The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics the objectivity of observations is not so clear, most markedly exposed in Wigner’s eponymous thought experiment where two observers can experience seemingly different realities. The question whether the observers’ narratives can be reconciled has only recently been made accessible to empirical investigation, through recent no-go theorems that construct an extended Wigner’s friend scenario with four observers. In a state-of-the-art six-photon experiment, we realize this extended Wigner’s friend scenario, experimentally violating the associated Bell-type inequality by five standard deviations. If one holds fast to the assumptions of locality and free choice, this result implies that quantum theory should be interpreted in an observer-dependent way.

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Photonic quantum technology increasingly uses frequency encoding to enable higher quantum information density and noise resilience. Pulsed time-frequency modes (TFM) represent a unique class of spectrally encoded quantum states of light that enable a complete framework for quantum information processing. Here, we demonstrate a technique for direct generation of entangled TFMencoded states in single-pass, tailored downconversion processes. We achieve unprecedented quality in state generation-high rates, heralding efficiency and state fidelity-as characterised via highly resolved time-of-flight fibre spectroscopy and two-photon interference. We employ this technique in a four-photon entanglement swapping scheme as a primitive for TFM-encoded quantum protocols.Generating entanglement in intrinsically highdimensional degrees of freedom of light, such as transverse and longitudinal spatial modes [1,2], or time and frequency, constitutes a powerful resource for photonic quantum technologies-photons that carry more information enable more efficient protocols [3,4]. Time-frequency encoding is intrinsically suitable for waveguide integration and fibre transmission [5,6], making it a promising choice for practical, high-dimensional quantum applications.Quantum information can be encoded either in discrete temporal or spectral modes (namely time-and frequency-bin encoding [6-9]) or in the spectral envelope of the singlephoton wavepackets-time-frequency mode (TFM) encoding [5,10]. TFM-encoded states arise naturally in parametric downconversion (PDC) sources, as TFMs are eigenstates of the PDC process and they span an infinite-dimensional Hilbert space. Conveniently, TFMs possess highly desirable properties: being centred around a target wavelength makes them compatible with fibre networks, they are robust against noise [11] and chromatic dispersion [12], their pulsed nature enables synchronisation and therefore multi-photon protocols and they offer intrinsically high dimensionality [10]. Manipulation and detection of TFMs is enabled by the quantum-pulse toolbox, where sum-and difference-frequency generation are used for reshaping and projecting the quantum states [5,10]. However, generating entangled TFMs in a controlled way can be very challenging [13][14][15][16][17], limiting their usefulness in realistic scenarios. Here, we overcome this problem exploiting domain-engineered nonlinear crystals [18,19] for generating TFM entanglement from standard ultrafast laser pulses in a single-pass PDC experiment. We experimentally validate this technique by benchmarking a maximally antisymmetric state at telecom wavelength † These two authors contributed equally.with near unity fidelity, and implement a four-photon entanglement swapping scheme. Our work complements the pulse-gate toolbox [5,10] for TFM quantum information processing, and establishes a standard for the generation of TFM quantum states of light while paving the way for more complex frequency encoding.In a PDC process, a pump photon probabilistically downconverts into t...