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Quantum transduction of optical photons from a superconducting qubit
Mohammad Mirhosseini,
1, 2,
Alp Sipahigil,
1, 2,
Mahmoud Kalaee,
1, 2, 3,
and Oskar Painter
1, 2, 3,
1
Kavli Nanoscience Institute and Thomas J. Watson, Sr., Laboratory of Applied Physics,
California Institute of Technology, Pasadena, California 91125, USA.
2
Institute for Quantum Information and Matter,
California Institute of Technology, Pasadena, California 91125, USA.
3
AWS Center for Quantum Computing, Pasadena, California 91125, USA.
(Dated: April 13, 2020)
Bidirectional conversion of electrical and optical
signals lies at the foundation of the global inter-
net. Such converters are employed at repeater
stations to extend the reach of long-haul fiber op-
tic communication systems and within data cen-
ters to exchange high-speed optical signals be-
tween computers. Likewise, coherent microwave-
to-optical conversion of single photons would en-
able the exchange of quantum states between re-
motely connected superconducting quantum pro-
cessors, a promising quantum computing hard-
ware platform [1]. Despite the prospects of quan-
tum networking [2], maintaining the fragile quan-
tum state in such a conversion process with su-
perconducting qubits has remained elusive. Here
we demonstrate the conversion of a microwave-
frequency excitation of a superconducting trans-
mon qubit into an optical photon. We achieve
this using an intermediary nanomechanical res-
onator which converts the electrical excitation
of the qubit into a single phonon by means of
a piezoelectric interaction [3], and subsequently
converts the phonon to an optical photon via
radiation pressure [4]. We demonstrate optical
photon generation from the qubit with a signal-
to-noise greater than unity by recording quan-
tum Rabi oscillations of the qubit through single-
photon detection of the emitted light over an op-
tical fiber. With proposed improvements in the
device and external measurement set-up, such
quantum transducers may lead to practical de-
vices capable of realizing new hybrid quantum
networks [2, 5], and ultimately, distributed quan-
tum computers [6, 7].
Recent developments with superconducting qubits
have demonstrated fast, high fidelity single- and two-
qubit gates, making them one of the leading platforms
for realizing quantum computers [1]. While the low-loss
environment of a superconductor and the strong single-
photon nonlinearity from the Josephson effect provide an
ideal combination for processing quantum information in
the microwave domain [8], optical photons are a natu-
ral choice for quantum networking tasks [9] where they
These authors contributed equally to this work.
opainter@caltech.edu; http://copilot.caltech.edu
provide low propagation loss in room-temperature envi-
ronments [10]. A coherent microwave-to-optical interface
can thus lead to hybrid architectures for quantum re-
peaters [2, 5] by connecting superconducting qubits and
ultra high-Q microwave cavities [11]– serving as logic and
memory registers– to flying optical qubits as a means of
long-distance information transfer. Although the process
of frequency conversion can be understood simply as a
noise-free and loss-less linear operation, an optical inter-
face for superconducting qubits has not been realized to
this date because of the technical challenges inherently
set by the vast frequency difference of microwave (
5
GHz) and telecom-band optical (
200 THz) photons.
Microwave-to-optical frequency conversion can be
achieved by bulk optical nonlinearities [12]. Alterna-
tively, effective nonlinearities can be realized by inter-
mediary degrees of freedom such as rare earth ions,
magnons, or phonons [13–15] that exhibit simultane-
ous electrical and optical susceptibilities. Employing en-
gineered nanomechanical resonators as such intermedi-
ary channels has been a particularly promising direc-
tion, where the pioneering work with integrated elec-
tromechanical and optomechanical systems in the past
decade has demonstrated electrical and optical prepa-
ration, control, and readout of mechanical modes near
their quantum ground state [3, 16, 17]. These demonstra-
tions, together with rapid developments in superconduct-
ing quantum circuits [8], have motivated recent experi-
mental efforts to combine electromechanical and optome-
chanical devices to build a microwave-to-optical quantum
transducer [18–22]. Although this approach has led to
impressive conversion efficiencies [23], all demonstrations
so far have been limited to classical signals due to a com-
bination of challenges associated with optically-induced
or thermal noise, small transduction bandwidths, and de-
vice integration complexities.
Here, we demonstrate optical photon generation from a
superconducting qubit and detection over an optical fiber
link. This is achieved using a monolithic platform that
integrates a transmon qubit with piezo-optomechanical
transducer components on the same microchip. In con-
trast to earlier work based on continuous wave linear
transducers, we use a pulsed scheme where we coherently
transfer the quantum state of a qubit into a nanome-
chanical mode via a piezoelectric swap operation, and
subsequently convert it to the optical domain using a
pulsed laser drive. This pulsed approach crucially sepa-
arXiv:2004.04838v1 [quant-ph] 9 Apr 2020
2
AlN mechanical mode
phonon waveguide
Si mechanical mode
a
b
c
5.25
0
800
0
4
5
d
g
pe
SC Qubit
mechanical resonator
optical resonator
G
om
= (
n
c
)
1/2
g
om
κ
m
κ
e,o
optical ber
5.25
5
e
CPW
RO res.
SC
qubit
transd.
Si optical WG
SQUID loop
tapered optical coupler
OMC
cavity
p-a
cavity
500
μ
m
qubit
RO res.
κ
e,q
κ
RO
g
RO
OMC cavity
piezo-optomechanical transducer
p-a
cavity
CPW
RO res. meander
optical mode
g
om
a
o
a
in
a
out
b
m
ω
m
/2
π
(GHz)
ω
m
/2
π
(GHz)
g
om
/2
π
(kHz)
g
pe
/2
π
(MHz)
FIG. 1.
Quantum transducer setup. a
, Schematic of the microwave-to-optical transduction process. The mechanical mode
(
ˆ
b
m
) couples to both the superconducting qubit (ˆ
σ
ge
=
|
g
〉〈
e
|
) and an optical mode (ˆ
a
o
) via piezoelectric (
g
pe
) and optome-
chanical (
g
om
) vacuum coupling rates, respectively.
b
, Numerically simulated resonant modes of the piezo-optomechanical
transducer.
c
, Simulated vacuum optomechanical (top) and piezoelectric (bottom) coupling rates to the hybridized mechanical
modes.
d
, Electrical circuit representation of the integrated qubit and transducer device.
e
, Optical micrograph of a pair
of fabricated devices, showing the readout resonator (green), transmon qubit (blue), transducer element (purple), and silicon
out-coupling waveguide (red). Corresponding zoomed in optical images of the different device sections are shown to the left
and right, with white scale bar corresponding to 10 microns. Electrical and optical readout is performed in reflection via the
CPW and a lensed optical fiber (not shown; optically coupled to the tapered silicon waveguide coupler), respectively. Figure
labels: Josephson-junction superconducting quantum interference device (SQUID); read-out resonator (RO res.); supercon-
ducting transmon qubit (SC qubit); optomechanical crystal cavity (OMC cavity); piezo-acoustic (
p
-
a
) cavity; silicon optical
waveguide (Si optical WG).
rates the electric and optical parts of the sequence, mak-
ing the experiment robust against light-induced noise in
the superconducting circuitry, and eliminates the need
for matching the interaction bandwidths (i.e. impedance
matching) on the microwave and optical sides [24].
Our device is based on a hybrid materials platform
consisting of thin-film aluminum nitride (AlN) sputter
deposited on a silicon-on-insulator wafer with a high re-
sistivity silicon (Si) device layer; this materials system
simultaneously provides for large piezoelectric and op-
tomechanical coupling coefficients and low electromag-
netic absorption in the microwave and (telecom) optical
frequency bands [4]. Using this integrated device plat-
form, we demonstrate the detection of single optical pho-
tons from a transmon qubit, and use them to register
quantum Rabi oscillations of the qubit via an entirely
optical measurement. We use the transmon as a sin-
gle photon source to directly quantify the overall added
noise photons in the process, finding a value (0
.
57
±
0
.
2)
below unity, which surpasses the threshold for faithful ex-
change of quantum information [25]. We discuss practical
avenues for achieving reduced noise and orders of magni-
tude improvement in the total system efficiency (10
5
),
and outline a clear path to future experiments involving
the remote entanglement of superconducting qubits via
optical photons.
Figure 1a shows the schematic of the system in our
experiment, where an intermediary mechanical mode is