Ultrastable near-unity-fidelity dynamic polarization encoding of deterministically generated single photons

The ability to reliably inscribe information on single photons is a crucial requirement for quantum applications such as quantum communication and measurement-based photonic quantum computation. Experimental implementations generally employ phase modulators in single-pass or Mach-Zehnder-interferometer configurations to encode information on photonic qubits. However, these approaches are intrinsically sensitive to environmental influences, which limits the achievable quantum error rates in practice, posing a major bottleneck in the transition of quantum applications with deterministic sources from the laboratory to the field. Inspired by the adaptation of Sagnac interferometer encoders for laser-based applications, we report a demonstration of a polarization encoder for single-photon qubits based on a free-space Sagnac interferometer. The device showcases inherent phase stability and overcomes previous error-rate limitations: Telecom-wavelength photons emitted by a quantum dot are modulated at a rate of 152 MHz and with a quantum bit error rate of 0.69±0.02%, marking the lowest value reported to date for MHz-rate information encoding on single photons by a factor of >3. This combination of low error rate, high modulation speed, stability, and operation with telecom-wavelength sub-Poissonian photons is an essential requirement for bringing quantum applications based on deterministic photon sources from the laboratory to the field and a significant step toward unlocking the full potential of a future quantum Internet.