[Paper Review] On-chip quantum interference between independent lithium niobate-on-insulator photon-pair sources

Robert J. Chapman, Tristan Kuttner+6 more|arXiv (Cornell University)|Apr 12, 2024

Photonic and Optical Devices4 citations

TL;DR

This paper demonstrates on-chip quantum interference between two independent photon-pair sources based on periodically poled lithium niobate-on-insulator (LNOI) waveguides. By integrating a programmable Mach-Zehnder interferometer with high-brightness SPDC sources, the authors achieve 96.8(3.6)% visibility quantum interference, enabling scalable, chip-integrated quantum photonic circuits with high indistinguishability and brightness.

ABSTRACT

Generating and interfering non-classical states of light is fundamental to optical quantum information science and technology. Quantum photonic integrated circuits provide one pathway towards scalability by combining nonlinear sources of non-classical light and programmable circuits in centimeter-scale devices. The key requirements for quantum applications include efficient generation of indistinguishable photon-pairs and high-visibility programmable quantum interference. Here, we demonstrate a lithium niobate-on-insulator (LNOI) integrated photonic circuit that generates a two-photon path-entangled state, and a programmable interferometer for quantum interference. We generate entangled photons with $\sim2.3 imes10^8$ pairs/s/mW brightness and perform quantum interference experiments on the chip with $96.8\pm3.6\%$ visibility. LNOI is an emerging photonics technology that has revolutionized high-speed modulators and efficient frequency conversion. Our results provide a path towards large-scale integrated quantum photonics including efficient photon-pair generation and programmable circuits for applications such as boson sampling and quantum communications.

Motivation & Objective

• To enable scalable, chip-integrated quantum photonic circuits by combining high-brightness photon-pair generation with programmable quantum interference.
• To overcome limitations of silicon and silicon nitride platforms by leveraging the strong nonlinear and electro-optic properties of lithium niobate-on-insulator (LNOI).
• To achieve high-visibility quantum interference between independently generated photons using on-chip interferometry, a key requirement for quantum advantage in boson sampling and quantum communications.
• To demonstrate that LNOI technology can support both efficient photon-pair generation and high-fidelity quantum interference in a single integrated platform.

Proposed method

• The chip integrates two identical periodically poled LNOI waveguides for spontaneous parametric down-conversion (SPDC), generating a path-entangled N00N state.
• A Mach-Zehnder interferometer (MZI) with thermo-optic phase shifters enables tunable quantum interference between the two output modes.
• Photon-pair generation is driven by a pump laser at 781 nm, producing correlated pairs at 1562 nm via type-II phase-matching.
• On-chip brightness is measured as ~2.3×10⁸ pairs/s/mW, significantly exceeding typical bulk SPDC sources.
• Quantum interference visibility is quantified via a controlled phase sweep of the N00N state and MZI, with output states measured using single-photon detectors.
• Off-chip Hong-Ou-Mandel (HOM) experiments verify photon indistinguishability, measuring a visibility of 83.2±0.1% with a dip width of 71.9(0.5) fs.

Figure 1: LNOI quantum photonic chip. a) The pump laser generates a path-entangled $N00N$ state across two periodically poled LNOI waveguides. The $N00N$ state phase $\phi$ is controlled with a thermo-optic phase shifter, and a MZI acts as a tunable beamsplitter to enable quantum interference. GC: G

Experimental results

Research questions

• RQ1 Can independent, on-chip photon-pair sources based on LNOI achieve high-visibility quantum interference?
• RQ2 What is the on-chip brightness of SPDC sources in periodically poled LNOI waveguides?
• RQ3 Can a single LNOI chip integrate both high-efficiency photon-pair generation and a programmable interferometer for quantum interference?
• RQ4 How does the visibility of on-chip quantum interference compare to off-chip HOM measurements of photon indistinguishability?
• RQ5 To what extent can LNOI technology enable scalable, integrated quantum photonic circuits for near-term quantum advantage?

Key findings

• The chip achieves 96.8(3.6)% visibility in on-chip quantum interference between independently generated photons, demonstrating high indistinguishability.
• On-chip photon-pair brightness reaches ~2.3×10⁸ pairs/s/mW, exceeding typical free-space SPDC sources, including those based on ppKTP.
• The Hong-Ou-Mandel experiment confirms photon indistinguishability with a visibility of 83.2±0.1%, limited by fiber beamsplitter bandwidth.
• The spectral bandwidth of the photons after fiber coupling is estimated at ~50 nm, consistent with the 71.9(0.5) fs HOM dip width.
• The observed visibility is reduced compared to on-chip interference, attributed to limited bandwidth and misalignment in off-chip components.
• The integration of SPDC sources and a programmable interferometer on a single LNOI chip enables a scalable platform for quantum information tasks like boson sampling and quantum metrology.

Figure 2: MZI classical characterization. Laser light at $1562\text{\,}\mathrm{n}\mathrm{m}$ is injected into one input mode of the MZI at a time. The phase $\theta$ is tuned and we monitor the optical power at both outputs of the MZI.