[Paper Review] On-chip nanomechanical filtering of quantum-dot single-photon sources

Xiaoyan Zhou, Ravitej Uppu+7 more|arXiv (Cornell University)|Oct 13, 2019

Mechanical and Optical Resonators42 references4 citations

TL;DR

This paper presents a monolithically integrated, nanomechanically tunable photonic crystal cavity filter in GaAs that enables on-chip spectral filtering of single photons from quantum dots at cryogenic temperatures. The filter achieves >10 nm tuning range via electro-mechanical displacement, enabling high-visibility single-photon emission under non-resonant excitation with g²(0) = 0.32 ± 0.05 and >18 dB extinction ratio, crucial for scalable, integrated quantum photonic circuits.

ABSTRACT

Semiconductor quantum dots in photonic integrated circuits enable scaling quantum-information processing to many single photons and quantum-optical gates. On-chip spectral filters are essential to achieve high-purity and coherent photon emission from quantum dots embedded in waveguides, without resorting to free-space optics. Such spectral filters should be tunable, to compensate for the inhomogeneous spectral distribution of the quantum dots transitions. Here, we report an on-chip filter monolithically integrated with quantum dots, that uses nanomechanical motion for tuning its resonant wavelength over 10 nm, enabling operation at cryogenic temperatures and avoiding cross-talk with the emitter. We demonstrate single-photon emission from a quantum dot under non-resonant excitation by employing only the on-chip filter. These results are key for the development of fully-integrated de-multiplexing, multi-path photon encoding schemes, and multi-emitter circuits.

Motivation & Objective

• To develop an on-chip, tunable spectral filter for quantum-dot single-photon sources that operates at cryogenic temperatures without cross-talk.
• To overcome the inhomogeneous spectral distribution of quantum dots (~30 nm) by enabling wide, precise tuning of the filter's resonant wavelength.
• To achieve high extinction ratio and low loss in a compact footprint for integration with multiple emitters on a single chip.
• To demonstrate single-photon emission filtering using only on-chip components, eliminating the need for off-chip optics.
• To enable scalable quantum photonic circuits by integrating tunable filtering with on-chip photon routing and detection.

Proposed method

• The filter is based on a coupled nanobeam photonic crystal cavity whose resonant wavelength is tuned via electro-mechanical displacement of the nanobeams.
• Voltage-induced mechanical motion changes the coupling strength between the nanobeams, shifting the cavity's resonant wavelength by >10 nm within 15 V bias.
• The device is monolithically integrated with InAs/GaAs quantum dots in a waveguide, enabling direct filtering of emitted photons.
• The system operates at temperatures below 10 K, ensuring compatibility with quantum dot operation and minimizing thermal noise.
• The filter's performance is characterized using a Hanbury Brown and Twiss (HBT) setup to measure g²(0) and assess single-photon purity.
• Theoretical models relate cavity quality factor (Q), intrinsic loss (Qi), and coupling rate (Qc) to transmission and bandwidth, guiding design optimization.

Experimental results

Research questions

• RQ1 Can a nanomechanical filter be monolithically integrated with quantum dots in GaAs to enable on-chip spectral filtering at cryogenic temperatures?
• RQ2 What is the achievable tuning range of the filter's resonant wavelength using electro-mechanical actuation?
• RQ3 Can the on-chip filter achieve sufficient extinction ratio and low loss to enable high-purity single-photon emission under non-resonant excitation?
• RQ4 How does the filter performance compare to off-chip filtering in terms of g²(0) and single-photon indistinguishability?
• RQ5 Can this filter design be scaled for integration with multiple quantum dot sources and multi-path quantum circuits?

Key findings

• The nanomechanical filter achieves a tuning range of >10 nm at a bias of 15 V, covering the typical inhomogeneous spectral distribution of quantum dots.
• The device demonstrates single-photon emission with g²(0) = 0.32 ± 0.05 when filtering emission from a quantum dot under non-resonant excitation.
• The on-chip filter achieves an extinction ratio >18 dB and a free-spectral range >15 nm, ensuring effective suppression of non-resonant emission.
• Transmission is measured at ~8%, with theoretical modeling indicating that improved fabrication could achieve T > 60% by increasing the intrinsic quality factor (Qi).
• The filter shows no detectable cross-talk with the quantum dot emitter, enabling coexistence in a single integrated circuit.
• The design is scalable and compatible with future integration of multiple emitters, on-chip demultiplexing, and Purcell-enhanced single-photon sources.