<s>
The	B-Device
nitrogen-vacancy	I-Device
center	I-Device
(	O
N-V	B-Device
center	I-Device
or	O
NV	B-Device
center	I-Device
)	O
is	O
one	O
of	O
numerous	O
point	O
defects	O
in	O
diamond	O
.	O
</s>
<s>
The	O
NV	B-Device
center	I-Device
's	O
electron	O
spin	O
,	O
localized	O
at	O
atomic	O
scales	O
,	O
can	O
be	O
manipulated	O
at	O
room	O
temperature	O
by	O
external	O
factors	O
such	O
as	O
magnetic	O
,	O
or	O
electric	O
fields	O
,	O
microwave	O
radiation	O
,	O
or	O
optical	O
light	O
,	O
resulting	O
in	O
sharp	O
resonances	O
in	O
the	O
intensity	O
of	O
the	O
photoluminescence	O
.	O
</s>
<s>
An	O
individual	O
NV	B-Device
center	I-Device
can	O
be	O
used	O
as	O
a	O
basic	O
unit	O
for	O
a	O
quantum	B-Architecture
computer	I-Architecture
,	O
a	O
qubit	O
,	O
and	O
used	O
for	O
quantum	O
cryptography	O
.	O
</s>
<s>
Further	O
potential	O
applications	O
in	O
novel	O
fields	O
of	O
electronics	O
and	O
sensing	O
include	O
spintronics	O
,	O
masers	O
,	O
and	O
quantum	B-Algorithm
sensors	I-Algorithm
.	O
</s>
<s>
If	O
the	O
charge	O
is	O
not	O
specified	O
the	O
term	O
"	O
NV	B-Device
center	I-Device
"	O
refers	O
to	O
the	O
negatively	O
charged	O
NV−	O
center	O
.	O
</s>
<s>
The	B-Device
nitrogen-vacancy	I-Device
center	I-Device
is	O
a	O
point	O
defect	O
in	O
the	O
diamond	O
lattice	O
.	O
</s>
<s>
This	O
NV−	O
state	O
is	O
what	O
is	O
commonly	O
,	O
and	O
somewhat	O
incorrectly	O
,	O
called	O
"	O
the	B-Device
nitrogen-vacancy	I-Device
center	I-Device
"	O
.	O
</s>
<s>
The	O
NV	B-Device
centers	I-Device
are	O
randomly	O
oriented	O
within	O
a	O
diamond	O
crystal	O
.	O
</s>
<s>
Nitrogen-vacancy	B-Device
centers	I-Device
are	O
typically	O
produced	O
from	O
single	O
substitutional	O
nitrogen	O
centers	O
(	O
called	O
C	O
or	O
P1	O
centers	O
in	O
diamond	O
literature	O
)	O
by	O
irradiation	O
followed	O
by	O
annealing	O
at	O
temperatures	O
above	O
700°C	O
.	O
</s>
<s>
Irradiation	O
produces	O
lattice	O
vacancies	O
,	O
which	O
are	O
a	O
part	O
of	O
NV	B-Device
centers	I-Device
.	O
</s>
<s>
Single	O
substitutional	O
nitrogen	O
produces	O
strain	O
in	O
the	O
diamond	O
lattice	O
;	O
it	O
therefore	O
efficiently	O
captures	O
moving	O
vacancies	O
,	O
producing	O
the	O
NV	B-Device
centers	I-Device
.	O
</s>
<s>
During	O
chemical	B-Algorithm
vapor	I-Algorithm
deposition	I-Algorithm
of	O
diamond	O
,	O
a	O
small	O
fraction	O
of	O
single	O
substitutional	O
nitrogen	O
impurity	O
(	O
typically	O
<	O
0.5	O
%	O
)	O
traps	O
vacancies	O
generated	O
as	O
a	O
result	O
of	O
the	O
plasma	O
synthesis	O
.	O
</s>
<s>
Such	O
nitrogen-vacancy	B-Device
centers	I-Device
are	O
preferentially	O
aligned	O
to	O
the	O
growth	O
direction	O
.	O
</s>
<s>
Many	O
of	O
them	O
already	O
have	O
sufficient	O
concentrations	O
of	O
grown-in	O
NV	B-Device
centers	I-Device
and	O
are	O
suitable	O
for	O
applications	O
.	O
</s>
<s>
Selection	O
of	O
a	O
certain	O
irradiation	O
dose	O
allows	O
tuning	O
the	O
concentration	O
of	O
produced	O
NV	B-Device
centers	I-Device
such	O
that	O
individual	O
NV	B-Device
centers	I-Device
are	O
separated	O
by	O
micrometre-large	O
distances	O
.	O
</s>
<s>
Then	O
,	O
individual	O
NV	B-Device
centers	I-Device
can	O
be	O
studied	O
with	O
standard	O
optical	O
microscopes	O
or	O
,	O
better	O
,	O
near-field	O
scanning	O
optical	O
microscopes	O
having	O
sub-micrometre	O
resolution	O
.	O
</s>
<s>
The	O
NV	B-Device
center	I-Device
has	O
a	O
ground-state	O
triplet	O
(	O
3A	O
)	O
,	O
an	O
excited-state	O
triplet	O
(	O
3E	O
)	O
and	O
two	O
intermediate-state	O
singlets	O
(	O
1A	O
and	O
1E	O
)	O
.	O
</s>
<s>
If	O
an	O
external	O
magnetic	O
field	O
is	O
applied	O
along	O
the	O
defect	O
axis	O
(	O
the	O
axis	O
which	O
aligns	O
with	O
the	O
nitrogen	O
atom	O
and	O
the	O
vacancy	O
)	O
of	O
the	O
NV	B-Device
center	I-Device
,	O
it	O
does	O
not	O
affect	O
the	O
ms	O
=	O
0	O
states	O
,	O
but	O
it	O
splits	O
the	O
ms	O
=	O
±1	O
levels	O
(	O
Zeeman	O
effect	O
)	O
.	O
</s>
<s>
Surrounding	O
spins	O
and	O
spin	O
–	O
orbit	O
interaction	O
will	O
modulate	O
the	O
magnetic	O
field	O
experienced	O
by	O
the	O
NV	B-Device
center	I-Device
.	O
</s>
<s>
which	O
suggested	O
the	O
use	O
of	O
the	O
NV	B-Device
center	I-Device
as	O
a	O
qubit	O
and	O
quantum	B-Algorithm
sensor	I-Algorithm
.	O
</s>
<s>
NV	B-Device
centers	I-Device
emit	O
bright	O
red	O
light	O
(	O
3E	O
→	O
3A	O
transitions	O
)	O
,	O
if	O
excited	O
off-resonantly	O
by	O
visible	O
green	O
light	O
(	O
3A	O
→	O
3E	O
transitions	O
)	O
.	O
</s>
<s>
At	O
room	O
temperature	O
the	O
NV	B-Device
center	I-Device
's	O
optical	O
spectrum	O
exhibits	O
no	O
sharp	O
peaks	O
due	O
to	O
thermal	O
broadening	O
.	O
</s>
<s>
However	O
,	O
cooling	O
the	O
NV	B-Device
centers	I-Device
with	O
liquid	O
nitrogen	O
or	O
liquid	O
helium	O
dramatically	O
narrows	O
the	O
lines	O
down	O
to	O
a	O
width	O
of	O
a	O
few	O
MHz	O
.	O
</s>
<s>
An	O
important	O
property	O
of	O
the	O
luminescence	O
from	O
individual	O
NV	B-Device
centers	I-Device
is	O
its	O
high	O
temporal	O
stability	O
.	O
</s>
<s>
change	O
their	O
charge	O
state	O
and	O
become	O
dark	O
)	O
after	O
emission	O
of	O
106	O
–	O
108	O
photons	O
,	O
bleaching	O
is	O
unlikely	O
for	O
NV	B-Device
centers	I-Device
at	O
room	O
temperature	O
.	O
</s>
<s>
Because	O
of	O
these	O
properties	O
,	O
the	O
ideal	O
technique	O
to	O
address	O
the	O
NV	B-Device
centers	I-Device
is	O
confocal	O
microscopy	O
,	O
both	O
at	O
room	O
temperature	O
and	O
at	O
low	O
temperature	O
.	O
</s>
<s>
The	O
diagram	O
on	O
the	O
right	O
shows	O
the	O
multi-electronic	O
states	O
of	O
the	O
NV	B-Device
center	I-Device
labeled	O
according	O
to	O
their	O
symmetry	O
(	O
E	O
or	O
A	O
)	O
and	O
their	O
spin	O
state	O
(	O
3	O
for	O
a	O
triplet	O
(	O
S=	O
1	O
)	O
and	O
1	O
for	O
a	O
singlet	O
(	O
S=	O
0	O
)	O
)	O
.	O
</s>
<s>
An	O
NV	B-Device
center	I-Device
in	O
the	O
ground	O
state	O
with	O
ms	O
=	O
0	O
will	O
be	O
excited	O
to	O
the	O
corresponding	O
excited	O
state	O
with	O
ms	O
=	O
0	O
due	O
to	O
the	O
conservation	O
of	O
spin	O
.	O
</s>
<s>
After	O
many	O
cycles	O
,	O
the	O
state	O
of	O
the	O
NV	B-Device
center	I-Device
(	O
independently	O
of	O
whether	O
it	O
started	O
in	O
ms	O
=	O
0	O
or	O
ms	O
=	O
±1	O
)	O
will	O
end	O
up	O
in	O
the	O
ms	O
=	O
0	O
ground	O
state	O
.	O
</s>
<s>
This	O
process	O
can	O
be	O
used	O
to	O
initialize	O
the	O
quantum	O
state	O
of	O
a	O
qubit	O
for	O
quantum	O
information	O
processing	O
or	O
quantum	B-Algorithm
sensing	I-Algorithm
.	O
</s>
<s>
Sometimes	O
the	O
polarisability	O
of	O
the	O
NV	B-Device
center	I-Device
is	O
explained	O
by	O
the	O
claim	O
that	O
the	O
transition	O
from	O
1E	O
to	O
the	O
ground	O
state	O
with	O
ms	O
=	O
±1	O
is	O
small	O
,	O
compared	O
to	O
the	O
transition	O
to	O
ms	O
=	O
0	O
.	O
</s>
<s>
Such	O
protocols	O
are	O
rather	O
important	O
for	O
the	O
practical	O
realization	O
of	O
quantum	B-Architecture
computers	I-Architecture
.	O
</s>
<s>
By	O
manipulating	O
the	O
population	O
,	O
it	O
is	O
possible	O
to	O
out	O
the	O
NV	B-Device
center	I-Device
into	O
a	O
more	O
sensitive	B-Algorithm
or	O
stable	O
state	O
.	O
</s>
<s>
Its	O
own	O
resulting	O
fluctuating	O
fields	O
may	O
also	O
be	O
used	O
to	O
influence	O
the	O
surrounding	O
nuclei	O
or	O
protect	O
the	O
NV	B-Device
center	I-Device
itself	O
from	O
noise	O
.	O
</s>
<s>
The	O
following	O
strong	O
interaction	O
results	O
in	O
so-called	O
spin	B-General_Concept
polarization	I-General_Concept
,	O
which	O
strongly	O
affects	O
the	O
intensity	O
of	O
optical	O
absorption	O
and	O
luminescence	O
transitions	O
involving	O
those	O
states	O
.	O
</s>
<s>
Strain	O
has	O
a	O
similar	O
effect	O
on	O
the	O
NV	B-Device
center	I-Device
as	O
electric	O
fields	O
.	O
</s>
<s>
There	O
is	O
an	O
additional	O
splitting	O
of	O
the	O
ms	O
=	O
±1	O
energy	O
levels	O
,	O
which	O
originates	O
from	O
the	O
hyperfine	O
interaction	O
between	O
surrounding	O
nuclear	O
spins	O
and	O
the	O
NV	B-Device
center	I-Device
.	O
</s>
<s>
Also	O
the	O
NV	B-Device
center	I-Device
's	O
own	O
spin	O
–	O
orbit	O
interaction	O
and	O
orbital	O
degeneracy	O
leads	O
to	O
additional	O
level	O
splitting	O
in	O
the	O
excited	O
3E	O
state	O
.	O
</s>
<s>
Temperature	O
and	O
pressure	O
directly	O
influence	O
the	O
zero-field	O
term	O
of	O
the	O
NV	B-Device
center	I-Device
leading	O
to	O
a	O
shift	O
between	O
the	O
ground	O
and	O
excited	O
state	O
levels	O
.	O
</s>
<s>
The	O
Hamiltonian	O
,	O
a	O
quantum	O
mechanical	O
equation	O
describing	O
the	O
dynamics	O
of	O
a	O
system	O
,	O
which	O
shows	O
the	O
influence	O
of	O
different	O
factors	O
on	O
the	O
NV	B-Device
center	I-Device
can	O
be	O
found	O
below	O
.	O
</s>
<s>
Although	O
it	O
can	O
be	O
challenging	O
,	O
all	O
of	O
these	O
effects	O
are	O
measurable	O
,	O
making	O
the	O
NV	B-Device
center	I-Device
a	O
perfect	O
candidate	O
for	O
a	O
quantum	B-Algorithm
sensor	I-Algorithm
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
switch	O
the	O
charge	O
state	O
of	O
the	O
NV	B-Device
center	I-Device
(	O
i.e.	O
</s>
<s>
The	O
spectral	O
shape	O
and	O
intensity	O
of	O
the	O
optical	O
signals	O
from	O
the	O
NV−	O
centers	O
are	O
sensitive	B-Algorithm
to	O
external	O
perturbation	O
,	O
such	O
as	O
temperature	O
,	O
strain	O
,	O
electric	O
and	O
magnetic	O
field	O
.	O
</s>
<s>
Due	O
to	O
the	O
relatively	O
small	O
size	O
of	O
nanodiamond	O
,	O
NV	B-Device
centers	I-Device
can	O
be	O
produced	O
by	O
irradiating	O
nanodiamond	O
of	O
100nm	O
or	O
less	O
with	O
medium	O
energy	O
H+	O
beam	O
.	O
</s>
<s>
The	O
NV	B-Device
center	I-Device
can	O
have	O
a	O
very	O
long	O
spin	O
coherence	O
time	O
approaching	O
the	O
second	O
regime	O
.	O
</s>
<s>
This	O
is	O
advantageous	O
for	O
applications	O
in	O
quantum	B-Algorithm
sensing	I-Algorithm
and	O
quantum	O
communication	O
.	O
</s>
<s>
)	O
of	O
the	O
NV	B-Device
center	I-Device
and	O
the	O
strong	O
phonon	O
sideband	O
in	O
its	O
emission	O
spectrum	O
.	O
</s>
<s>
Both	O
issues	O
can	O
be	O
addressed	O
by	O
putting	O
the	O
NV	B-Device
center	I-Device
in	O
an	O
optical	O
cavity	O
.	O
</s>
<s>
Also	O
one	O
of	O
the	O
outstanding	O
properties	O
of	O
the	O
NV	B-Device
center	I-Device
was	O
demonstrated	O
,	O
namely	O
room-temperature	O
optically	O
detected	O
magnetic	O
resonance	O
.	O
</s>
