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Tunable Quantum Optics in Index-Guiding Fibres

Jonas Hammer

Published Jan 1, 2021• 0 citations
OpenAlex

Abstract

The spontaneous decay of a pump photon into a tripartite entangled photon state (photon triplets) has been extensively studied in theory. Observation of these photon triplets at optical frequencies is of interest for fundamental science and future applications of quantum optics, such as quantum computing and quantum cryptography. This decay is known as spontaneous third-order parametric down-conversion (TOPDC). Experimentally, TOPDC has not been demonstrated in the optical frequency domain. TOPDC is enabled by the third-order nonlinearity of the material in which the interaction takes place. In optical fibres, the effective nonlinearity of the glass is increased manifold. The reason for this is the long light-matter interaction length and the tight confinement of light in the waveguide. Optical fibres are therefore the ideal platform for the observation of TOPDC, provided that phase matching can be achieved. In this thesis, the use of submicrometre fibre tapers is studied in the context of TOPDC, both in experiment and theory. Comparing different fibre-based TOPDC sources, fibre tapers rank among the highest in terms of expected triplet generation rate. At the same time, the background noise generated in these fibres is very low, which leads to a high expected three-photon coincidence-to-accidental ratio (CAR) on the order of 7000. In the experiments related to TOPDC, the precision of manufacturing fibre tapers is studied. A novel gas-based mechanism for dispersion tuning is demonstrated. Without this mechanism the manufacturing precision of the fibre tapers would not suffice to observe TOPDC in an experiment. The tuning mechanism is demonstrated using third-harmonic generation, which is the inverse process of TOPDC. The resonant third-harmonic radiation generated around 460nm can be tuned at a rate of −0.118nm/bar. This is achieved by introducing pressurised argon in the taper environment. The background noise from fibre tapers is characterised. The effect of the tuning mechanism on the three-photon-state and the signal-to-noise ratio is studied in theory. In another part of the thesis, gas-filled suspended-core fibres (SCFs) are used as a spectrally tunable source of entangled photon pairs (biphotons). Generation of biphotons via spontaneous four-wave mixing (FWM) in fibres offers great advantages over the generation in bulk crystals, which is the commonly employed technique. The most important advantages are the excellent control of the spatial mode structure, the extra degrees of freedom for dispersion design and the possibility to integrate the fibres into existing networks. Yet, fibre biphoton sources lack the convenient spectral tunability of crystal-based sources. Introducing pressurised gas into the hollow channels of the SCF is demonstrated to be a convenient and broadband spectral tuning mechanism. The FWM-sidebands can be continuously tuned at a rate of −0.3 THz/bar, in the quantum and in the seeded scenario. A high two-photon-CAR exceeding 10000 is measured for the biphoton state, and the frequency entangled joint-spectral intensity is measured. Finally, seeded FWM is used to characterise the core size of high-NA submicrometre fibres to a nanometre precision. If ultrafast pulses are used instead of nanosecond pulses, this will present a non-destructive and high precision characterisation technique for such fibres. The results of this work present an important step towards realising TOPDC at optical frequencies in the near future. The experimental realisation of a broadly tunable biphoton source combines the convenient tuning mechanism found in crystal-based sources with the advantages of fibre-based biphoton generation. Such tunable fibre biphoton sources can therefore be competitive with crystal-based systems for quantum optics applications.

Related papers

Quantum State Generation with Nonlinear Integrated Photonics: Conventional and Unconventional Approaches

Milica Banić

In this thesis we study the generation of non-classical states of light using nonlinear integrated photonics. We focus first on third-order parametric down-conversion (TOPDC), a process in which pump photons can be down-converted into photon triplets, and which could serve as a source of non-Gaussian states and tripartite discrete-variable entanglement. We investigate the possibility of implementing TOPDC in conventional integrated structures. By comparing the estimated triplet generation rates in a waveguide and a microring resonator, we establish that non-resonant structures appear to be a more promising platform for a near-term demonstration of TOPDC, due to the high generation bandwidth enhancing the rate of triplet generation. We estimate generation rates of ~10 triplets per second for a 1-cm-long waveguide driven by a 100 mW CW pump, a rate that is comparable to or higher than those quoted for other proposals. We then estimate the non-Gaussianity that might be expected in a supermode of the field generated by our sample structures. The predicted non-Gaussianity is negligible in both structures, for different reasons: The generation rate in the ring is too low, while the field generated by the waveguide is highly multimode. We conclude that although a simple SiN waveguide may be a suitable source for a first demonstration of optical TOPDC, the design will need to be improved significantly for observations of non-Gaussianity or tripartite entanglement in the generated light. Indeed, state generation protocols based on conventional spontaneous four-wave mixing sources and postselection appear to be more promising in the near term. We adopt this approach to design integrated sources of postselected multipartite states in a frequency bin encoding scheme. We introduce sources of W states and GHZ states, with kHz generation rates expected for picojoule pump pulses. This platform appears to be promising for the generation of more arbitrary states, including high-dimensional ones consisting of entangled qudits. We review a graph-based strategy that may be useful for designing other devices, and for understanding the limitations of this platform.

•Jun 1, 2024•0 citations

Tailored Spontaneous Four-Wave Mixing in Sinusoidally-Tapered Fibres

Mohammed F. Saleh

Quasi-phase-matching periodically-tapered waveguides (PTWs) can enable efficient on-demand third-order parametric interactions using right combinations of the tapering period and modulation amplitude [1]. Similar to periodically-poled ferroelectric crystals, this new technique eliminates the stringent constraints imposed by conventional methods on the frequencies, mode profiles, and polarisations of the interacting photons. An example of PTWs is the sinusoidally-tapered fibres that have been exploited in manuiplating supercontinuum generation and modulation instability [2]. In this work, I have developed a rigorous quantum model to investigate spontaneous four-wave mixing (SFWM) inside these tapered waveguides [3]. The right combinations between the modulation amplitude Δd and tapering period ΛTthat enhances the expected number of photons (N̂s) at the targeted wavelengths are portrayed in Fig. 1(a), for fibres with same number of periods M. The values of (N̂s) are normalised to the case when Δd = 0, to quantify the enhancement in photon-pairs generation using the PTW-technique in comparison to uniform fibres. For only M = 50, (N̂s) is remarkably enhanced by 35 dB. The output spectrum of the photon-pairs is featured as a narrow sinc-function with very weak sidelobes that are significantly diminished for large number of periods, as depicted in Fig. 1(b). The 2D representation of the spectrum as a function of the photon-pairs wavelengths (λs, λi) is shown in panel (c). In this plot, the pump is assumed to be a monochromatic at a frequency satisfies the energy conservation. The corresponding (N̂s) for a Gaussian-pulse pump source with an input energy 1 nJ and a full-width-half-maximum 4 ps is portrayed in Fig. 1(d). Using the Schmidt decomposition analysis, the spectral-purity is 0.74. This shows the ability of the PTW-technique in producing highly-efficient relatively-pure single photons at any on-demand frequencies without applying any bandpass filters. This work will also open a new direction of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in third-order nonlinear guided structures.

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 1, 2019•0 citations

Generation of photon pairs by stimulated emission in ring resonators

Milica Banić, Marco Liscidini, John E. Sipe

Third-order parametric downconversion (TOPDC) describes a class of nonlinear interactions in which a pump photon is converted into a photon triplet. This process can occur spontaneously or it can be stimulated by seeding fields. Here we show that stimulated TOPDC (StTOPDC) can be exploited for the generation of quantum correlated photon pairs. We model StTOPDC in a microring resonator, predicting observable pair generation rates in a microring engineered for third-harmonic generation, and we examine the peculiar features of this approach when compared with second-order spontaneous parametric downconversion and spontaneous four-wave mixing. We conclude that if the experimental difficulties associated with implementing StTOPDC can be overcome, it may soon be possible to demonstrate this process in resonant integrated devices.

arXiv:2107.06339•Optics Letters•Feb 23, 2022•5 citations

Generation of Non-Classical States of Light Using Photonics Crystal Fibres

Nicolas Y. Joly

Generation of spontaneous third-order down conversion as the reverse of third harmonic generation remains a challenging task, not yet demonstrated. We present different possible strategies involving gas-filled hollow-core photonic crystal fibres, hybrid fibres and tapered fibres in gas.

European Conference on Optical Communication•Sep 1, 2018•0 citations

Towards photon pair generation via spontaneous four-wave mixing in nonlinear metasurfaces

Son, Changjin

Entangled photons have acted as a workhorse in many quantum technologies, including cryptography, imaging, sensing, and computation. With the growing demands of quantum technology, the entangled photon pair sources are required to be compact and integrable. For the last few years, the field of quantum optics in the nanoscale has demonstrated photon pair generation in ’flat’ optics such as thin layers, Van der Waals crystals, micro-, nano-antennas, and metasurfaces. The word ’flat’ is used because such optical platforms have sub-wavelength thicknesses. Due to the short travel length inside the sources, they can possess multiple functions that their bulky counterparts cannot have. This doctoral thesis addresses photon pair generation in ’flat’ optics via second- and third-order nonlinear processes, which are spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM). The main aims of the thesis are to understand how the photon pair generation is realized via SPDC and SFWM on subwavelength-scale platforms, and to make a bridge between photon pair generation via two nonlinear processes under the presence of optical resonances. The thesis covers two experimental parts related to SPDC and SFWM, and one future expectation of the SFWM in metamaterials. The first part of the thesis addresses one of the multi-functionalities that has recently been reported: the emission direction control of the photon pairs. A 150 nm-thick gallium phosphide (GaP) metasurface is used as a source. The metasurface supports a quasi-bound state in the continuum (quasi-BIC) resonance, enhancing the electric field inside the material. I generate the entangled photons through the SPDC process. The generated photons are in two different wavelengths under the given pump wavelength: signal photon at the resonance, and idler photon out of the resonance. The photon pair generation rate is enhanced by 66 times compared to the unpatterned GaP slab since the vacuum field in the signal mode is enhanced by the resonance. The generated photons obey the transmission spectrum of the metasurface. As a result, the signal photons at resonance are mostly reflected, and the idler photons are transmitted after generation. This result is the first work of photon demultiplexing by a metasurface. The second part of the thesis covers the photon pair generation through SFWM in thin nonlinear films. Silicon-rich silicon nitride (SiN) layers are selected as the sources. The layers have an intriguing characteristic that the third-order nonlinear susceptibility (chi-3) differs with the nitrogen doping rate. I generate the photon pairs through chi-3 variant SiN layers. The thicknesses of the samples are all sub-wavelength level, having a few hundred nanometers. Time-energy correlation measurements are conducted for all samples, and the second-order correlation functions (g(2)(0)) are calculated. The maximum g(2)(0) of the samples exceeds 2. Interestingly, I observed two-photon interference effects between photon pairs generated in the layers and the fused-silica substrate, which allows me to estimate the chi-3 values of the samples by knowing the reference: chi-3 value of the substrate. The last part of the thesis introduces designs of metasurfaces and possible experimental schemes for photon pair generation through SFWM. Yet, due to its weak light-matter interaction in third-order nonlinear processes, the photon pair generation through SFWM has not been observed in metasurfaces. However, it is fairly possible to enhance the vacuum fields at the output photon modes and generate photon pairs because preliminary work in unpatterned slabs has already been observed in this thesis. Finally, I introduce a fabricated metasurface. It needs improvements, but it is quite promising for the ongoing project.

•Jan 1, 2026•0 citations

Modelling spontaneous four-wave mixing in periodically tapered waveguides

Mohammed F. Saleh

A periodically tapered waveguides technique is an emerging potential route to establish quasi-phase-matching schemes in third-order nonlinear materials for efficient on-demand parametric interactions. In this paper, I investigate this method in enhancing spontaneous photon-pair emission in microstructured fibres and planar waveguides with sinusoidally varying cross sections. To study this process for continuous and pulsed-pump excitations, I have developed a general robust quantum model that takes into account self- and cross-phase modulations. The model shows a great enhancement in photon-pair generation in waveguides with a small number of tapering periods that are feasible via the current fabrication technologies. I envisage that this work will open a new area of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in nonlinear guided structures.

arXiv:1901.06302•Optics Express•Apr 15, 2019•7 citations
The spontaneous decay of a pump photon into a tripartite entangled photon state (photon triplets) has been extensively studied in theory. Observation of these photon triplets at optical frequencies is of interest for fundamental science and future applications of quantum optics, such as quantum computing and quantum cryptography. This decay is known as spontaneous third-order parametric down-conversion (TOPDC). Experimentally, TOPDC has not been demonstrated in the optical frequency domain. TOPDC is enabled by the third-order nonlinearity of the material in which the interaction takes place. In optical fibres, the effective nonlinearity of the glass is increased manifold. The reason for this is the long light-matter interaction length and the tight confinement of light in the waveguide. Optical fibres are therefore the ideal platform for the observation of TOPDC, provided that phase matching can be achieved. In this thesis, the use of submicrometre fibre tapers is studied in the context of TOPDC, both in experiment and theory. Comparing different fibre-based TOPDC sources, fibre tapers rank among the highest in terms of expected triplet generation rate. At the same time, the background noise generated in these fibres is very low, which leads to a high expected three-photon coincidence-to-accidental ratio (CAR) on the order of 7000. In the experiments related to TOPDC, the precision of manufacturing fibre tapers is studied. A novel gas-based mechanism for dispersion tuning is demonstrated. Without this mechanism the manufacturing precision of the fibre tapers would not suffice to observe TOPDC in an experiment. The tuning mechanism is demonstrated using third-harmonic generation, which is the inverse process of TOPDC. The resonant third-harmonic radiation generated around 460nm can be tuned at a rate of −0.118nm/bar. This is achieved by introducing pressurised argon in the taper environment. The background noise from fibre tapers is characterised. The effect of the tuning mechanism on the three-photon-state and the signal-to-noise ratio is studied in theory. In another part of the thesis, gas-filled suspended-core fibres (SCFs) are used as a spectrally tunable source of entangled photon pairs (biphotons). Generation of biphotons via spontaneous four-wave mixing (FWM) in fibres offers great advantages over the generation in bulk crystals, which is the commonly employed technique. The most important advantages are the excellent control of the spatial mode structure, the extra degrees of freedom for dispersion design and the possibility to integrate the fibres into existing networks. Yet, fibre biphoton sources lack the convenient spectral tunability of crystal-based sources. Introducing pressurised gas into the hollow channels of the SCF is demonstrated to be a convenient and broadband spectral tuning mechanism. The FWM-sidebands can be continuously tuned at a rate of −0.3 THz/bar, in the quantum and in the seeded scenario. A high two-photon-CAR exceeding 10000 is measured for the biphoton state, and the frequency entangled joint-spectral intensity is measured. Finally, seeded FWM is used to characterise the core size of high-NA submicrometre fibres to a nanometre precision. If ultrafast pulses are used instead of nanosecond pulses, this will present a non-destructive and high precision characterisation technique for such fibres. The results of this work present an important step towards realising TOPDC at optical frequencies in the near future. The experimental realisation of a broadly tunable biphoton source combines the convenient tuning mechanism found in crystal-based sources with the advantages of fibre-based biphoton generation. Such tunable fibre biphoton sources can therefore be competitive with crystal-based systems for quantum optics applications.
In this thesis we study the generation of non-classical states of light using nonlinear integrated photonics. We focus first on third-order parametric down-conversion (TOPDC), a process in which pump photons can be down-converted into photon triplets, and which could serve as a source of non-Gaussian states and tripartite discrete-variable entanglement. We investigate the possibility of implementing TOPDC in conventional integrated structures. By comparing the estimated triplet generation rates in a waveguide and a microring resonator, we establish that non-resonant structures appear to be a more promising platform for a near-term demonstration of TOPDC, due to the high generation bandwidth enhancing the rate of triplet generation. We estimate generation rates of ~10 triplets per second for a 1-cm-long waveguide driven by a 100 mW CW pump, a rate that is comparable to or higher than those quoted for other proposals. We then estimate the non-Gaussianity that might be expected in a supermode of the field generated by our sample structures. The predicted non-Gaussianity is negligible in both structures, for different reasons: The generation rate in the ring is too low, while the field generated by the waveguide is highly multimode. We conclude that although a simple SiN waveguide may be a suitable source for a first demonstration of optical TOPDC, the design will need to be improved significantly for observations of non-Gaussianity or tripartite entanglement in the generated light. Indeed, state generation protocols based on conventional spontaneous four-wave mixing sources and postselection appear to be more promising in the near term. We adopt this approach to design integrated sources of postselected multipartite states in a frequency bin encoding scheme. We introduce sources of W states and GHZ states, with kHz generation rates expected for picojoule pump pulses. This platform appears to be promising for the generation of more arbitrary states, including high-dimensional ones consisting of entangled qudits. We review a graph-based strategy that may be useful for designing other devices, and for understanding the limitations of this platform.
Quasi-phase-matching periodically-tapered waveguides (PTWs) can enable efficient on-demand third-order parametric interactions using right combinations of the tapering period and modulation amplitude [1]. Similar to periodically-poled ferroelectric crystals, this new technique eliminates the stringent constraints imposed by conventional methods on the frequencies, mode profiles, and polarisations of the interacting photons. An example of PTWs is the sinusoidally-tapered fibres that have been exploited in manuiplating supercontinuum generation and modulation instability [2]. In this work, I have developed a rigorous quantum model to investigate spontaneous four-wave mixing (SFWM) inside these tapered waveguides [3]. The right combinations between the modulation amplitude Δd and tapering period ΛTthat enhances the expected number of photons (N̂s) at the targeted wavelengths are portrayed in Fig. 1(a), for fibres with same number of periods M. The values of (N̂s) are normalised to the case when Δd = 0, to quantify the enhancement in photon-pairs generation using the PTW-technique in comparison to uniform fibres. For only M = 50, (N̂s) is remarkably enhanced by 35 dB. The output spectrum of the photon-pairs is featured as a narrow sinc-function with very weak sidelobes that are significantly diminished for large number of periods, as depicted in Fig. 1(b). The 2D representation of the spectrum as a function of the photon-pairs wavelengths (λs, λi) is shown in panel (c). In this plot, the pump is assumed to be a monochromatic at a frequency satisfies the energy conservation. The corresponding (N̂s) for a Gaussian-pulse pump source with an input energy 1 nJ and a full-width-half-maximum 4 ps is portrayed in Fig. 1(d). Using the Schmidt decomposition analysis, the spectral-purity is 0.74. This shows the ability of the PTW-technique in producing highly-efficient relatively-pure single photons at any on-demand frequencies without applying any bandpass filters. This work will also open a new direction of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in third-order nonlinear guided structures.
Third-order parametric downconversion (TOPDC) describes a class of nonlinear interactions in which a pump photon is converted into a photon triplet. This process can occur spontaneously or it can be stimulated by seeding fields. Here we show that stimulated TOPDC (StTOPDC) can be exploited for the generation of quantum correlated photon pairs. We model StTOPDC in a microring resonator, predicting observable pair generation rates in a microring engineered for third-harmonic generation, and we examine the peculiar features of this approach when compared with second-order spontaneous parametric downconversion and spontaneous four-wave mixing. We conclude that if the experimental difficulties associated with implementing StTOPDC can be overcome, it may soon be possible to demonstrate this process in resonant integrated devices.
Entangled photons have acted as a workhorse in many quantum technologies, including cryptography, imaging, sensing, and computation. With the growing demands of quantum technology, the entangled photon pair sources are required to be compact and integrable. For the last few years, the field of quantum optics in the nanoscale has demonstrated photon pair generation in ’flat’ optics such as thin layers, Van der Waals crystals, micro-, nano-antennas, and metasurfaces. The word ’flat’ is used because such optical platforms have sub-wavelength thicknesses. Due to the short travel length inside the sources, they can possess multiple functions that their bulky counterparts cannot have. This doctoral thesis addresses photon pair generation in ’flat’ optics via second- and third-order nonlinear processes, which are spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM). The main aims of the thesis are to understand how the photon pair generation is realized via SPDC and SFWM on subwavelength-scale platforms, and to make a bridge between photon pair generation via two nonlinear processes under the presence of optical resonances. The thesis covers two experimental parts related to SPDC and SFWM, and one future expectation of the SFWM in metamaterials. The first part of the thesis addresses one of the multi-functionalities that has recently been reported: the emission direction control of the photon pairs. A 150 nm-thick gallium phosphide (GaP) metasurface is used as a source. The metasurface supports a quasi-bound state in the continuum (quasi-BIC) resonance, enhancing the electric field inside the material. I generate the entangled photons through the SPDC process. The generated photons are in two different wavelengths under the given pump wavelength: signal photon at the resonance, and idler photon out of the resonance. The photon pair generation rate is enhanced by 66 times compared to the unpatterned GaP slab since the vacuum field in the signal mode is enhanced by the resonance. The generated photons obey the transmission spectrum of the metasurface. As a result, the signal photons at resonance are mostly reflected, and the idler photons are transmitted after generation. This result is the first work of photon demultiplexing by a metasurface. The second part of the thesis covers the photon pair generation through SFWM in thin nonlinear films. Silicon-rich silicon nitride (SiN) layers are selected as the sources. The layers have an intriguing characteristic that the third-order nonlinear susceptibility (chi-3) differs with the nitrogen doping rate. I generate the photon pairs through chi-3 variant SiN layers. The thicknesses of the samples are all sub-wavelength level, having a few hundred nanometers. Time-energy correlation measurements are conducted for all samples, and the second-order correlation functions (g(2)(0)) are calculated. The maximum g(2)(0) of the samples exceeds 2. Interestingly, I observed two-photon interference effects between photon pairs generated in the layers and the fused-silica substrate, which allows me to estimate the chi-3 values of the samples by knowing the reference: chi-3 value of the substrate. The last part of the thesis introduces designs of metasurfaces and possible experimental schemes for photon pair generation through SFWM. Yet, due to its weak light-matter interaction in third-order nonlinear processes, the photon pair generation through SFWM has not been observed in metasurfaces. However, it is fairly possible to enhance the vacuum fields at the output photon modes and generate photon pairs because preliminary work in unpatterned slabs has already been observed in this thesis. Finally, I introduce a fabricated metasurface. It needs improvements, but it is quite promising for the ongoing project.
A periodically tapered waveguides technique is an emerging potential route to establish quasi-phase-matching schemes in third-order nonlinear materials for efficient on-demand parametric interactions. In this paper, I investigate this method in enhancing spontaneous photon-pair emission in microstructured fibres and planar waveguides with sinusoidally varying cross sections. To study this process for continuous and pulsed-pump excitations, I have developed a general robust quantum model that takes into account self- and cross-phase modulations. The model shows a great enhancement in photon-pair generation in waveguides with a small number of tapering periods that are feasible via the current fabrication technologies. I envisage that this work will open a new area of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in nonlinear guided structures.