QUANTUM OPTICS
Tunable and state-preserving frequency conversion
of single photons in hydrogen
R. Tyumenev^1 , J. Hammer^1 , N. Y. Joly2,1*, P. St. J. Russell^1 ,D.Novoa3,4,1
In modern quantum technologies, preservation of the photon statistics of quantum optical states upon
frequency conversion holds the key to the viable implementation of quantum networks, which often
require interfacing of several subsystems operating in widely different spectral regions. Most current
approaches offer only very small frequency shifts and limited tunability, while suffering from high
insertion loss and Raman noise originating in the materials used. We introduce a route to quantum-
correlationÐpreserving frequency conversion using hydrogen-filled antiresonant-reflecting photonic
crystal fibers. Transient optical phonons generated by stimulated Raman scattering enable selective
frequency up-conversion by 125 terahertz of the idler photon of an entangled pair, with efficiencies up to
70%. This threshold-less molecular modulation process preserves quantum correlations, making it
ideal for applications in quantum information.
L
ight quanta are efficient and reliable car-
riers of information because they barely
interact with the environment. In this
regard, the manipulation of quantum
states of light such as single photons or
entangled photon pairs without affecting their
quantum properties has attracted major in-
terest in the last two decades ( 1 , 2 ). Whereas
operations on spatial and temporal photonic
degrees of freedom can be carried out using
passive devices ( 3 , 4 ), other modifications such
as frequency conversion must be performed
in active systems via nonlinear interactions
( 5 – 13 ). Quantum-correlation–preserving fre-
quency conversion is essential in modern
quantum technologies requiring quantum
light sources operating at different frequen-
cies, but is quite difficult to achieve. First, the
quality of the quantum source is degraded by
almost unavoidable addition of noise from
Raman scattering or fluorescence during in-
teractions with the media within which the
light propagates. Second, apart from the spe-
cial case of thin, high-gain media ( 14 ), most
approaches based on second- or third-order
nonlinear processes require stringent phase-
matching conditions to be satisfied, which in
the common case of dispersive solid materials
effectively restricts the spectral range of opera-
tion to a few selected frequencies and greatly
narrows the bandwidth over which the conver-
sion can efficiently take place.
Gas-filled antiresonant-reflecting photonic
crystal fiber (ARR-PCF) allows highly efficient
nonlinear frequency conversion by a combi-
nation of pressure-adjustable dispersion and
nonlinearity, with diffraction-free low-loss
broadband guidance ( 15 – 17 ). When filled with
a Raman-active diatomic gas such as hydrogen,
transient optical phonons or coherence waves
of synchronous collective molecular motion
can be excited in the gaseous core via stimu-
lated Raman scattering (SRS). They can be
subsequently used for threshold-less frequency
up-conversion of arbitrary signals via molec-
ular modulation ( 18 , 19 ), provided momentum
conservation is satisfied ( 20 ).
Here we report the use of Raman coherence
waves excited in H 2 -filled ARR-PCFs for effi-
cient quantum frequency conversion of a single
photon from a highly correlated photon pair
(abiphoton;Fig.1A).Astheup-conversion
process depends strongly on phase matching
between the interacting single photons and
coherence waves, the signal is almost free from
any background noise caused by incoherent
molecular modulation. Spontaneous Raman
scattering from the solid microstructure is
also negligible because the light-glass overlap
is extremely low (< 0.01%) (17, 21). In addition,
other parasitic nonlinear effects such as in-
stantaneous four-wave mixing are also large-
ly suppressed in the gaseous core owing to
the low effective nonlinearity at the pump
powers used.
In SRS-based quantum molecular modula-
tion, a strong pump pulse launched in the fun-
damental core mode (Fig. 1B) is scattered by
hydrogen molecules in the core, giving rise to a
Stokes signal down-shifted by the Raman fre-
quencynR~125 THz [i.e., the frequency of the
main vibrational transition of theQ-branch of
H 2 ( 22 )]. The pump-Stokes beat note then drives
the molecular vibrations, stimulating exponen-
tial amplification of the Stokes light and caus-
ing excitation of a quantum coherence wave
of collective molecular vibrations. The result
(Fig. 1A) is a moving refractive index gratingcapable of threshold-less Doppler frequency
shifting of any subsequent, arbitrarily weak
signal, providing phase matching is satisfied,
i.e., dephasingθ¼jðbuibiÞ–bcwj≈0( 20 ),
wherebi,bui, andbcware the propagation
constants of the idler photon, its up-shifted
counterpart, and the coherence wave, respec-
tively. In this regard, the“S-shaped”dispersion
landscape of gas-filled ARR-PCFs ( 20 ) (Fig. 1C)
allows selective intramodal phase-matched
conversion of any pair of frequencies from
the ultraviolet to the mid-infrared—a working
range not achievable by any other existing
technology—provided they spectrally differ
bynR. In our system,θ≈0 occurs at ~70 bar
(Fig. 1C).
To verify these predictions experimental-
ly, we split the output of a laser delivering
1064-nm pulses of 3.8-ns duration into two
arms. A small fraction was used to generate
the entangled biphotons at 849 nm (signal)
and 1425 nm (idler) through spontaneous
four-wave mixing ( 23 , 24 ) in a suspended-
core fiber (fig. S2). The signal photons were
directly sent to a superconducting single-
photon detector (SSPD) and the idler pho-
tons, along with the remaining pump power,
were launched into a 60-cm-long, single-
ring–type ARR-PCF with 60-mm core diameter
and ~270-nm average capillary-wall thickness
(Fig. 1B). These design parameters ensured
that loss-inducing anticrossings between the
core mode and resonances in the glass-walled
capillaries ( 21 ) lay spectrally away from any
wavelength of interest. The launch efficiencies
for the pump in the fundamental mode (Fig. 1B)
reached 96%, which helped to mitigate noise
arising from unwanted effects in the glass
microstructure. Both fiber ends were enclosed
in hermetically sealed gas cells. The out-
coupled idler photons, whether frequency
up-converted to 894 nm or not, were sent to
a second SSPD, and coincidences between
the two channels were recorded using a time
tagger ( 24 ), enabling access to the second-
order correlation functiong(2)(t), wheret¼
titsis the difference between the idler (ti)
and signal (ts) arrival times. The set-up is
sketched in fig. S3.
We first measured the quantum conversion
efficiency of the up-shifting process (Fig. 2A),
defined ash¼½ 1 ðÞIRC=I 100, whereIRC
andIare the idler count rates with and with-
out Raman coherence prepared in the system
(controlledbythepresenceof115-mJ pump
pulses; Fig. 2B). This is a valid measure of
conversion efficiency in an effective two-level
system such as the hydrogen molecule because
no other viable scattering routes (such as rota-
tional SRS ( 12 ) or vibrational down-shifting)
are available to the idler photons. As shown
in Fig. 2A,hreaches 70% at 70 bar, in excel-
lent agreement with the theory, and grad-
ually falls at other pressures owing to strongSCIENCEscience.org 6 MAY 2022•VOL 376 ISSUE 6593 621
(^1) Max-Planck Institute for the Science of Light, Staudtstrasse
2, 91058 Erlangen, Germany.^2 Department of Physics,
Friedrich-Alexander-Universität, Staudtstrasse 2, 91058
Erlangen, Germany.^3 Department of Communications
Engineering, Engineering School of Bilbao, University of the
Basque Country (UPV/EHU), Torres Quevedo 1, 48013
Bilbao, Spain.^4 IKERBASQUE, Basque Foundation for Science,
Plaza Euskadi 5, 48009 Bilbao, Spain.
*Corresponding author. Email: [email protected]
Present address: Interherence GmbH, Henkestrasse 91, 91052
Erlangen, Germany.
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