The generation of optical quantum states on an integrated platform will enable low cost and accessible advances for quantum technologies such as secure communications and quantum computation. We demonstrate that integrated quantum frequency combs (based on high-Q microring resonators made from a CMOS-compatible, high refractive-index glass platform) can enable, among others, the generation of heralded single photons, cross-polarized photon pairs, as well as bi- and multi-photon entangled qubit states over a broad frequency comb covering the S, C, L telecommunications band, constituting an important cornerstone for future practical implementations of photonic quantum information processing

Caspani, Lucia

Chu, Sai T.

Kues, Michael

Little, Brent E.

Morandotti, Roberto

Morandotti, Roberto (morandotti@emt.inrs.ca)

Moss, David J.

Reimer, Christian

Roztocki, Piotr

Wetzel, Benjamin

English

Enlighten

    Reimer, C., Kues, M., Roztocki, P., Wetzel, B., Little, B. E., Chu, S. T., Caspani, L., Moss, D. J. and Morandotti, R. (2017) Generation of Complex Quantum States Via Integrated Frequency Combs. In: 2017 Design, Automation & Test in Europe Conference & Exhibition (DATE), Lausanne, Switzerland, 27-31 Mar 2017, pp. 336-337. ISBN 9783981537086 (doi:10.23919/DATE.2017.7927012)  This is the author’s final accepted version.  There may be differences between this version and the published version. You are advised to consult the publisher’s version if you wish to cite from it.     http://eprints.gla.ac.uk/144503/            Deposited on: 24 July 2017              Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Generation of Complex Quantum States  via Integrated Frequency Combs  Christian Reimer1, Michael Kues1,2, Piotr Roztocki1, Benjamin Wetzel1,3,  Brent E. Little4, Sai T. Chu5, Lucia Caspani6, David J. Moss7, Roberto Morandotti1,8,*  1INRS-EMT, 1650 Boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada; 2School of Engineering, University of Glasgow, Rankine Building, Oakfield Avenue, Glasgow G12 8LT, UK; 3Department of Physics and Astronomy, University of Sussex, Falmer, Brighton BN1 9QH, UK; 4State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Science, Xi'an, China; 5City University of Hong Kong, Department of Physics and Material Science, Tat Chee Avenue, Hong Kong, China; 6 Institute of Photonics, Department of Physics, University of Strathclyde, Glasgow G4 0NW, UK; 7Center for Micro-Photonics, Swinburne University of Technology, Hawthorne, Victoria 3122, Australia; 8Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China Email: morandotti@emt.inrs.ca   Abstract— The generation of optical quantum states on an integrated platform will enable low cost and accessible advances for quantum technologies such as secure communications and quantum computation. We demonstrate that integrated quantum frequency combs (based on high-Q microring resonators made from a CMOS- compatible, high refractive-index glass platform) can enable, among others, the generation of pure heralded single photons, cross-polarized photon pairs, as well as bi- and multi-photon entangled qubit states over a broad frequency comb covering the S, C, L telecommunications band, constituting an important cornerstone for future practical implementations of photonic quantum information processing. Keywords— Quantum optics; Integrated optics devices; Nonlinear optics, four-wave mixing I. INTRODUCTION With the realization of commercial solutions for quantum cryptography, it is foreseeable that reliable, low cost and scalable on-chip sources of single and heralded photons will represent a key enabling technology for quantum communications [1]. The requirements of such sources differ for different applications, but usually include long-term operational stability and insensitivity to environment, compatibility with quantum memories, operation at telecomm wavelengths (around 1550nm) and compatibility with large-scale electronic chip production standards (CMOS). In addition further characteristics such as frequency multiplexing to enable high dimensional multi-user operation or polarization diversity to implement polarization operations are highly desirable to enable versatile applications. Finally, the generation of multiple and large entangled states will open further applications in quantum metrology [2] and computation [3], adding to the large amount of possible applications for versatile and scalable quantum sources. The search for integrated sources has attracted considerable attention [4] from the scientific community, where difficulties arise when sources need to satisfy several requirements at the same time. For example, quantum memories (typically based on atomic transitions and thus requiring linewidths on the order of 100MHz), while most quantum communication or computation protocols require either pure single photons, or entangled states with high fidelity. We demonstrate, that by adjusting the pump configuration, integrated quantum frequency combs (based on high-Q microring resonators made from Hydex [5]) can generate pure heralded single photons, cross-polarized photon pairs, as well as multiple entangled photons pairs over a broad frequency comb covering the full S, C and L telecommunication band, with photon frequencies centered at standard telecommunication channels spaced by 200 GHz. II. GENERATION OF PURE SINGLE MODE PHOTONS When pump in a self-locked, intra-cavity operation [6], a highly stable integrated source of multiplexed heralded single photons is demonstrated, operating continuously for several weeks with less than 5% fluctuation and without any active stabilization [6]. We performed coincidence measurements for combinations of different signal/idler wavelengths located around the pump wavelength. Clear coincidence peaks are visible on all symmetric channel pairs, while no coincidences are measured between non-diagonal elements of the frequency matrix [6]. For a pump power of 15 mW at the ring input we obtained CAR values between 12.8 and 32.4, and pair generation rates between 14 and 29 Hz per channel (simultaneously). The signal/idler coherence time is determined using time-resolved coincidence measurements, resulting in a measured value of Δν=110 MHz, consistent with the linewidth of the ring resonator (considering the time jitter of the detectors). This narrow linewidth, allowed by the high Q-factor of the microring, makes this device extremely appealing for applications such as quantum memories or quantum repeaters. III. GENERATION OF CROSS-POLARIZED PHOTON PAIRS By pumping two resonances of orthogonal polarization modes, we introduce a new type of spontaneous four-wave mixing (FWM) to the toolbox of integrated photonics [7]. In particular, we demonstrated the first realization of type-II spontaneous FWM (in analogy of type-II spontaneous parametric down conversion in second order media), which allows to directly generate orthogonally polarized photon pairs on a chip [7]. By properly designing the waveguide dimensions it is possible to tailor the resonances of both polarizations (TE and TM) of the microring resonator in order to generate a frequency offset between TE and TM modes. In this way they are non-symmetric with respect to the stimulated FWM bands, thus suppressing the stimulated process completely. If at the same time the dispersion of TE and TM modes is controlled to achieve similar free spectral ranges, the requirement of energy conservation and phase matching for an efficient spontaneous FWM process can be achieved.  When pumped at low power, individual orthogonally polarized photon pairs are generated inside the micro-ring resonator due to type-II spontaneous FWM, which are then separated by a polarizing beam splitter. Using single photon detectors and a time-to-digital converter, a clear photon coincidence peak with a coincidence-to-accidental ratio around 10 at 2mW pump power was measured between orthogonally polarized photon pairs. The clear photon coincidence peak confirms that the FWM process is indeed spontaneous and seeded by the vacuum fluctuations, underlying the fact that the stimulated FWM is successfully suppressed. When the pump power is further increased, the output power increases quadratically until it reaches the optical parametrical oscillation threshold at 14mW, after which the output scales linearly with the pump power [7]. IV. GENERATION OF ENTANGLED PHOTON PAIRS By changing the pump configuation to excite the resonator with coherent double-pulses, it becomes possible generate time-bin entangled photon pairs [8]. An imbalanced phase-stabilized Michelson interferometer was used to generate such double pulses. The double pulses were then coupled into the ring resonator, where their center frequency was matched with a bandpass filter to a single ring resonance. Due to the high field enhancement and high nonlinearity, photon pairs were generated through SFWM on several frequency channels (corresponding to the ring resonances) symmetrically with respect to the pump frequency. To measure the entanglement through quantum interference, we used a second imbalanced stabilized Michelson interferometer with the path length difference matched to the first interferometer. After passing through the second interferometer, we filtered the ring resonance frequencies and measured the arrival times of the photon pairs of 5 channels symmetric to the pump frequency with two single photon detectors [8]. Using time-stamps together with the reference of the pulsed laser (to tag the double pulse) allowed us to post-select the relevant photon events leading to quantum interference (i.e. photons are generated in the first pulse and take the long way in the second interferometer or photons are generated in the second pulse and take the short way in the interferometer, both leading to the same arrival times). For these events, the coincidence rate depends on the interferometer phase, which was adjusted with a piezoactuator-based phase-shifter [8]. By varying the phase of the second interferometer it was possible to measure quantum interference [8]. With a visibility of 83% (without background correction) we obtain a violation of the Clauser-Horne-Shimony-Holt (Bell-like) inequality [9], clearly proving the observation of time-bin entanglement from our integrated photon pair source. The violation of this inequality has been observed in all the 5 channels of frequency pairs symmetric to the pump, thus underlying the simultaneous generation of multiplexed time-bin entangled photon pairs [8]. V. GENERATION OF MULTI-PHOTON ENTANGLED STATES After confirming the presence of entanglement in the multiple lines, as described above, we performed quantum state tomography and confirmed the generation of qubit entangled Bell states [8]. In order to combine these multiple Bell states to multi-photon entangled states, it is important that all photon pairs are generated from the same excitation field, and that the generated photons have the same bandwidth as the pump field. In this configuration, which is intrinsically given by the resonance characteristic of the ring resonator, the temporal modes of the generated Bell states cannot be distinguished and can thus be combined to multi-photon product states. In particular, we selected four frequency modes, which are symmetric to the excitation frequency, individually forming two Bell states, which are then combined to a four-photon entangled state [8]. We confirm the generation of this four-photon state through four-photon quantum interference, where each photon is passed through an unbalanced interferometer with a path length difference equal to the time-bin separation. Following the post-selection of four-photon coincidences, quantum interference was measured with a visibility of 89% without background correction [8]. We furthermore performed quantum state tomography, where the calculated fidelity of 64%, confirms that the measured density matrix is close to the ideal case. REFERENCES [1] H.J. Kimble,“The quantum internet,” Nature 453, 1023 (2008) [2] M. Kolobov, “The spatial behavior of nonclassical light,” Rev. Mod. Phys. 71, 1539 (1999) [3] P. Walther et al., “Experimental one-way quantum computing,” Nature. 434, 169 (2005) [4] D. Bonneau, J. W. Silverstone, M. G. Thompson, in Silicon Photonics III, L. Pavesi, D. J. Lockwood, Eds. (Springer, ed. 3rd, 2016), pp. 41–82. [5] D. J. Moss et al., “New CMOS-compatible platforms based on silicon nitride and Hydex for nonlinear optics,” Nature Photon. 7, 597 (2013). [6] C. Reimer et al., “Integrated frequency comb source of heralded single photons,” Opt. Express. 22, 1023 (2014) [7] C. Reimer et al., “Cross-polarized photon-pair generation and bi-chromatically pumped optical parametric oscillation on a chip,” Nat. Commun. 6, 8236 (2015). [8] C. Reimer et al., “New CMOS-compatible platforms based on silicon nitride and Hydex for nonlinear optics,” Science 351, 1176 (2016). [9] J. F. Clauser, et al., “Proposed Experiment to Test Local Hidden-Variable Theories,” Phys. Rev. Lett. 23, 880 (1969). 

Generation of Complex Quantum States Via Integrated Frequency Combs

Enlighten: Publications

 
 
 
 
Reimer, C., Kues, M., Roztocki, P., Wetzel, B., Little, B. E., Chu, S. T., 
Caspani, L., Moss, D. J. and Morandotti, R. (2017) Generation of Complex 
Quantum States Via Integrated Frequency Combs. In: 2017 Design, 
Automation & Test in Europe Conference & Exhibition (DATE), Lausanne, 
Switzerland, 27-31 Mar 2017, pp. 336-337. ISBN 9783981537086 
(doi:10.23919/DATE.2017.7927012) 
 
This is the author’s final accepted version. 
 
There may be differences between this version and the published version. 
You are advised to consult the publisher’s version if you wish to cite from 
it. 
 
 
 
 
http://eprints.gla.ac.uk/144503/ 
     
 
 
 
 
 
 
Deposited on: 24 July 2017 
 
 
 
 
 
 
 
 
 
 
 
 
 
Enlighten – Research publications by members of the University of Glasgow 
http://eprints.gla.ac.uk 
Generation of Complex Quantum States  
via Integrated Frequency Combs 
 
Christian Reimer1, Michael Kues1,2, Piotr Roztocki1, Benjamin Wetzel1,3,  
Brent E. Little4, Sai T. Chu5, Lucia Caspani6, David J. Moss7, Roberto Morandotti1,8,* 
 
1INRS-EMT, 1650 Boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada; 2School of Engineering, University of Glasgow, Rankine 
Building, Oakfield Avenue, Glasgow G12 8LT, UK; 3Department of Physics and Astronomy, University of Sussex, Falmer, Brighton BN1 9QH, 
UK; 4State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Science, 
Xi'an, China; 5City University of Hong Kong, Department of Physics and Material Science, Tat Chee Avenue, Hong Kong, China; 6 Institute of 
Photonics, Department of Physics, University of Strathclyde, Glasgow G4 0NW, UK; 7Center for Micro-Photonics, Swinburne University of 
Technology, Hawthorne, Victoria 3122, Australia; 8Institute of Fundamental and Frontier Sciences, University of Electronic Science and 
Technology of China, Chengdu 610054, China 
Email: morandotti@emt.inrs.ca 
 
 
Abstract— The generation of optical quantum states on an 
integrated platform will enable low cost and accessible advances 
for quantum technologies such as secure communications and 
quantum computation. We demonstrate that integrated quantum 
frequency combs (based on high-Q microring resonators made 
from a CMOS- compatible, high refractive-index glass platform) 
can enable, among others, the generation of pure heralded single 
photons, cross-polarized photon pairs, as well as bi- and multi-
photon entangled qubit states over a broad frequency comb 
covering the S, C, L telecommunications band, constituting an 
important cornerstone for future practical implementations of 
photonic quantum information processing. 
Keywords— Quantum optics; Integrated optics devices; 
Nonlinear optics, four-wave mixing 
I. INTRODUCTION 
With the realization of commercial solutions for quantum 
cryptography, it is foreseeable that reliable, low cost and 
scalable on-chip sources of single and heralded photons will 
represent a key enabling technology for quantum 
communications [1]. The requirements of such sources differ 
for different applications, but usually include long-term 
operational stability and insensitivity to environment, 
compatibility with quantum memories, operation at telecomm 
wavelengths (around 1550nm) and compatibility with large-
scale electronic chip production standards (CMOS). In addition 
further characteristics such as frequency multiplexing to enable 
high dimensional multi-user operation or polarization diversity 
to implement polarization operations are highly desirable to 
enable versatile applications. Finally, the generation of 
multiple and large entangled states will open further 
applications in quantum metrology [2] and computation [3], 
adding to the large amount of possible applications for versatile 
and scalable quantum sources. The search for integrated 
sources has attracted considerable attention [4] from the 
scientific community, where difficulties arise when sources 
need to satisfy several requirements at the same time. For 
example, quantum memories (typically based on atomic 
transitions and thus requiring linewidths on the order of 
100MHz), while most quantum communication or computation 
protocols require either pure single photons, or entangled states 
with high fidelity. 
We demonstrate, that by adjusting the pump configuration, 
integrated quantum frequency combs (based on high-Q 
microring resonators made from Hydex [5]) can generate pure 
heralded single photons, cross-polarized photon pairs, as well 
as multiple entangled photons pairs over a broad frequency 
comb covering the full S, C and L telecommunication band, 
with photon frequencies centered at standard 
telecommunication channels spaced by 200 GHz. 
II. GENERATION OF PURE SINGLE MODE PHOTONS 
When pump in a self-locked, intra-cavity operation [6], a 
highly stable integrated source of multiplexed heralded single 
photons is demonstrated, operating continuously for several 
weeks with less than 5% fluctuation and without any active 
stabilization [6]. We performed coincidence measurements for 
combinations of different signal/idler wavelengths located 
around the pump wavelength. Clear coincidence peaks are 
visible on all symmetric channel pairs, while no coincidences 
are measured between non-diagonal elements of the frequency 
matrix [6]. For a pump power of 15 mW at the ring input we 
obtained CAR values between 12.8 and 32.4, and pair 
generation rates between 14 and 29 Hz per channel 
(simultaneously). The signal/idler coherence time is 
determined using time-resolved coincidence measurements, 
resulting in a measured value of Δν=110 MHz, consistent with 
the linewidth of the ring resonator (considering the time jitter 
of the detectors). This narrow linewidth, allowed by the high 
Q-factor of the microring, makes this device extremely 
appealing for applications such as quantum memories or 
quantum repeaters. 
III. GENERATION OF CROSS-POLARIZED PHOTON PAIRS 
By pumping two resonances of orthogonal polarization 
modes, we introduce a new type of spontaneous four-wave 
mixing (FWM) to the toolbox of integrated photonics [7]. In 
particular, we demonstrated the first realization of type-II 
spontaneous FWM (in analogy of type-II spontaneous 
parametric down conversion in second order media), which 
allows to directly generate orthogonally polarized photon pairs 
on a chip [7]. By properly designing the waveguide dimensions 
it is possible to tailor the resonances of both polarizations (TE 
and TM) of the microring resonator in order to generate a 
frequency offset between TE and TM modes. In this way they 
are non-symmetric with respect to the stimulated FWM bands, 
thus suppressing the stimulated process completely. If at the 
same time the dispersion of TE and TM modes is controlled to 
achieve similar free spectral ranges, the requirement of energy 
conservation and phase matching for an efficient spontaneous 
FWM process can be achieved.  When pumped at low power, 
individual orthogonally polarized photon pairs are generated 
inside the micro-ring resonator due to type-II spontaneous 
FWM, which are then separated by a polarizing beam splitter. 
Using single photon detectors and a time-to-digital converter, a 
clear photon coincidence peak with a coincidence-to-accidental 
ratio around 10 at 2mW pump power was measured between 
orthogonally polarized photon pairs. The clear photon 
coincidence peak confirms that the FWM process is indeed 
spontaneous and seeded by the vacuum fluctuations, 
underlying the fact that the stimulated FWM is successfully 
suppressed. When the pump power is further increased, the 
output power increases quadratically until it reaches the optical 
parametrical oscillation threshold at 14mW, after which the 
output scales linearly with the pump power [7]. 
IV. GENERATION OF ENTANGLED PHOTON PAIRS 
By changing the pump configuation to excite the resonator 
with coherent double-pulses, it becomes possible generate 
time-bin entangled photon pairs [8]. An imbalanced phase-
stabilized Michelson interferometer was used to generate such 
double pulses. The double pulses were then coupled into the 
ring resonator, where their center frequency was matched with 
a bandpass filter to a single ring resonance. Due to the high 
field enhancement and high nonlinearity, photon pairs were 
generated through SFWM on several frequency channels 
(corresponding to the ring resonances) symmetrically with 
respect to the pump frequency. To measure the entanglement 
through quantum interference, we used a second imbalanced 
stabilized Michelson interferometer with the path length 
difference matched to the first interferometer. After passing 
through the second interferometer, we filtered the ring 
resonance frequencies and measured the arrival times of the 
photon pairs of 5 channels symmetric to the pump frequency 
with two single photon detectors [8]. Using time-stamps 
together with the reference of the pulsed laser (to tag the 
double pulse) allowed us to post-select the relevant photon 
events leading to quantum interference (i.e. photons are 
generated in the first pulse and take the long way in the second 
interferometer or photons are generated in the second pulse and 
take the short way in the interferometer, both leading to the 
same arrival times). For these events, the coincidence rate 
depends on the interferometer phase, which was adjusted with 
a piezoactuator-based phase-shifter [8]. By varying the phase 
of the second interferometer it was possible to measure 
quantum interference [8]. With a visibility of 83% (without 
background correction) we obtain a violation of the Clauser-
Horne-Shimony-Holt (Bell-like) inequality [9], clearly proving 
the observation of time-bin entanglement from our integrated 
photon pair source. The violation of this inequality has been 
observed in all the 5 channels of frequency pairs symmetric to 
the pump, thus underlying the simultaneous generation of 
multiplexed time-bin entangled photon pairs [8]. 
V. GENERATION OF MULTI-PHOTON ENTANGLED STATES 
After confirming the presence of entanglement in the 
multiple lines, as described above, we performed quantum state 
tomography and confirmed the generation of qubit entangled 
Bell states [8]. In order to combine these multiple Bell states to 
multi-photon entangled states, it is important that all photon 
pairs are generated from the same excitation field, and that the 
generated photons have the same bandwidth as the pump field. 
In this configuration, which is intrinsically given by the 
resonance characteristic of the ring resonator, the temporal 
modes of the generated Bell states cannot be distinguished and 
can thus be combined to multi-photon product states. In 
particular, we selected four frequency modes, which are 
symmetric to the excitation frequency, individually forming 
two Bell states, which are then combined to a four-photon 
entangled state [8]. We confirm the generation of this four-
photon state through four-photon quantum interference, where 
each photon is passed through an unbalanced interferometer 
with a path length difference equal to the time-bin separation. 
Following the post-selection of four-photon coincidences, 
quantum interference was measured with a visibility of 89% 
without background correction [8]. We furthermore performed 
quantum state tomography, where the calculated fidelity of 
64%, confirms that the measured density matrix is close to the 
ideal case. 
REFERENCES 
[1] H.J. Kimble,“The quantum internet,” Nature 453, 1023 (2008) 
[2] M. Kolobov, “The spatial behavior of nonclassical light,” Rev. 
Mod. Phys. 71, 1539 (1999) 
[3] P. Walther et al., “Experimental one-way quantum computing,” 
Nature. 434, 169 (2005) 
[4] D. Bonneau, J. W. Silverstone, M. G. Thompson, in Silicon 
Photonics III, L. Pavesi, D. J. Lockwood, Eds. (Springer, ed. 
3rd, 2016), pp. 41–82. 
[5] D. J. Moss et al., “New CMOS-compatible platforms based on 
silicon nitride and Hydex for nonlinear optics,” Nature Photon. 
7, 597 (2013). 
[6] C. Reimer et al., “Integrated frequency comb source of heralded 
single photons,” Opt. Express. 22, 1023 (2014) 
[7] C. Reimer et al., “Cross-polarized photon-pair generation and 
bi-chromatically pumped optical parametric oscillation on a 
chip,” Nat. Commun. 6, 8236 (2015). 
[8] C. Reimer et al., “New CMOS-compatible platforms based on 
silicon nitride and Hydex for nonlinear optics,” Science 351, 
1176 (2016). 
[9] J. F. Clauser, et al., “Proposed Experiment to Test Local 
Hidden-Variable Theories,” Phys. Rev. Lett. 23, 880 (1969).
 



	The generation of optical quantum states on an integrated platform will enable low cost and accessible advances for quantum technologies such as secure communications and quantum computation. We demonstrate that integrated quantum frequency combs (based on high-Q microring resonators made from a CMOS-compatible, high refractive-index glass platform) can enable, among others, the generation of heralded single photons, cross-polarized photon pairs, as well as bi- and multi-photon entangled qubit states over a broad frequency comb covering the S, C, L telecommunications band, constituting an important cornerstone for future practical implementations of photonic quantum information processing. &copy; 2017 IEEE.</p

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Generation of complex quantum states via integrated frequency combs

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Generation of Complex Quantum States Via Integrated Frequency Combs

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