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arXiv:2406.13406 · Optica Quantum

Photon number distribution of squeezed light from a silicon nitride microresonator measured without photon number resolving detectors

Emanuele Brusaschi, Massimo Borghi, Marcello Bacchi, Marco Liscidini, Mattéo Galli, Daniele Bajoni

Published Jun 21, 2024• 4 citations
arXivHTMLPDF

Abstract

The measurement of the photon number distribution (PND) allows one to extract metrics of non-classicality of fundamental and technological relevance, but in principle it requires the use of detectors with photon number resolving (PNR) capabilities. In this work we reconstruct the PND of two-mode pulsed squeezed light generated from a silicon nitride microresonator using threshold detectors and variable optical attenuations. The PNDs are characterized up to ∼1.2 photons per pulse, through which we extracted an on-chip squeezing level of 6.2(2) dB and a noise reduction factor of −3.8(2) dB. The PNDs are successfully reconstructed up to a Hilbert space dimension of 6 × 6. The analysis performed on the photon number basis allows us to characterize the influence of a spurious thermal background field that spoils the photon number correlations. We evaluate the impact of self- and cross-phase modulation on the generation efficiency in case of a pulsed pump, and validate the results through numerical simulations of the master equation of the system.

DOI
10.1364/opticaq.528566

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The measurement of the photon number distribution (PND) allows one to extract metrics of non-classicality of fundamental and technological relevance, but in principle it requires the use of detectors with photon number resolving (PNR) capabilities. In this work we reconstruct the PND of two-mode pulsed squeezed light generated from a silicon nitride microresonator using threshold detectors and variable optical attenuations. The PNDs are characterized up to ∼1.2 photons per pulse, through which we extracted an on-chip squeezing level of 6.2(2) dB and a noise reduction factor of −3.8(2) dB. The PNDs are successfully reconstructed up to a Hilbert space dimension of 6 × 6. The analysis performed on the photon number basis allows us to characterize the influence of a spurious thermal background field that spoils the photon number correlations. We evaluate the impact of self- and cross-phase modulation on the generation efficiency in case of a pulsed pump, and validate the results through numerical simulations of the master equation of the system.
Measuring the photon number distribution (PND) of optical states allows one to extract metrics of non-classicality of fundamental and technological relevance, but requires in principle the use of detectors with photon number resolving capabilities (PNR). Here we reconstruct the PND of two-mode pulsed squeezed light generated from a silicon nitride microresonator using threshold detectors and variable optical attenuations. The PNDs are characterized up to ∼1.2\sim 1.2∼1.2 photons/pulse, corresponding to an on-chip squeezing level of 6.2(2) dB6.2(2) \ \mathrm{dB}6.2(2) dB and a noise reduction factor (photon number difference squeezing) of 3.8(2) dB3.8(2) \ \mathrm{dB}3.8(2) dB. The analysis performed on the photon-number basis allows us to additionally characterize the influence of a spurious thermal background field that reduces the photon number correlations.
Silicon nitride microresonators driven by strong pump pulses can generate squeezed light in a dominant spectral-temporal mode, a central resource for continuous-variable quantum computation. In the high parametric gain regime, several effects, including self- and cross-phase modulation as well as time-ordering corrections, become significant and can degrade source performance. In this work, we comprehensively investigate the generation of squeezed light from a silicon nitride resonator under pulsed pumping, spanning from low to high parametric gain up to approximately 16 photons/pulse. We experimentally study how the average photon number and the first- and second-order correlations of the squeezed marginal modes evolve with increasing pulse energy, across various frequency detunings and pulse durations. Furthermore, we analyze the errors introduced by multipair emissions in estimating the joint temporal intensity via time-resolved coincidence measurements. We propose and demonstrate an error-correction strategy based on the marginal distributions of time-resolved multiphoton events. Our results provide a practical strategy for optimizing the gain and the temporal mode structure of pulsed squeezed light sources in microresonators, elucidating the physical mechanisms and limitations that govern source performance in the high-gain regime.
The work presented in this thesis aims to integrate sources and detectors of nonclassical states of light into a single photonic chip. Correlated pairs of single photons are among the most used sources in many quantum optics and quantum information experiments but their integration with detectors still remains a challenging task. In the first part of the thesis we exploit the generation of correlated pairs of photons in the visible spectrum using an integrated Silicon Nitride microring resonator. As the fabrication process of our sources is fully compatible with back-end CMOS technology, we also discuss the unique possibility of integration with silicon avalanche photodectors to provide a platform for commercially available, fully integrated, analogue quantum simulator working at room temperature. The drawback of universal quantum computation with single photons is its probabilistic nature that increases the technological complexity to obtain complete on chip integration of all necessary components to prepare, manipulate and measure quantum states of light. On the other hand, quantum information processing with continuous variables is completely deterministic. Therefore, in the second part of this work we report design, fabrication and experimental verification of an integrated source of broadband squeezed state of light on a photonic chip. We numerically investigate the amount of obtained squeezing and spurious noise that prevents us to observe shot noise reduction at short sideband frequencies. Additionally, we propose a design to extend our work towards hybridization with discrete variables to obtain qubits that can be protected against errors.
Integration of quantum optical technique on-chip is crucial for large scale applications of quantum technology, which were proven in a free space environment to be superior to the corresponding classical technology. Squeezed states of light can be used for enhancing the sensitivity of quantum sensors and for fault-tolerant quantum computing. Although chip-based squeezed light generation has advanced significantly, practical impact remains limited because coupling losses between the chip and off-chip detectors destroy delicate quantum correlations, restricting the amount of observed squeezing. Here, we overcome this limitation by implementing the idea of on-chip quantum measurement with the aid of a parametric amplifier and applying it to the squeezed state generated by a silicon nitride (SiN) microring resonator. In our scheme, two matched SiN micro-rings are sequentially constructed. The first ring generates a squeezed state, whereas the second ring acts as a high-gain parametric amplifier (PA) that measures the squeezed state before the light experiences significant off-chip loss. This architecture is inherently loss-tolerant: the amplifier elevates the quantum noise well above the vacuum level, making the measurement insensitive to downstream losses. We directly observe a quantum noise reduction of 4.6 dB from the first ring, despite a chip-to-fiber coupling loss exceeding 5 dB. This work also demonstrates the first monolithic SU(1,1) interferometer with an estimated 5 dB signal-to-noise enhancement compared to traditional linear interferometers, and thus establishes a practical pathway for chip-based quantum sensors.
The photon number distribution is reconstructed from the measured statistics of vacuum states in single-photon detection. Numerical results are presented for the coherent, thermal, number and squeezed-vacuum states. The quantum efficiency of single-photon detection should be high for the successful evaluation of the number state. The quantum efficiency provides the marginal influence for the evaluation of the coherent, thermal and squeezed-vacuum states. However, the accuracy of the reconstructed photon number distribution of the squeezed-vacuum state is low even though the quantum efficiency is unity. For a more accurate reconstruction, we must obtain a priori information on the photon number distribution of the squeezed-vacuum state.