Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Acta Physica Sinica

Experimental research on generating and splitting degenerate correlated photon pairs in Sagnac fiber loop

Yang Lei, Liu Nan-Nan, Xiaoying Li

Published Jan 1, 2016• 2 citations
Publisher

Abstract

Degenerate correlated photon pairs (DCPPs) have been widely used in quantum information science,especially in the areas of quantum computation,quantum state control and precision measurement,which are typically generated in a (2) nonlinear crystal through the spontaneous parametric down-conversion.However,such a source is not compatible with optical fiber as large coupling losses occur when the pairs are launched into it,which restricts its direct application to quantum information processing system.More recently,DCPP generation from spontaneous four-wave mixing in (3) optical fiber has aroused strong interest,due to its advantages of compatibility with existing fiber networks and free of alignment.The process of generating DCPP in fiber can be described as follows:two pump photons at different frequencies p1 and p2 scatter through the (3) nonlinearity to create a pair of identical photons at the mean frequency c,such that p1+p2=2c.Because the collinear tensor component xxxx(3) in a Kerr nonlinear medium is 3 times as large as the tensor component xyxy(3),the co-polarized four-wave mixing is preferred,which means the two pump photons and new-born twin photons are both co-polarized.Therefore,it is very challenging to deterministically separate the fiber-based DCPP,since the twin photons share the same properties in all degrees of freedom:frequency,polarization and spatial.Sagnac fiber loop (SFL),composed of a piece of nonlinear fiber and 50/50 coupler,is presented as the splitter for DCPP based on the reversed Hong-Ou-Mandel quantum interference of counter-propagating DCPPs.The SFL can be configured as a total reflector,total transmitter or equally transmissive and reflective state,which sets the differential phases of counter-propagating DCPPs meeting at 50/50 coupler to be ,0 and -,respectively.In order to satisfy the differential phase requirement for completely splitting the DCPP,the SFL is always set to be equally transmissive and reflective state,however,the polarization-mode matching of counter-propagating DCPPs is not easily achieved due to the disturbance of fiber birefringence.According to the Jones matrix derivation of DCPP propagating in the SFL,the polarization mode of counter-propagating DCPPs when interference at 50/50 coupler is automatically matched,if the SFL is set as a total reflector or total transmitter.In experimental scheme,utilizing the SFL as a total reflector,the 1.1 nm bandwidth and 1544.53 nm central wavelength DCPPs are generated by two pulsed light beams pumping the 300 m dispersion-shifted fiber in the SFL.Using the two pieces of single mode fiber connecting the 300 m dispersion-shifted fiber and 50/50 coupler,whose length difference is fixed at 3.3 m,the differential phase of counter-propagating DCPPs highly dependent on the dispersion properties of single mode fiber is managed at 2 for fully distributing DCPPs into which degrades the fidelity of DCPP source.The measured ratio of coincidence to accidental-coincidence of DCPPs from one port is approximately 1.8:1,which indicates that the coincidence counts mainly originate from accidental coincidence counts and extra coincidence counts from photon bunching and there are not any DCPPs outputting from one port.Meanwhile,the ratio of best measured coincidence to accidental-coincidence of DCPPs from two ports reaches 47:1,when the average power of two pumps is fixed at 0.026 mW.The experimental results demonstrate that the high purity and fully spatial separation DCPPs are successfully prepared in optical fibers,which is a very useful tool for realizing various quantum information systems.How the spatial state of outputting DCPPs depends on the length difference between single-mode fiber and detuning wavelength is also discussed in detail.

DOI
10.7498/aps.65.194202

Related papers

All-fiber photon-pair source for practical quantum communications

Prem Kumar, Marco Fiorentino B, Paul L. Voss +1 more

Q uantum entanglement refers to the nonclassical dependency of physically separated quantum systems. It is an essential resource that must be freely available for implementing many of the novel functions of quantum information processing, such as database searching, clock synchronization, teleportation, computing and cryptography.1 The efficient generation and transmission of quantum entanglement is therefore of prime importance. With the ubiquitous standard optical fiber serving as the transmission medium and the widespread availability of efficient active and passive fiber devices, technological synergy between the generation and propagation components of the overall quantum network can be achieved by deploying sources of entanglement that rely on the nonlinearity of the fiber itself. Such fiber-based sources of quantum entanglement would also have the advantage of modal purity over their crystal counterparts,2,3 which is very important for realizing complex networks that involve many quantum operations. In our quest to develop fiber-based devices for quantum information processing, we have recently demonstrated an allfiber source of quantum-correlated photon pairs.4 The operating physical mechanism in this source is nondegenerate fourphoton mixing, wherein two pump photons scatter through the Kerr nonlinearity of the fiber to create simultaneous signal and idler photons. Our experiment is conducted near the zero-dispersion wavelength of standard dispersion-shifted fiber, where such scattering is enhanced owing to phase matching of the photon wave functions. It must be pointed out here that in a conventional wavelength-divisionmultiplexed classical optical communication line, one strives to suppress the fourphoton or four-wave mixing process, which otherwise causes cross talk between the wavelength channels and sets limits on the total data capacity of the communication line. In our experiment, the four-photon mixing takes place in a nonlinear-fiber Sagnac interferometer (NFSI), which we previously used to generate quantum-correlated twin beams in the fiber.5 The pump is a mode-locked train of ~3-ps-long pulses. The pulsed operation serves two important purposes: the NFSI amplifier can be operated at low average powers and the production of the signal/ idler fluorescence photons is confined to well-defined temporal windows, allowing use of a gated detection scheme to increase the signal-to-noise ratio. To measure the nonclassical (i.e., quantum) correlations between the signal and the idler photons, one must effectively suppress the pump photons from reaching the detectors. Since a typical pump pulse contains ~108 photons and we are interested in detecting ~0.01 signal (idler) photons/pulse, a pump-to-signal (idler) rejection ratio in excess of 100 dB is required. We achieved this specification by sending the fluorescence photons through a free-space double-grating spectral filter that separates the signal and idler photons from each other and from the pump photons not rejected by the NFSI. To demonstrate the quantum nature of the four-photon scattering at the single photon level, we assembled a photonFigure 1.C oincidence rates as a function of the single-photon rates in two different cases: signalidler fluorescence produced by a pump pulse (diamonds) and signal-idler fluorescence produced by two consecutive pump pulses (triangles). T he line represents the calculated accidental counts. T he inset shows a plot of the detected idler photons as a function of the injected pump photons (hollow circles). A second-order polynomial is shown to fit the experimental data. T he contributions of the dark counts, linear scattering and quadratic scattering are plotted separately as well. counting apparatus for detecting the signal and idler photons in coincidence. This apparatus is based on commercial InGaAs avalanche photodiodes operating in the gated Geiger mode.4 In the inset in Fig. 1, we show the number of scattered photons, NS, versus the number of pump photons, NP, injected into the NFSI. We fit the experimental data with a second-order polynomial fit, including the number of dark counts during the gate interval, ND. The fit clearly shows that the quadratic scattering ( ∝NP) in the fiber can dominate over the residual linear scattering ( ∝NP) of the pump, owing to imperfect filtering. The main body of Fig. 1 shows the coincidence-counting results. The diamonds represent the rate of coincidence counts versus the rate of the signal (idler) photons generated during the same pump pulse. The triangles represent the measured coincidence rate versus the signalphoton count rate when the signal is delayed with respect to the idler by one pulse period. For two independent photon sources, each with a count rate RS <<1, the accidental coincidence rate RC is given by RC = RS 2, regardless of the photon statistics of the sources. This quadratic relation is plotted as the solid line in Fig. 1, which fits the delayed-coincidence data (triangles) very well. These measurements show that while the fluorescence photons produced by the adjacent pump pulses are independent, those coming from the same pump pulse show a strong correlation, which is a signature of their non-classical behavior. This work was supported by the Department of Defense Multidisciplinary University Research Initiative program administered by the Army Research Office under Grants DAAD19-00-1-0177 and DAAD19-00-1-0469, and by the U.S. Office of Naval Research under Grant N00014-91-J-1268.

•Jan 1, 2002•5 citations

Generation of 15 μm discrete frequency-entangled two-photon state in polarization-maintaining fibers

Qiang Zhou, Wei Zhang, Chenzhi Yuan +2 more

In this Letter, the generation of a 1.5 μm discrete frequency-entangled two-photon state is realized based on a piece of commercial polarization-maintaining fiber (PMF). It is connected with a polarization beam splitter to realize a modified Sagnac fiber loop (MSFL). Correlated two-photon states are generated through a spontaneous four-wave-mixing process along the two propagation directions of the MSFL, and output from the MSFL with orthogonal polarizations. Their quantum interference is realized through a 45° polarization collimation between polarization axes of PMFs inside and outside the MSFL, while their phase difference is controlled by the polarization state of the pump light. The frequency-entangled property of the two-photon state is demonstrated by a spatial quantum beating experiment with a fringe visibility of 98.2±1.3%, without subtracting the accidental coincidence counts. The proposed scheme generates a 1.5 μm discrete frequency-entangled two-photon state in a polarization-maintaining way, which is desired in practical quantum light sources.

arXiv:1403.1917•Optics Letters•Mar 27, 2014•14 citations

High-fidelity transmission of quantum polarization states through birefringent optical fibers

Takeshi Ozeki

High fidelity transmission of quantum polarization states is essential for realizing quantum networks. The degenerate ${\ensuremath{\Psi}}^{\ensuremath{-}}$ state is an eigenstate of a transmission fiber. In this paper we propose a method for preparing the degenerate ${\ensuremath{\Psi}}^{\ensuremath{-}}$ state using spontaneous four wave mixing (SFWM) in a fiber Sagnac loop excited by two orthogonally polarized pump waves. This configuration includes a 45\ifmmode^\circ\else\textdegree\fi{} Faraday rotator, which creates a relative phase of $\ensuremath{\pi}$ between the two counterpropagating entangled photon pairs with indistinguishability both in frequencies and generation paths. We confirm that only the amplitude is degraded in the output wave form of nonlinear sum frequency generation in the Bell state analyzer after propagation through the fiber due to the polarization mode dispersion.

Physical Review A•Aug 21, 2006•7 citations

Fiber-based frequency-degenerate polarization entangled photon pair sources for information encoding

Feng Zhu, Wei Zhang, Yidong Huang

In this paper, a generation scheme of telecom band frequency-degenerate polarization entangled photon pairs is proposed and demonstrated experimentally. It is based on the vector spontaneous four wave mixing (SFWM) process in a Sagnac fiber loop, in which two frequency-degenerate and polarization orthogonal biphoton states have equal probabilities for generating along the clockwise and counter-clockwise directions. The quantum interference between them at the 50:50 fiber coupler of the fiber loop separates the two frequency-degenerate photons in a pair, leading to the generation of polarization entanglement. The raw fringe visibilities of the two-photon interferences under two non-orthogonal polarization bases are 91% and 90%, respectively. Information can be encoded on the generated photon pairs using the polarization entangled Bell states, which requires the frequency-degenerate property. It is demonstrated by a simplified Bell state measurement with a fringe visibility of 83%.

Optics Express•Oct 25, 2016•17 citations

All-fiber photon-pair source at telecom wavelengths

Erik Nicolai Christensen, Mario A. Usuga Castaneda, Karsten K. Rottwitt

Single photon sources are a key element for quantum computing, quantum key distribution (QKD) and quantum communications. In particular, producing single photons at telecommunications wavelengths is valuable for QKD protocols and would enable realizing the quantum internet. The preferred method for their generation has long been spontaneous down conversion in bulk crystals, which suffers from connection loss to fiber networks. In-fiber spontaneous four-wave mixing provides a viable alternative as a photon pair source due to being compatible with existing fiber networks. We present an all-fiber photon pair source based on degenerate four-wave mixing in a 400 m Highly-Nonlinear fiber, with signal and idler wavelengths generated at 1552.5 nm and 1557 nm respectively. The source consists of CW pump laser operating at 1554.75 nm, which is slightly detuned from the zero group velocity dispersion wavelength into the normal dispersion regime. After pair generation in the highly-nonlinear fiber, three arrayed waveguide gratings are employed to spatially separate signal and idler, and provides a 120 dB pump power reduction. Firstly the source is modelled and experimentally characterized in the well known classical regime of stimulated four-wave mixing. The effect of fiber cooling on spontaneous Raman scattering is modelled and characterized, and a 30% reduction in spontaneous emission is found when cooling the fiber to -77°C. In the low power regime the coincidence to accidental count ratio is simulated and measured. An increase in the coincidence to accidental count ratio is observed when cooling the fiber.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Feb 20, 2017•0 citations

Silicon quantum photonics for pair photon sources and wavelength conversion

Bryn A. Bell, Jie He, Chunle Xiong +1 more

In this talk, I will discuss two applications of nonlinear quantum photonics in silicon nanowires: correlated photon pair sources and a single-photon wavelength converter. Correlated photon pairs can be generated in a nonlinear silicon waveguide through the spontaneous four-wave mixing (SFWM) process, where two photons from a bright pump laser are annihilated, and a two photons are spontaneously created at different wavelengths (the signal and idler). However, indistinguishable photon pairs are of particular interest for two-photon interference experiments, meaning that the signal and idler should have the same wavelength and be entirely identical to each other. Indistinguishable pairs can be generated with the SFWM process. where two pump lasers create a degenerate signal and idler at the frequency mid-way between them. Since the two photons are indistinguishable, it is not trivial to divide them into separate waveguides so as to manipulate them individually. I will present recent results concerning a silicon quantum splitter, where degenerate pairs are created in a ring resonator which is pumped in both directions. The ring is coupled to a sagnac loop interferometer, which combines the two directions at a 50 : 50 coupler. Here, a quantum interference effect (reverse-Hong Ou Mandel interference) splits the photons to two output waveguides. This device is ultra-compact and the use of a sagnac loop means that the phase is automatically set correctly for the interference. I will also discuss progress on single photon wavelength conversion in silicon nanowires using the four-wave mixing Bragg scattering (FWMBS) process. Here, two intense pump beams are used to modulate a weak signal, causing it to shift to the idler by the frequency difference between the pumps. In principle, this can allow coherent, noiseless frequency conversion of single photon signals. Fig. 1(b) shows output spectra from a 6mm silicon nanowire when the frequency shift is varied 100 GHz to 400 GHz. Multiple peaks are visible, corresponding to up-conversion and down-conversion of the input signal.

Progress in Electromagnetic Research Symposium•Aug 1, 2016•0 citations
Degenerate correlated photon pairs (DCPPs) have been widely used in quantum information science,especially in the areas of quantum computation,quantum state control and precision measurement,which are typically generated in a (2) nonlinear crystal through the spontaneous parametric down-conversion.However,such a source is not compatible with optical fiber as large coupling losses occur when the pairs are launched into it,which restricts its direct application to quantum information processing system.More recently,DCPP generation from spontaneous four-wave mixing in (3) optical fiber has aroused strong interest,due to its advantages of compatibility with existing fiber networks and free of alignment.The process of generating DCPP in fiber can be described as follows:two pump photons at different frequencies p1 and p2 scatter through the (3) nonlinearity to create a pair of identical photons at the mean frequency c,such that p1+p2=2c.Because the collinear tensor component xxxx(3) in a Kerr nonlinear medium is 3 times as large as the tensor component xyxy(3),the co-polarized four-wave mixing is preferred,which means the two pump photons and new-born twin photons are both co-polarized.Therefore,it is very challenging to deterministically separate the fiber-based DCPP,since the twin photons share the same properties in all degrees of freedom:frequency,polarization and spatial.Sagnac fiber loop (SFL),composed of a piece of nonlinear fiber and 50/50 coupler,is presented as the splitter for DCPP based on the reversed Hong-Ou-Mandel quantum interference of counter-propagating DCPPs.The SFL can be configured as a total reflector,total transmitter or equally transmissive and reflective state,which sets the differential phases of counter-propagating DCPPs meeting at 50/50 coupler to be ,0 and -,respectively.In order to satisfy the differential phase requirement for completely splitting the DCPP,the SFL is always set to be equally transmissive and reflective state,however,the polarization-mode matching of counter-propagating DCPPs is not easily achieved due to the disturbance of fiber birefringence.According to the Jones matrix derivation of DCPP propagating in the SFL,the polarization mode of counter-propagating DCPPs when interference at 50/50 coupler is automatically matched,if the SFL is set as a total reflector or total transmitter.In experimental scheme,utilizing the SFL as a total reflector,the 1.1 nm bandwidth and 1544.53 nm central wavelength DCPPs are generated by two pulsed light beams pumping the 300 m dispersion-shifted fiber in the SFL.Using the two pieces of single mode fiber connecting the 300 m dispersion-shifted fiber and 50/50 coupler,whose length difference is fixed at 3.3 m,the differential phase of counter-propagating DCPPs highly dependent on the dispersion properties of single mode fiber is managed at 2 for fully distributing DCPPs into which degrades the fidelity of DCPP source.The measured ratio of coincidence to accidental-coincidence of DCPPs from one port is approximately 1.8:1,which indicates that the coincidence counts mainly originate from accidental coincidence counts and extra coincidence counts from photon bunching and there are not any DCPPs outputting from one port.Meanwhile,the ratio of best measured coincidence to accidental-coincidence of DCPPs from two ports reaches 47:1,when the average power of two pumps is fixed at 0.026 mW.The experimental results demonstrate that the high purity and fully spatial separation DCPPs are successfully prepared in optical fibers,which is a very useful tool for realizing various quantum information systems.How the spatial state of outputting DCPPs depends on the length difference between single-mode fiber and detuning wavelength is also discussed in detail.
Q uantum entanglement refers to the nonclassical dependency of physically separated quantum systems. It is an essential resource that must be freely available for implementing many of the novel functions of quantum information processing, such as database searching, clock synchronization, teleportation, computing and cryptography.1 The efficient generation and transmission of quantum entanglement is therefore of prime importance. With the ubiquitous standard optical fiber serving as the transmission medium and the widespread availability of efficient active and passive fiber devices, technological synergy between the generation and propagation components of the overall quantum network can be achieved by deploying sources of entanglement that rely on the nonlinearity of the fiber itself. Such fiber-based sources of quantum entanglement would also have the advantage of modal purity over their crystal counterparts,2,3 which is very important for realizing complex networks that involve many quantum operations. In our quest to develop fiber-based devices for quantum information processing, we have recently demonstrated an allfiber source of quantum-correlated photon pairs.4 The operating physical mechanism in this source is nondegenerate fourphoton mixing, wherein two pump photons scatter through the Kerr nonlinearity of the fiber to create simultaneous signal and idler photons. Our experiment is conducted near the zero-dispersion wavelength of standard dispersion-shifted fiber, where such scattering is enhanced owing to phase matching of the photon wave functions. It must be pointed out here that in a conventional wavelength-divisionmultiplexed classical optical communication line, one strives to suppress the fourphoton or four-wave mixing process, which otherwise causes cross talk between the wavelength channels and sets limits on the total data capacity of the communication line. In our experiment, the four-photon mixing takes place in a nonlinear-fiber Sagnac interferometer (NFSI), which we previously used to generate quantum-correlated twin beams in the fiber.5 The pump is a mode-locked train of ~3-ps-long pulses. The pulsed operation serves two important purposes: the NFSI amplifier can be operated at low average powers and the production of the signal/ idler fluorescence photons is confined to well-defined temporal windows, allowing use of a gated detection scheme to increase the signal-to-noise ratio. To measure the nonclassical (i.e., quantum) correlations between the signal and the idler photons, one must effectively suppress the pump photons from reaching the detectors. Since a typical pump pulse contains ~108 photons and we are interested in detecting ~0.01 signal (idler) photons/pulse, a pump-to-signal (idler) rejection ratio in excess of 100 dB is required. We achieved this specification by sending the fluorescence photons through a free-space double-grating spectral filter that separates the signal and idler photons from each other and from the pump photons not rejected by the NFSI. To demonstrate the quantum nature of the four-photon scattering at the single photon level, we assembled a photonFigure 1.C oincidence rates as a function of the single-photon rates in two different cases: signalidler fluorescence produced by a pump pulse (diamonds) and signal-idler fluorescence produced by two consecutive pump pulses (triangles). T he line represents the calculated accidental counts. T he inset shows a plot of the detected idler photons as a function of the injected pump photons (hollow circles). A second-order polynomial is shown to fit the experimental data. T he contributions of the dark counts, linear scattering and quadratic scattering are plotted separately as well. counting apparatus for detecting the signal and idler photons in coincidence. This apparatus is based on commercial InGaAs avalanche photodiodes operating in the gated Geiger mode.4 In the inset in Fig. 1, we show the number of scattered photons, NS, versus the number of pump photons, NP, injected into the NFSI. We fit the experimental data with a second-order polynomial fit, including the number of dark counts during the gate interval, ND. The fit clearly shows that the quadratic scattering ( ∝NP) in the fiber can dominate over the residual linear scattering ( ∝NP) of the pump, owing to imperfect filtering. The main body of Fig. 1 shows the coincidence-counting results. The diamonds represent the rate of coincidence counts versus the rate of the signal (idler) photons generated during the same pump pulse. The triangles represent the measured coincidence rate versus the signalphoton count rate when the signal is delayed with respect to the idler by one pulse period. For two independent photon sources, each with a count rate RS <<1, the accidental coincidence rate RC is given by RC = RS 2, regardless of the photon statistics of the sources. This quadratic relation is plotted as the solid line in Fig. 1, which fits the delayed-coincidence data (triangles) very well. These measurements show that while the fluorescence photons produced by the adjacent pump pulses are independent, those coming from the same pump pulse show a strong correlation, which is a signature of their non-classical behavior. This work was supported by the Department of Defense Multidisciplinary University Research Initiative program administered by the Army Research Office under Grants DAAD19-00-1-0177 and DAAD19-00-1-0469, and by the U.S. Office of Naval Research under Grant N00014-91-J-1268.
In this Letter, the generation of a 1.5 μm discrete frequency-entangled two-photon state is realized based on a piece of commercial polarization-maintaining fiber (PMF). It is connected with a polarization beam splitter to realize a modified Sagnac fiber loop (MSFL). Correlated two-photon states are generated through a spontaneous four-wave-mixing process along the two propagation directions of the MSFL, and output from the MSFL with orthogonal polarizations. Their quantum interference is realized through a 45° polarization collimation between polarization axes of PMFs inside and outside the MSFL, while their phase difference is controlled by the polarization state of the pump light. The frequency-entangled property of the two-photon state is demonstrated by a spatial quantum beating experiment with a fringe visibility of 98.2±1.3%, without subtracting the accidental coincidence counts. The proposed scheme generates a 1.5 μm discrete frequency-entangled two-photon state in a polarization-maintaining way, which is desired in practical quantum light sources.
High fidelity transmission of quantum polarization states is essential for realizing quantum networks. The degenerate ${\ensuremath{\Psi}}^{\ensuremath{-}}$ state is an eigenstate of a transmission fiber. In this paper we propose a method for preparing the degenerate ${\ensuremath{\Psi}}^{\ensuremath{-}}$ state using spontaneous four wave mixing (SFWM) in a fiber Sagnac loop excited by two orthogonally polarized pump waves. This configuration includes a 45\ifmmode^\circ\else\textdegree\fi{} Faraday rotator, which creates a relative phase of $\ensuremath{\pi}$ between the two counterpropagating entangled photon pairs with indistinguishability both in frequencies and generation paths. We confirm that only the amplitude is degraded in the output wave form of nonlinear sum frequency generation in the Bell state analyzer after propagation through the fiber due to the polarization mode dispersion.
In this paper, a generation scheme of telecom band frequency-degenerate polarization entangled photon pairs is proposed and demonstrated experimentally. It is based on the vector spontaneous four wave mixing (SFWM) process in a Sagnac fiber loop, in which two frequency-degenerate and polarization orthogonal biphoton states have equal probabilities for generating along the clockwise and counter-clockwise directions. The quantum interference between them at the 50:50 fiber coupler of the fiber loop separates the two frequency-degenerate photons in a pair, leading to the generation of polarization entanglement. The raw fringe visibilities of the two-photon interferences under two non-orthogonal polarization bases are 91% and 90%, respectively. Information can be encoded on the generated photon pairs using the polarization entangled Bell states, which requires the frequency-degenerate property. It is demonstrated by a simplified Bell state measurement with a fringe visibility of 83%.
Single photon sources are a key element for quantum computing, quantum key distribution (QKD) and quantum communications. In particular, producing single photons at telecommunications wavelengths is valuable for QKD protocols and would enable realizing the quantum internet. The preferred method for their generation has long been spontaneous down conversion in bulk crystals, which suffers from connection loss to fiber networks. In-fiber spontaneous four-wave mixing provides a viable alternative as a photon pair source due to being compatible with existing fiber networks. We present an all-fiber photon pair source based on degenerate four-wave mixing in a 400 m Highly-Nonlinear fiber, with signal and idler wavelengths generated at 1552.5 nm and 1557 nm respectively. The source consists of CW pump laser operating at 1554.75 nm, which is slightly detuned from the zero group velocity dispersion wavelength into the normal dispersion regime. After pair generation in the highly-nonlinear fiber, three arrayed waveguide gratings are employed to spatially separate signal and idler, and provides a 120 dB pump power reduction. Firstly the source is modelled and experimentally characterized in the well known classical regime of stimulated four-wave mixing. The effect of fiber cooling on spontaneous Raman scattering is modelled and characterized, and a 30% reduction in spontaneous emission is found when cooling the fiber to -77°C. In the low power regime the coincidence to accidental count ratio is simulated and measured. An increase in the coincidence to accidental count ratio is observed when cooling the fiber.
In this talk, I will discuss two applications of nonlinear quantum photonics in silicon nanowires: correlated photon pair sources and a single-photon wavelength converter. Correlated photon pairs can be generated in a nonlinear silicon waveguide through the spontaneous four-wave mixing (SFWM) process, where two photons from a bright pump laser are annihilated, and a two photons are spontaneously created at different wavelengths (the signal and idler). However, indistinguishable photon pairs are of particular interest for two-photon interference experiments, meaning that the signal and idler should have the same wavelength and be entirely identical to each other. Indistinguishable pairs can be generated with the SFWM process. where two pump lasers create a degenerate signal and idler at the frequency mid-way between them. Since the two photons are indistinguishable, it is not trivial to divide them into separate waveguides so as to manipulate them individually. I will present recent results concerning a silicon quantum splitter, where degenerate pairs are created in a ring resonator which is pumped in both directions. The ring is coupled to a sagnac loop interferometer, which combines the two directions at a 50 : 50 coupler. Here, a quantum interference effect (reverse-Hong Ou Mandel interference) splits the photons to two output waveguides. This device is ultra-compact and the use of a sagnac loop means that the phase is automatically set correctly for the interference. I will also discuss progress on single photon wavelength conversion in silicon nanowires using the four-wave mixing Bragg scattering (FWMBS) process. Here, two intense pump beams are used to modulate a weak signal, causing it to shift to the idler by the frequency difference between the pumps. In principle, this can allow coherent, noiseless frequency conversion of single photon signals. Fig. 1(b) shows output spectra from a 6mm silicon nanowire when the frequency shift is varied 100 GHz to 400 GHz. Multiple peaks are visible, corresponding to up-conversion and down-conversion of the input signal.