Description
It is well-established that measurements on spatially separated qubits exhibit correlations that violate statistical limitations arising from local realism. These limitations are mathematically expressed by Bell-type inequalities. A violation of a Bell-type inequality is referred to as Bell nonlocality. In 2002, Collins, Gisin, Linden, Massar and Popescu (CGLMP) developed a generalisation of the two-dimensional Bell-type inequality for qudits $-$ discrete $d-$dimensional systems. Foundationally, qudits are interesting because they manifest stronger violations of Bell-type inequalities than two-dimensional qubits. Furthermore, unlike qubits, the maximally entangled qudit state is not maximally nonlocal $-$ it does not produce the maximum violation of a Bell-type inequality. From a practical perspective, demonstrations of Bell nonlocality in qudit systems are a crucial resource for emerging quantum technologies, as it provides a strict certification of high-dimensional entanglement.
Photons are an attractive system for experimental demonstrations of CGLMP inequality violations because they have naturally high-dimensional properties at room temperature. The spatial, temporal, and spectral structure of light are just some of the degrees-of-freedom that can provide an orthogonal basis spanning a theoretically infinite-dimensional Hilbert space.
In this work, we harness the temporal degree-of-freedom of photons to establish Bell nonlocality via a direct measurement of the joint spectral intensity (JSI). Prior to the work of Leach et al. in 2025, the JSI was thought to lack the phase information needed for demonstrations of nonlocality. However, discrete time-bins modulate the JSI such that all the phase information required for the CGLMP inequality can be reconstructed. Using this method, we prepare both maximally entangled qudit states and maximally nonlocal qudit states, via complex spectral shaping of the pump, to certify Bell nonlocality.
| I am the presenting author | Yes |
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