Can a single photon's twisted light survive a colour change?
Single photons carrying twisted-light orbital angular momentum were shifted from telecom infrared to green light while keeping their spatial shape, quantum correlations and superposition coherence.
Source
Orbital angular momentum photonic quantum interface
Study at a glance
- Design
- Other — Tabletop nonlinear-optics experiment: cavity-enhanced sum-frequency generation in a PPKTP crystal converting OAM-carrying light from 1560 nm to 525 nm, first with classical and attenuated light, then with heralded single photons.
- N
- No sample size; results are measured conversion efficiencies, cross-correlations and interference visibilities for OAM values l = 0, 1 and 2.
- Population
- Heralded single photons from spontaneous parametric down-conversion and attenuated laser light carrying orbital angular momentum
- Outcome
- Conversion efficiency, preservation of spatial mode shape, signal-idler cross-correlation, and superposition interference visibility/fidelity
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What they did
The team pumped a nonlinear PPKTP crystal inside a bow-tie ring cavity with a strong 795 nm beam so that 1560 nm signal light would combine with it to make 525 nm light (sum-frequency generation). They first tested bright and then very dim laser light carrying different amounts of orbital angular momentum (OAM), then used heralded single photons from a down-conversion source, including photons in superpositions of opposite OAM values. They imaged the output with a single-photon camera and measured coincidences with the partner (idler) photon.
What they found
With classical light, the quantum conversion efficiency fell as OAM increased, from 0.224 for the Gaussian mode to 0.0833 for l = 1 and 0.0296 for l = 2, matching their theory based on beam overlap. Converted single photons kept their donut and petal shapes, and the cross-correlation with the idler stayed well above the classical limit of 2 (for example 15.5 for l = 1). Superposition interference visibilities of about 89% gave process fidelities of 0.94 for l = 1 and 0.95 for l = 2.
The limits
What it doesn't show
Efficiency is low (about 8% for l = 1) and drops quickly for higher OAM, so the interface was only shown for l up to 2; higher orders are proposed but not demonstrated. The converted photons were not actually connected to a quantum memory or another quantum system, so the 'interface' role is anticipated rather than tested. Measured correlations deviated from theory at the beam centre, which the authors attribute to misalignment and Raman noise, and filtering through a single-mode fibre adds loss.
Key terms
- Orbital angular momentum (OAM) of light
- A property of beams with a helical phase front; a beam with index l carries l units of ħ per photon and has a donut-shaped intensity.
- Sum-frequency generation (SFG)
- A second-order nonlinear process where two photons combine in a crystal to produce one photon at the sum of their frequencies.
- Heralded single photon
- A photon from a pair source whose presence is signalled by detecting its partner photon.
- Cross-correlation g(2)
- A normalised coincidence rate between two light fields; values above 2 indicate non-classical correlation for these sources.
- Fidelity
- How close the output quantum state is to the ideal; here computed from interference visibility as (1+V)/2.
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Quiz yourself
What wavelength change did the interface perform?
Common questions
Why convert photons from 1560 nm to visible wavelengths at all?
Telecom-band photons travel well in optical fibre, but most quantum memories work at visible or near-infrared wavelengths, so a converter is needed to link them.
Why is conversion less efficient for higher OAM?
Higher-OAM beams have larger donut profiles that overlap less with the Gaussian pump beam inside the crystal, reducing the nonlinear interaction.
How do they know quantum properties survived conversion?
The idler cross-correlation stayed well above the classical bound and superposition interference had high visibility, giving fidelities around 0.94-0.95.
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