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Introduction to entanglement

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School of Physics and Astronomy

FACULTY OF MATHEMATICAL AND PHYSICAL SCIENCES

Introduction to entanglement

Jacob Dunningham

Paraty, August 2007

School of Physics and Astronomy

FACULTY OF MATHEMATICAL AND PHYSICAL SCIENCES

Vlatko pic

October 2004

1

www.quantuminfo.org

School of Physics and Astronomy

FACULTY OF MATHEMATICAL AND PHYSICAL SCIENCES

Vlatko pic

October 2005

October 2004

1

9

www.quantuminfo.org

School of Physics and Astronomy

FACULTY OF MATHEMATICAL AND PHYSICAL SCIENCES

Vlatko pic

October 2005

October 2006

October 2004

1

9

~ 25

www.quantuminfo.org

School of Physics and Astronomy

FACULTY OF MATHEMATICAL AND PHYSICAL SCIENCES

October 2010 (projected)

- Lecture1: Introduction to entanglement:
- Bell’s theorem and nonlocality
- Measures of entanglement
- Entanglement witness
- Tangled ideas in entanglement

- Lecture1: Introduction to entanglement:
- Bell’s theorem and nonlocality
- Measures of entanglement
- Entanglement witness
- Tangled ideas in entanglement
- Lecture 2: Consequences of entanglement:
- Classical from the quantum
- Schrodinger cat states

- Lecture1: Introduction to entanglement:
- Bell’s theorem and nonlocality
- Measures of entanglement
- Entanglement witness
- Tangled ideas in entanglement
- Lecture 2: Consequences of entanglement:
- Classical from the quantum
- Schrodinger cat states
- Lecture 3: Uses of entanglement:
- Superdense coding
- Quantum state teleportation
- Precision measurements using entanglement

Both speakers yesterday referred to how

Schrödinger coined the term “entanglement” in 1935 (or earlier)

- "When two systems, …… enter into temporary physical interaction due to known forces between them, and …… separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives [the quantum states] have become entangled."
- Schrödinger (Cambridge Philosophical Society)

Both speakers yesterday referred to how

Schrödinger coined the term “entanglement” in 1935 (or earlier)

- Superpositions:
- Superposed correlations:
- Entanglement
- (pure state)

- Tensor Product:

Entangled

Separable

- Separable states (with respect to the subsystems
- A, B, C, D, …)

- Separable states (with respect to the subsystems
- A, B, C, D, …)
- Everything else is entangled
- e.g.

- 1935: Einstein, Podolsky, Rosen - QM is not complete
- Either:
- Measurements have nonlocal effects on distant parts of the system.
- QM is incomplete - some element of physical reality cannot be accounted for by QM - ‘hidden variables’

An entangled pair of particles is sent to Alice and Bob. The spin in measured in the z, x (or any other) direction.

The measurement Alice makes instantaneously affects Bob’s….nonlocality? Hidden variables?

- 1964: John Bell derived an inequality that must be obeyed if the system has local hidden variables determining the outcomes.
- CHSH:
- S = |E(a,b) - E(a, b’) + E(a’,b) + E(a’,b’)| <= 2

- 1964: John Bell derived an inequality that must be obeyed if the system has local hidden variables determining the outcomes.
- CHSH:
- S = |E(a,b) - E(a, b’) + E(a’,b) + E(a’,b’)| <= 2

a

b

a’

b’

Alice’s axes: a and a’

Bob’s axes: b and b’

- 1964: John Bell derived an inequality that must be obeyed if the system has local hidden variables determining the outcomes.
- CHSH:
- S = |E(a,b) - E(a, b’) + E(a’,b) + E(a’,b’)| <= 2

a

0o (a)’ + + + + - - - -

45o (b)’ + + + - - - - +

90o (a’) + + - - - - + +

135o (b’) + - - - - + + +

b

a’

b’

Alice’s axes: a and a’

Bob’s axes: b and b’

S = +1 - (-1) +1 -1 = 2

S = +1 -(+1) +1 +1 = 2

a

- S = |E(a,b) - E(a, b’) + E(a’,b) + E(a’,b’)| <= 2

b

Without local hidden variables, e.g. for Bell states

a’

b’

E(a,b) = cos

E(a,b’) = cos = - sin

E(a’,b) = cos= sin

E(a’,b’) = cos

S = | 2 cos sin

When =45o, we have S = > 2

i.e no local hidden variables

- Bipartite pure states:

Schmidt decomposition

Positive, real coefficients

- Bipartite pure states:

Schmidt decomposition

Positive, real coefficients

Reduced density operators

Same coefficients

Measure of mixedness

- Bipartite pure states:

Schmidt decomposition

Positive, real coefficients

Reduced density operators

Same coefficients

Measure of mixedness

Unique measure of entanglement (Entropy)

- Consider the Bell state:

- Consider the Bell state:

This can be written as:

- Consider the Bell state:

This can be written as:

Maximally entangled (S is maximised for two qubits)

“Monogamy of entanglement”

- Bipartite mixed states:

- Average over pure state entanglement that makes up the mixture
- Problem: infinitely many decompositions and each leads to a different entanglement
- Solution: Must take minimum over all decompositions (e.g. if a decomposition gives zero, it can be created locally and so is not entangled)

- Bipartite mixed states:

- Average over pure state entanglement that makes up the mixture
- Problem: infinitely many decompositions and each leads to a different entanglement
- Solution: Must take minimum over all decompositions (e.g. if a decomposition gives zero, it can be created locally and so is not entangled)

Entanglement of formation

von Neumann entropy

Minimum over all realisations of:

- An entanglement witness is an observable that distinguishes entangled states from separable ones

- An entanglement witness is an observable that distinguishes entangled states from separable ones

Theorem: For every entangled state, there exists a Hermitian operator, A, such that Tr(A)<0 and Tr(A)>=0 for all separable states,

Corollary: A mixed state, , is separable if and only if:

Tr(A)>=0

- An entanglement witness is an observable that distinguishes entangled states from separable ones

Theorem: For every entangled state, there exists a Hermitian operator, A, such that Tr(A)<0 and Tr(A)>=0 for all separable states,

Corollary: A mixed state, , is separable if and only if:

Tr(A)>=0

Thermodynamic quantities provide convenient (unoptimised) EWs

- Covalent bonding relies on entanglement of the electrons e.g. H2

Lowest energy (bound) configuration

Overall wave function is antisymmetric so the spin part is:

Entangled

The energy of the bound state is lower than any separable state - witness

Covalent bonding is evidence of entanglement

- Covalent bonding relies on entanglement of the electrons e.g. H2

NOTE: It is not at all clear that this entanglement could be used in quantum processing tasks.

You will often hear people distinguish “useful” entanglement from other sorts

The energy of the bound state is lower than any separable state - witness

Covalent bonding is evidence of entanglement

- State tomography
- Bell’s inequalities
- Entanglement witnesses (EW)

- State tomography
- Bell’s inequalities
- Entanglement witnesses (EW)

- It can give rise to macroscopic effects
- It can occur at finite temperature (i.e. the system need not be in the ground state)
- We do not need to know the state to detect entanglement
- It can occur for a single particle

- It can give rise to macroscopic effects
- It can occur at finite temperature (i.e. the system need not be in the ground state)
- We do not need to know the state to detect entanglement
- It can occur for a single particle

Let’s consider an example that exhibits all these features….

Molecule of the Year

Molecule of the Year

Overall state:

Atoms are not entangled

Use Entanglement Witnesses for free quantum fields

e.g. Bosons

Use Entanglement Witnesses for free quantum fields

e.g. Bosons

“Biblical” operators - more on these later…..

Use Entanglement Witnesses for free quantum fields

e.g. Bosons

Want to detect entanglement between regions of space

- Particle in a box of length L

where

- In each dimension:

- Particle in a box of length L

where

- In each dimension:

- For N separable particles in a d-dimensional box of length L, the minimum energy is:

- M spatial regions of length L/M

- M spatial regions of length L/M

- Internal energy, temperature, and equation of state

- Internal energy, temperature, and equation of state

- The critical temperature for BEC in an homogeneous trap is:

Comparing with the onset of entanglement across the system

These differ only by a numerical factor of about 2 !

Entanglement as a phase transition

- Typical numbers:

This gives:

In experiments, the temperature of the BEC is typically:

Entanglement in a BEC (even though it can be written as a product state of each particle)

- A reservoir of entanglement - changes the state of the BEC

Ref: I. Bloch et al., Nature 403, 166 (2000)

- The Munich experiment demonstrates long-range order (LRO)

Interference term Phase coherence

- It is tempting to think that LRO and entanglement are the same

- The Munich experiment demonstrates long-range order (LRO)

Interference term Phase coherence

- It is tempting to think that LRO and entanglement are the same

A GHZ-type state is clearly entangled:

BUT

- They are, however, related Ongoing research

- 1. Entanglement does not depend on how we divide the system
- 2. A single particle cannot be ‘entangled’
- 3. Nonlocality and entanglement are the same thing

- Entanglement depends on what the subsystems are

- Entanglement depends on what the subsystems are

Entangled

- “Superposition is the only mystery in quantum mechanics”

R. P. Feynman

What about entanglement?

- “Superposition is the only mystery in quantum mechanics”

R. P. Feynman

What about entanglement?

Instead of the superposition of a single particle, we can think of the entanglement of two different variables:

- “Superposition is the only mystery in quantum mechanics”

R. P. Feynman

What about entanglement?

Instead of the superposition of a single particle, we can think of the entanglement of two different variables:

Is this all just semantics?

Can we measure any real effect, e.g. violation of Bell’s inequalities?

- Single photon incident on a 50:50 beam splitter:

- Single photon incident on a 50:50 beam splitter:

- Single photon incident on a 50:50 beam splitter:

Entangled “Bell state”

Entanglement must be due to the single particle state

- “The term ‘particle’ survives in modern physics but very little of its classical meaning remains. A particle can now best be defined as the conceptual carrier of a set of variates. . . It is also conceived as the occupant of a state defined by the same set of variates... It might seem desirable to distinguish the ‘mathematical fictions’ from ‘actual particles’; but it is difficult to find any logical basis for such a distinction. ‘Discovering’ a particle means observing certain effects which are accepted as proof of its existence.”
- A. S. Eddington, Fundamental Theory, (Cambridge University Press., Cambridge, 1942) pp. 30-31.

- “The term ‘particle’ survives in modern physics but very little of its classical meaning remains. A particle can now best be defined as the conceptual carrier of a set of variates. . . It is also conceived as the occupant of a state defined by the same set of variates... It might seem desirable to distinguish the ‘mathematical fictions’ from ‘actual particles’; but it is difficult to find any logical basis for such a distinction. ‘Discovering’ a particle means observing certain effects which are accepted as proof of its existence.”
- A. S. Eddington, Fundamental Theory, (Cambridge University Press., Cambridge, 1942) pp. 30-31.

We need a field theory treatment of entanglement

- Nonlocality implies position distinguishability, which is not necessary for entanglement

- Confusion arises because Alice and Bob are normally spatially separated

- Nonlocality implies position distinguishability, which is not necessary for entanglement

- Confusion arises because Alice and Bob are normally spatially separated

Example:

This state is local, but can be considered to have entanglement

1

2

PBS

- What is entanglement
- Bell’s theorem and nonlocality
- Measures of entanglement
- Entanglement witness in a BEC
- Confusing concepts in entanglement