a limit on nonlocality in any world in which communication complexity is not trivial
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A limit on nonlocality in any
world in which communication
complexity is not trivial
IFT6195Alain Tapp
In collaboration with… Gilles Brassard Harry Buhrman Naoh Linden André Allan Methot Falk Unger
Quant-ph/0508042
Motivation What would be the consequences if the non local collerations in our world were stronger than the one given by quantum mechanics?
Theoretical computer science? Foundation of physics? Philosophy?
Perfect Non Local Boxes
Alice Bob
NLB
yxba
a byx
NLB and communication
One bit of communication is enough toimplement a NLB.
Alice sends a to Bob and output x=0 Bob outputs bay
NLB and communication
NLBs does not allow for communication.
We can have a perfect box for which x and y are uniformly distributed and independent of (respectively) a and b.
NLB, classical deterministic strategies
yes yes yes no
yes yes no yes
yes no yes yes
no yes yes yes
0 0 0 a b 0 ba0 01 00 11 1
NLB classical implementation There is a probabilistic strategy with succes probability ¾ on all input.
There is no classical déterministic strategy with success proportion greater than ¾.
There is no probabilistic strategy with success probability greater than ¾. ¾
Alice and Bob have the same strategy.If input=0 applies otherwiseMeasure and output the result.This strategy works on all inputs with probability:
)cos()sin(
)sin()cos()(
R
NLB quantum strategy
)16/( R
11002
1
)16/3( R
%85)8/(cos2
NLB quantum strategy
%85)8/(cos2
Tsirelson proved in 1980 that this is optimal whatever the entanglement shared by the players.
Bell theorem
The classical upper bound and the quantum lower bound do not match.
We can derive an inequality from this that provides a Bell theorem proof.
This is known as the CHSH inequality.
4/3 )8/(cos %85 2
Classical Communication Complexity
Alice Bobk
Rz }1,0{
x y
),( yxf
Quantum Communication Complexity
Alice Bobk
11
2
100
2
1
x y
),( yxf
The classical/quantum probabilistic communication complexity of f, C(f)/Q(f) is the amount of classical communication required by the best protocol that succeeds on all input with probability at least when the players have unlimited prior classical/quantum correlation.
Communication Complexity
2/1
Inner product (IP)
)2(mod
)()()(
),(
1
3211
2121
n
iii
nn
nn
yxyx
yxyxyxyx
yyyyxxxx
yxyxIP
Inner product (IP)
)()(
)1()(
nIPQ
OnIPC
Most functions are difficultFor most functions f
)1()(
)1()(
OnfQ
OnfC
Equality
0),EQ(
1),EQ(
yxyx
yxyx
Alice and Bob each have a very large file and they want to know if it is exactly the same.
How much do they need to communicate?
Equality
nRz 1,0
Alice Bobzymb yx
zxma
bm
ba mm Output
Equality
2)EQ( C
2
1
1
yzxzPyx
yzxzPyx
By repeating the protocol twice we have success probability of at least ¾.
Scheduling
0),(
)()()(),(
1
3211
2121
n
iii
nn
nn
yxyxS
yxyxyxyxS
yyyyxxxx
Alice and Bob want to find a time where they are both available for a meeting.
Scheduling
)()(
)()(
nSQ
nSC
Raz separationThere exists a problem such that:
))(log()(
)log()(
4/1
nOSQ
n
nSC
IP using NLB
)()(
)()()(
)()()(
),(),(
2121
2211
3211
nn
nn
nn
iiii
iiii
BBBAAAyx
BABABAyx
yxyxyxyx
yxBA
yxNLBBA
Perfect NLB implies trivial CC
ba xxx
ba xxx
0,1 ba xxx
Any function can be computed with a serie of AND gates and negations.
Distributed bit
Input bit
Negation:
AND Two NLBs
Outcome Bob sends to Aliceba yyy by
AND
))(())((
)()()()(
)()(
),(),(
),(),(
2121
22
11
22
11
bbaa
bbabbaaa
baba
ba
ba
ba
ba
yxBBAAyxyx
yxyxyxyxyx
yyxxyx
xyBA
yxBA
xyNLBBA
yxNLBBA
Main result
1)(, 6
1
2
1* fCfNLB
In any world where non local boxes can be implemented with accuracy larger than 0.91 communication complexity is trivial.
CC with a bias We say that a function f can be computed with a bias if Alice and Bob can produce a distributed bit z such that
2
1]),([ zyxfP
ba zzz
CC with a biasEvery function can be computed with a bias.
Alice’s input: xBob’s input: yAlice and Bob share z
Alice outputs a=f(x,z)Bob outputs b=0 if y=z and a random bit otherwise.
2
1
2
1
2
11
2
1 ]),([
nnbayxfP
Idea We want a bounded bias.
Let’s amplify the bias.
Repetition and majority?
IdeaMaj
Maj Maj Maj
Maj Maj Maj Maj Maj Maj Maj Maj Maj
)(~
)(~
)(~
xfxfxf )(~
)(~
)(~
xfxfxf )(~
)(~
)(~
xfxfxf )(~
)(~
)(~
xfxfxf )(~
)(~
)(~
xfxfxf )(~
)(~
)(~
xfxfxf )(~
)(~
)(~
xfxfxf )(~
)(~
)(~
xfxfxf )(~
)(~
)(~
xfxfxf
)(~xf
Non local majority
),,(
2 iff 1),,(
332211
333
222
111
321321
babababa
ba
ba
ba
ba
xxxxxxMajyy
yyy
xxx
xxx
xxx
xxxxxxMajy
NLM > 5/6 If NLM can be computed with probability stricly greather than 5/6 than every fonction can be computed with a bounded bias.
Below that treshold NLM makes things worst.
NLM > 5/6
pphsp
s
pppqpppqph
q
p
)(2/1
2
1
312
3
2
1
))1()1(3)(1())1(3()(
0)( 6/5
)0( 2/1
3223
Non local equality
),,(
iff 0),,(
332211
333
222
111
321321
babababa
ba
ba
ba
ba
xxxxxxNLEyy
yyy
xxx
xxx
xxx
xxxxxxNLEy
NLE implies NLM
),,(
),,(
332211
321
321
332211
babababa
bbbbb
aaaaa
babababa
xxxxxxMajzz
xxxyz
xxxyz
xxxxxxNLEyy
2 NLB implies NLE
213221
213221
33222211
3221
321
213222
322111
333222111
))()((
))()((
) () (
)()(
),,(
) ,1(
),(
bbbbbb
aaaaaa
babababa
bbaaba
bbaaba
bababa
zzxxxx
zzxxxx
xxxxxxx
xxxx
xxxNLE
xxxxNLBzz
xxxxNLBzz
xxxxxxxxx
To conclude the proof
6
5
6
5
6
1
2
1 MajNLENLB
•Compute f several times with a bias•Use a tree of majority to improve the bias.•Bob sends his share of the outcome to Alice.
Open question
Show some unacceptable consequences of correlations epsilon-stronger than the one predicted by quantum mechanics.
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