of 6
Research Article
Vol. 12, No. 1 / January 2025 /
Optica
99
High-efficiency low-noise optomechanical crystal
photon-phonon transducers
Sameer Sonar,
1
,
2
,
Utku Hatipoglu,
1
,
2
,
Srujan Meesala,
1
,
2
David P. Lake,
1
,
2
Hengjiang Ren,
1
,
2
,
3
AND
Oskar Painter
1
,
2
,
4
,
*
1
Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA
2
Kavli Nanoscience Institute and Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125,
USA
3
Current address: Anyon Computing Inc., Emeryville, California 94608, USA
4
AWS Center for Quantum Computing, Pasadena, California 91125, USA
These authors contributed equally to this work.
*opainter@caltech.edu
Received 9 August 2024; revised 30 November 2024; accepted 1 December 2024; published 17 January 2025
Optomechanical crystals (OMCs) enable coherent interactions between optical photons and microwave acoustic
phonons, and represent a platform for implementing quantum transduction between microwave and optical signals.
Optical-absorption-induced thermal noise at cryogenic (millikelvin) temperatures is one of the primary limitations
of performance for OMC-based quantum transducers. Here, we address this challenge with a two-dimensional sili-
con OMC resonator that is side-coupled to a mechanically detached optical waveguide, realizing a six-fold reduction
in the heating rate of the acoustic resonator compared to prior state-of-the-art, while operating in a regime of high
optomechanical-backaction and millikelvin base temperature. This reduced heating translates into a demonstrated
phonon-to-photon conversion efficiency of 93.1
±
0.8% at an added noise of 0.25
±
0.01 quanta, representing a sig-
nificant advance toward quantum-limited microwave-optical frequency conversion and optically controlled quantum
acoustic memories.
© 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
https://doi.org/10.1364/OPTICA.538557
1. INTRODUCTION
Optomechanical crystals (OMCs) are periodic dielectric struc-
tures engineered to co-localize light and acoustic vibrations on
the wavelength scale [1]. OMCs in one-dimensional silicon
nanobeams have been used to demonstrate MHz-scale inter-
action rates between single photons and phonons confined
in telecom-wavelength optical and GHz-frequency acoustic
modes, respectively [2]. In recent years, these devices have enabled
advances in quantum acoustics [3,4], and microwave-optical
quantum transduction [5–8]. In these applications, OMCs are
operated at temperatures
0.1 K to ensure negligible thermal
occupation in GHz frequency acoustic modes. However, optical
excitation of OMCs in this temperature regime is accompanied by
local heating due to weak parasitic absorption of laser light. This
effect results in elevated thermal occupation and reduced coher-
ence of the acoustic mode [3,9], and limits the performance of
OMCs as quantum memories and transducers. For instance, in the
context of microwave-optical quantum transduction, operation
of OMCs in the quantum coherent regime is currently possible
in pulsed mode with reduced laser power, albeit at the expense of
reduced efficiency for the optomechanical scattering process, and
ultimately, a reduced entanglement generation rate for quantum
networking applications [5,6].
Two-dimensional (2D) device geometries [10–13], which
offer increased thermal contact with the substrate, are a promising
approach to mitigate the detrimental effects of optical-absorption-
induced heating in OMCs at millikelvin temperatures. These
structures rely on a phononic bandgap to protect the acoustic mode
of interest from clamping losses while leveraging a larger density
of states at frequencies above the gap to allow high-frequency ther-
mal phonons to escape out of the OMC cavity. However, a large
thermal contact area can also lead to a significant influx of thermal
phonons from sites of optical absorption external to the cavity.
Such a phenomenon was noted in our prior work on 2D-OMCs in
Ref. [11], where optical absorption in the coupling waveguide was
found to be the dominant source of thermal noise in the acoustic
mode. In principle, thermal isolation of the OMC cavity from the
coupling optical waveguide could be achieved through mechanical
detachment of both structures from one another while maintain-
ing evanescent optical coupling. However, these requirements are
challenging to engineer in a 2D geometry since abrupt termination
of the OMC can result in significantly increased optical loss via
parasitic edge modes. In this work, we overcome this challenge
with a 2D-OMC with a novel side-coupled waveguide design and
demonstrate a significant improvement in phonon-to-photon
conversion performance over previous state-of-the-art [11]. We
observe a six-fold reduction in the heating rate of the acoustic
2334-2536/25/010099-06 Journal © 2025 Optica Publishing Group
Research Article
Vol. 12, No. 1 / January 2025 /
Optica
100
resonator in the regime of high optomechanical backaction and
millikelvin base temperatures. Under continuous-wave (pulsed)
optical excitation, we show through optomechanical sideband
thermometry that the device can perform phonon-to-photon con-
version with an internal efficiency of 98.6
±
0.2% (93.1
±
0.8%)
and an added noise of 0.28
±
0.01 quanta (0.25
±
0.01 quanta)
at the highest optical powers used in our experiments. Our results
indicate that side-coupled 2D-OMCs can enable high-fidelity
quantum-coherent operations with acoustic modes and advance
the performance of optomechanical quantum transducers and
memories.
2. DESIGN OF THE SIDE-COUPLED 2D-OMC
CAVITY
The OMC cavity in this work is designed on a 220 nm thick silicon
device layer of a silicon-on-insulator (SOI) substrate. Figure 1(a)
shows a helium ion microscope image of a fabricated device. The
insets show (from left to right) unitcells of a snowflake, fish-bone
waveguide, coupling waveguide, and the photonic crystal mir-
ror. The snowflake crystal provides a pseudo-bandgap for TE-like
optical guided waves and a full bandgap for all acoustic mode polar-
izations, whereas the fish-bone structure is engineered to maximize
the combined photoelastic and moving boundary contributions
to the optomechanical coupling. The coupling waveguide is
designed with a half-snowflake unitcell to evanescently couple to
the optical mode of the OMC cavity. The detailed design process
and mode analysis underlying this configuration are presented in
Supplement 1, Section 6. The gap between the two structures
can be controlled to set the external coupling rate of the optical
cavity. To facilitate measurements of the OMC in reflection, the
coupling waveguide is terminated with a photonic crystal mirror.
The orange inset on the right shows the supercell for the combined
setup with the cavity and coupling waveguide. The corresponding
optical bandstructure is shown in Fig. 1(b). The shaded region
represents the relevant optical bandgap. The cavity (waveguide)
band of interest is shown in solid red (blue). The corresponding
electric field profile for cavity (waveguide) mode is shown on the
right labeled 1 (2).
Figure 1(c) illustrates the simulated acoustic breathing mode
with a frequency of 10.3 GHz, and the transverse electric field
distribution of the fundamental optical mode with a wavelength
of 1550 nm. The energy of the optical mode is predominantly
localized in the air gaps of the fish-bone structure to enhance
optomechanical coupling due to the moving boundaries. For an air
gap size of 70 nm, this design provides a vacuum optomechanical
coupling rate,
g
OM
/
2
π
=
1.1 MHz, in simulation. In principle,
fabrication of smaller gaps (e.g., 20 nm) can allow for
g
OM
/
2
π
up
to 2.5 MHz. More details are provided in Supplement 1, Section 7.
The devices are patterned using electron beam lithography and
reactive ion etching, and are suspended by etching the underlying
buried oxide layer with hydrofluoric acid etch. We present here the
characterization of two devices, which we refer to as device I and
II in our discussion. All measurements are performed in a dilution
refrigerator, with the samples mounted to the mixing chamber
plate at a temperature of
T
f
10 mK. The optical and acoustic
mode parameters of both devices are tabulated in Supplement 1,
Section 1. The primary difference between the two devices is their
coupling to the external chip environment. For device I, the intrin-
sic acoustic damping rate,
γ
0
/
2
π
, is measured to be 21.46 kHz,
whereas device II is better isolated from the chip environment
with a
γ
0
/
2
π
=
0.97 kHz. We attribute the difference in
γ
0
/
2
π
between the two devices to a difference in the as-fabricated feature
sizes. The effectiveness of the snowflake acoustic bandgap region
in suppressing acoustic radiation from the central cavity region
depends on the frequency alignment of the localized mode and
the acoustic bandgap, and is highly sensitive to the feature size and
shape. A scanning electron microscope image of device I indicates
that the patterned features are slightly more rounded and extended
than the ideal design, whereas device II replicates the design more
faithfully. Although unintentional, these as-fabricated differences
in the two devices provide further information about how they
thermalize with their environment.
3. OPTICAL-ABSORPTION-INDUCED HOT BATH
Previous measurements on OMC devices in the dilution refrigera-
tor have shown that the acoustic mode thermalizes to temperatures
(a)
(b)
(c)
Fig.1.
Side-coupled 2D optomechanical crystal (OMC) cavity. (a) Helium-ion microscope image of a representative device with insets indicating salient
features from left to right: unitcells of the 2D snowflake lattice, central fish-bone waveguide, optical coupling waveguide, and the optical waveguide mir-
ror. The orange inset on the right shows the supercell of the geometry used to simulate optical and acoustic bandstructures. (b) Simulated optical bandstruc-
ture of the supercell. The solid red (blue) band has energy predominantly in the cavity (coupling waveguide). Transverse electric field profiles of these optical
modes at the X-point are shown on the right. Dashed lines in the bandstructure indicate other guided modes. (c) FEM simulations of the acoustic (left; total
displacement) and optical (right; transverse electric field) modes of the 2D-OMC cavity with acoustic resonance frequency,

m
/
2
π
=
10.3 GHz, and opti-
cal resonance wavelength,
λ
=
1550 nm, respectively.
Research Article
Vol. 12, No. 1 / January 2025 /
Optica
101
(b)
(a)
(c)
(d)
(e)
Fig. 2.
Characterization of optical-absorption-induced hot bath. (a) Schematic showing interactions of the acoustic resonator with various baths con-
sidered in our heating model. (b) Schematic of measurement setup for time-resolved measurements of the hot bath using single-photon counting on the
optical sideband generated by thermal motion of the acoustic resonator. (c) Measurement of the transient thermal occupation of the acoustic resonator,
n
?
m
,
in response to a rectangular optical pulse on resonance with the optical cavity (pulse duration
τ
d
=
50
μ
s, repetition rate,
R
=
1 kHz, and peak intracavity
photon occupation,
n
c
=
385). The black line represents an exponential fit to the observed data with the characteristic rate,
γ
p
+
γ
0
, and steady-state ther-
mal occupation,
n
m
. Here
γ
0
is the intrinsic damping rate of the acoustic resonator and
γ
p
is the coupling rate to the optical-absorption-induced hot bath.
(d) Thermal occupation of the hot bath,
n
p
estimated from measurements of
n
m
performed at varying optical power, shown on the
x
-axis in units of peak
intra-cavity photon occupation,
n
c
. For comparison,
n
p
curves for 1D-OMC [3], butt-coupled 2D-OMC [11] are shown. (e) Variation of
γ
p
/
2
π
with
n
c
.
The data point marked with an arrow in (d) and (e) corresponds to the data in (c) for
n
c
=
385.
well below 100 mK [3]. In our study, we model this connection to
the cold substrate by a coupling to a cold bath with occupancy
n
f
(
<
10
3
), at an intrinsic acoustic damping rate,
γ
0
, as shown in
Fig. 2(a). Under excitation with laser fields, optical-absorption-
induced heating from the optical cavity and coupling waveguide
is modeled by considering a hot bath at a thermal occupation,
n
p
(corresponding to a bath temperature,
T
p
), coupled to the acoustic
mode at a rate
γ
p
. The acoustic resonator experiences optome-
chanical backaction at a rate
γ
OM
. We operate our devices in a
low-power optical regime where radiation pressure shot noise is
negligible. When the laser is tuned to the red motional sideband of
the optical cavity (detuning
1
=−

m
), a parametric beamsplitter
interaction allows us to operate the device as a linear, bi-directional
converter between quantum states in the acoustic and optical
modes at a rate
γ
OM
=
4
g
2
OM
n
c
t
, where
n
c
is the intracavity
photon occupation and
κ
t
is the total linewidth of the optical
resonance [14].
As a first step towards characterizing the optical-absorption-
induced hot bath we operate the laser on resonance with the optical
cavity (
1
=
0). Under this detuning condition the electric field
amplitude in the waveguide is negligible compared to the cavity,
and the optomechanical backaction rate is zero. This enables
us to isolate optical absorption within the cavity alone without
the effects of heating due to optical absorption in the waveguide
or cooling due to optomechanical backaction. We measure the
heating dynamics of the acoustic mode by detecting optical pho-
tons scattered from the laser field onto a motional sideband of
the optomechanical cavity. In steady state, the occupation of the
acoustic mode,
n
m
, is expected to be an average of the thermal
occupations of the hot and the cold baths, weighted by the cou-
pling rates of the acoustic mode to the respective baths, as given by
the relation
n
m
=
γ
p
n
p
+
γ
0
n
f
γ
p
+
γ
0
.
(1)
Figure 2(b) shows the schematic of the measurement setup
for
n
m
. We send laser pulses with pulse duration
τ
d
=
50
μ
s at
a repetition rate
R
=
1 kHz to the device in the dilution refrig-
erator via a circulator. The optical signal reflected from the
device is directed to a Fabry–Perot filter setup that suppresses
the pump pulses and transmits photons generated on the motional
sideband of the optomechanical cavity to a superconducting
nanowire single-photon detector. Figure 2(c) shows the time-
dependent occupation of the acoustic mode measured from
heating induced by a square laser pulse with a peak power corre-
sponding to
n
c
=
385. The rate of increase in the occupation is
used to infer
γ
p
+
γ
0
, whereas the steady state occupation is used to
infer
n
m
. Finally, the value of
γ
0
is measured independently from
ringdown measurements [3], thereby allowing us to extract the
parameters
γ
p
and
n
p
of the hot bath from Eq. (1).
Figure 2(d) shows
n
p
as a function of
n
c
for devices I and II along
with results from similar measurements performed previously on
1D-OMCs [3], and butt-coupled 2D-OMCs [11]. The vertical
axis on the right represents the corresponding bath temperature
T
p
. The solid lines around the experimental data points for devices
I and II are fits to the power law
n
p
=
An
k
c
. We find that the fits to
the hot bath occupancies for devices I and II are 2.2
×
n
0.31
c
and
2.9
×
n
0.21
c
, respectively. For comparison, we have also shown the
fits to
n
p
(
n
c
)
for previously measured butt-coupled 2D-OMC
(purple solid line) and 1D-OMC (green solid line) devices, corre-
sponding to 1.1
×
n
0.3
c
and 7.94
×
n
0.33
c
, respectively. As expected,
the thermal conductance seems to be highest for the butt-coupled
2D-OMC, slightly lower for the side-coupled 2D-OMC devices
that are restricted to approximately the half-plane, and lowest
Research Article
Vol. 12, No. 1 / January 2025 /
Optica
102
for the 1D-OMCs. Of note, the power law of the side-coupled
device II of this work (
k
=
0.21) deviates from other measured
1D and 2D-OMCs, and does not seem to follow from the simple
phonon-bottleneck model of Ref. [3]. This suggests there may be
more to the underlying conductance of heat from these structures.
While a systematic study of this effect is beyond the scope of this
article, we note that previous studies on phononic crystal structures
have observed varying power laws for thermal conductance [15],
and disorder-dependent thermal conductance [16,17], which may
offer insights into the observed
n
p
power laws in this work.
Figure 2(e) shows the variation of
γ
p
with
n
c
. For device
II, we find the power-law
γ
p
/
2
π
=
4.3
(
kHz
)
×
n
0.39
c
when
n
c
<
1000 and
γ
p
/
2
π
=
8.25
(
kHz
)
×
n
0.29
c
when
n
c
>
1000.
The power-law exponent of 0.39 in the regime of low optical power
is significantly different from 0.6 and 0.66 observed previously
for butt-coupled 2D-OMC and 1D-OMC devices, respectively.
This reduction can be explained by the reduced
n
p
power expo-
nent of 0.21, together with a two-dimensional phonon bath (see
Supplement 1, Section 10). For device I, in the regime of high
optical power with
n
c
>
1000,
γ
p
/
2
π
=
8.7
(
kHz
)
×
n
0.29
c
. For
n
c
<
1000, the measurement error on
γ
p
for device I is larger due to
a higher intrinsic damping rate,
γ
0
, thereby reducing the reliability
of a fit in this regime. We observe that the power-law exponent for
γ
p
in the regime of high optical power is identical for devices I and
II in this study, and also in close agreement with the butt-coupling
geometry.
4. PHONON-TO-PHOTON TRANSDUCTION UNDER
CONTINUOUS-WAVE LASER EXCITATION
After characterizing the optical-absorption-induced hot bath, we
test the device with the laser tuned to the red motional sideband
of the optical cavity (
1
=−

m
), relevant for phonon-photon
transduction. The thermal occupation of the acoustic mode,
n
th
,
due to optical-absorption-induced heating adds finite noise to any
transduced signal. Other performance metrics of interest for such a
transducer are the conversion efficiency,
η
OM
, and bandwidth,
γ
m
.
In continuous-wave operation, the conversion efficiency is given
by
η
OM
=
γ
OM
γ
OM
+
γ
0
+
γ
p
. We calibrate
γ
OM
for different
n
c
using
electromagnetically induced transparency [18] [see Supplement 1,
Fig. S3(a) for details]. Due to the low
γ
0
in our devices, we expect
high conversion efficiency for modest values of
n
c
. Specifically,
operation at
n
c
13 and 3 is expected to yield
η
OM
50% in
devices I and II, respectively. The bandwidth of the transducer
is limited by the total acoustic linewidth, which is dominated by
the optomechanical backaction
γ
m
γ
OM
in the regime of high
conversion efficiency.
To characterize the transducer-added noise under continuous-
wave laser excitation, we use the measurement setup shown
schematically in Fig. 3(a), where we perform single-photon count-
ing at the optical cavity resonance frequency. The data in Figs. 3(b)
and 3(c) show
n
th
measured with varying
n
c
for devices I and II,
respectively. In the high-power regime, specifically for
n
c
>
500,
the
n
th
data has been corrected for the limited bandwidth of the
sideband filter when the acoustic bandwidth
γ
m
starts to approach
the sideband filter linewidth (see Supplement 1, Section 3 for
details). The top
x
-axis displays the corresponding
γ
OM
for side-
coupled devices. The green and purple curves in these panels
show results from similar measurements of thermal occupancy
of the acoustic mode performed previously on 1D-OMCs [3]
and butt-coupled 2D-OMCs [11]. We observe that side-coupled
2D-OMCs allow for lower thermal noise across a wide range of
input optical powers. A minimum
n
th
of 0.28
±
0.01 is achieved
for device I at
n
c
=
2030, corresponding to an optomechanical
transduction efficiency of 98.6
±
0.2%, and a bandwidth of
γ
OM
/
2
π
6 MHz.
We model the thermal occupancy
n
th
using the heating model
introduced in the previous section. However, in contrast with the
measurements with
1
=
0 in the previous section, the majority
of the incident power is reflected under detuned operation with
1
=−

m
, and parasitic optical absorption in the coupling wave-
guide can contribute substantially to the hot bath. To include this
(b)
(a)
(d)
(c)
Fig.3.
Phonon-to-photon transduction under continuous-wave excitation. (a) Schematic of measurement setup showing single-photon counting of up-
converted photons at the optical resonance frequency with the OMC pumped continuously on the red-detuned sideband (
1
=−

m
) of the optical reso-
nance. Measured thermal phonon occupancy,
n
th
, with varying optical power, shown on the bottom
x
-axis in units of intra-cavity photon occupation,
n
c
,
and on the top-axis in units of optomechanical transduction rate,
γ
OM
. Results are shown on separate charts for (b) device I, and (c) device II. Filled circles
are data points whereas the solid line indicated with
β
=
0
μ
w
1
is the modeled
n
th
dependence using Eq. (2). For comparison, model curves are shown for
a butt-coupled 2D-OMC (
β
=
15
μ
W
1
) [11], and 1D-OMC [3]. Dashed lines indicate the
n
c
value for optomechanical transduction efficiency
η
OM
=
50% and 90%. For
n
c
=
1, on-chip input powers for devices I and II are
P
in
=
0.20
μ
W and 0.28
μ
W, respectively. (d) Estimated heating rate of the acous-
tic resonator
γ
p
n
p
/
2
π
as a function of
n
c
under
1
=−

m
for different devices plotted for their measured value of
β
.
Research Article
Vol. 12, No. 1 / January 2025 /
Optica
103
effect in our heating model, we define an effective photon occupa-
tion associated with the coupling waveguide,
n
wg
, varying linearly
with the input power as
n
wg
=
β
P
in
for some fixed constant,
β
, and
the input power,
P
in
. We then add the contributions from both
cavity and waveguide components and define the parameters of the
modified hot bath as
n
p
[
n
c
]→
n
p
[
n
c
+
β
P
in
]
, and similarly for
γ
p
. The thermal occupation of the acoustic mode is then given by
n
th
=
γ
p
[
n
c
]
n
p
[
n
c
]+
γ
0
n
f
γ
p
[
n
c
]+
γ
0
+
γ
OM
[
n
c
]
.
(2)
For small
n
c
, Eq. (2) is dominated by heating from the hot bath
γ
p
n
p
and exhibits an increase in
n
th
with increasing
n
c
. Conversely
for large
n
c
,
n
th
decreases with increasing
n
c
as a consequence of
backaction cooling
γ
OM
. The turnaround point between these
two regimes is influenced by parasitic optical absorption in the
coupling waveguide. For example, the solid blue and orange curves
in Figs. 3(b) and 3(c) show the predicted thermal phonon occu-
pancy if the waveguide-heating contribution,
β
, were set to zero.
We see that the experimental data from both devices I and II is in
reasonable agreement with the
β
=
0 curves. This indicates that
waveguide-related heating is negligible in side-coupled 2D-OMC
geometry. In comparison, for the butt-coupled 2D-OMCs,
β
was
measured to be 15
μ
W
1
[11].
While
n
th
in Eq. (2) depends on the optomechanical device
parameters
γ
0
and
γ
OM
, improvements purely based on geometric
modifications of the device platform under detuned operation
1
=−

m
can be studied using the heating rate
γ
p
n
p
/
2
π
[
n
c
]
,
and is plotted in Fig. 3(d) for various devices. For 1D-OMC, the
heating rate scales linearly (
γ
p
n
p
n
c
), similar to the cooling rate
(
γ
OM
n
c
), which results in the saturation of
n
th
to a few phonon
levels as
n
c
increases. 2D geometries are expected to have a lower
magnitude of the heating rate due to larger thermal contact with
the cold bath. However, due to a large
β
=
15
μ
W
1
, the butt-
coupling design exhibits almost identical heating performance
(
γ
p
n
p
n
0.9
c
) to that of 1D-OMC for
n
c
<
300, and only becomes
sublinear for
n
c
>
300 (
γ
p
n
p
n
0.6
c
). In contrast, for side-coupled
geometries, the heating rate scales sub-linearly (
γ
p
n
p
n
0.6
c
) for
all
n
c
range measured and has a lower magnitude. Specifically, for
device II, the heating rate is reduced by approximately six-fold
compared to the butt-coupling design for
n
c
>
300.
5. PHONON-TO-PHOTON TRANSDUCTION UNDER
PULSED LASER EXCITATION
Pulsed transduction schemes are often preferred due to the delayed
heating response of the acoustic resonator [9], which allows for the
initialization of the optomechanical transduction pulse prior to the
onset of heating. We characterize the performance of side-coupled
2D-OMC as a transducer in pulsed mode by sending short rectan-
gular pulses on the red sideband. A pulse duration of
τ
d
=
500 ns is
selected to account for the finite rise time of the sideband filters (
200 ns). The transduction efficiency in the pulsed mode is given by
η
OM
=
γ
OM
γ
OM
+
γ
0
+
γ
p
(
1
e
0
+
γ
p
+
γ
OM
d
)
.
(3)
Figure 4 shows the internal transduction noise
̄
n
for device II in
a pulsed scheme along with similar measurements performed pre-
viously on 1D-OMC [3] and butt-coupling design [11] at
n
c
=
10.
The
n
c
value corresponding to the optomechanical conversion
efficiency
η
OM
=
10%, 50%, and 90% for device II, are indicated
Fig. 4.
Phonon-to-photon transduction under pulsed laser excitation.
Internal added noise,
̄
n
(red data points), as a function of the peak intra-
cavity pump photon number,
n
c
. All noise measurements are performed
on device II with rectangular optical pump pulses with a pulse width
of 500 ns at a repetition rate of 250 Hz. For comparison,
̄
n
is shown for
butt-coupling design [11], and 1D-OMC [3]. Dashed lines indicate
the
n
c
value for transduction efficiency
η
OM
=
10%, 50%, and 90% for
device II. For
n
c
=
1, on-chip input power is
P
in
=
0.28
μ
W.
with dashed lines. Given the low intrinsic linewidth of the acoustic
resonator for this device (
γ
0
/
2
π
=
0.97 kHz), we used a repetition
rate of 250 Hz to allow the acoustic mode to sufficiently ther-
malize to the cold bath between successive optical pulses. For the
highest optical power we could send to the device (
n
c
=
444), we
measured
̄
n
=
0.25
±
0.01, which corresponds to a transduction
efficiency of
η
OM
=
93.1
±
0.8%. This constitutes a significant
enhancement across transduction metrics over 1D-OMC with
(
n
c
,
̄
n
OM
)
'
(
10
,
0.4
,
4%
)
[3].
6. DISCUSSION
The side-coupled 2D-OMC design presented here reduces optical-
absorption-induced thermal noise in OMCs, a major obstacle in
quantum application requiring operation at millikelvin tempera-
tures. We anticipate that further reduction in thermal noise can be
achieved through surface passivation techniques [19], potentially
minimizing optical absorption via surface defect reduction.
This result unlocks new possibilities for more quantum-
coherent applications using OMCs. The compact form factor
and long-lived acoustic mode of such devices are attractive for
applications such as quantum memory for telecom photons [4].
While we achieved a low intrinsic decay rate of 0.97 kHz for the
acoustic resonator, we anticipate that embedding the 2D-OMC in
a cross phononic shield may lead to drastically lower intrinsic decay
rates [3,11]. The low thermal occupation of the acoustic resonator
presents an opportunity to investigate two-level systems (TLS) in
amorphous solids [3,20,21], further expanding the potential of
OMCs in quantum technologies.
The improved performance of our side-coupled 2D-OMC
design in pulsed operation promises significant advancements in
single-photon heralding and remote entanglement generation.
For a realistic pulse repetition rate of 10 kHz and a total detection
efficiency of 5%, we estimate a single-photon heralding rate of
465 Hz, a substantial increase compared to the 20 Hz achievable
with 1D-OMCs. Furthermore, in a two-node remote entangle-
ment experiment, we project a photon coincidence rate of 21 Hz,
representing a
500-fold enhancement over the 0.04 Hz rate
achievable with 1D-OMCs.
Research Article
Vol. 12, No. 1 / January 2025 /
Optica
104
In the context of microwave-to-optical quantum trans-
duction, side-coupled 2D-OMC could be integrated into
piezo-optomechanical transducers by designing a piezo-acoustic
cavity that couples the electrical circuit to the optomechanical
circuit through a phononic waveguide [5]. Such transducers offer
the potential for large-bandwidth, low-noise, and near-unity
efficiency conversion between microwave and optical signals.
While this study has focused on improved thermal handling in
silicon-based devices, piezo-optomechanical transducers uti-
lize heterogeneously integrated platforms with optically robust
microwave resonators [6–8], potentially involving distinct heating
mechanisms. Nevertheless, our findings establish a performance
benchmark for microwave-to-optical transduction. By connect-
ing such a transducer to an off-chip qubit module [22–24], high
optical powers can be employed without compromising super-
conducting qubit coherence. Additionally, techniques like atomic
force microscope (AFM) nano-oxidation can be used to precisely
match frequencies between remote piezo-optomechanical systems
[25], paving the way for optically mediated remote entanglement
of superconducting qubit nodes.
Funding.
Amazon Web Services; National Science Foundation (PHY-
1125565); Army Research Office (W911NF-18-1-0103); U.S. Department
of Energy Office of Science National Quantum Information Science Research
Centers (Q-NEXT, DE-AC02-06CH11357); Institute for Quantum Information
and Matter (IQIM); Gordon and Betty Moore Foundation; Kavli Nanoscience
Institute at Caltech; AWS Center for Quantum Computing.
Acknowledgment.
The authors thank Matthew Shaw and Boris Korzh
for providing single photon detectors in this work. We thank Piero Chiappina
for helpful discussions. S.M. acknowledges support from the IQIM Postdoctoral
Fellowship. Amazon Web Services (AWS) provided partial funding support for this
work through a sponsored research grant.
Disclosures.
O.P. is currently employed by AWS as Director of their quan-
tum hardware program.
Data availability.
Data underlying the results presented in this paper are not
publicly available at this time but may be obtained from the corresponding author
upon reasonable request.
Supplemental document.
See Supplement 1 for supporting content.
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