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the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 1/24
(Continuous Variables) Quantum Optics, QuantumInformation and Relativistic Quantum Information
David Edward Bruschi
York Centre of Quantum TechnologiesDepartment of Physics
University of Yorkthe (now Brexited) United Kingdom
XVII August MMXVII
Partially based on: Introductory Quantum Optics, C. C. Gerry and P. L. Knight.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 2/24
1 IntroductionHigh energy physicsLow energy physics
2 Quantum OpticsQuantum Optics premisesQuantum Optics implementations
3 Convariance matrix formalismGaussian statesEntanglement of bipartite Gaussian statesExamples with Gaussian states
4 (Quantum) Information TheoryQuantum Information theory: teleportationConclusions
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 3/24
Introduction
High energy physics
Quantum Electro Dynamics (QED)
Relativistic and quantum Lagrangian LQED for matter interacting with light
LQED = ψ (i γµDµ −m) ψ − 1
4Fµν Fµν .
- γµ are Dirac matrices;
- ψ a bispinor field of spin-1/2 particles (e.g. electron-positron field);
- ψ ≡ ψ†γ0, called ”psi-bar”, also referred to as the Dirac adjoint;
- Dµ ≡ ∂µ + i e Aµ + i e Bµ is the gauge covariant derivative;
- e is the coupling constant, i.e., the electric charge of the bispinor field;
- m is the mass of the electron or positron;
- Aµ is the covariant four-potential of the electromagnetic fieldgenerated by the electron itself;
- Bµ is the external field imposed by external source;
- Fµν = ∂µAν − ∂νAµ is the electromagnetic field tensor.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 4/24
Introduction
High energy physics
Quantum Electrodynamics
High energy physics - Interacting theory
- Construct vertex from the interaction coupled-term LI ∼ e ψ γµ ψ Aµ;
- Coupling strength λ ∼ e;
- Energy, momentum, charge must be conserved in physical processes;
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 5/24
Introduction
Low energy physics
Semiclassical theory
Low energy physics of light and matter
In the low energy physics regime we can safely ignore details of light-matterinteraction contained in LQED. We can employ a semiclassical theory.
Classical and quantum fields (the latter with [aks , a†k′s′ ] = δ3(k − k ′) δss′)
A(x , t) =∑s
∫d3k ek,s
[Ak,s e
i (k·x−ωk t) + A∗k,s e−i (k·x−ωk t)
],
A(x , t) =∑s
∫d3k ek,s
[ak,s e
i (k·x−ωk t) + a†k,s e−i (k·x−ωk t)
].
Classical field
Classical Hamiltonian of e.m. field:
H0 =∑s
∫d3x ω(k)A∗k,s Ak,s .
Quantized field
Quantum Hamiltonian of e.m. field
H0 =∑s
∫d3k ω(k) a†k,s ak,s + E0.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 5/24
Introduction
Low energy physics
Semiclassical theory
Low energy physics of light and matter
In the low energy physics regime we can safely ignore details of light-matterinteraction contained in LQED. We can employ a semiclassical theory.
Classical and quantum fields (the latter with [aks , a†k′s′ ] = δ3(k − k ′) δss′)
A(x , t) =∑s
∫d3k ek,s
[Ak,s e
i (k·x−ωk t) + A∗k,s e−i (k·x−ωk t)
],
A(x , t) =∑s
∫d3k ek,s
[ak,s e
i (k·x−ωk t) + a†k,s e−i (k·x−ωk t)
].
Classical field
Classical Hamiltonian of e.m. field:
H0 =∑s
∫d3x ω(k)A∗k,s Ak,s .
Quantized field
Quantum Hamiltonian of e.m. field
H0 =∑s
∫d3k ω(k) a†k,s ak,s + E0.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 5/24
Introduction
Low energy physics
Semiclassical theory
Low energy physics of light and matter
In the low energy physics regime we can safely ignore details of light-matterinteraction contained in LQED. We can employ a semiclassical theory.
Classical and quantum fields (the latter with [aks , a†k′s′ ] = δ3(k − k ′) δss′)
A(x , t) =∑s
∫d3k ek,s
[Ak,s e
i (k·x−ωk t) + A∗k,s e−i (k·x−ωk t)
],
A(x , t) =∑s
∫d3k ek,s
[ak,s e
i (k·x−ωk t) + a†k,s e−i (k·x−ωk t)
].
Classical field
Classical Hamiltonian of e.m. field:
H0 =∑s
∫d3x ω(k)A∗k,s Ak,s .
Quantized field
Quantum Hamiltonian of e.m. field
H0 =∑s
∫d3k ω(k) a†k,s ak,s + E0.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 6/24
Quantum Optics
Quantum Optics premises
Moving to Quantum Optics
Quantized electric and magnetic fields
E (x , t) =i∑s
∫d3k ek,s
[ak,s e
i (k·x−ωk t) + a†k,s e−i (k·x−ωk t)
]B(x , t) =
i
c
∑s
∫d3k
(k|k |× ek,s
)[ak,s e
i (k·x−ωk t) + a†k,s e−i (k·x−ωk t)
].
We now make important considerations:
i) Most quantum optical situations, coupling of field to matter is throughelectric field interacting with a dipole moment or through somenonlinear type of interaction involving powers of the electric field;
ii) Focus on the electric field E (x , t);
iii) Magnetic field is “weaker” than the electric field by a factor of 1c ;
iv) Field couples to the spin magnetic moment of the electrons;
v) This interaction is negligible for essentially all the aspects of QuantumOptics that we are concerned with;
vi) Negligible spatial variation of field over dimensions of atomic system.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 7/24
Quantum Optics
Quantum Optics premises
From the electromagnetic field to modes of light
Dipole approximation:
E (x , t) ∼ ex
[a e−i ωk t + a† e i ωk t
], [a, a†] = 1.
For our purposes: replace field operator E (x , t) by single bosonic mode a.
Significant states of one-mode a
Vacuum state |0〉: a |0〉 = 0.Number state |n〉: a† a |n〉 = n |n〉,
|n〉 =
(a†)n
√n!|0〉.
Coherent state |α〉:
|α〉 = exp[α a− α∗ a†] |0〉.
Single-mode squeezed state |s〉:
|s〉 = exp[s a† 2 − s∗ a2] |0〉.
Significant states of two-modes a,c
Vacuum state |0〉: a |0〉 = c |0〉 = 0Number state |n,m〉:[a† a + b† b] |n,m〉 = (n + m) |n,m〉,
|n,m〉 =
(a†)n
√n!
(b†)m
√m!|0〉.
Two-mode squeezed state |r〉:
|r〉 = exp[r a† b† − r∗ a b] |0〉.
We have [a, b] = [a, b†] = 0.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 7/24
Quantum Optics
Quantum Optics premises
From the electromagnetic field to modes of light
Dipole approximation:
E (x , t) ∼ ex
[a e−i ωk t + a† e i ωk t
], [a, a†] = 1.
For our purposes: replace field operator E (x , t) by single bosonic mode a.
Significant states of one-mode a
Vacuum state |0〉: a |0〉 = 0.Number state |n〉: a† a |n〉 = n |n〉,
|n〉 =
(a†)n
√n!|0〉.
Coherent state |α〉:
|α〉 = exp[α a− α∗ a†] |0〉.
Single-mode squeezed state |s〉:
|s〉 = exp[s a† 2 − s∗ a2] |0〉.
Significant states of two-modes a,c
Vacuum state |0〉: a |0〉 = c |0〉 = 0Number state |n,m〉:[a† a + b† b] |n,m〉 = (n + m) |n,m〉,
|n,m〉 =
(a†)n
√n!
(b†)m
√m!|0〉.
Two-mode squeezed state |r〉:
|r〉 = exp[r a† b† − r∗ a b] |0〉.
We have [a, b] = [a, b†] = 0.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 8/24
Quantum Optics
Quantum Optics premises
Phase space representation of interesting states
|α〉 = exp[α a− α∗ a†] |0〉 = e−|α|22∑
nαn√n!|n〉 .
ρ(T ) = 1cosh2 r
∑n tanh2 n r |n〉〈n| , tanh r = e
− ~ωkB T .
|s〉 = exp[s a† 2 − s∗ a2] |0〉 = 1cosh s
∑n tanhn s |2 n〉 .
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 9/24
Quantum Optics
Quantum Optics implementations
Operations in Quantum Optics
Time evolution / transformation of states ρ: ρ(λ) = U†(λ) ρ(0)U(λ),
Linear unitary operations: effectively reduced to
Free evolution/phase shifting:U(t) = exp[−i ωa t(= φ)a† a].
Beam splitting:U(θ) = exp[−i θ (a† b + a b†)].
Single mode squeezing:U(s) = exp[(s a†,2 − s∗a2)].
Two mode squeezing:U(r) = exp[(r a† b† − r∗a b)].
φ
r
a ' = e−iφaa
a
b
s a a ' = cosh(s)a+ sinh(s)a†
a ' = cosh(r)a+ sinh(r)b†
b ' = cosh(r)b+ sinh(r)a†
a
b
b ' = cos(θ )b− sin(θ )a
a ' = cos(θ )a+ sin(θ )b
Unitary Evolution / Transformation
Non-linear unitary operations: effectively reduced to
PDC: U(ξ) = exp[ξ a† b† c − ξ∗ a b c†]. ξ c
a ' = cosh(ξc )a+ sinh(ξc )b†
b ' = cosh(ξc )b+ sinh(ξc )a†
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 9/24
Quantum Optics
Quantum Optics implementations
Operations in Quantum Optics
Time evolution / transformation of states ρ: ρ(λ) = U†(λ) ρ(0)U(λ),
Linear unitary operations: effectively reduced to
Free evolution/phase shifting:U(t) = exp[−i ωa t(= φ)a† a].
Beam splitting:U(θ) = exp[−i θ (a† b + a b†)].
Single mode squeezing:U(s) = exp[(s a†,2 − s∗a2)].
Two mode squeezing:U(r) = exp[(r a† b† − r∗a b)].
φ
r
a ' = e−iφaa
a
b
s a a ' = cosh(s)a+ sinh(s)a†
a ' = cosh(r)a+ sinh(r)b†
b ' = cosh(r)b+ sinh(r)a†
a
b
b ' = cos(θ )b− sin(θ )a
a ' = cos(θ )a+ sin(θ )b
Unitary Evolution / Transformation
Non-linear unitary operations: effectively reduced to
PDC: U(ξ) = exp[ξ a† b† c − ξ∗ a b c†]. ξ c
a ' = cosh(ξc )a+ sinh(ξc )b†
b ' = cosh(ξc )b+ sinh(ξc )a†
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 9/24
Quantum Optics
Quantum Optics implementations
Operations in Quantum Optics
Time evolution / transformation of states ρ: ρ(λ) = U†(λ) ρ(0)U(λ),
Linear unitary operations: effectively reduced to
Free evolution/phase shifting:U(t) = exp[−i ωa t(= φ)a† a].
Beam splitting:U(θ) = exp[−i θ (a† b + a b†)].
Single mode squeezing:U(s) = exp[(s a†,2 − s∗a2)].
Two mode squeezing:U(r) = exp[(r a† b† − r∗a b)].
φ
r
a ' = e−iφaa
a
b
s a a ' = cosh(s)a+ sinh(s)a†
a ' = cosh(r)a+ sinh(r)b†
b ' = cosh(r)b+ sinh(r)a†
a
b
b ' = cos(θ )b− sin(θ )a
a ' = cos(θ )a+ sin(θ )b
Unitary Evolution / Transformation
Non-linear unitary operations: effectively reduced to
PDC: U(ξ) = exp[ξ a† b† c − ξ∗ a b c†]. ξ c
a ' = cosh(ξc )a+ sinh(ξc )b†
b ' = cosh(ξc )b+ sinh(ξc )a†
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 10/24
Quantum Optics
Quantum Optics implementations
Quantum Optics laboratory
Now use BCH: eA eB = eA+B+ 12 [A,B]+... .
Figure: Figure from phys.org
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 11/24
Convariance matrix formalism
Gaussian states
Gaussian states of light
Picking experimentally realisable states of light
i) Very few states can be realised in optics laboratories (i.e., not |981〉);
ii) Employable states are prepared using linear optics;
iii) These state can be manipulated with linear optics.
We choose to work with Gaussian states
Gaussian states
i) Have Gaussian Wigner function;
ii) Wigner function is positive;
iii) Are defined by finite d.o.f.;
iv) Produced in every Q.O. lab.
W (ξ) =1
π2
∫R2 N
d2NX χs(κ) e i X Ω ξ
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 11/24
Convariance matrix formalism
Gaussian states
Gaussian states of light
Picking experimentally realisable states of light
i) Very few states can be realised in optics laboratories (i.e., not |981〉);
ii) Employable states are prepared using linear optics;
iii) These state can be manipulated with linear optics.
We choose to work with Gaussian states
Gaussian states
i) Have Gaussian Wigner function;
ii) Wigner function is positive;
iii) Are defined by finite d.o.f.;
iv) Produced in every Q.O. lab.
W (ξ) =1
π2
∫R2 N
d2NX χs(κ) e i X Ω ξ
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 11/24
Convariance matrix formalism
Gaussian states
Gaussian states of light
Picking experimentally realisable states of light
i) Very few states can be realised in optics laboratories (i.e., not |981〉);
ii) Employable states are prepared using linear optics;
iii) These state can be manipulated with linear optics.
We choose to work with Gaussian states
Gaussian states
i) Have Gaussian Wigner function;
ii) Wigner function is positive;
iii) Are defined by finite d.o.f.;
iv) Produced in every Q.O. lab.
W (ξ) =1
π2
∫R2 N
d2NX χs(κ) e i X Ω ξ
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 12/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
- N modes of light a1, ..., aN ;
- Introduce X := (a1, ..., aN , a†1, ..., a
†N)T ;
- Commutation relations: [an, a†m] = i Ωnm.
- Symplectic form Ω: Ωnm = diag(−i ,−i , ..., i , i , ...).
- First moments d : d := 〈X〉ρ.
- Second moments σ: σnm := 〈Xn, X†m〉ρ − 2 〈Xn〉ρ〈X†m〉ρ.
Linear transformations
- Quadratic in an, a†m;
- N-mode linear represented by 2N × 2N symplectic matrix S .
- Symplectic: S†ΩS = Ω.
- Unitary U = exp[−i H t]: represented by S = exp[−i F (t)ΩH].Here H = 1
2 XT H X.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 13/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
Gaussian states are defined by only first and second moments: they aredefined univocally by the covariance matrix σ and the first moments d .
Williamson Theorem
σ = S† ν⊕ S
Here ν⊕ = diag(ν1, ν2, ..., ν1, ν2, ...) is the Williamson form of σ andνk ≥ 1 are the symplectic eigenvalues of σ.
Purity
N.B. The state σ is pure iff νk = 1 for all k. The symplectic eigenvaluesare νk = coth( ~ωk
Kb Tk).
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 13/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
Gaussian states are defined by only first and second moments: they aredefined univocally by the covariance matrix σ and the first moments d .
Williamson Theorem
σ = S† ν⊕ S
Here ν⊕ = diag(ν1, ν2, ..., ν1, ν2, ...) is the Williamson form of σ andνk ≥ 1 are the symplectic eigenvalues of σ.
Purity
N.B. The state σ is pure iff νk = 1 for all k. The symplectic eigenvaluesare νk = coth( ~ωk
Kb Tk).
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 13/24
Convariance matrix formalism
Gaussian states
Covariance matrix formalism
Gaussian states
Gaussian states are defined by only first and second moments: they aredefined univocally by the covariance matrix σ and the first moments d .
Williamson Theorem
σ = S† ν⊕ S
Here ν⊕ = diag(ν1, ν2, ..., ν1, ν2, ...) is the Williamson form of σ andνk ≥ 1 are the symplectic eigenvalues of σ.
Purity
N.B. The state σ is pure iff νk = 1 for all k . The symplectic eigenvaluesare νk = coth( ~ωk
Kb Tk).
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 14/24
Convariance matrix formalism
Gaussian states
Quantum optics with covariance matrix formalism
Gaussian state evolution/transformations
σ(λ) = S†(λ)σ(0) S(λ) .
Gaussian vs. non-Gaussian states
Non-Gaussian / Hilbert
i) N modes;
ii) Infinite d.o.f;
iii) Tensor product Hilbert space;
iv) Unitary linear operators;
v) Trace operation: infinite sums.
vi) Entanglement: very difficultmeasures to compute.
Gaussian / CM formalism
i) N modes;
ii) Finite d.o.f.;
iii) Direct sum CM construction;
iv) 2N × 2N symplectic matrices;
v) Trace operation: deleterows/columns.
vi) Entanglement: see next.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 14/24
Convariance matrix formalism
Gaussian states
Quantum optics with covariance matrix formalism
Gaussian state evolution/transformations
σ(λ) = S†(λ)σ(0) S(λ) .
Gaussian vs. non-Gaussian states
Non-Gaussian / Hilbert
i) N modes;
ii) Infinite d.o.f;
iii) Tensor product Hilbert space;
iv) Unitary linear operators;
v) Trace operation: infinite sums.
vi) Entanglement: very difficultmeasures to compute.
Gaussian / CM formalism
i) N modes;
ii) Finite d.o.f.;
iii) Direct sum CM construction;
iv) 2N × 2N symplectic matrices;
v) Trace operation: deleterows/columns.
vi) Entanglement: see next.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 15/24
Convariance matrix formalism
Entanglement of bipartite Gaussian states
Entanglement of Gaussian states
Separability and entanglement
The state ρAB separable if exists ρAB = ρA ⊗ ρB . If not, it is entangled.EPR state: |Ψ〉 = 1√
2[|01〉+ |10〉].
Symplectic eigenvalues νk ≥ 1: spectrum of i Ωσ.
Symplectic spectrum of the partial transpose
Compute spectrum νk of i ΩP† σP . Here P implements partialtransposition of one mode.
Of these eigenvalues νk take the smallest (in absolute value), called ν−.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 15/24
Convariance matrix formalism
Entanglement of bipartite Gaussian states
Entanglement of Gaussian states
Separability and entanglement
The state ρAB separable if exists ρAB = ρA ⊗ ρB . If not, it is entangled.EPR state: |Ψ〉 = 1√
2[|01〉+ |10〉].
Symplectic eigenvalues νk ≥ 1: spectrum of i Ωσ.
Symplectic spectrum of the partial transpose
Compute spectrum νk of i ΩP† σP . Here P implements partialtransposition of one mode.
Of these eigenvalues νk take the smallest (in absolute value), called ν−.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 15/24
Convariance matrix formalism
Entanglement of bipartite Gaussian states
Entanglement of Gaussian states
Separability and entanglement
The state ρAB separable if exists ρAB = ρA ⊗ ρB . If not, it is entangled.EPR state: |Ψ〉 = 1√
2[|01〉+ |10〉].
Symplectic eigenvalues νk ≥ 1: spectrum of i Ωσ.
Symplectic spectrum of the partial transpose
Compute spectrum νk of i ΩP† σP . Here P implements partialtransposition of one mode.
Of these eigenvalues νk take the smallest (in absolute value), called ν−.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 16/24
Convariance matrix formalism
Examples with Gaussian states
An example: using Hilbert space formalism
Start from the vacuum state |0〉. Use linear optics and two-mode squeezethe vacuum with U(r) := exp[−r (a† b† − a b)]. (Lots of) algebra:
|Ψ(r)〉 = U(r) |0〉 =∑n
tanhn r
cosh r|n, n〉.
The state ρ(r) is:
ρ(r) = |Ψ(r)〉〈Ψ(r)| =∑n,m
tanhn+m r
cosh2 r|n, n〉〈m,m|.
Partial transpose mode b is
ρ(r) = |Ψ(r)〉〈Ψ(r)| =∑n,m
tanhn+m r
cosh2 r|n,m〉〈m, n|.
Choose measure: Negativity N :=Tr(√ρ† ρ)−12
More algebra: N = e2 r−12 .
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 17/24
Convariance matrix formalism
Examples with Gaussian states
An example: using Covariance Matrix formalism
Start from the vacuum state σ0. Use linear optics and two-mode squeezethe vacuum with S(r). (Little) algebra:
σ0 =
1 0 0 00 1 0 00 0 1 00 0 0 1
, S(r) =
cosh r 0 0 sinh r
0 cosh r sinh r 00 sinh r cosh r 0
sinh r 0 0 cosh r
.
The state σ(r) := S†(r)σ0 S(r) is:
σ(r) =
cosh 2r 0 0 sinh 2r
0 cosh 2r sinh 2r 00 sinh 2r cosh 2r 0
sinh 2r 0 0 cosh 2r
.
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 18/24
Convariance matrix formalism
Examples with Gaussian states
An example: using Covariance Matrix formalism
Partial transpose mode b is implemented by P and obtain P† σP
P =
1 0 0 00 0 0 10 0 1 00 1 0 0
, P† σP =
cosh 2r sinh 2r 0 0sinh 2r cosh 2r 0 0
0 0 cosh 2r sinh 2r0 0 sinh 2r cosh 2r
.
Compute the spectrum of i ΩP† σP, which is
e2 r , ν− = e−2 r ,−e2 r ,−e−2 r , .
Choose measure: Negativity N := 1−ν−2 ν−
.
Simply obtain: N = e2 r−12 .
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 19/24
(Quantum) Information Theory
Quantum Information
Information theory aims at understanding how to...
Store
Transmit
Decode Quantify
Employ
Secure
...information
Classical IT Discrete QI CV QI
Systems Bits (“0” and “1”) Qbits (|0〉 and |1〉) Modes of lightDimensions 2N 2N Infinite
Gates Classical Boolean Unitaries Linear unitariesMeasurements Magnetic readout Projective Projective
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 19/24
(Quantum) Information Theory
Quantum Information
Information theory aims at understanding how to...
Store
Transmit
Decode Quantify
Employ
Secure
...information
Classical IT Discrete QI CV QI
Systems Bits (“0” and “1”) Qbits (|0〉 and |1〉) Modes of lightDimensions 2N 2N Infinite
Gates Classical Boolean Unitaries Linear unitariesMeasurements Magnetic readout Projective Projective
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 20/24
(Quantum) Information Theory
Quantum Information theory: teleportation
Discrete Variables
i) Generate EPR. One qubit to A, other to B;
ii) Bell measurement at A of EPR pair qubitand the qubit (|φ〉) to be teleported. Yieldone of four measurement outcomes,encoded in two classical bits;
iii) Using the classical channel, the two bitsare sent from A to B. (Speed less than c);
iv) Result of measurement at A, EPR pairqubit at B in one of four possible states.Of these, one identical to the originalquantum state |φ〉, other three are closelyrelated. Which of these four possibilitiesactually obtains is encoded in the twoclassical bits. Knowing this, the qubit atlocation B is modified to result in a qubitidentical to |φ〉.
Figure: Figure fromWikipedia
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 21/24
(Quantum) Information Theory
Quantum Information theory: teleportation
Continuous Variables
i) Generate EPR-like state: two-modesqueezed state;
ii) CV version of Bell measurement.Beam split modes a and “in”.Homodyne detect quadratures x−and p+;
iii) Classical channel, same as before;
iv) Similar as before. Use classicalinformation to perform extradisplacement. Obtain initial stateρin.
Figure: Protocol and figure based onSection IV.B in Laser Physics 16,1418 (2006)
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 22/24
(Quantum) Information Theory
Quantum Information theory: teleportation
Relativistic Quantum Teleportation
i) Generate usual squeezed state;
ii.i) Homodyne measurement at A of localmode and the mode to be teleported;
ii.ii) Rob moves! Field inside Rob’s cavity isaffected by motion. Mode from squeezedstate is mixed to other modes;
iii) Using the classical channel, the two bitsare sent from A to R. (Speed less than c);
iv) Result like before but depends also onmotion. Figure: PRL 110, 113602
(2013)
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 23/24
(Quantum) Information Theory
Conclusions
Conclusions
Information Theory Thermodynamics Quantum Mechanics
Relativity
Quantum Information
Black Hole Thermodyn. QFT in curved spacetime
Relativistic Quantum Information
the Orthodox Academy of Crete - (Continuous Variables) Quantum Optics, Quantum Information and Relativistic Quantum Information. 24/24
(Quantum) Information Theory
Conclusions
Acknowledgments
Thank you