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Quantum Games
Quantum Strategies in Classical Games
Presented by Yaniv Carmeli
1
Talk Outline

Introduction



PQ Games


Game Theory
Why quantum games?
PQ penny flip
2x2 Games

Quantum strategies
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Game Theory

The study of decision making of competing
agents is conflict situations.





Economic problems
Diplomatic relations
Social sciences
Biology
Engineering
John von Neumann
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Why Quantum Games?

Attempt to understand the source of the advantages of
quantum computation.

Quantum algorithms as games.


Quantum communication as a game.


Which problems are solvable more efficiently using quantum
algorithms?
The objective is to maximize effective communication
Quantum cryptography as a game

Eve’s objective is to learn the contents of the conversation.
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Game Theory - Terminology



Players
Moves
Strategy

Instructions to the player how to react to all
scenarios of the game.


Pure strategy – Always play a given move.
Mixed strategy – Probabilistic choice of moves.
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Game Theory – Terminology (Cont.)

Utility


Numerical measure of the desirability of an outcome.
Payoff Matrix

Gives the utility for all the players and for all game
outcomes.
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Game Theory – Terminology (Cont.)

A Nash Equilibrium (NE)

A combination of strategies from which no player can
improve his payoff by unilateral change of strategy.
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PQ Coin Flip
P
Q
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Coin Flip (Cont.)

The Game:
 A penny is placed in a box head up.
 Q can choose to flip or not to flip.
 P can choose to flip or not to flip.
 Q can choose to flip or not to flip
 At the end: If the coin is head up, Q wins,
else P wins.
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Coin Flip (Cont.)



No deterministic solution.
Best mixed strategy: Flip with probability ½, don’t flip with
probability ½.
 Expected payoff: 0.
General probabilistic strategy: Flip with probability p, don’t
flip with probability 1-p.
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PQ Coin Flip
P
Q
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Coin Flip – Quantum Representation

The coin is represented by a qubit, where
represents head up, and
represents tail.

Initial state:

Flipping the coin:

Not flipping:

Probabilistic strategy:
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Coin Flip – A Quantum Player

A quantum player is allowed any unitary strategy.

Q’s first operation is

After Picard’s mixed strategy:
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Coin Flip (Cont.)

What if the game was to end here?

If Q were to employ a strategy for which
Picard could get an expected payoff of
by selecting p=0 (or p=1).

If Picard were to choose
Q could get an expected payoff of
by selecting a=1 (or b=1)

NE is:
Where
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Coin Flip (Cont.)
NE is:
where

This represent the same results as in a classic
game.

A quantum player has no advantage
if he has only one move, and there
is no entanglement involved.
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Coin Flip (Cont.)


Q has a winning strategy:

After Q’s first move:

After P’s move:

After Q’s second move:
The mixed/quantum equilibria:
with exp. payoff of 1 to Q.
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Coin Flip (Cont.)

What about a game with two quantum players?
Consider an arbitrary pair of quantum strategies
 If
Q can improve his expected payoff
by choosing

If
P can improve his
expected payoff by choosing
No Equilibrium!
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Coin Flip – Bad Example?

Can be implemented classically – not an example for
superiority of a quantum player (S.J. van Enk, 2000).

Classical implementations are not scalable – quantum
implementations are (D.A. Meyer, 2000).

It’s like losing a game of chess and
saying: “If we would have played on
a larger board, I would have won”.
(S.J. van Enk, 2000).
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Game Theory – Terminology (Cont.)

A Pareto optimal outcome

An outcome from which no player can increase his
utility without reducing the utility of another player.
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2x2 Games


2 players
Each has the choice between 2 pure strategies.
The choice has to be made without communication,
and before knowing the opponent’s chosen move.
 The payoff matrix is known.
Assumptions on the players:
 Rationality - Players aim to maximize their payoffs
 Each player knows that other players are rational


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The Prisoners’ Dilemma

Two suspects held in separate cells are charged with a
major crime. However, there is not enough evidence.

Both suspects are told the following policy:
 If neither confesses - both will be convicted of a minor
offense and get one year in jail.
 If both confess - both will be
sentenced to six years.
 If one confesses but the other
does not, then the confessor will
be released, but the other will be
sentenced to jail for nine years.
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The Prisoners’ Dilemma

Each player has the choice between 2 strategies:
 C (Cooperate)
 D (Defect)

Nash Equilibrium: [D,D]
Pareto Optimal Strategy: [C,C]

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PD – Another Version

Two firms, Reynolds and Philip, share a market.
Each firm earns $60M from its customers if neither
do advertising.
Advertising costs a firm $20M.

Advertising captures $30M from the competitor.


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PD – Third Version

Two cyclists halfway in a race, with the rest of the
cyclists far behind them.

Each has two options: Taking the lead, where there is
no shelter from the wind (C), or staying behind and
riding in the other’s slipstream (D).

If they both make no effort to stay ahead, the rest of
the cyclists will catch up. If one takes the lead, he
works much harder and the other cyclist is likely to win.
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PD – Yet Another Version

Two states engaged in an arms race.

They both have two options, either to increase
military expenditure (D) or to make an agreement
to reduce weapons (C).

Neither state can be certain that the other one will
keep to such an agreement; therefore, they both
incline towards military expansion.
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Chicken

Each player has the choice between 2 strategies:
 C (Cooperate \ Swerve)
 D (Defect \ Don’t Swerve)

Nash Equilibria: [C,D],[D,C]
Pareto Optimal Strategy: [C,C]

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Battle of The Sexes

Each player has the choice between 2 strategies:
 O (Opera)
 T (Television)

Nash Equilibria: [O,O],[T,T]
Pareto Optimal Strategy: [O,O],[T,T]

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The Dating Problem




Alice may want a date with Bob, but if he
doesn’t want a date with her, she doesn’t want
him to know that she was interested.
Bob may want a date with Alice, but if she
doesn’t want a date with him, he doesn’t want
her to know that he was interested.
Is there still hope for the shy Alice
and Bob?
Is this a 2x2 game?
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Quantum 2x2 Games




The players receive two qubits (one for each) in a
known initial state.
Each player manipulates his qubit according to his
chosen move.
At the end, both qubits are measured using a
predetermined known basis.
The expected payoff is determined according
to the payoff matrix:
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Quantum 2x2 Games (Cont.)

Observation: If the qubits’ initial state is not entangled,
there is no advantage over a classical player utilizing a
mixed strategy.

What if the initial state was the maximally entangled
state
?
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Quantum Prisoners’ Dilemma

For the rest of this section we consider the Prisoners’
Dilemma with the initial state:
and the basis for measurement:
The payoff of final state :
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One Parameter Set of Strategies

When Alice and Bob select their strategies from S(CL)
(local rotations with one parameter):

Note that:
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One Parameter Set of Strategies

The expected payoff:
The same as mixed strategies in the classical version.
(Like choosing to cooperate with probability
).

Equilibrium is still [D,D].
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Two Parameter Set of Strategies

Now Alice and Bob select their strategies from S(TP)
(the two-parameter set of operators):

Note that:
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Two Parameter Set of Strategies

The expected payoff:
There is a new unique equilibrium [Q,Q], where:
Since
, it is also the optimal
solution (Given without proof).
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Two Parameter Set of Strategies

Proof that [Q,Q] is an equilibrium:

If Bob uses Q, then for every strategy of Alice U(,)

the same argument holds for every strategy of Bob,
when Alice uses Q.

Q is unique – given without proof.
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General Unitary Operations as Strategies

If Alice and Bob select their strategies from S(GU)
(the set of general local unitary operations):

For every move of Bob
(where a,b,c,d are
appropriate complex numbers), there exists a move for
Alice

, s.t.
.
The same argument is true for Bob, so there is no
Nash Equilibrium in pure strategies.
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General Unitary Operations as Strategies

There exists a NE in mixed strategies:

Expected payoff for both is 2.5.
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General Unitary Operations as Strategies


Problem: It is not the only equilibrium.

For example: The following is also an equilibrium

It has the same property as the previous one.
And there are more…
Which of them will be chosen?
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General Unitary Operations as Strategies



There exist probabilities
operators
s.t.:
and unitary
If Alice will choose “R”, Bob’s actions will not
change the state of the quantum system anymore.
[R,R] is the single NE which is the only one that gives
an expected payoff of 2.25 for both players
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General Unitary Operations as Strategies



Focal points are outcomes which are distinguished
from others on the basis of some characteristics which
are not included in the formalism of the model. Those
characteristics may distinguish an outcome as a result
of some psychological or social process and may even
seem trivial, such as the names of the actions.
If there are more that one NEs and one of them can be
considered a focal equilibrium, then it is the one that
will be chosen.
The NE is [R,R]
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Is It Really That Good?

Can we say we have cracked the prisoners’ dilemma?
The solution [D,D] is not a NE anymore!

The effect of Q can be described classically:


If Alice chooses Q and Bob chooses C or D then his choice is
changed to the one he didn’t pick (and vice versa).
If both choose Q – the payout is as if both cooperated.
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Is It Really That Good?

In non-cooperative games players are not allowed to
communicate, and the use of correlated random
variables is not allowed.
Letting Alice and Bob use an entangled state, means
they are using the correlations present in such a state.
This goes against the spirit of the prisoners’ dilemma.
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