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Multimode Quantum Interference of Photons in Multiport Integrated Devices

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Multimode Quantum Interference of Photons in Multiport Integrated Devices

Alberto Peruzzo et al. Nat Commun.

Abstract

Photonics is a leading approach in realizing future quantum technologies and recently, optical waveguide circuits on silicon chips have demonstrated high levels of miniaturization and performance. Multimode interference (MMI) devices promise a straightforward implementation of compact and robust multiport circuits. Here, we show quantum interference in a 2 × 2 MMI coupler with visibility of V=95.6 ± 0.9%. We further demonstrate the operation of a 4 × 4 port MMI device with photon pairs, which exhibits complex quantum interference behaviour. We have developed a new technique to fully characterize such multiport devices, which removes the need for phase-sensitive measurements and may find applications for a wide range of photonic devices. Our results show that MMI devices can operate in the quantum regime with high fidelity and promise substantial simplification and concatenation of photonic quantum circuits.

Figures

Figure 1
Figure 1. MMI devices.
(a) Schematic representation of a 4×4 MMI integrated chip. (b) Simulation of classical light propagation in the device shown schematically in a. Light is launched into input waveguide 2, and MMI in the central region results in equal intensity in each of the four output waveguides, via self imaging. Analogous behaviour is observed for injection of light in each of the other input waveguides.
Figure 2
Figure 2. Experimental setup for two photon quantum interference measurements in MMI devices.
The parametric down-conversion source includes two Filter A (2nm FWHM in the 2×2 MMI measurements, 0.5nm FWHM in the 4x4 MMI measurements) to ensure single photons are indistinguishable. To increase the coherence length of the photons in the 2×2 MMI measurement, Filter B (0.5nm FWHM) was inserted the setup. CW, continuous wave; BiBO, bismuth borate; PM, polarization-maintaining; SM, single mode; APDs, silicon single-photon avalanche photodiodes.
Figure 3
Figure 3. Quantum interference in a 2×2 MMI coupler.
(a) The measured HOM dip for 2 nm filters, corresponding to a dip FWHM of 239 μm. (b) The measured HOM dip for the same device and source, but with an additional 0.5 nm filter inserted into one output, resulting in a dip FWHM of 296 μm. Error bars are given by Poissonian statistics. The blue data show the measured rate of accidental counts. The visibilities for the 2×2 MMI, reported in the main text, are corrected for these accidentals.
Figure 4
Figure 4. Quantum interference in a 4×4 MMI coupler.
Coincidence counts of two photons at the output ports of a 4×4 MMI device as the arrival time of the photons is varied. The different graphs represent the six possible input states in the splitter: (a) |11〉12, (b) |11〉13, (c) |11〉14, (d) |11〉23, (e) |11〉24, (f) |11〉34. The FWHM of ∼800 μm is as expected for the 0.5 nm interference filters used. The visibilities for the 4×4 MMI, reported in the main text, are not corrected for accidentals.
Figure 5
Figure 5. Quantum interference matrix of a 4×4 MMI coupler.
Measured (a) and reconstructed (b) visibility matrices Vijkl of the non-classical interference between two photons injected in input waveguides i and j and detected in output waveguides k and l of a 4×4 MMI splitter. Positive visibilities correspond to a HOM-like dip, negative to peaks. The errors of the measured visibilities vary between 1.3 and 3.6%.

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