two up to six modes is analyzed. The results show that most of the 1 versus n-mode and m versus n-mode states are entangled, and the amount of entanglement can be regulated due to the competitions of mode components caused by different interaction strengths of co-existing FWMs. Our study could be applied for any multimode Gaussian states with a quadratic Hamiltonian.