Delft University of Technology
Quantum technologies with optically interfaced solid-state spins
Awschalom, David D.; Hanson, Ronald; Wrachtrup, Jörg; Zhou, Brian B.
DOI
10.1038/s41566-018-0232-2
Publication date
2018
Document Version
Accepted author manuscript
Published in
Nature Photonics
Citation (APA)
Awschalom, D. D., Hanson, R., Wrachtrup, J., & Zhou, B. B. (2018). Quantum technologies with optically
interfaced solid-state spins.
Nature Photonics
,
12
(9), 516-527. https://doi.org/10.1038/s41566-018-0232-2
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Quantum Technologies with Optically Interfaced Solid-State Spins 1
David D. Awschalom
1,2,†
, Ronald Hanson
3,4
, Jörg Wrachtrup
5,6
, Brian B. Zhou
1,*
2
1
Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA 3
2
Institute for Molecular Engineering and Materials Science Division, Argonne National 4
Laboratory, Argonne, Illinois 60439, USA 5
3
QuTech, Delft University of Technology, P. O. Box 5046, 2600 GA Delft, The Netherlands 6
4
Kavli Institute of Nanoscience, Delft University of Technology, P. O. Box 5046, 2600 GA Delft, 7
The Netherlands 8
5
Institute for Quantum Science and Technology IQST, and 3. Physikalisches Institut, University 9
of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany 10
6
Max Planck Institute for Solid State Research, Heisenbergstraße, 1 70569 Stuttgart, Germany 11
* Present address: Department of Physics, Boston College, Chestnut Hill, Massachusetts 12
02467, USA 13
†
email: awsch@uchicago.edu 14
15
Spins of impurities in solids provide a unique architecture to realize quantum 16
technologies. A quantum register of electron and nearby nuclear spins in the lattice 17
encompasses high-fidelity state manipulation and readout, long-lived quantum memory, 18
and long-distance transmission of quantum states by optical transitions that coherently 19
connect spins and photons. These features, combined with solid-state device engineering, 20
establish impurity spins as promising resources for quantum networks, information 21
processing, and sensing. Focusing on optical methods for the access and connectivity of 22
single spins, we review recent progress in impurity systems such as color centers in 23
diamond and silicon carbide, rare-earth ions in solids, and donors in silicon. We project a 24
possible path to chip-scale quantum technologies through sustained advances in 25
nanofabrication, quantum control, and materials engineering. 26
27
Driven by the quest for efficiency, modern technologies developed through persistent 28
miniaturization. Devices such as transistors, magnetic memories, and lasers advanced by 29
reducing the number of electrons used per gate, bit, or output photon. This progression’s arrival 30
at the quantum limit now inspires a new class of information processing hardware that starts with 31
the quantum coherence of single charges, spins, or photons and grows by harnessing the 32
inseparable connections among them. This reversal from scaling down to building up lies at the 33
heart of radical technologies that promise breakthroughs in computational power, 34
communications security, and sensor detection limit. 35
Solid-state spins are a promising platform for realizing these quantum advantages 36
because of their robustness to decoherence and compatibility with scalable device engineering
1
. 37
In particular, this review focuses on optically addressed electron and nuclear spins at impurities 38
in crystals. In recent years, pioneering experiments have isolated single spins at these atomic-39
scale impurities and demonstrated high-fidelity initialization, manipulation, and readout of their 40
quantum states
2
. These advances at the single-qubit level establish a critical foundation, but the 41
connectivity among multiple qubits is required to unlock their full potential. We highlight the 42
capacity of hybrid quantum registers formed by an electron spin coupled to multiple nuclear spins 43
in its proximity. Electron spins readily sense and interface to the outside environment, while 44
nuclear spins provide well-isolated quantum memories. These complementary functionalities, 45
accessed through the generation of entangled states, enable an array of applications, including 46
photonic memories
3
, quantum repeaters
4
, error-correction
5
, and enhanced quantum sensing
6
. 47
We concentrate on solid-state spins that utilize optical electronic transitions to fulfill several 48
of the DiVincenzo criteria for quantum information processing
7
. Optical pumping can directly 49
initialize the electron spin and its coupled nuclear spins, or alternatively, coherent manipulations 50
can transfer optically generated electron spin polarization to nuclear memories
8,9
. Additionally, 51
spin-dependent optical cycles correlate spin information to photon emission, enabling sensitive 52
readout of spin states. Such remarkable optical properties of defect systems have been combined 53
with techniques adapted from atomic physics and magnetic resonance to empower experiments 54
on single electron and nuclear spins at ambient conditions, surpassing limitations in the original 55
fields. Moreover, spin-selective optical transitions, accessed at cryogenic temperatures, 56
coherently map between the quantum states of local spins and propagating photons
10,11
. This 57
light-matter interface establishes each electron as a quantum gateway to distribute and process 58
entanglement between distant registers in a quantum network. 59
We aim t
o provide an introduction and broad update on optically-active impurity systems, 60
emphasizing the partnership between electron and nuclear spins. We first describe the framework 61
for manipulating hybrid quantum registers in the context of the prototype defect system, the 62
nitrogen-vacancy (NV) center in diamond
8
. We briefly review the optical and coherence properties 63
of the NV electron spin, which provides access to the entire register. This discussion identifies 64
the nuclear spin bath as the dominant source of decoherence but leads to the opportunity to 65
control selected nuclear memories via their distinct hyperfine interaction. We then overview 66
emerging impurity systems, including alternative color centers in diamond and silicon carbide, 67
rare-earth ions in solids, and optically-active donors in silicon. These platforms offer unique 68
advantages, such as in their optical properties or integrability with electronic or photonic devices 69
and stand to benefit from techniques developed for the NV center. In a latter part, we focus on 70
technological applications of registers of quantum memories, ranging from quantum 71
communication, computing, and sensing. We conclude our review by looking ahead to future 72
challenges and progress with impurity spins in solids. 73
We remark that spins in self-assembled
12,13
and gate-defined quantum dots
14
share many 74
parallel directions with impurity spins, including achievement of extended coherence times and 75
enhanced light-matter coupling to enable multi-qubit scaling and single photon nonlinearities. The 76
rapidly advancing state-of-the-art in this field is however beyond the scope of our discussion. 77
Likewise, we will overlook two-dimensional material systems, such as transition metal 78
dichalcogenides and hexagonal boron nitride, that have recently emerged as hosts for single 79
quantum emitters
15–17
. For these materials, explorations toward using the valley or spin degree of 80
freedom of excitons or defect states as qubits are still in their infancy but could open functionalities 81
for quantum photonics, optoelectronics, and sensing unattainable in bulk materials. 82
The NV center in diamond 83
Consisting of a substitutional nitrogen impurity adjacent to a missing carbon atom, the negatively 84
charged NV center in diamond traps six electrons at localized atomic-like states, protected from 85
charge scattering by diamond’s wide bandgap (Fig. 1a). NV centers display room-temperature 86
quantum coherence, spin-photon entanglement, and functionality inside engineered 87
nanostructures, establishing their versatility for quantum information processing and nanoscale 88
sensing. 89
The electron spin and its optical interface. The NV electron spin can be off-resonantly excited 90
from its spin-triplet ground state (GS) to a spin-triplet, orbital-doublet excited state (ES) via 91
phonon-assisted optical absorption
8
. Due to a nonradiative, spin-flip decay channel that 92
preferentially couples to the
= ±1 sublevels of the excited state, repeated optical cycling 93
initializes the electron spin into the
= 0 level (~90% polarization)
18
. Concurrently, off-resonant 94
excitation of |
= ±1
results in ~30% lower photoluminescence (PL) than |
= 0
, allowing 95
optical determination of the spin state at room temperature
19
. Higher fidelity initialization and 96
readout are obtained by cooling diamond below 10 K, where distinct spin-selective, zero-phonon 97
optical transitions are resolved
18
(Fig. 1b). Resonant optical pumping of a spin-mixed transition 98
(e.g. |
= ±1
→ |
) fully initializes the NV into |
= 0
(>99.7% polarization)
18
. Alternatively, 99
by resonant excitation of a cycling transition (|
= 0
→ |
or |
) and optimizing photon 100
collection efficiency, the electron spin state can be determined without averaging multiple 101
preparations (>97% fidelity averaged for |
= 0
and |
= ±1
)
20
. Such single-shot 102
measurements can be non-demolition to allow initialization of electron and nuclear spins by 103
projective measurement
18
. Moreover, these spin-dependent optical transitions and their 104
polarization selection rules form the basis for spin-photon entanglement
10,11
. 105
Aided by diamond’s high Debye temperature and low spin-orbit coupling, NV centers possess 106
long spin-lattice relaxation times
that reach ~5 ms at room temperature and exceed hours at 107
cryogenic temperatures (~25 mK)
21,22
. In high quality samples grown by chemical vapor 108
deposition, low concentrations of paramagnetic impurities leave the bath of
13
C nuclear spins 109
(1.1% natural abundance) as the dominant magnetic noise
19
. This dephasing can be mitigated in 110
isotopically enriched materials (>99.99%
12
C). For single spins in isotopically purified samples, 111
the inhomogeneous dephasing time
, reflecting temporal magnetic fluctuations, exceeds 100 112
µs at room temperature
23
. Dynamical decoupling further filters the noise spectrum and extends 113
spin coherence to the homogeneous dephasing time
of several milliseconds at room 114
temperature
24
and nearly seconds at low temperature, limited by direct lattice contributions to spin 115
dephasing (
0.5
)
25
(Fig. 1c). These remarkable coherence times underpin the technological 116
promise of NV centers, extending the range of its access to nearby nuclear spins and enhancing 117
its sensitivity to environmental influences. 118
Strongly-coupled nuclear spins. While the nuclear bath represents the main contribution to 119
electron spin decoherence, individual nuclear spins with isolated interactions offer a resource for 120
quantum memories and multi-qubit entanglement
9,26,27
. Strongly-coupled nuclear spins, such as 121
the intrinsic N forming the NV center and proximal
13
C atoms, possess hyperfine couplings larger 122
than the electron spin resonance (ESR) linewidth, set by the dephasing rate 1
(Fig. 1a,d). For 123
samples with natural isotope abundance, strongly-coupled nuclei typically occur within 1 nm from 124
the electron and have hyperfine couplings from 300 kHz to 130 MHz, where the latter value 125
corresponds to a
13
C on a nearest neighbor lattice site
28
. For these nuclei, narrowband microwave 126
(MW) pulses at the distinct ESR transition frequencies (Fig. 1d) perform rotations of the electron 127
spin conditional on the nuclear spin state (e.g., controlled NOT gate, C
n
NOT
e
). Moreover, radio-128
frequency (RF) pulses can directly drive nuclear spin transitions (
= ±1) conditional on the 129
electron spin manifold (e.g. C
e
NOT
n
)
29
. Nuclear rotations can alternatively be implemented by 130