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Integrated photonic platform for quantum information with continuous variables
Francesco Lenzini, Jiri Janousek, Oliver Thearle, Matteo Villa, Ben Haylock, Sachin Kasture, Liang Cui, Hoang-Phuong Phan, Dzung Viet Dao, Hidehiro Yonezawa, Ping Koy Lam, Elanor H. Huntington, Mirko Lobino
TL;DR
The paper characterizes fabrication, transmission, detection-efficiency, and noise-calibration procedures for an integrated periodically poled waveguide platform. It reports wavelength-dependent transmission and propagation-loss estimates, while accounting for optical losses, detector efficiency, and shot-noise uncertainty; the measurement uses orthogonal quadratures despite small phase offsets.
Problem
The paper examines how an integrated periodically poled waveguide platform can be fabricated and characterized for optical quantum measurements.
Method
The platform combines tapered periodically poled waveguides, electrically controlled directional couplers and phase shifters, calibrated homodyne detection, and shot-noise measurements.
Results
61% transmission at 1550 nm and 40% at 775 nm were measured, with estimated propagation losses of 0.14 dB/cm and 0.55 dB/cm, respectively.
Takeaways & Limitations
The characterization combines device losses, detector contributions, and shot-noise calibration to support the reported measurements.
Takeaways & Limitations
The inseparability quadratures had offsets of -0.09 rad and -0.10 rad from the squeezed and anti-squeezed positions, although they remained orthogonal within the measurement error.
Abstract
from arXiv · showhide
Integrated quantum photonics provides a scalable platform for the generation, manipulation, and detection of optical quantum states by confining light inside miniaturized waveguide circuits. Here we show the generation, manipulation, and interferometric stage of homodyne detection of non-classical light on a single device, a key step towards a fully integrated approach to quantum information with continuous variables. We use a dynamically reconfigurable lithium niobate waveguide network to generate and characterize squeezed vacuum and two-mode entangled states, key resources for several quantum communication and computing protocols. We measure a squeezing level of -1.38+-0.04 dB and demonstrate entanglement by verifying an inseparability criterion I=0.77+-0.02<1. Our platform can implement all the processes required for optical quantum technology and its high nonlinearity and fast reconfigurability makes it ideal for the realization of quantum computation with time encoded continuous variable cluster states.
METHODS Fabrication of the chip
The chip uses proton-exchanged, periodically poled waveguides with tapered mode transitions and electrically controlled directional couplers and phase shifters.
- METHODS Fabrication of the chip: 1.85 µm proton exchange, annealing, and reverse proton exchange define the waveguide fabrication process.The two thermal treatments were performed at 328 °C for 8 and 10 hours, respectively.
- METHODS Fabrication of the chip: 2.5 µm input channels support near-single-mode pumping at 775 nm, while adiabatic tapers expand and contract the channels for quasi-phase matching and 1550 nm single-mode operation.The poling-region width increases to 8 µm over 7 mm and decreases to 6 µm over 1.5 mm afterward.
- METHODS Fabrication of the chip: 16.12 µm-period electric-field poling with a 50:50 duty cycle provides the periodically poled structure.The device also includes aluminium electrodes on a 200 nm SiO2 buffer layer.
- METHODS Fabrication of the chip: Directional couplers and 12 mm electrodes implement reconfigurable splitting and phase control on the chip.Coupler centre separations range from 10.6 to 11.3 µm, with lengths of 3.5 or 6.1 mm.
Propagation losses
Waveguide transmission and estimated propagation losses were characterized at both the signal and pump wavelengths.
- Propagation losses: 61% transmission at 1550 nm and 40% at 775 nm were measured after correcting for Fresnel losses.The measurements used the second and fifth waveguides for the signal wavelength and the two central inputs for the pump wavelength.
- Propagation losses: 0.14 dB/cm signal-wavelength and 0.55 dB/cm pump-wavelength propagation losses were estimated from numerical mode-overlap calculations.Signal-wavelength propagation loss could not be directly measured from the central inputs because 1550 nm light is only weakly guided in the first tapered section.
Detection efficiencies
Detection efficiencies were estimated by combining chip, filtering, coupling, detector, and shot-noise-clearance contributions.
- Detection efficiencies: Detection-efficiency estimates include 0.14 dB/cm propagation loss, 0.5% directional-coupler loss, pump-filter losses of 20% and 14%, and 13% Fresnel loss.The filtering losses apply to waveguides 1 and 2, respectively.
- Detection efficiencies: The estimate also incorporates 99% quantum efficiency and 17 dB shot-noise clearance measured at 4 mW local-oscillator power.These terms are combined with the optical losses to estimate η1 and η2 from Eq. 1.
Shot-noise levels
Shot-noise levels were measured by periodically blocking the pump beams and evaluating variance over repeated acquisition windows.
- Shot-noise levels: Five 0.4 ms time windows were used to calculate the shot-noise variance for each data acquisition.A motorized optical chopper periodically blocked the pump-beam power.
- Shot-noise levels: ±0.025 dB was added to all reported uncertainties as the standard error of the shot-noise-level evaluation.
Driving voltage
The chip electrodes were driven by three dual-channel arbitrary waveform generators operating in burst mode with a shared photodiode trigger.
- Three dual-channel arbitrary waveform generators drove the chip electrodes in burst mode.A common trigger was generated by a photodiode at the optical chopper output.
- The generators used a common trigger generated by a photodiode at the optical chopper output.
- Phase shifters, DC5, and DC1 for entanglement generation were driven directly with voltages in the ±10 V range.
Squeezing and anti-squeezing levels
Squeezing and anti-squeezing levels were obtained by fitting each noise trace with a time-dependent function, using least-squares uncertainty estimates.
- Squeezing and anti-squeezing levels were evaluated by fitting each noise trace with a specified function.
- The fitting function uses acquisition time t and fitting parameters a and ϕ.
- Uncertainties were estimated as standard errors of coefficients calculated by least-square fitting.
Inseparability criterion
The inseparability analysis derives summed and subtracted quadrature variances from two homodyne photocurrents and accounts for shot noise, averaging, and phase offsets.
- Summed and subtracted quadrature variances were calculated from photocurrents i1 and i2 measured by two homodyne detectors.
- The calculation uses the shot-noise levels of the two homodyne detectors as reference variances.
- Noise variances were obtained by averaging four points centered on squeezed and anti-squeezed quadrature positions.
- Finite scanning speed offset the quadratures used for the inseparability criterion by -0.09 rad and -0.10 rad, yet they remained orthogonal within measurement error.
Homodyne detectors
The experiment used matched photodetectors with custom photodiodes, high stated efficiency, and dual-amplifier readout configurations.
- The homodyne detectors used two matched photodetectors with custom ordered photodiodes from Laser Components.
- Each photodetector used a dual-amplifier configuration with separate DC-coupled transimpedance and AC/DC signal paths.
- The photodiodes had efficiencies above 99% and dark currents above 20 pA.
SHG cavity
The second-harmonic-generation cavity is a free-space bow-tie resonator built around a PPKTP nonlinear crystal. It uses resonant locking, temperature stabilization, and focused mode geometry to support efficient nonlinear conversion.
- Cavity configuration: The SHG cavity uses a free-space bow-tie configuration with a 15 mm PPKTP crystal positioned between two concave mirrors.The concave mirrors have a 50 mm radius of curvature and form an approximately 27 μm beam waist at the crystal.
- Cavity stabilization: The cavity is locked to resonance using the Pound–Drever–Hall technique and uses anti-reflection coatings at the second-harmonic wavelength.The crystal faces are wedged and anti-reflection coated at both wavelengths to reduce parasitic interference.
- Phase matching: A Peltier-controlled oven stabilizes the PPKTP crystal near its approximately 40°C optimum phase-matching temperature.Temperature stabilization maintains the condition required for nonlinear frequency conversion.
- Conversion performance: Injecting 1 W of fundamental optical power produces second-harmonic light with 80% conversion efficiency.The generated second-harmonic field exits through one of the concave mirrors.