Near-infrared Hong-Ou-Mandel interference on a silicon quantum photonic chip.
ABSTRACT Near-infrared Hong-Ou-Mandel quantum interference is observed in silicon nanophotonic directional couplers with raw visibilities on-chip at 90.5%. Spectrally-bright 1557-nm two-photon states are generated in a periodically-poled KTiOPO4 waveguide chip, serving as the entangled photon source and pumped with a self-injection locked laser, for the photon statistical measurements. Efficient four-port coupling in the communications C-band and in the high-index-contrast silicon photonics platform is demonstrated, with matching theoretical predictions of the quantum interference visibility. Constituents for the residual quantum visibility imperfection are examined, supported with theoretical analysis of the sequentially-triggered multipair biphoton, towards scalable high-bitrate quantum information processing and communications. The on-chip HOM interference is useful towards scalable high-bitrate quantum information processing and communications.
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ABSTRACT: Photon sources are fundamental components for any quantum photonic technology. The ability to generate high count-rate and low-noise correlated photon pairs via spontaneous parametric down-conversion using bulk crystals has been the cornerstone of modern quantum optics. However, future practical quantum technologies will require a scalable integration approach, and waveguide-based photon sources with high-count rate and low-noise characteristics will be an essential part of chip-based quantum technologies. Here, we demonstrate photon pair generation through spontaneous four-wave mixing in a silicon micro-ring resonator, reporting separately a maximum coincidence-to-accidental (CAR) ratio of 602 ± 37 (for a generation rate of 827kHz), and a maximum photon pair generation rate of 123 MHz ± 11 kHz (with a CAR value of 37). To overcome free-carrier related performance degradations we have investigated reverse biased p-i-n structures, demonstrating an improvement in the pair generation rate by a factor of up to 2 with negligible impact on CAR.Optics Express 11/2013; 21(23):27826-34. · 3.53 Impact Factor
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ABSTRACT: Although interference is a classical-wave phenomenon, the superposition principle, which underlies interference of individual particles, is at the heart of quantum physics. An interaction-free measurements (IFM) harnesses the wave-particle duality of single photons to sense the presence of an object via the modification of the interference pattern, which can be accomplished even if the photon and the object haven't interacted with each other. By using the quantum Zeno effect, the efficiency of an IFM can be made arbitrarily close to unity. Here we report an on-chip realization of the IFM based on silicon photonics. We exploit the inherent advantages of the lithographically written waveguides: excellent interferometric phase stability and mode matching, and obtain multipath interference with visibility above 98%. We achieved a normalized IFM efficiency up to 68.2%, which exceeds the 50% limit of the original IFM proposal.04/2014;
Near-infrared Hong-Ou-Mandel interference on a
silicon quantum photonic circuit
Xinan Xu1, Zhenda Xie1, Jiangjun Zheng1, Junlin Liang1, Tian Zhong2, Mingbin Yu3, Serdar
Kocaman1 , Guo-Qiang Lo3, Dim-Lee Kwong3, Dirk R. Englund4, Franco N. C. Wong2, and Chee Wei
1 Optical Nanostructures Laboratory, Center for Integrated Science and Engineering,
Solid-State Science and Engineering, and Mechanical Engineering, Columbia University, New York, NY 10027
2 Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts
3The Institute of Microelectronics, 11 Science Park Road, Singapore, Singapore 117685
4 Quantum Photonics Laboratory, Columbia University, New York, NY 10027
Author e-mail address: email@example.com, firstname.lastname@example.org
Abstract: Near-infrared Hong-Ou-Mandel quantum interference is observed in silicon
nanophotonic directional couplers with raw visibilities on-chip at 90.5%.
Spectrally-bright 1557-nm two-photon states are generated in a periodically-poled
KTiOPO4 waveguide chip, serving as the entangled photon source and pumped with a
self-injection locked laser, for the photon statistical measurements. Efficient four-port
coupling in the communications C-band and in the high-index-contrast silicon photonics
platform is demonstrated, with matching theoretical predictions of the quantum
interference visibility. Constituents for the residual quantum visibility imperfection are
examined, supported with theoretical analysis of the sequentially-triggered multipair
biphoton contribution and techniques for visibility compensation, towards scalable
high-bitrate quantum information processing and communications.
OCIS codes: (190.4410) Nonlinear Optics, parametric processes; (270.5585) Quantum information
and processing; (270.5290) Photon Statistics; (230.7370) Waveguides.
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In recent years, quantum information has been popular for its robust applications on cryptography [1–
5], computation [6–8] and communication [9,10], and chip-scale cavity quantum electrodynamics 
involving single photons and single excitons [12–17]. Working with biphoton or multiphoton states and
atom-photon interactions, entanglement in various degrees of freedom [17–20], such as time-energy [21,22],
spatial-momentum, and polarization  has been utilized to harness the efficiency and complexity of
quantum information processing. In parallel, quantum secure communications with various protocols [1–
3,5,10,24–27], has been proposed to enhance the security of channels and networks. Recent breakthrough
experiments are typically achieved in free-space , while recent theoretical in-roads on photon transport
on-chip [17,29–33] have led in studies on quantum information processing and communication. Emerging
measurements of entangled photons on-chip [34–39] have benefited from the arrayed scalability in the
nanophotonics platform and potentially robust phase-sensitivity of chip-scale samples albeit with the
challenges of device nanofabrication, design, and low-fluence single photon level measurements against
chip-scale Rayleigh-scattering photon and coupling losses. In the silica system with remarkable phase
control, visibilities up to 98.2% were observed ; in the compact silicon system, raw visibilities up to 80%
were observed . Most chip-scale measurements have been performed at the visible wavelengths and
with bulk nonlinear crystal sources, although there are some recent instances at near-infrared and
telecommunications wavelengths [40–42].
Here we report observations of near-infrared Hong-Ou-Mandel (HOM) quantum interference in
chip-scale silicon nanophotonics circuits, introducing the biphoton experiments to the integrated optics
regime. Employing spectrally-bright type-II periodically-poled KTiOPO4 waveguides (PPKTP) as the
entangled photon source, we demonstrate raw quantum visibilities up to 90.5% on-chip – one of the highest
visibilities observed in the silicon CMOS-compatible platform. Furthermore, we evaluate the various
sources of residual distinguishability including multiphoton pairs, chip-scale excess loss and non-ideal
splitting ratios, and polarization effects. The observed interference visibility matches our theoretical
predictions, for the different symmetric and asymmetric integrated directional couplers examined.
2. Near-infrared Hong-Ou-Mandel experimental setup
Figure 1 illustrates the experimental setup. A 1-cm periodically-poled KTiOPO4 waveguide  from
AdvR serves as the source for indistinguishable photons ; in this case, the waveguide is poled and
designed for quasi-phase-matching and high-fluence spontaneous parametric downconversion (SPDC) at
approximately 1556-nm to 1558-nm wavelengths. We use a relatively high power (100-mW; QPhotonics
QLD-780-80S) semiconductor laser diode as the pump for sufficiently high biphoton rates at approximately
107 per second, to compensate for losses in the fiber and free-space chip coupling setup. The laser is
thermally-tuned and stabilized by self-injection locking to 778.9-nm, which is exactly half of the center
working wavelength of the PPKTP waveguide. The temperature of the PPKTP waveguide is typically
controlled to ~ 25C for optimal phase matching. A long-pass-filter with cutoff at 1064-nm (Semrock
BLP01-1064R-25) blocks pump photons after the SPDC process, and a band-pass filter with 3-nm
(Semrock NIR01-1570/3-25) passes the non-degenerate biphoton states. The polarization controller right
before the fiber-based PBS is used to tune the polarization so that the fiber-based polarization beamsplitter
(PBS) spatially separates the correlated photons. In one branch, a tunable delay is realized by a
retroreflector (Thorlabs PS971-C) and a picomotor stage with loss less than 1-dB. In both branches,
polarization controllers are introduced to respectively change the polarization of each channel to match the
transverse magnetic (TM) mode for coupling into the chip waveguides (Figure 1b).
The chip coupling setup is built with six aspheric lenses, each mounted on individual three-axis
precision stages. The two input and output beams are separated by a D-shaped mirror after 60 cm
divergence to avoid crosstalk. Single and coincidence measurements are performed by two InGaAs single
photon Geiger-mode avalanches detectors D1 and D2 from Princeton Lightwave, with ~ 300 ps gate widths
and ~ 20% detection efficiencies. The clock of D1 is set to 15 MHz, and its output signal triggers D2. This
allows the coincidence rate to be read directly from the D2 counting rate, with the optical delay calibrated to
compensate the electronic delay.