Long coherence time of spin qubits in 12C enriched polycrystalline
chemical vapor deposition diamond
K. D. Jahnke, B. Naydenov, T. Teraji, S. Koizumi, T. Umeda et al.
Citation: Appl. Phys. Lett. 101, 012405 (2012); doi: 10.1063/1.4731778
View online: http://dx.doi.org/10.1063/1.4731778
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i1
Published by the American Institute of Physics.

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APPLIED PHYSICS LETTERS 101, 012405 (2012)

Long coherence time of spin qubits in 12C enriched polycrystalline chemical
vapor deposition diamond
K. D. Jahnke,1 B. Naydenov,1,a) T. Teraji,2 S. Koizumi,2 T. Umeda,3 J. Isoya,3 and F. Jelezko1
1

Institute for Quantum Optics, Ulm University, Ulm 89081, Germany
National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
3
Research Center for Knowledge Communities, University of Tsukuba, Tsukuba, Ibaraki 305-8550, Japan
2(Received 6 March 2012; accepted 21 May 2012; published online 3 July 2012)
Single defects in diamond and especially negatively charged nitrogen vacancy (NV) centers arevery promising quantum systems with wide applications in physics and biology. It was shown that
their coherence properties can be strongly improved by growing ultrapure diamond with low
concentration of parasitic spins associated with nitrogen electron spins and nuclear spins related to13
C carbon isotope. Here we report a high quality 12C-enriched polycrystalline chemical vapor
deposition diamond material with properties comparable with single crystals. We find single NVs
in the grains of this material, which show extremely long electron spin coherence time T2 > 2 ms.
C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731778]
V

During the last decade diamond has attracted attention
as one of the promising materials for quantum technology
applications.1 It was shown that single optically active
defects (color centers) can be detected and controlled using
combination of optical microscopy and magnetic resonancetechniques.2 Negatively charged nitrogen vacancy centers(NVs) in diamond have been intensively studied, since they
have an application as quantum bits (qubits)3–6 and for ultrasensitive detection of electric7 and magnetic fields8–10 withnanometer spatial resolution. These defect centers in diamond consist of a substitutional nitrogen atom next to a vacancy at the adjacent lattice site. This system has a triplet
ground state with a strong optical transition towards excitedtriplet states and shows dependence of the fluorescence emission on spin state, allowing the detection of spin states associated with single defects.11 The spin coherence time T2
associated with the ground state of NV defects depends onthe concentration of paramagnetic centers in the diamond lattice. It was shown that mainly isolated neutral substitutional
nitrogen (P1 center12) limits the coherence time of NV
defects to a few microseconds in nitrogen-rich type-Ib diamond, grown by high pressure high temperature (HPHT)
method.13 Recently, there has been great improvement in thequality of diamond crystals grown using chemical vapor deposition (CVD) technique where the nitrogen concentration
has been reduced below 0.1 ppb.14,15 In such an electron spin
free environment the coherence time (0.65 ms) is limited
only by the magnetic fluctuation related to the 13C (I ¼ 1/2,
natural abundance 1.1%) nuclear spin bath.16 T2 can be
improved even further if the nuclear spin possessing isotope
13
C is depleted and T2 ¼ 1.8 ms has been already demonstrated for ultra-high pure homoepitaxially grown single
CVD crystal (SC) with 13C ¼ 0.3%.17 The size of these crystals is limited by the availability of HPHT substrate.
Recently it was shown that 13C spin bath limited coherencetime can also be achieved for polycrystalline diamond material.14 In this letter we demonstrate that isotopically enricheda)Electronic mail: boris.naydenov@uni-ulm.de.

0003-6951/2012/101(1)/012405/5/$30.00

polycrystalline diamond material can achieve record T2 comparable with that of CVD SC diamond. Polycrystalline CVD
diamond films, which are typically obtained when nondiamond substrates are used, can be grown over large areas,
thus, obtained in large quantities with lower cost. This material is inhomogeneous, consisting of grains (small single
crystals) and grain boundaries. However, some physicalproperties of polycrystalline materials are dominantly determined by the grains and not by the grain boundaries. Thus,
large-area polycrystalline diamond is technologically important for applications using the unique physical properties of
diamond.15 Each specific application requires a growth condition which optimizes a given property, such as hardness,transparency, thermal, and electronic conductance.15 The
physical properties of the grains in polycrystalline diamondare expected to be strongly affected by impurities anddefects, as in the case of single crystal. Since the coherencetime of the NV center is sensitive to the presence of paramagnetic impurities, defects and the isotope enrichment in
the lattice, native-NV, which is a grown-in defect in CVD diamond, is a useful probe for characterizing the quality of thegrains. In this study, National Institute for Materials Science(NIMS) home-made microwave plasma-activated chemical
vapor deposition (MPCVD) system that is well designed
to minimize external leakage was used. In addition, 12C
enriched methane containing 3 ppm nitrogen was passed
through a methane purifier that reduces the residual impurity
level being less than 10 ppb. Secondary ion mass spectrometry (SIMS) measurements proved that 12C enrichment of thepolycrystalline diamond film is 99.998%.18 Free-standing
film (thickness 230 lm) removed from the molybdenum substrate was used. The nucleation surface is relatively smoothand the growth surface is rough, reflecting the evolution of
the surface morphology as the film growth. The size of thegrains in this material is around 30 lm, as estimated fromlaser microscope (Keyence Ltd., VK-9700) measurements.
A typical picture is shown in Fig. 1. In polycrystalline
CVD diamond, preferential orientation of the grains is often
observed.15,19 At the initial stage of the growth, the grains

101, 012405-1

C 2012 American Institute of Physics
V

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012405-2

Jahnke et al.

FIG. 1. Laser microscope picture of the polycrystalline diamond. Grains
with a size of around 30 lm are easily distinguishable.

are randomly oriented when the density of nucleation sites
on the substrate is high. However, the grains aligned alongfavored orientations grow faster than the rest and fill up thevolume first. As a result, the grains which reach a size of a
few tens of lm at the growth surface, exhibit preferential orientation. In thicker films, columnar growth occurs. This preferred orientation depends on the substrate and the growth
conditions such as the CH4/H2 ratio, the addition of nitrogento the gas mixture, and the substrate temperature. Highly oriented [100]-textured films were obtained in a polycrystalline
CVD diamond grown on Si (100).19,20 In our samples, thegrains have their crystal lattice preferentially aligned withthe [110] axis perpendicular to the substrate surface.18 In thecase of [110] preferential orientation, the in-plane orientations of the grains are random.19 The facets appearing on thesurface are not necessarily (110) parallel to the plane of thefilm and can be (111) which is tilted by 35 from the growth
direction.19 The details of synthesis of 12C enriched polycrystalline CVD diamond films and x-ray diffraction (XRD)
studies will be described elsewhere.18 The isotopically engineered diamond material was characterized using optical microscopy techniques. The fluorescence of the NV centers has
been measured on a custom built confocal microscope withan oil immersion objective (NA ¼ 1.4) and excitation laser
with a wavelength of k ¼ 532 nm. Scattered laser light was
blocked by a 640 nm long-pass filter. Figure 2 shows a typical fluorescence image of the growth side of a polycrystalline
diamond sample. As it is often observed in this type of mate-

FIG. 2. Scanned confocal fluorescence image of a polycrystalline CVD diamond (right) and a zoom into a 20 lm  20 lm region (left). Red and blue
color indicates low and high fluorescence respectively. In the left part some
single NVs are visible.

Appl. Phys. Lett. 101, 012405 (2012)

rial, grain boundaries show strong fluorescence signal owing
to increased incorporation of impurities and defects in theseregions.14 They also show disordered structure containing
carbon dangling bonds and hydrogen impurities. The size ofthe individual grains is in a good agreement with the electron
microscopy data. Regions far from boundaries show much
lower fluorescence background with distinct fluorescencespots related to single NV defects. In order to investigate
their optical and spin properties, fluorescence correlation
spectroscopy and optically detected magnetic resonance(ODMR) techniques were used. Two avalanche photo diodes(APD) in a Hanbury Brown and Twiss configuration wereused as detectors, allowing us to measure the second order
fluorescence intensity correlation function g(2)(s), thus estimating the number of emitters in a confocal spot. These
measurements confirm (data not shown) that individual fluorescent spots indeed correspond to emission from single NV
defects.
A polycrystalline sample with similar quality and NV
concentration has been previously reported.14 In this earlier
work the carbon material had the natural abundance of 12C,
which limits the electron spin coherence time to T2 ¼ 0.6 ms.
Isotopical engineering of the diamond allows further
improvement of the T2. Coherence properties of single NV
centers were measured using pulsed ODMR. The NV electron spin system can be described by the following
Hamiltonian:12
H ¼ glB B0 Sz þ D Sz  SðS þ 1Þ þ EðS2x  S2y Þ þ SAI:
3
With the Bohr magneton lB , g-factor of the electron sping  2:002, the components of the NV electron spin (S ¼ 1)
Si for i ¼ x, y, z, the applied constant magnetic field B0 , theaxial zero field splitting (ZFS) D ¼ 2867 MHz, the non-axial
ZFS due to strain E and the hyperfine interaction to the nitrogen nuclear spin (I ¼ 1) A ¼ 2.2 MHz. Owing to the fastintersystem crossing from the excited state magnetic sublevels with magnetic quantum number mS ¼ 61, a high degreeof spin polarization in the ground state can be achieved aftera few microseconds of optical excitation. Since the fluorescence intensity of the mS ¼ 0 state is higher, magnetic resonance can be detected by measuring the photon emission rate
when sweeping the microwave frequency.21 A typical
ODMR spectrum of a single NV center in polycrystalline diamond is plotted in Fig. 3. The spectral line is a doublet centered at the ZFS frequency  ZFS ¼ D. The spectrum wasmeasured in zero magnetic field. The two peaks correspondto the mS ¼ þ1 $ mS ¼ 0 and mS ¼ 1 $ mS ¼ 0 transitions which are not degenerated due to non-zero non-axial
strain caused by disorder present in the diamond lattice. The
red dashed line is a simulation with E ¼ 12 MHz. All theNVs we analyzed showed significant strain, with an average
splitting of Eavg ¼ 20ð7Þ MHz (based on the statistics of 4centers). Despite of the observed strain, the coherence timeof the single NVs is remarkably long for this type of the material. A typical Hahn echo decay from a single NV inside
the grains is plotted in Fig. 4. All measured T2 values for different NVs are longer than a millisecond, with an average
T2 ¼ 1.6 ms. The longest T2 we measured was T2 ¼ 2.1 ms,

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012405-3

Jahnke et al.

FIG. 3. ODMR spectrum of a single NV center. Due to non-axial strain E
the degeneracy of the mS ¼ 61 levels is lifted and the line is split.

which is slightly longer than the recently reported coherencetime of single defects in 12C enriched (99.7%) enriched
monocrystalline diamond material.17 This result is a bit surprising since a higher concentration of impurities in the polycrystalline CVD diamond is expected compared to themono-crystal. The relaxation time T1 of the NV is in average
5.3 ms (data not shown) and is also comparable with NVs in
SC diamond. The T2 longer than 0.65 ms, which is the best
one obtained so far in natural abundance samples, requires
12
C enrichment. Ensemble measurements of SC CVD diamond with 12C natural abundance show T2 > 0.6 ms which islimited by the 13C nuclear spin bath, where a low concentration of P1-centers (electron spin S ¼ 1/2) of 1015 cm3
(10 ppb corresponds to 1:76  1015 cm3 ) is necessary.22 Prolonging T2 by 12C enrichment requires low total nitrogenconcentration in the lattice, while 12C enriched methane has
a substantially higher amount (>3 ppm) of nitrogen impurities compared to methane with natural abundance. The longT2 of 2 ms in our 12C enriched polycrystalline CVD dia-Appl. Phys. Lett. 101, 012405 (2012)

mond suggests that the in-grain concentration of P1 is significantly lowered.
The concentration of these paramagnetic centers inside
the grains was measured by continuous wave (CW) electron
paramagnetic resonance (EPR) on a Bruker E500 spectrometer operating at the microwave frequency of 9.65 GHz.
Several films (with total weight 60.8 mg) glued in a stack
on both sides of high-purity silica glass plate were used as asample. It is expected that the P1 concentration should be
different between the grains and their boundaries andbetween the randomly-oriented grains at the initial growthstage and those grown up to a preferential orientation.
Instead of obtaining the average concentration of P1 fromthe whole sample, it is necessary to extract its signal selectively from the preferentially oriented grains. It is likelythat the P1 center in these grains, where its concentration islow, has longer T2 compared to T2 in other parts of the sample. Moreover, it is expected that the EPR spectrum fromthere should show a hint for the crystal orientation. EPR
spectroscopy of the P1 centers was previously used to characterize preferential orientation of the grains of polycrystalline CVD diamond films.20,23 Usually a slow passage
technique is applied for the quantitative measurements, but
it lacks sensitivity if the relaxation and coherence times are
long. In this case a rapid passage method is used for determining low concentrations (0.1 ppb to 1 ppm) of P1
centers.24
We have used this technique to measure the amountof P1 in preferentially oriented grains. This center has C3v
symmetry, where the unpaired electron occupies the antibonding orbital of one of the four N-C bonds. The antibonding character is lowered by the elongation of the N-Cbond length. There are four symmetry-related sites corresponding to four C-C directions. The EPR spectrum of P1
consists of three lines arising from hyperfine coupling to
14
N (I ¼ 1, natural abundance 99.64%). The central line isnearly isotropic with a small g-anisotropy (gjj ¼ 2:0024,
g? ¼ 2:0025).25 The positions of the outer lines are predominantly determined by the anisotropy of the hyperfine
splitting which is in the first-order given by
Aeff ðhÞ ¼ ðA2jj cos2 h þ A2? sin2 hÞ1=2 ;FIG. 4. Hahn echo decay of an NV center in polycrystalline diamond(markers). The solid line is a stretched exponential fit to EðsÞ
¼ a  exp½ðs=T2 Þ2  þ b.

where Ajj ¼ 4:093 mT, A? ¼ 2:920 mT, and h is the angle
between the symmetry-axis and the external magnetic field.
In a single crystal at an arbitrary direction of the magnetic
field, EPR spectrum of P1 consists of four sets of outer lines
corresponding to four sites, since h is different among them.
For [110]-oriented films, when the magnetic field is perpendicular to the film plane, it is expected that the EPR spectrum
is similar to one taken for Bjj[110] for single crystal sample
in which the hyperfine spectrum consists of two sets: one
from two sites with h ¼ 90 and the other from two sites withh ¼ 35.26 . The two sites in each set are magnetically equivalent in this orientation. As shown in Fig. 5(a), the EPR spectrum taken with the magnetic field perpendicular to the film
plane is similar to that of single crystal spectrum taken withthe magnetic field along [110]. In the oriented polycrystalline CVD diamond, there is a distribution of the angles
between the crystal axis of each grain and the direction

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012405-4

Jahnke et al.

Appl. Phys. Lett. 101, 012405 (2012)teristic feature of preferential orientation, where the in-grain
concentration is estimated to be 4 ppb. The coherence timeof the single NV defects is comparable with reported T2 of
NVs in a SC CVD diamond. Thus, high quality (i.e., lowconcentrations of paramagnetic impurities and defects) of
the grains has been proven. The material can be produced inlarge quantities with lower costs, compared to SC CVD diamond. This opens many interesting possibilities, for example
the production of large amounts of nanodiamonds containing
NVs with good properties by milling the diamond grains.

FIG. 5. Rapid passage cw-EPR spectrum of 12C enriched polycrystalline
CVD film at room temperature. Central line is cut off. (a) Magnetic field (B)perpendicular to the film plane, (b) B parallel to the film plane.

This work was partly supported by the DFG (projects:
FOR1482, FOR1493, SFB/TR21), the Strategic International
Collaborative Research Program (Nanoelectronics) fromJapan Science and Technology Agency, EU via project DIAMANT and the Grant-in-Aid for Scientific Research fromthe Japan Society for the Promotion of Science, Japan (No.
18760241).
1

perpendicular to the film plane. Thus, the positions of theouter 14N hyperfine lines (mI ¼ 61) are additionally broadened due to the misalignment of the grain axis with respect
to the normal to the film plane. Here the grain axis is [110]of the crystal lattice of each grain. The misalignment morestrongly affects the line positions of the sites corresponding
to h ¼ 35.26 compared to those with h ¼ 90 , resulting in
broader lines. As shown in Fig. 5(b), where the magnetic
field is parallel to the film plane, a line shape corresponding
to random orientations of the grains is observed as expected.
Thus, the EPR spectrum extracted by the rapid passage arises
from P1 centers located in the preferentially oriented grains,and from the signal intensity their concentration can be estimated. As shown in Fig. 2, the volume contribution of thegrain boundaries is relatively small and the preferential oriented grains dominate after growing 10% of the film. By
using the whole volume of the sample, the average concentration of P1 of the preferentially oriented grains is determined to be as low as 4 ppb, explaining the long coherencetime of a single NV center in these regions.
In a slow passage condition (microwave power 8 lW),
0
the EPR spectrum reveals the H1 center with g ¼ 2.0029(data not shown). A weak hyperfine interaction with nearbyhydrogen is revealed by the 1H nuclear-spin flip transitions.
This center is likely to be a carbon dangling bond in the grainboundaries where hydrogen is incorporated.
In summary, we have demonstrated a long coherencetime T2 for single NV centers located in the grains of polycrystalline CVD diamond grown from 12C 99.999% enrichedmethane. The measured T2 exceeding 1 ms requires both
depleting the 13C isotope and lowering the concentration of
P1 which is the dominant form of nitrogen impurity in CVD
diamond. Since the 12C enriched methane contains a substantial amount of nitrogen impurity (>3 ppm), it is challenging
to obtain a low nitrogen concentration in 12C enriched CVD
diamond, especially in polycrystalline material. We observe
single NV defects indicating that concentration of nitrogen
incorporated during the crystal growth is lower than ppm
level and concentration of NV defects is on the order of
0.1 ppb. The EPR spectrum using rapid passage techniqueshows P1 located inside the grains which exhibits a charac-

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