Can a chip change the colour of single photons without loss?
An on-chip lithium niobate modulator shifted single photons' frequency by up to about 641 GHz and squeezed their bandwidth 18-fold, without adding noticeable loss or noise.
Source
Spectral control of nonclassical light pulses using an integrated thin-film lithium niobate modulator
Study at a glance
- Design
- Other — Lab experiment: heralded single photons from down-conversion were phase-modulated by a double-pass thin-film lithium niobate modulator for spectral shearing and time-lens compression, with spectra and two-photon interference measured.
- N
- No sample count; a single integrated modulator device characterised in several configurations.
- Population
- Telecom-band (~1560 nm) heralded single-photon pulses and one double-pass thin-film lithium niobate phase modulator
- Outcome
- Single-photon frequency shift, spectral bandwidth after compression, and Hong-Ou-Mandel interference visibility
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What they did
The authors made pairs of photons near 1560 nm by spontaneous parametric down-conversion and sent one photon through a thin-film lithium niobate phase modulator whose waveguide passes the microwave electrode twice. Timing the photon to the steep slope of the sinusoidal microwave drive produced a linear phase ramp (spectral shearing), while timing it to the drive's trough gave a quadratic phase that acts as a time lens. They measured single-photon spectra by dispersive time-of-flight and tested whether shifted photons stayed indistinguishable using Hong-Ou-Mandel interference.
What they found
Frequency shifts reached ±641 GHz (±5.2 nm), about three times larger than earlier bulk-modulator demonstrations, with no added insertion loss beyond coupling drift. Two photons initially 154 GHz apart interfered with only 23.1% visibility, but after shifting one of them the visibility rose to 90.5%. Used as a time lens, the modulator compressed a photon's bandwidth from 807 GHz to 43.1 GHz, a factor of 18.7.
The limits
What it doesn't show
Only one device was tested, and its low half-wave voltage occurs only at specific drive frequencies because of the double-pass loopback, so it is not a broadband modulator. Timing jitter between laser and microwave drive broadened the shifted spectra, and the quoted interference visibilities are background-subtracted (raw values were lower). The bulk waveshaper used for dispersion was off-chip, and the method has not been shown for narrowband photons from quantum emitters or as a partial frequency beam splitter.
Key terms
- Spectral shearing
- Shifting all frequencies of a pulse by the same amount by applying a phase that changes linearly in time.
- Time lens
- A quadratic-in-time phase that acts on pulses the way a lens acts on beams, allowing spectral compression when combined with dispersion.
- Half-wave voltage (Vπ)
- The drive voltage needed to shift an optical phase by π; lower means a more efficient modulator.
- Hong-Ou-Mandel interference
- Two identical photons meeting at a beam splitter leave together, causing a dip in coincidences whose depth measures indistinguishability.
- Spontaneous parametric down-conversion
- A nonlinear process in which a pump photon splits into a correlated signal and idler pair.
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Quiz yourself
Which phase profile turns the modulator into a time lens?
Common questions
Why is lithium niobate thin film better than bulk modulators here?
Tighter confinement lowers the half-wave voltage, so available microwave power produces many multiples of π phase and thus larger frequency shifts.
How do they know the photons were not damaged by shifting?
After shifting, the photon interfered with its partner at high visibility, which only happens if the two are nearly indistinguishable and no noise was added.
What limits the size of the frequency shift?
The authors say the frequency synthesiser and amplifier, plus timing jitter, rather than the chip's own bandwidth or power handling.
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