Structured light
Can one laser beam mimic three-way quantum entanglement?
Open access · cc by · source: Europe PMC
A specially tuned laser produces a single beam whose ray paths, directions and polarisations are linked in an eight-dimensional pattern, and with simple modulation it reproduces all eight classical analogues of three-party GHZ entangled states.
Study at a glance
- Design
- Other — Laboratory optics experiment: off-axis pumped frequency-degenerate laser cavity producing ray-wave vector beams, external SLM and iris modulation, and a Bell-state projection tomography.
- N
- No sample size; eight GHZ states were generated and characterised.
- Population
- Laser beams from a frequency-degenerate solid-state laser cavity
- Outcome
- Beam patterns, polarisation structure and fidelity of eight classical GHZ states
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Key findings
The laser produced an eight-dimensional non-separable vector beam, and all eight GHZ states were generated with measured density matrices agreeing closely with theory, with fidelities above 90%. The approach scales: other cavity ratios allow, for example, a 12-dimensional space, and adding orbital angular momentum could extend to four or five parties.
Methodology
The authors tuned a laser cavity to a degenerate condition in which the output beam behaves both as one wave and as a bundle of rays making four round trips. Using the birefringence of a c-cut crystal and the pump position and power, they marked different rays with different polarisations, giving three linked degrees of freedom. They then used irises, wave plates and a spatial light modulator to convert the beam into each of the eight classical GHZ states, and verified them with a new Bell-projection tomography.
Limitations
This is classical entanglement, meaning non-separability within a single beam; it does not create entanglement between separate particles and cannot violate locality or be used for genuine quantum communication. Higher-dimensional cases beyond eight dimensions are described in supplementary theory rather than demonstrated in the main experiment. Fidelity was estimated with a new tomography method introduced by the authors, so it has not been independently benchmarked.
How this study connects
Role on claims
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One beam can hold correlated path, direction and polarization patterns.
A tuned laser produced an eight-dimensional non-separable vector beam and reproduced all eight classical analogues of three-party GHZ states with fidelities above 90%.
Evidence for the claim as stated.
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