More is Better: Applications of a Multi-Output Quantum Pulse Gate

The quantum pulse gate (QPG) is a device that allows us to implement projective measurements onto arbitrary time-frequency superpositions [1]. Given the rising interest in the spectral-temporal degree of freedom of quantum light as a means for encoding high-dimensional quantum states for quantum metrology, communications, and computing, the QPG has become a versatile tool during the last decade. The QPG is based on dispersion-engineered frequency conversion in a periodically poled lithium niobate waveguide. At its operating point, the input quantum light propagates through the QPG with the same group velocity as a strong pump pulse. Tailoring the complex spectrum of the latter and detecting the converted output light of the QPG with single-photon detectors then implements the projective measurement. While the QPG has demonstrated its capabilities in several proof-of-concept experimental demonstrations, its true potential has been held back by the fact that, in the legacy configuration, only one single projection could be realized at any one time. Although this is sufficient for, for instance, super-resolved time-frequency metrology, it does not allow for the implementation of high-dimensional quantum key distribution or frequency-encoded quantum networks.

In this presentation, I will discuss an advanced version of the legacy QPG, the so-called multi-output QPG (mQPG) [2]. This device combines the operation of the QPG with a tailored phase-matching function, which provides several output frequency channels. Each channel can implement its own projection, while frequency-resolved photon counting then yields the results for all projections simultaneously. We have used this device to implement a new pulse characterization scheme, which we dubbed FIREFLY [3]. In contrast to existing schemes, FIREFLY is fully compatible with single-photon input states, can operate with partially unknown reference pulses, and retrieves not only the spectral shape but also the coherence information of the light under investigation. The mQPG is further the basis for frequency-encoded quantum networks, where we have demonstrated that we can interfere squeezed quantum states of light that are encoded in up to six frequency bins in used-defined, fully configurable, frequency-encoded quantum networks [4, 5].

The mQPG is expected to enable advanced applications for future high-dimensional photonic quantum technologies that are compatible with standard single-mode fibre architecture.

[1] B. Brecht et al, Phys. Rev. A 90, 030302(R) (2014)
[2] L. Serino et al, PRX Quantum 4, 020306 (2023)
[3] A. Bhattacharjee et al, arXiv:2504.08607 (2025)
[4] P. Folge et al, PRX Quantum 5, 040329 (2024)
[5] P. Folge et al, in preparation