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Three-Photon Entanglement

Final Report Summary - THREEPLE (Three-Photon Entanglement)

THREEPLE, namely “Three-photon entanglement”, aims at investigating the generation of entanglement shared among three photons via a direct process. Typically, entanglement configurations focused on the generation of photon pairs, through the processes of spontaneous parametric down conversion or spontaneous four-wave mixing. The first occurs in second order (χ(2)) nonlinear media and relies on the annihilation of a pump photon and the subsequent generation of two daughter photons (signal and idler). Similarly, spontaneous four-wave mixing generates signal and idler pairs, yet it occurs in third order (χ(3)) nonlinear media via the annihilation of two pump photons. However, high photon number entanglement has also been investigated as resource for fundamental investigations, related, e.g. to the study of local realism and nonstatistical predictions of quantum mechanics, as well as from a technological point of view, for its applications in quantum information and quantum metrology. The generation of high photon number entanglement typically relied on the combination of two or more entangled photon pairs, through, e.g. probabilistic post-selection. THREEPLE instead focused on the direct generation of three-photon entanglement in the process of third order spontaneous down conversion where a pump photon is annihilated in a χ(3) medium and three photons are generated in a single process. THREEPLE relied on a two-fold approach: i) on the one hand we investigated the use of non-standard pump beams, e.g. Bessel, for satisfying the underlining momentum conservation conditions required to achieve efficient third order spontaneous down conversion; ii) on the other hand we exploited integrated optics for the generation of high photon number entanglement.
On this last topic, building on the know-how developed by the fellow in the Third Country, we investigated the onset of novel nonlinear processes in integrated microring resonators, allowing the first demonstration of direct generation of cross-polarised photon pairs and bichromatically pumped optical parametric oscillator on-chip [1]. The ability to fully control the polarisation degree of freedom in integrated structures, where typically only one polarisation at the time is considered, paves the way for the generation of more complex multi-mode states [2].
These results were the precursors of further studies, related to the possibility of generating time-bin entangled photons distributed over a frequency comb, owing to the intrinsic multiplexing capabilities of integrated microresonators. We demonstrated the generation of entangled photons distributed over as many as five signal-idler frequency pairs, limited only by the available frequency filters, with tens of frequency channels in principle available in the standard telecom bands, around 1550 nm [3]. The access to multiple entangled pairs on the same chip allowed us also to investigate multiphoton, four-photon entanglement, as evidenced by the visibility of the fringes in quantum interference experiments shown in Figure 1(a). This work was featured on several press releases, including Physics World [4], Optics and Photonics News “Optics in 2016” issue [5], OSA Newsroom [6], as well as in the Photonics Spectra issue “2015 Trends – Miniaturization” [7].
The investigation on the use of non-standard pump beams, led us to the definition of a phase matched configuration for third order spontaneous down conversion in bulk crystal, using Bessel pump beams. Our theoretical and numerical investigations show that phase matching can be achieved for different pump wavelengths. The generation rate in standard Kerr media, such as glasses, diamond, BaF2, KBr and others, is constrained by the low nonlinearity and short coherence length, which limits the suitable interaction region. To overcome the shortcomings of the available nonlinear materials we investigated the possibility to artificially increase the nonlinear effects. We studied the nonlinear enhancement at the ϵ-near-zero (ENZ) wavelength, that is, the wavelength at which the real part of the permittivity of a material reaches a value close to zero. We first studied silver-glass artificial media (metamaterials), obtained by an engineered sequence of subwavelength thickness layers, for achieving ENZ wavelength in the near infrared, at the operation wavelength of standard lasers (800 nm). With this arrangement, we observed ultrafast metal-to-dielectric transitions [8], yet with a limited nonlinear enhancement consequent of the inherent dispersion of the silver nonlinearity. To overcome this, we considered transparent conductive oxide thin films that naturally exhibit the ENZ condition at wavelengths accessible to the laser systems at our disposal. We identified Aluminium-doped zinc oxide (AZO) as the best material in this class (ENZ at ~1300nm). The specific linear and nonlinear properties of AZO allowed us to observe a six-fold enhancement of the nonlinear index, shown in Figure 1(b), with refractive index changes as high as 500% [9]. To address the requirements set by the generation of quantum states of light we devised an innovative dual excitation scheme, which allowed us to combine both intraband and interband nonlinearities and achieve novel functionalities, including a higher modulation bandwidth and optical spectral tuning [10].

The results achieved within THREEPLE have been widely disseminated at international conferences (nearly 40 contributions, including 18 invited talks, 1 keynote and 1 postdeadline talk), by publications in high-impact peer-reviewed journals (including 1 Science [3], 1 Physical Review Letters [9], 1 accepted paper in Nature Communications [10], 1 invited review paper in Nanophotonics [2], and 1 review paper recently accepted in Light: Science & Applications [11]), by seminars at different national and international institutions, by several press releases (e.g. [4-7]), as well as on the project webpage

[1] C. Reimer, M. Kues, L. Caspani et al., “Cross-polarized photon-pair generation and bi-chromatically pumped optical parametric oscillation on a chip,” Nat. Commun. 6, 8236 (2015).
[2] L. Caspani et al., “Multifrequency sources of quantum correlated photon pairs on-chip: a path toward integrated Quantum Frequency Combs,” Nanophotonics 5, 351–362 (2016).
[3] C. Reimer et al. (L. Caspani) “Generation of multiphoton entangled quantum states by means of integrated frequency combs,” Science 351, 1176–1180 (2016).
[8] R. M. Kaipurath, M. Pietrzyk, L. Caspani et al., “Optically induced metal-to-dielectric transition in Epsilon-Near-Zero metamaterials,” Sci. Rep. 6, 27700 (2016).
[9] Caspani et al., “Enhanced Nonlinear Refractive Index in ϵ-Near-Zero Materials,” Phys. Rev. Lett. 116, 233901 (2016).
[10] M. Clerici et al. (including L. Caspani), “Controlling hybrid nonlinearities in transparent conducting oxides via two-colour excitation”, Nat. Commun., accepted 2017.
[11] L. Caspani et al., “Integrated sources of photon quantum states based on nonlinear optics”, Light Sci. Appl., accepted (2017).