Science - USA (2022-05-06)

dephasing and Raman gain suppression ( 25 ),
the latter being responsible for the total can-
cellation of the modulation process below
~50 bar (fig. S4).
The excellent quantitative agreement be-
tween experimental results and numerical
modeling (Fig. 2A) ( 24 ) allows us to gain in-
sight into the quantum molecular modulation
process in the fiber. In particular, the spatio-
temporal evolution of the Raman coherence
can be studied, something not accessible in
the experiments. As illustrated in Fig. 3A, at
70 bar the Raman coherence builds up along
the fiber, peaking at ~30 cm. From this point
on, the single idler photons are predominant-
ly transformed into anti-Stokes idler photons
at 894 nm (Fig. 3B), with conversion to Stokes
photons remaining five orders of magnitude
lower (Fig. 3C). In the last section of the fiber,
the pump pulses are substantially depleted

and the coherence gradually decays away

(Fig.3A).Despitethehighpressures,back-

ward SRS does not play any role in the dynam-

ics (fig. S5).

It is also important to verify that the pre-

existing quantum correlations in the original

biphoton pairs are preserved during single-

pump molecular modulation and subsequent

propagation of the up-shifted anti-Stokes pho-

tons through the ARR-PCF. To do so, we mea-

sured the second-order correlation function

g(2)(t) of the original biphotons (Fig. 4A) and

compared the result with the second-order

correlations established between the up-shifted

idler and the unmodified signal (Fig. 4B).

The strong resemblance of bothg(2)(t) peaks

(Fig. 4C), normalized to account for acciden-

tal coincidences caused by the pulsed nature

of the biphoton source ( 24 , 26 ), as shown in

fig. S6, indicates that fragile quantum cor-

relations remain intact during the Raman

molecular modulation process. It is impor-

tant to highlight that the system is, in prin-

ciple,fullyreconfigurablebyusingother

Raman-active molecular degrees of freedom

or different gas species and mixtures ( 25 ).

Furthermore, because state-of-the-art ARR-

PCFs feature extraordinarily low attenuation

( 27 ) and polarization-maintaining properties

( 28 , 29 ), these fiber-based solutions might also

provide environmentally insulated transport

of the modulated biphotons, making these

quantum devices far more versatile than any

other alternative to date.

We have demonstrated that quantum mo-

lecular modulation based on SRS in gas-filled

ARR-PCFs can be used for efficient, quantum-

correlation–preserving frequency conversion

of biphoton pairs. The approach is indepen-

dent of the quality and nature of the quantum

622 6 MAY 2022•VOL 376 ISSUE 6593 science.orgSCIENCE

Fig. 1. Fiber-based Raman molecular modulation

of quantum light sources.(A) Illustration of the

nonlinear conversion process of single idler photons

from biphotons. (B) Near-field mode profile of

the pump (1064 nm) imaged by a charge-coupled

device camera and overlaid with a scanning electron

micrograph of the transverse fiber microstructure.

(Right) theoretical intensity profiles (linear scale)

of fundamental modes at the wavelengths of

the Stokes signal (1911 nm), the biphoton idler

(1425 nm), and its up-shifted anti-Stokes (894 nm)

and down-shifted Stokes (3511 nm) bands, obtained

via finite-element modeling of a perfect structure.

(C) Frequency versus wave vector diagrams at

three different pressures. To reveal the subtle

S-shaped profile, the modal propagation constants

are offset bybref¼ðÞ 2 pn=cneffðÞnref, whereneffis the

modal index atnref= 600 THz andnis the optical

frequency. The arrows represent the coherence

waves (solid) and the corresponding single-photon

transitions (dashed), which perfectly match at

~70 bar and are otherwise highly dephased. The

filled-in circles mark the zero-dispersion points.

150

200

250

300

350

–2.0 –1.9 –1.8 –1.7 –1.6 –1.5

80 bar 70 bar 60 bar

Pump (1064 nm)

Stokes (1911 nm)

Biphoton idler (1425 nm)

Up-shifted idler (894 nm)

νR

50 μm

~270 nm 894 nm 1911 nm

1425 nm 3511 nm

1064 nm

Frequency (THz)

• ref(mm–1)

A B

C

Fig. 2. Pressure dependence of the quantum

conversion efficiency.(A) Conversion efficiency

as a function of gas pressure. The experimental

results (symbols) are in good agreement with

numerical simulations (dashed line) based on

Maxwell-Bloch equations. (B) Estimate of the con-

version efficiency based on the drop in idler

count rate at 1425 nm at 70 bar. When the pump

signal is unblocked after 25 s of measurement time

(shaded green area), the excited optical phonons

yield a clear drop in the idler count rate (see fig. S1

for the behavior of the up-shifted idler). Only dark

counts are registered (shaded purple area) when

all the optical signals are blocked.

Experiment

(^0606468727680)
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Simulation
0 10 20 30 40 50
Time (s)
400
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0
Idler count rate (s
–1
)
2000
70% drop
Dark counts
A B Pump OFF Pump ON
Quantum conversion efficiency η (%)
Pressure (bar)
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