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Optically addressing single rare-earth ions in a nanophotonic cavity
Tian Zhong,
1, 2, 3
Jonathan M. Kindem,
2, 3
John G. Bartholomew,
2, 3
Jake Rochman,
2, 3
Ioana Craiciu,
2, 3
Varun
Verma,
4
Sae Woo Nam,
4
Francesco Marsili,
5
Matthew D. Shaw,
5
Andrew D. Beyer,
5
and Andrei Faraon
2, 3,
1
Institute of Molecular Engineering, University of Chicago, Chicago, IL 60637, USA
2
Kavli Nanoscience Institute and Thomas J. Watson, Sr., Laboratory of Applied Physics,
California Institute of Technology, Pasadena, California 91125, USA.
3
Institute for Quantum Information and Matter,
California Institute of Technology, Pasadena, California 91125, USA.
4
National Institute of Standards and Technology,
325 Broadway, MC 815.04, Boulder, Colorado 80305, USA
5
Jet Propulsion Laboratory, California Institute of Technology,
4800 Oak Grove Drive, Pasadena, California 91109, USA
(Dated: March 21, 2018)
Detection and control of single rare-earth dopants in solids is an important step towards quan-
tum devices that take full advantage of the outstanding coherence properties of rare-earth ions in
both optical and spin degrees of freedom. Coupling the 4f-4f transitions of ions to photonic res-
onators with highly confined optical modes provides an effective approach to overcome their weak
photoluminescence emission and poor photon collection efficiency, which have so far hindered the
experimental progress on optical isolation and control of single rare-earth emitters. Here we demon-
strate a nanophotonic platform based on a yttrium orthovanadate (YVO) photonic crystal nanobeam
resonator coupled to spectrally resolved individual neodymium (Nd
3+
) ions. The strong emission
enhancement in the nanocavity enables optical addressing of single Nd
3+
ions. The ions show near-
radiatively-limited single photon emissions. The measured high coupling strength between a single
photon and the ion allows for optical Rabi oscillations and a high coupling cooperativity, which
could enable optically controlled spin qubits, quantum logic gates, and spin-photon interfaces in
future quantum networks.
Rare-earth dopants in solids exhibit long-lived coher-
ence in both optical and spin degrees of freedom [1, 1].
The effective shielding of their 4f electrons leads to
optical and radio-frequency transitions with less sensi-
tivity to electronic and magnetic noise in their crys-
talline surroundings at cryogenic temperatures. Signif-
icant progress in rare-earth based quantum technologies
has led to ensemble-based optical quantum memories [3–
6] and coherent transducers [7], with promising perfor-
mance as efficient quantum light-matter interfaces for
quantum networks. On the other hand, addressing sin-
gle ions has remained an outstanding challenge, with the
progress hindered by long optical lifetimes of rare-earth
ions and resultant faint photoluminescence (PL). So far,
only a few experiments have succeeded in isolating indi-
vidual praseodymium [8–10], Cerium [11–13], and erbium
[14, 15] ions, though majority of them were not probing
ions via their highly coherent 4f-4f optical transitions.
Recently, several works have demonstrated significant en-
hancement of spontaneous emissions of rare-earth emit-
ters coupled to a nanophotonic cavity [6, 15–17], among
which [6, 16] also showed little detrimental effect on the
coherence properties of ions in nanodevices. These re-
sults point at a viable approach to efficiently detect and
coherently control individual ions in a chip-scale archi-
tecture.
Here we demonstrate a nanophotonic platform based
on a yttrium orthovanadate (YVO) photonic crystal
nanobeam resonator coupled to spectrally resolved in-
dividual neodymium (Nd) ions. While the system acts
as an ensemble quantum memory when operating at the
center of the inhomogeneous line [6], it also enables direct
optical addressing of single Nd
3+
in the tails of the inho-
mogeneous distribution, which show strongly enhanced,
near-radiatively-limited single photon emissions. A mea-
sured vacuum Rabi frequency of 2
π
×
28.5 MHz signifi-
cantly exceeds the linewidth of a Nd
3+
ion, potentially
allowing coherent manipulation of single spins with op-
tical pulses. Unlike prior experiments [8–13], this tech-
nique does not hinge on spectroscopic details of a specific
type of ion and can be readily extended to other rare-
earths or defect centers. It opens up new opportunities
of spectroscopy on single ions that are distinct from con-
ventional ensemble measurements, which offers a probe
for local nanoscopic environment around individual rare-
earth ions and may lead to new quantum information
processing, interconnect and sensing devices.
Our experiment builds upon a triangular nanobeam
photonic crystal resonator [16, 18] that was fabricated
in a nominally 50 parts per million (ppm) doped Nd:YVO
crystal using focused ion beam (FIB) milling [18]. The
device is a one-sided cavity, as the input (left mirror in
Fig.1(a,b)) has a lower reflectivity. The optical coupling
in/out of the device was implemented via a 45
-angled
coupler [16]. An aspheric doublet matches the mode of
the single mode fiber to that of the nanobeam waveguide
(Fig.1(a)). The coupling efficiency was optimized to 19%
(from fiber to waveguide) using a 3-axis nano-positioner.
arXiv:1803.07520v1 [quant-ph] 20 Mar 2018
2
-300
-200
-100
0
100
200
300
400
+340698 (GHz)
0
1
-60
-40
-20
0
20
40
GHz
10
15
20
25
30 35 40 45
+340703 (GHz)
10
2
10
3
PL counts
N
10
1
0.1
T = 20 mK
c
SNSPD
aspheric doublet
0
-1
1
a
b
b
c
κ
sc
κ
in
690 nm
4
I
9/2
4
F
3/2
1
2
4
3
c axis
a
b
B = 390 mT
60
1
4
1’
4’
a
b
c
d
resolved singles
FIG. 1: (a) Schematics of the experiment in a dilution refrig-
erator. Scale bar is 1
μ
m. (b) SEM images of the one-sided
nanobeam photonic crystal cavity in YVO fabricated using
FIB. Lower panel shows simulated the TM fundamental mode
profile, which has the polarization aligned to dipoles of Nd
3+
along the crystallographic c axis. (c) Cavity reflection spec-
trum (upper) and Nd
3+
photoluminescence (PL) spectrum
(lower). Insets show the applied magnetic field and resulting
Zeeman levels and transitions. PL from ions in the bulk sub-
strate (1’ and 4’) appear red-shifted from ions coupled to the
cavity (1 and 4). (d) Atomic spectra density versus detuning
on the shorter wavelength tail of the inhomogeneous distri-
bution. The shaded area shows the projected atomic shot
noise.
The nanocavity fundamental mode volume is
V
mode
=
0.056
μ
m
3
(simulated) with a measured quality factor
Q
= 3,900 (energy decay rate
κ
= 2
π
×
90 GHz). The
waveguide-cavity coupling
κ
in
through the input mirror
was 45% of
κ
. The device was cooled to
20 mK base
temperature in a dilution refrigerator, though the device
effective temperature was estimated to be around 500
mK (by comparing the ground Zeeman level populations
from the PL spectra). The elevated temperature was at-
tributed to the diminishing thermal conductance in the
nanobeam. The laser for probing the ions was modu-
lated by two double-pass acousto-optic (AOM) modula-
tors, and delivered to the sample via a single-mode fiber.
The reflected signal from the device was sent via a circu-
lator to a superconducting nanowire single photon detec-
tor (SNSPD) that measured a 82% detection efficiency
at 880 nm and
<
2 Hz dark counts [6]. The SNSPD
was mounted in the same fridge at the 100 mK stage.
The overall photon detection efficiency including trans-
mission from the cavity to the detector and the detection
efficiency was 3.6% (Supplementary material [19]).
A typical cavity reflection spectrum when it was tuned
nearly on resonance with the Nd
4
F
3
/
2
(
Y
1
)
4
I
9
/
2
(
Z
1
)
transition at 880 nm is shown in Fig.1(c). A 390 mT mag-
netic field was applied along the crystallographic a axis
of YVO, giving rise to split Zeeman levels and four possi-
ble optical transitions [3] (labelled 1-4) shown in the in-
set. Symmetry considerations impose that the 2, 3 cross
transitions are forbidden, and the 1, 4 transitions are
close to cyclic [4, 6]. The PL emission spectrum (with a
200-ns pulsed resonant laser excitation) is shown in the
lower part of Fig.1(c). Two weak lines labelled 1’ and 4’
were identified as emissions from Nd ions in the bulk sub-
strate, which are red-detuned from ions coupled to the
cavity by 2.5 GHz. This shift is due to a static strain in
the nanobeam, which makes it easier to spectrally sepa-
rate the ions in the cavity from the bulk. For subsequent
experiments, we focus on the shorter wavelength tail of
the inhomogeneous distribution. Figure 1(d) plots the
resonant PL against frequency detuning from the peak
of line 1 (340703.0 GHz). The PL and thus the atomic
spectral density (
N
ions per excitation pulse bandwidth)
fits with a power law of
N
2
.
9
, where ∆ is the de-
tuning from the center of line 1. The 2.9 power exponent
corresponds to a strain-induced broadening according to
[22]. Statistical fine structures (SFS) [23] were also ev-
ident. By fitting the SFS with the projected shot noise
of
N
(i.e.
N
indicated as the shaded area), it is pro-
jected that discrete single ion spectra (
N <
1) emerge at
a detuning
>
25 GHz.
To search for spectrally resolved single ions, we
scanned the center frequency of a 200-ns resonant ex-
citation pulse around
30 GHz blue detuning from the
peak of line 1, and measured the PL integrated over 5
μ
s after the excitation. The repetition rate of the ex-
citation pulses was 25 kHz, and the integration time
was 20 seconds at each frequency. The laser was fre-
quency stabilized to a vacuum can reference cavity at-
taining a narrowed linewidth of
<
5 kHz and a long term
drift
<
100 kHz/day. Figure 2(a) shows the measured PL
over a few GHz range. A handful of peaks, such as the
one with close-up shown in Fig.2(c), were possible sin-