of 2
FM1A.7.pdf
CLEO 2019 © OSA 2019
Toward Microwave
-
to
-
Optical Conversion using Erbium
Doped Crystals
and Integrated Resonators
Jake Rochman,
1,2
John G. Bartholomew,
1,2
Ioana Cra
i
ciu,
1,2
Chuting Wang,
1,2
Tian Xie,
1,2
Jonathan M.
Kindem,
1,2
Keith Schwab,
1,2
Andrei Faraon
1,2
1
Kavli Nanoscience Institute and Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pas
adena, California
91125, USA
2
Institute for Quantum Information and Matter, California Institute of Technolog
y, Pasadena, California 91125, USA
jrochman@caltech.edu
Abstract:
We present
progress towards
a
bidirectional coherent
microwave
-
to
-
optical photon
converter using
an ensemble of rare
-
earth ions
coupled to
integrated photonic and microwave
resonators
.
OCIS codes:
160.5690
,
270.1670
,
350.4238
.
1. Introduction
Future quantum networks will benefit from an optical interconnect to
entangle distant superconducting circuits via
optical photons.
An e
nsemble of rare
-
earth ions (REIs)
simultaneously
coupled to
an
optical and microwave cavit
y
at
dilution
refrigerator
temperatures
offer
s
a promising
system
to achieve bidirectional coherent
conversion between
microwave
and
optical
photons
.
Conversion
can be realized by
utilizing isolated three level systems within the energy
structure of the REIs, where the coherence of the ensemble generated from incoming
m
icrowave (optical)
photons
with frequency
ω
(
ω
)
can be mapped to
optical (microwave)
photons using an optical pump with Rabi
frequency,
Ω
, as shown in Figure 1a.
A
high
-
efficiency conver
ter
using REI ensembles
can be achieved
by using a Raman
heterodyne scheme w
hen the
system has
sufficiently large optical and microwave cooperativities,
mode
overlap and
optical
pump power
[
1
]
.
REI
ensembles
have
previously
been shown
to
have strong coupling to both optical and microwave fields independently
[
2
,
3
].
Here,
we present an integrated platform using
microwave and
o
ptical
resonators
fabricated on top of our REI doped
substrate
in order to optimize the mode overlap and cavity mode volumes to achieve high
-
efficiency conversion with
low optical pump power.
We show
prel
iminary
results of the platform fabricated on sapphire substrates to characterize
the performance of the resonators.
Figure 1: (a) An energy diagram of the microwave to optical converter. An incoming microwave photon at frequency
ω
induces a coherence
between
|
1
and
|
2
. This coherence is upconverted to the optical frequency
ω
using the pump with Rabi frequency
Ω
. (b)
A
n SEM image
of
the
optical and microwave resonators
.
(c) A
higher magnification image
of the inductive component
of the microwave resonator next to the
optical resonator
(rectangle indicated in (b))
. The two photonic crystal mirrors
of the optical resonator
are at the ends of the inductive component.
(d) A zoom in
image
of one of the photonic crystal mirrors located
at the ends of the cavit
y
(rectangle indicated in (c))
.
FM1A.7.pdf
CLEO 2019 © OSA 2019
2. Results
For our microwave to optical converter, t
he integrated platform is composed of a superconducting microwave
resonator and an amorphous silicon photonic resonator coupled to an erbium ensemble doped in
a
yttrium orthosilicate
(YSO) substrate.
In
order
to maximize the field overlap, we arrange the o
ptical cavity next to the inductive component
of the microwave cavity such that th
e two resonators
couple to a similar ensemble of ions.
Erbium’s optical transitions
are at telecom wavelengths (~15
4
0 nm)
, which enables the use of silicon photonics to
make
an
optical resonator on the YSO surface
for evanescent coupling to the REIs
[4]
.
The optical resonator consists
of a long waveguide
between
two photonic crystal mirrors
(Figure
1
c
)
.
The photonic crystal mirror
s
consist of
periodic
holes etched into the wa
veguide structure
(Figure
1
d)
.
G
ratings couplers
are used
to couple light from free
-
space
optics
.
The microwave resonator consists of an interdigitated capacitor and a
narrow
waveguide with
high
inductance.
Next to the high inductance waveguide
is a metal
-
free gap where the optical circuit can be positioned in close
proximity.
The size of the capacitor is tuned to reach a resonant frequency of ~5 GHz.
Coupling to the resonator is
achieved using a bus waveguide that capacitively couples to the res
onator.
In order to test the platform
and the fabrication process
, the resonators were first fabricated on a sapphire substrate,
which has similar permittivities to YSO at both optical and microwave frequencies and low losses.
Both resonators
are fabricat
ed on top of the substrate permitting the capability of transferring the process to different substrates.
Niobium films are first sputtered on the substrate and
patterned
via dry etching
with
a
SF
6
-
base
d
chemistry
. Next
,
amorphous silicon films are deposited with
plasma enhanced chemical vapor deposition
and
patterned
using with a
pseudo
-
B
osch process
.
The quality factors of the both the optical and microwave resonators were measured. The optical resonators were
mea
sured at room temperature using a confocal microscope and the microwave resonators were measured in dilution
fridge at <30 mK
using a network analyzer
. Optical resonances show quality factors
of ~
110,000 at a wavelength of
153
1
nm
(Figure 2
a
)
. Microwave re
sonances show
internal
quality factors
of
~
9
0,000 at a frequency of 5 GHz
(Figure
2
b
)
.
Figure 2: (a)
The transmission spectrum of
a high
-
Q mode of
the photonic crystal cavity. Resonance fitting of the under
-
coupled cavity gives a
total Q of
110,000.
(b)
The transmission spectrum of the
bus waveguide coupled to the
microwave resonator. Resonance fitting gives an internal
Q of 91,000.
3. Conclusion
Our platform enables resonators with high quality factors in both the optical and microwave domain, while maintaining
sufficient overlap between the optical and microwave fields required for the magneto
-
optical converter to achieve
high efficiency.
4. Re
ferences
[1]
.
L.A.
Williamson , Y.H. Chen, and J.J. Longdell,
Magneto
-
optic modulator with unit quantum efficiency.
Phys
.
Rev
.
Lett
.
, 2014.
113
(20): p.
203601.
[2]
. T.
Zhong, J.M. Kindem, J. Rochman, and A. Faraon,
Interfacing broadband photonic qubits to on
-
chip cavity
-
protected rare
-
earth
ensembles.
Nat. Comm., 2017.
8
: p. 14107.
[3]
. S.
Probst, H. Rotzinger, S. Wünsch, P. Jung, M. Jerger, M. Siegel, A.V. Ustinov, P.A. Bushev,
Anisotropic Rare
-
Earth Spin Ensemble
Strongly Coupled to a Superconducting Resonator
. Phys. Rev. Lett., 2013.
110
: p. 157001.
[4]
.
E.
Miyazono, I. Craiciu, A. Arbabi, T. Zhong, and A. Faraon,
Coupling erbium dopants in yttrium orthosilicate to silicon photonic resonators
and waveguides.
Optic
s Express, 2017.
25
(3): p. 2863
-
2871.