<s>
In	O
quantum	B-Architecture
computing	I-Architecture
,	O
a	O
qubit	O
is	O
a	O
unit	O
of	O
information	O
analogous	O
to	O
a	O
bit	O
(	O
binary	O
digit	O
)	O
in	O
classical	O
computing	O
,	O
but	O
it	O
is	O
affected	O
by	O
quantum	O
mechanical	O
properties	O
such	O
as	O
superposition	O
and	O
entanglement	O
which	O
allow	O
qubits	O
to	O
be	O
in	O
some	O
ways	O
more	O
powerful	O
than	O
classical	O
bits	O
for	O
some	O
tasks	O
.	O
</s>
<s>
Qubits	O
are	O
used	O
in	O
quantum	B-Application
circuits	I-Application
and	O
quantum	B-Device
algorithms	I-Device
composed	O
of	O
quantum	O
logic	O
gates	O
to	O
solve	O
computational	O
problems	O
,	O
where	O
they	O
are	O
used	O
for	O
input/output	B-General_Concept
and	O
intermediate	O
computations	O
.	O
</s>
<s>
A	O
physical	B-Device
qubit	I-Device
is	O
a	O
physical	O
device	O
that	O
behaves	O
as	O
a	O
two-state	O
quantum	O
system	O
,	O
used	O
as	O
a	O
component	O
of	O
a	O
computer	O
system	O
.	O
</s>
<s>
A	O
logical	O
qubit	O
is	O
a	O
physical	O
or	O
abstract	O
qubit	O
that	O
performs	O
as	O
specified	O
in	O
a	O
quantum	B-Device
algorithm	I-Device
or	O
quantum	B-Application
circuit	I-Application
subject	O
to	O
unitary	O
transformations	O
,	O
has	O
a	O
long	O
enough	O
coherence	O
time	O
to	O
be	O
usable	O
by	O
quantum	O
logic	O
gates	O
(	O
c.f.	O
</s>
<s>
,	O
most	O
technologies	O
used	O
to	O
implement	O
qubits	O
face	O
issues	O
of	O
stability	O
,	O
decoherence	O
,	O
fault	B-General_Concept
tolerance	I-General_Concept
and	O
scalability	B-Architecture
.	O
</s>
<s>
Because	O
of	O
this	O
,	O
many	O
physical	B-Device
qubits	I-Device
are	O
needed	O
for	O
the	O
purposes	O
of	O
error-correction	B-Error_Name
to	O
produce	O
an	O
entity	O
which	O
behaves	O
logically	O
as	O
a	O
single	O
qubit	O
would	O
in	O
a	O
quantum	B-Application
circuit	I-Application
or	O
algorithm	O
;	O
this	O
is	O
the	O
subject	O
of	O
quantum	B-Device
error	I-Device
correction	I-Device
.	O
</s>
<s>
Thus	O
,	O
contemporary	O
logical	O
qubits	O
typically	O
consist	O
of	O
many	O
physical	B-Device
qubits	I-Device
to	O
provide	O
stability	O
,	O
error-correction	B-Error_Name
and	O
fault	B-General_Concept
tolerance	I-General_Concept
needed	O
to	O
perform	O
useful	O
computations	O
.	O
</s>
<s>
A	O
quantum	B-Device
algorithm	I-Device
can	O
be	O
instantiated	O
as	O
a	O
quantum	B-Application
circuit	I-Application
.	O
</s>
<s>
A	O
logical	O
qubit	O
specifies	O
how	O
a	O
single	O
qubit	O
should	O
behave	O
in	O
a	O
quantum	B-Device
algorithm	I-Device
,	O
subject	O
to	O
quantum	O
logic	O
operations	O
which	O
can	O
be	O
built	O
out	O
of	O
quantum	O
logic	O
gates	O
.	O
</s>
<s>
However	O
,	O
issues	O
in	O
current	O
technologies	O
preclude	O
single	O
two-state	O
quantum	O
systems	O
,	O
which	O
can	O
be	O
used	O
as	O
physical	B-Device
qubits	I-Device
,	O
from	O
reliably	O
encoding	O
and	O
retaining	O
this	O
information	O
for	O
long	O
enough	O
to	O
be	O
useful	O
.	O
</s>
<s>
Therefore	O
,	O
current	O
attempts	O
to	O
produce	O
scalable	B-Architecture
quantum	B-Architecture
computers	I-Architecture
require	O
quantum	B-Device
error	I-Device
correction	I-Device
,	O
and	O
multiple	O
(	O
currently	O
many	O
)	O
physical	B-Device
qubits	I-Device
must	O
be	O
used	O
to	O
create	O
a	O
single	O
,	O
error-tolerant	O
logical	O
qubit	O
.	O
</s>
<s>
Depending	O
on	O
the	O
error-correction	B-Error_Name
scheme	O
used	O
,	O
and	O
the	O
error	O
rates	O
of	O
each	O
physical	B-Device
qubit	I-Device
,	O
a	O
single	O
logical	O
qubit	O
could	O
be	O
formed	O
of	O
up	O
to	O
1,000	O
physical	B-Device
qubits	I-Device
.	O
</s>
<s>
The	O
approach	O
of	O
topological	B-Device
qubits	I-Device
,	O
which	O
takes	O
advantage	O
of	O
topological	O
effects	O
in	O
quantum	O
mechanics	O
,	O
has	O
been	O
proposed	O
as	O
needing	O
many	O
fewer	O
or	O
even	O
a	O
single	O
physical	B-Device
qubit	I-Device
per	O
logical	O
qubit	O
.	O
</s>
<s>
Topological	B-Device
qubits	I-Device
rely	O
on	O
a	O
class	O
of	O
particles	O
called	O
anyons	O
which	O
have	O
spin	O
that	O
is	O
neither	O
half-integral	O
(	O
fermions	O
)	O
nor	O
integral	O
(	O
bosons	O
)	O
,	O
and	O
therefore	O
obey	O
neither	O
the	O
Fermi	O
–	O
Dirac	O
statistics	O
nor	O
the	O
Bose	O
–	O
Einstein	O
statistics	O
of	O
particle	O
behavior	O
.	O
</s>
<s>
Anyons	O
exhibit	O
braid	B-Application
symmetry	I-Application
in	O
their	O
world	O
lines	O
,	O
which	O
has	O
desirable	O
properties	O
for	O
the	O
stability	O
of	O
qubits	O
.	O
</s>
