Nanophotonic quantum storage at telecommunications
wavelength
Ioana Craiciu,
1, 2
Mi Lei,
1, 2
Jake Rochman,
1, 2
Jonathan M.
Kindem,
1, 2
John G. Bartholomew,
1, 2
Evan Miyazono,
1, 2
Tian Zhong,
1, 2,
∗
Neil Sinclair,
3, 4
and Andrei Faraon
1, 2,
†
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
Division of Physics, Mathematics and Astronomy,
California Institute of Technology, Pasadena, California 91125, USA
4
Alliance for Quantum Technologies,
California Institute of Technology, Pasadena, California 91125, USA
(Dated: April 18, 2019)
Abstract
Quantum memories for light are important components for future long distance quantum net-
works. We present on-chip quantum storage of telecommunications band light at the single photon
level in an ensemble of erbium-167 ions in an yttrium orthosilicate photonic crystal nanobeam
resonator. Storage times of up to 10
μ
s are demonstrated using an all-optical atomic frequency
comb protocol in a dilution refrigerator under a magnetic field of 380 mT. We show this quantum
storage platform to have high bandwidth, high fidelity, and multimode capacity, and we outline a
path towards an efficient erbium-167 quantum memory for light.
∗
Currently at: Institute of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA
†
Corresponding author: faraon@caltech.edu
1
arXiv:1904.08052v1 [quant-ph] 17 Apr 2019
Optical quantum memories can aid processes involving the transfer of quantum informa-
tion via photons, with applications in long distance quantum communication and quantum
information processing [1–5]. Rare-earth ions in crystals are a promising solid-state platform
for optical quantum memories due to their long-lived optical and spin transitions that are
highly coherent at cryogenic temperatures [6, 7]. Among rare-earth ions, only erbium has
been shown to possess highly coherent optical transitions in the telecommunications C-band,
which allows for integration of memory systems with low loss optical fibers and integrated
silicon photonics [8].
Fixed delay quantum storage for less than 50 ns at telecommunications wavelengths has
been demonstrated in erbium-doped fibers [9], and lithium niobate waveguides [10] at ef-
ficiencies approaching 1%. The protocol used in both cases, the atomic frequency comb
(AFC), requires spectrally selective optical pumping [11]. The storage efficiencies in these
works were limited in part by the lack of suitable long-lived shelving states in the erbium ions
in these hosts. Moving to isotopically purified erbium-167 in a yttrium orthosilicate host
(YSO) offers the prospect of long-lived shelving states in the form of hyperfine levels [12].
While optical storage has been realized in erbium-doped YSO [13–15], including efficiencies
approaching 50% at storage times of 16
μ
s (revival of silenced echo protocol [15]) quantum
storage has yet to be demonstrated in this material.
In this work, we demonstrate on-chip quantum storage of telecommunications light at
the single photon level. We used a nanophotonic crystal cavity milled directly in
167
Er
3+
doped YSO (
167
Er
3+
:YSO) to couple to an ensemble of erbium ions and realize quantum
storage using the AFC protocol [11]. The cavity increased the absorption of light by the ion
ensemble, allowing on-chip implementation of the memory protocol [16]. By working in a
dilution refrigerator and using permanent magnets to apply a field of 380 mT, we accessed a
regime in which the ions have optical coherence times of
∼
150
μ
s and long lived spin states
to allow spectral tailoring. For the shortest measured storage time of 165 ns, we achieved
an efficiency of 0
.
2%, with lower efficiencies for longer storage times, up to 10
μ
s. We
demonstrated storage of multiple temporal modes and measured a high fidelity of storage,
exceeding the classical limit. Lastly, we identified the limits on the storage efficiency and
proposed avenues for overcoming them to achieve an efficient
167
Er
3+
quantum memory for
light.
Memories using spectral tailoring such as the AFC protocol require a long-lived level
2
within the optical ground state manifold, where population can be shelved. Hyperfine levels
in the optical ground state in
167
Er
3+
:YSO have been shown to have long lifetimes at 1.4 K
and a magnetic field of 7 T[12]. In general, these levels can be long-lived when the erbium
electron spin is frozen, which occurs when
~
ω
e
k
B
T
, where
ω
e
is the electron Zeeman
splitting [12]. In this work, we satisfied this inequality by using a moderate magnetic field
of 380 mT parallel to the D
1
axis of the crystal (
ω
e
= 2
π
×
80 GHz) and a nanobeam
temperature of
∼
400 mK. The nanobeam temperature was estimated via the electron spin
population at a lower magnetic field [17, 18]. The hyperfine lifetime in the bulk crystal was
measured to be 29 minutes, enabling the long-lived, spectrally selective optical pumping
required for the AFC protocol (see supplementary material [17]).
Figures 1a-c describe the nanoresonator used in this experiment. A triangular nanobeam
photonic crystal cavity [19] was milled in a YSO crystal doped with isotopically purified
167
Er
3+
(92% purity) at a nominal concentration of 50 ppm. The width of the nanobeam
was 1
.
5
μ
m, and the length
∼
20
μ
m. The slots in the nanobeam created a photonic crystal
bandgap, and the periodic pattern (lattice constant = 590 nm, groove width = 450 nm) was
modified quadratically in the center to create a cavity mode. Figure 1a shows a scanning
electron micrograph of the nanobeam and Fig
.
1b shows a finite element analysis simulation
of the TM cavity mode.
The coherence time of the 1539 nm optical transition, which provides an upper bound
on all-optical storage time, was measured to be 149
μ
s
±
4
μ
s in the nanobeam. The bulk
optical coherence time of this transition under similar cooling conditions was measured to
be 760
μ
s
±
41
μ
s. The reduction in coherence time as measured in the nanobeam is likely
caused by a combination of higher temperature in the nanobeam during measurement and
the impact of the focused ion beam milling process [17]. We note that the fabrication
method has not significantly impacted the coherence properties of ions in similar devices
[20, 21], although the bulk crystal coherence times measured in those works were also lower,
preventing a direct comparison. The optical coherence time did not limit the storage time
achieved in this work.
Figure 1d shows a schematic of the optical testing setup. Figure 1c shows the reflection
spectrum of the nanobeam cavity, which has a measured loaded quality factor of 7
×
10
3
.
The cavity was tuned onto resonance with the 1539 nm transition of the
167
Er
3+
ions by
freezing nitrogen gas onto the nanobeam at cryogenic temperatures [22]. The coupling of
3