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Journal of Physics Conference Series

Thermo-optical properties of silicon nitride Mach-Zehnder interferometer for the on-chip quantum random number generator

A. I. Prokhodtsov, Vadim V. Kovalyuk, Pavel P. An, Alexander D. Golikov, Yu. V. Konyshev, Roman Shakhovoy, Violetta Sharoglazova, Alexander Udaltsov, Yury Kurochkin, Gregory N Gol'tsman

Published Dec 1, 2021• 0 citations
Publisher

Abstract

Abstract Here we study the thermo-optical properties of an on-chip silicon nitride Mach-Zehnder interferometer (MZI). The spectral shift of the MZI is associated with a change in the chip temperature. This can be explained by a change in the splitting ratio of the directional couplers, as well as a significant change in phase difference between waveguide arms. We experimentally found a phase shift of 2π when heated by 1.67 °C and changes in resonant wavelength at different temperatures (dλ/dT) equal 12.0 pm/°C, theoretically obtained a formula for an arbitrary splitting ratio of the directional couplers in an MZI, and determined the temperature stability required to the device operation inside a quantum cryptography system.

DOI
10.1088/1742-6596/2086/1/012164

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Abstract Here we study the thermo-optical properties of an on-chip silicon nitride Mach-Zehnder interferometer (MZI). The spectral shift of the MZI is associated with a change in the chip temperature. This can be explained by a change in the splitting ratio of the directional couplers, as well as a significant change in phase difference between waveguide arms. We experimentally found a phase shift of 2π when heated by 1.67 °C and changes in resonant wavelength at different temperatures (dλ/dT) equal 12.0 pm/°C, theoretically obtained a formula for an arbitrary splitting ratio of the directional couplers in an MZI, and determined the temperature stability required to the device operation inside a quantum cryptography system.
Abstract In this work, we experimentally studied silicon nitride Mach-Zehnder interferometer (MZI) with two directional couplers and 400 ps optical delay line for telecom wavelength 1550 nm. We achieved the extinction ratio in a range of 0.76-13.86 dB and system coupling losses of 28-44 dB, depending on the parameters of directional couplers. The developed interferometer is promising for the use in a compact random number generator for the needs of a fully integrated quantum cryptography system, where compact design, as well as high generation speed, are needed.
Abstract In this work we describe phase modulators for several Mach-Zehnder interferometers (MZI) on silicon nitride platform for telecomm wavelength (1550 nm). We obtained current-voltage and phase-voltage curves for these modulators. MZI are needed for experimental realisation of various quantum receivers that can distinguish weak coherent states of light with extremely low error. Thermo-optical (TO) modulation is ensured by microheaters on one of the arms of MZI, which enables the change of the refractive index of the material with temperature. This approach allows to apply the necessary voltage to the golden microheaters to obtain the required phase change. For the on-chip microheaters we demonstrate the dependence of the phase shift on the voltage applied to our on-chip microheaters.
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This thesis investigates the phenomena of phase stability in a fiber-optical MZI (Mach-Zehnder Interferometer). The MZI is a key building block of optical systems for use in experiments with both continuous-wave light and with single photons. By splitting incoming light into two beams and allowing it to interfere with itself, an interference pattern is visible at the output, and this phenomena can be used to code information. This is the operating principle in, for example, QKD (Quantum Key Distribution) experiments. This interference requires coherence that is higher than the length difference between the beams that the incoming light is split into. Particularly the phase of the beams must be equal to achieve constructive interference. If one beam is phase-shifted (with respect to the other) due to the light having traversed a longer path, only partially constructive interference is achieved. If the phase shift also varies with time this leads to a system where experiments can no longer reliably be performed. Sources of these fluctuations are thermal, acoustic or mechanical. Fiber-optical interferometers are particularly sensitive to path length fluctuations of the waveguides as the fiber-optic medium contracts and elongates with temperature, and also has a larger surface area for circulating air to mechanically disturb the waveguides than bulk optics interferometers. In this thesis, a solution to environment-induced phase drift is presented by evaluating implementations of feedback algorithms for automatic control. The algorithms PID (Proportional-, Integral-, Derivative controller) and an ICA (IncrementalControl Algorithm) have been investigated and the performance of these controllers has been compared when used with, and without, optical enclosures. The algorithms are implemented in an FPGA (Field-Programmable Gate Array) and the controller actuates an electro-optical phase modulator that can add a phase shift to one of the light beams in the MZI. This thesis shows that significant improvement in the optical stability can be achieved with active control compared to an interferometer without active phase control.
The design, simulation and measurements of an on-chip Mach-Zehnder interferometer operating in the microwave regime are described. Using microwave signals in microfabricated superconducting Al waveguides, the concept of an interferometer is transferred from optics to on-chip. Tuning of the path length of one of the interferometer arms is executed through the tunable inductance of a SQUID. By placing one or more SQUIDs in the waveguide structure and by varying the magnetic flux through the SQUID loop, the total SQUID inductance can be tuned. In this way, a phase difference leading to destructive or constructive interference at the interferometer output can be achieved. Thorough software simulations were performed to determine the different design parameters, assign a desired working frequency and provide a reference for comparison with experimental results. Measurements at 300 mK show an effective working frequency close to the simulations, with a deviation smaller than 0.05 GHz. The behavior of the interferometer is very similar to the simulations as well. The on-off ratio exceeds 40 dB.
Integrated Mach-Zehnder (MZ) quantum optic circuit is one of most promising approaches for generation and reconfiguration of complex quantum interference. Here, multi photons quantum entanglement is formulated in terms of thermooptic phase change in MZ interferometer having 3 dB two mode interference couplers (MZ-TMI). The manipulation of quantum entanglement is demonstrated theoretically, opening a way to design silicon based quantum optic technology platform for quantum processing applications. Our results also provide multi-photon quantum interference with high fabrication tolerance and fidelity of 0.98 ± 0.01 within smaller dimension than previous implementation of MZ component.