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Long coherence times at 300 K for nitrogen-vacancy center spins Long coherence times at 300 K for nitrogen-vacancy center spins
in diamond grown by chemical vapor deposition in diamond grown by chemical vapor deposition
John S. Colton
john_colton@byu.edu
T. A. Kennedy
J. E. Butler
R. C. Linares
P.J. Doering
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Original Publication Citation Original Publication Citation
Long Coherence Times at 3K for Nitrogen-Vacancy Center Spins in Diamond Grown by Chemical
Vapor Deposition, T.A. Kennedy, J.S. Colton, J.E. Butler, R.C. Linares, and P.J. Doering, Appl.
Phys. Lett. 83, 419 (23). The original version may be found at: http://apl.aip.org/resource/1/
applab/v83/i2/p419_s1
BYU ScholarsArchive Citation BYU ScholarsArchive Citation
Colton, John S.; Kennedy, T. A.; Butler, J. E.; Linares, R. C.; and Doering, P.J., "Long coherence times at 300
K for nitrogen-vacancy center spins in diamond grown by chemical vapor deposition" (2003).
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Long coherence times at 300 K for nitrogen-vacancy center spins in
diamond grown by chemical vapor deposition
T. A. Kennedy, J. S. Colton, J. E. Butler, R. C. Linares, and P. J. Doering
Citation: Appl. Phys. Lett. 83, 4190 (2003); doi: 10.1063/1.1626791
View online: http://dx.doi.org/10.1063/1.1626791
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v83/i20
Published by the American Institute of Physics.
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Long coherence times at 300 K for nitrogen-vacancy center spins
in diamond grown by chemical vapor deposition
T. A. Kennedy,
a)
J. S. Colton,
b)
and J. E. Butler
Naval Research Laboratory, Washington, DC 20375
R. C. Linares and P. J. Doering
Apollo Diamond, Sherborn, Massachusetts 01770
共Received 7 July 2003; accepted 19 September 2003兲
Electron-spin-echo experiments reveal phase-memory times as long as 58
sat300Kfor
nitrogen-vacancy centers in chemical vapor deposition 共CVD兲 single crystals. The spins were
optically polarized and optically detected. Two high-quality CVD samples were studied. From the
current results, it is not clear whether these phase-memory times represent a fundamental limit or are
limited by an external source of decoherence. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1626791兴
A great deal of interest and effort is focused on develop-
ing quantum information technologies. These make use of
superpositions of quantum states, and long coherence times
are critically important since decoherence represents a loss of
quantum information. Among the semiconductor systems be-
ing considered, modulation doped II–VI semiconductors
1
and GaAs two-dimensional electron-gases grown on 关110兴
surfaces
2
have shown spin lifetimes around 1 ns for conduc-
tion electrons at room temperature. Carbon-based materials,
such as fullerenes, nanotubes and diamond, hold a special
place because the low spin-orbit interaction and strong cova-
lent bonding lead to very long lifetimes for spin states. The
high Debye temperature of diamond also suggests the possi-
bility of high temperature operation.
The nitrogen-vacancy pair 共NV center兲 in diamond has
additional special qualities that are suited to quantum infor-
mation applications. First, the negatively charged state has an
allowed optical transition with a zero-phonon line at 638 nm
that is highly stable.
3
This has enabled the use of the NV
center in diamond nanocyrstals to serve as the single photon
source in a demonstration of quantum key distribution.
4
Sec-
ond, the NV center has an electronic spin in its ground state
that can be polarized
5
and detected
6
using absorption and
emission by the optical transition. This combination has been
extensively studied in spectroscopy and has led to different
suggestions for applications in quantum information.
7–10
Re-
cently the detection of the state of a single spin has been
demonstrated using the NV center in diamond.
11
Previous work on coherence times in diamond has been
at low temperatures and has made use of natural and high-
pressure, high-temperature 共HPHT兲 samples.
6
Recently, there
has been great progress in the quality of diamond single
crystals grown by chemical vapor deposition 共CVD兲.
12,13
In
this work, spin coherence times of 50
s at 300 K are re-
ported for single crystal samples of CVD-grown diamond.
The single crystal samples were grown epitaxially on
single-crystal substrates using CVD.
12
The substrates were
subsequently removed to produce free standing single crys-
tals with typical dimensions of 4⫻4⫻1mm
3
. Photolumines-
cence and Raman characterization was done with a Renishaw
S-2000 spectrometer and microscope. The spin lifetimes
were measured using optically detected electron spin echoes
in a homebuilt spectrometer at a microwave frequency of 35
GHz.
9
HPHT grown diamonds have concentrations of neutral,
substitutional nitrogen (N
s
0
) of 20–200 ppm. When these
crystals are electron-irradiated and annealed, the vacancies
migrate to the substitutional N to produce NV centers. Be-
cause of the large excess of substitutional nitrogen, the NV
centers are predominantly in their negative charge state. The
NV
⫺
are readily detectable through their strong lumines-
cence under blue or green excitation with a zero phonon line
at 638 nm and intense vibronic sidebands extending to lower
wavelengths 共see Fig. 1兲.
a兲
Electronic mail: kennedy@bloch.nrl.navy.mil
b兲
Present address: Physics Department, U.W. La Crosse, La Crosse, WI
54601.
FIG. 1. Photoluminescence data for three samples with 488 nm excitation.
The zero-phonon lines 共ZPL兲 for NV
0
and NV
⫺
are indicated. The HPHT
sample was electron irradiated and annealed. The CVD samples are as
grown. The sharp line at 522 nm is due to Raman scattering.
APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 20 17 NOVEMBER 2003
41900003-6951/2003/83(20)/4190/3/$20.00 © 2003 American Institute of Physics
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In contrast to the HPHT diamonds, CVD-grown thin
films and single crystals have much lower N concentrations
and contain native NV
⫺
centers. The N
s
0
concentrations
range from 0.05 to 10 ppm. The NV concentrations range
from 0.1% to 10% of the
关
N
s
0
兴
.
14
From the photolumines-
cence 共PL兲, it can be inferred that the NV centers are more
equally distributed between the neutral charge state, which
exhibits PL with a ZPL at 575 nm, and the negative charge
state. The PL for the two CVD crystals used in this work is
shown in Fig. 1. The highly pure CVD crystals are ideal to
investigate the spin coherence times for NV
⫺
in diamond.
The ground state of the NV
⫺
center has an electronic
spin S⫽ 1. The spin Hamiltonian is
H⫽ g

H"S⫹ D
关
S
z
2
⫺
共
1/3
兲
S
共
S⫹ 1
兲
兴
, 共1兲
with g⫽2.0028 and D⫽ 960⫻ 10
⫺ 4
cm
⫺ 1
with the z axis
along a 具111典 direction.
5
The electron paramagnetic reso-
nance 共EPR兲 or optically detected magnetic resonance line
for the NV
⫺
center in a high-purity diamond is inhomoge-
neously broadened by magnetic interactions with other spins
through dipolar interactions and electric interactions with
other charges through the D term in the Hamiltonian. The
static effects lead to dephasing over the ensemble of spins.
The dynamic parts of the spin interactions include spin-
lattice relaxation and any changes in the local environment
that produce changes in the resonant frequency 共spectral dif-
fusion兲. Microwave pulses on a time scale that is fast with
respect to these dynamic interactions allow them to be
probed. In particular, a spin-echo experiment eliminates the
static parts of the inhomogeneous broadening resulting in a
decay that falls to 1/e in a time defined as the phase-memory
time.
15
The phase-memory time is limited by the dynamic
parts of the defect’s interaction with its environment that
produce true decoherence. Decoherence denotes loss of
quantum information.
The spin-echo experiments were performed under the
following conditions. The sample was photoexcited with 70
mW of 532 nm light from a Spectra Millenium Xs laser and
focused to a spot size with a diameter of 0.1 mm. The NV
⫺
emission was detected using a long-pass filter and a Si pho-
todiode. The sample was placed in the electromagnet of a
Bruker ESP 300 spectrometer in a 35 GHz cavity with opti-
cal access. The microwaves were provided by an Agilent
E8254A signal generator at a power level of 63 mW. The
pulse patterns were generated with an Interface Technology
RS690 word generator. For this cavity and microwave power
level, a
pulse takes 1.5
s.
To determine the phase-memory time, a set of spin ech-
oes was generated by choosing values for the initial delay
1
and then taking data through a range of delays
2
chosen to
pass through
1
and reveal the echo. See the inset to Fig. 2.
The data for sample CVD1 is shown in the main part of this
figure. An exponential is superposed on the data whose value
in this case is 26
s. The phase-memory time is twice this
value, 52
s, to account for the initial decay
1
. A slightly
longer value was obtained for sample CVD2. See Table I.
These phase-memory times are remarkably long for
spins in solids. The longest reported phase-memory time for
the NV
⫺
center is 80
s at small magnetic fields and a tem-
perature of 1.4 K.
16
This work shows that comparable times
are available in state-of-the-art CVD single crystals at room
temperature. Coupled with the accessibility of the spin
through the optical transition of the center, this provides a
great resource for exploitation in quantum information.
Is a value around 50
s the ultimate time possible for
NV
⫺
at room temperature? Consideration of what limits the
phase-memory in the present experiments leaves unclear
whether somewhat longer times may be possible. The discus-
sion can be divided into sources of the decoherence involv-
ing the NV
⫺
centers themselves and other sources.
The ultimate limit for phase memory is the spin-lattice
relaxation of the NV
⫺
itself. Spin-lattice relaxation is
strongly temperature dependent but this is moderated in dia-
mond by the high Debye temperature and small spin-orbit
interaction. An EPR measurement found T
1
to be 1.2 ms for
T⫽80 K and B⫽0.3 T.
17
While the temperature and field
differ from those in the present work, it may be that spin-
lattice relaxation limits T
M
at room temperature. A second
source of relaxation is phonon-modulation of the crystal field
关
D term in Eq. 共1兲兴 and this is again something that should
be important at high temperatures. Third, there are dipolar
interactions between the NV centers. A preliminary check for
this mechanism using instantaneous spectral diffusion
18
showed only a weak contribution to the measured time. To
FIG. 2. Electron-spin-echoes for sample CVD1 taken at 35 GHz and 300 K.
The pulse sequence is shown in the inset and contains the second
/2 pulse
required for optical detection. The decay curve is exponential with a time of
26
s. The phase-memory time is 2⫻26
s⫽52
s.
TABLE I. Phase memory times (T
M
) for different diamond samples. In
diamond, 1 ppm equals 1.77⫻ 10
17
cm
⫺ 3
.
Sample
Irradiation
fluence
(cm
⫺ 2
)
关
NV
⫺
兴
共ppm兲
关
N
s
0
兴
共ppm兲
Temperature
共K兲
T
M
共
s兲
1b natural
a
Not given Not given 5–50 1.5 80
CVD1 0 0.01 0.34 300 52
CVD2 0 Not detected
in EPR
0.03 300 58
HPHT 1⫻ 10
17
2.9 30 100 6.2
a
See Ref. 16.
4191Appl. Phys. Lett., Vol. 83, No. 20, 17 November 2003 Kennedy
et al.
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summarize, it is possible that interactions involving the NV
center and the pure lattice are currently limiting the phase
memory.
The dominant impurity in the CVD single crystals is N
occurring as an isolated substitutional defect.
14
EPR mea-
surements were performed to determine the concentration of
the neutral charge state (N
s
0
) in the samples used 共see Table
I兲. While the fluctuations in the spin state of the substitu-
tional nitrogen change the local field for NV
⫺
centers and
produce decoherence, fluctuations arising from continuous,
intense photoexcitation can be more important.
11
The energy
of the green light is sufficient to ionize the N
s
0
, producing N
s
⫹
and free electrons. The fluctuating charge associated with
this process couples to the fine-structure term in the spin
Hamitonian of the NV
⫺
, again changing the local field and
causing spectral diffusion. Increasing the optical power den-
sity by a factor of 4 increases the phase-relaxation rate by
about 50%. This confirms the importance of relaxation
caused by the continuous photoexcitation. Using pulses of
light for the polarization and readout would eliminate the
problem.
Another external source of decoherence is the imperfec-
tion in the measuring system itself. The synthesized signal
source is stable to the degree required but there are problems
with the stability of the magnetic fields. Even in our best
magnet, a field drift could be detected and it is possible that
this limits the phase-memory time.
In summary, long coherence times have been observed
at room temperature for the NV
⫺
center in CVD-grown,
single-crystal diamond samples. The long times coupled with
the optical ability to polarize and readout the spin state are
fundamental properties that are necessary for quantum infor-
mation processing. This observation in CVD-grown materi-
als enhances the possibility for a technology using these
properties.
J.S.C. was a NRL–NRC Research Associate. The au-
thors thank M. Newton for some of the quantitative measure-
ments of defect concentrations.
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