Science - USA (2022-05-06)

Molecular modulation changes photon

color while preserving quantum

correlations

GRAPHIC: KELLIE HOLOSKI/

SCIENCE

science.org SCIENCE

cal applications ( 2 ). Quantum informatics,
quantum sensing, and quantum metrol-
ogy use specially prepared states of light
to achieve sensitivity, precision, and speed
beyond the classical limits. Quantum micro-
spectroscopy, for example, allows a more ef-
ficient and faster collection of spectroscopic
fingerprints ( 3 , 4 ) with a spatial resolution
well beyond the diffraction limit ( 5 ). Many
of these existing and potential applications
can benefit from the ability to transpose
photon states to new, distinct frequencies.
For the past 20 years, researchers have
intensified their efforts to develop prac-
tical and efficient fiber-based sources of
quantum, entangled states of light. The
next step in developing a quantum network
is to produce an efficient interface device
that can transform the photon state, for
example, to induce a shift in its frequency.

When the frequency shift is small, this can
be easily achieved with standard telecom-
munication modulators, which typically
operate at gigahertz (GHz) frequencies
and rely on refractive index modulation.
This can be done by applying an alter-
nating voltage to an electro-optic crystal
whose refractive index changes as a func-
tion of voltage. If the conversion efficiency
of such a modulator is sufficiently high,
it can convert the delicate quantum state
of light and produce an equivalent state
at a slightly different frequency. However,
if the required frequency shift is much
larger, this approach becomes impractical,
and a different method is needed.
Molecular modulation can shift the fre-
quency of entangled photons by up to 100
terahertz (THz)—10,000 times more than
the GHz range offered by crystal modula-
tors. Molecular motion produces refrac-
tive index variation, which in turn results
in phase modulation of light, where the

phase, which specifies the timing of the

wave cycle, is shifted. Molecular modula-

tors have been used to produce single-cy-

cle laser pulses ( 6 – 8 ), as well as precisely

controlled nonsinusoidal optical fields ( 9 ).

The method relies on the variability of

the bulk refractive index of a medium as

its molecules vibrate: A typical molecule

becomes more susceptible to polarization

as it is stretched. When all the molecules

inside an ensemble stretch and compress

in unison, the overall refractive index var-

ies accordingly. The basic nature of this

process, referred to as a coherent Raman

effect, has been understood for many de-

cades ( 10 ). However, a qualitatively differ-

ent phenomenon emerges when the degree

of coherence of the molecular vibrations

reaches a certain value ( 6 , 11 , 12 ). Then, the

molecular modulator can function simi-

larly to a telecom modulator, but operating

at a much higher frequency.

Typically, a modulator produces a spec-

trum of multiple modulation sidebands,

where the outgoing light contains photons

in multiple frequency bands. This is useful

for encoding a signal onto the light wave, but

if the frequency needs to be shifted to a dif-

ferent range, then a single-sideband modu-

lator must be used ( 13 , 14 ). Tyumenev et al.

achieve single-sideband conversion by using

the special dispersion properties of their

microstructured fiber (see the figure). Their

fiber has a hollow core filled with hydrogen

gas, and a cladding that surrounds the core

with a porous glass structure. The combina-

tion of the microstructure and the hydrogen

molecules provides the fiber with its unusual,

customizable photon transmission and dis-

persion characteristics, which determine

the phase delay of various frequency compo-

nents of light propagating through the fiber.

In principle, the glass microstructure and the

hydrogen gas pressure can be varied to exert

control over the relative phases of the light

waves, which determine how these waves

interact with each other and with the vibrat-

ing hydrogen molecules. This control enables

the optical frequency conversion to only one

modulation sideband. Tyumenev et al. dem-

onstrated their fiber-based modulator in a

proof-of-principle experiment using a low-

intensity quantum light source, and achieved

a 125-THz frequency change. This conversion

process is linear in the intensity of the probe

light, and therefore is expected to preserve

quantum correlations. This was verified by

the authors in their measurements of photon

correlations after the conversion.

But is there anything surprising about

this achievement? Why wouldn’t a hypo-

thetical petahertz modulator preserve the

quantum properties of modulated light in

the same way as a GHz modulator does ( 15 )?

Although in a world of spherical cows this

may indeed be expected, the picture is much

more complex in reality. Inside the molecu-

lar modulator created by Tyumenev et al.,

one of the light fields that drive the coher-

ent molecular motion is produced inside

the modulator and is seeded by quantum-

mechanical random-phased vacuum-field

fluctuations. This field is therefore noisier

compared to a perfectly well-behaved and

predictable classical field, leading to po-

tential phase uncertainties for the up-con-

verted photons. Tyumenev et al. rule out the

potentially detrimental effect of these phase

uncertainties by presenting experimental

evidence of the modulator preserving quan-

tum correlations. A tunable, highly efficient,

fiber-based frequency converter is ideal for

applications in the field of quantum infor-

mation. Thus, this latest demonstration is

an important step toward enabling practi-

cal applications in quantum-enhanced in-

formation processing and sensing. j

REFERENCES AND NOTES

1. R. Tyumenev et al., Science 376 , 621 (2022).


2. M. O. Scully, M. S. Zubairy, Quantum Optics (Cambridge
   Univ. Press, 1997).


3. S. Mukamel et al., J. Phys. At. Mol. Opt. Phys. 53 , 072002
   (2020).


4. A. Svidzinsky et al., Phys. Rev. Res. 3 , 043029 (2021).


5. M. D. Al-Amri et al., in Advances in Atomic, Molecular, and
   Optical Physics, P. Berman et al., Eds. (Elsevier, 2012),
   vol. 61, p. 409.


6. A. V. Sokolov et al., J. Mod. Opt. 52 , 285 (2005).


7. M. Y. Shverdin et al., Phys. Rev. Lett. 94 , 033904 (2005).


8. S. Baker et al., Nat. Photonics 5 , 664 (2011).


9. H.-S. Chan et al., Science 331 , 1165 (2011).


10. E. Garmire et al., Phys. Rev. Lett. 11 , 160 (1963).


11. S. E. Harris, A. V. Sokolov, Phys. Rev. A 55 , R4019 (1997).


12. J. Q. Liang et al., Phys. Rev. Lett. 85 , 2474 (2000).


13. A. V. Sokolov et al., Opt. Lett. 26 , 728 (2001).


14. J. Zheng, M. Katsuragawa, Sci. Rep. 5 , 8874 (2015).


15. M. Katsuragawa et al., Phys. Rev. A 65 , 025801 (2002).

ACKNOWLEDGMENTS

Support from the Welch Foundation (grant A-1547) is grate-

fully acknowledged.

10.1126/science.abo2358

INSIGHTS | PERSPECTIVES

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Photons

Molecular modulator

Using a hollow-core

microstructured fiber

filled with hydrogen

gas molecules that

vibrate in unison, the

device changes the

wavelength for one of

the photons while

preserving the

quantum correlations

within the pair.

Quantum entanglement

In an entangled photon pair,

the two photon states are

perfectly correlated but

individually undetermined.

However, if one of the photon

states is measured, the other

is immediately revealed.

576 6 MAY 2022 • VOL 376 ISSUE 6593