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Fibonacci anyons & Topological quantum Computers
By Christos Charalambous
Christos Charalambous 1/45 Fibonacci anyons & Topological Quantum Computing Fibonacci anyons & Topological Quantum Computers 13/05/13
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1. Quantum Computer Quantum Computing Decoherence
2. โAnyโ-ons Abelian anyons Non-abelian anyons Physical realization of non-abelian anyons Fractional Quantum Hall effect Geometric phase Topological Quantum Computer
3. Computing using anyons Braiding Fusion Recap: Fusion rules for minimal models Fusion space F & R matrices The unitary B-matrix Compatibility equations: Pentagon and Hexagon equations
4. Fibonacci model Fibonacci/Yang Lee model Chern-Simons theory Recap of WZW models
5. Summary
Contents
Fibonacci anyons & Topological Quantum Computing Fibonacci anyons & Topological Quantum Computers Christos Charalambous 2/45 13/05/13
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1. Quantum Computer Quantum Computing Decoherence
2. โAnyโ-ons Abelian anyons Non-abelian anyons Physical realization of non-abelian anyons Fractional Quantum Hall effect Geometric phase Topological Quantum Computer
3. Computing using anyons Braiding Fusion Recap: Fusion rules for minimal models Fusion space F & R matrices The unitary B-matrix Compatibility equations: Pentagon and Hexagon equations
4. Fibonacci model Fibonacci/Yang Lee model Chern-Simons theory Recap of WZW models
5. Summary
Contents
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Classical computer: limited computational power
3d magnet
Interference โ Quantum physics could speed up processes
Qubit: โฃ ๐ > = ๐ผ โฃ 0 > +๐ฝ โฃ 1 > where ๐2 + ๐ฝ2 = 1, ๐, ๐ฝ โ โ
Hilbert spaces: ๐1 โ ๐ป1 , ๐2 โ ๐ป2
โ ๐12 = ๐๐๐ฯ๐๐,๐= 1,2 โจฯ๐ โ ๐ป1โจ๐ป2
classical: m bits โ 2๐ states
quantum: m qubits โ 2๐ basis states
Entanglement: speeding up classical algorithms
Quantum computation
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โข Quantum Circuits: unitary operators that act on a Hilbert space, generated by n qubits, whose states encode the information we want to process.
Quantum Circuits are composed of elementary Quantum gates.
โข Universality: existence of a universal set of quantum gates, the elements of which can perform any unitary evolution in SU(N) with arbitrary accuracy
Need:
1. Singe qubit rotation gates that can span SU(2): 2. A Two - qubit entangling gate: ๐ โ ๐๐(2)
Examples of quantum algorithms: โ Deutsch algorithm
โ Shorโs factoring algorithm
Quantum computation
๐
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where โจ = addition mod 2
time
x=0 โ y unchanged
x=1 โ ๐ฆ = 0 โ ๐ฆ = 1๐ฆ = 1 โ ๐ฆ = 0
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Deutsch Algorithm
โข Boolean Function F: Constant (f(0)=f(1)) or Balanced (f(0)โ f(1))?
โข Requires only 1 measurement to answer while classically it takes 2 evaluations of F
Decoherence
Very easy for errors to appear in the system due to interactions with the environment:
Examples: 1. Bit flips: โฃ 0 > โ โฃ 1 > , โฃ 1 > โ โฃ 0 >
2. Phase flips: 1
2(โฃ 0 >+โฃ 1 >) โ
1
2(โฃ 0 >โโฃ 1 >)
Quantum computation
Fibonacci anyons & Topological Quantum Computing
H
H ๐ผ๐ญ
0
1
H
H
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๐ป =1
2
1 11 โ1
Single qubit rotation gate called Hadamard :
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Goal: encode information in an environment independent way
Idea: Topological properties are insensitive to local perturbations
Quantum computation
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TIME
Vu Pham
1. Quantum Computer Quantum Computing Decoherence
2. โAnyโ-ons Abelian anyons Non-abelian anyons Physical realization of non-abelian anyons Fractional Quantum Hall effect Geometric phase Topological Quantum Computer
3. Computing using anyons Braiding Fusion Recap: Fusion rules for minimal models Fusion space F & R matrices The unitary B-matrix Compatibility equations: Pentagon and Hexagon equations
4. Fibonacci model Fibonacci/Yang Lee model Chern-Simons theory Recap of WZW models
5. Summary
Contents
Fibonacci anyons & Topological Quantum Computing Christos Charalambous 8/45 Fibonacci anyons & Topological Quantum Computers 13/05/13
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Exchanging:
โฃ ๐๐ด >โฃ ๐๐ต > โ ๐๐๐ โฃ ๐๐ต >โฃ ๐๐ด >
Winding:
โฃ ๐๐ด >โฃ ๐๐ต > โ (๐๐๐)2 โฃ ๐๐ด >โฃ ๐๐ต >
In 3D: (๐๐๐)2 = ๐ผ
โBoson: ฮธ=2ฯ+2ฯn โฃ ๐๐ด๐ต > โ โฃ ๐๐ต๐ด >
โFermion: ฮธ=ฯ+2ฯn โฃ ๐๐ด๐ต > โ โโฃ ๐๐ต๐ด >
Move to 2D:
Any ฮธ โ โAnyโ-ons
Exchanging=braiding
๐ถ1(๐ก1)
๐ถ2(๐ก2)
๐ถ0(๐ก0)
A
B
Abelian anyons
Fibonacci anyons & Topological Quantum Computing Christos Charalambous 9/45 Fibonacci anyons & Topological Quantum Computers 13/05/13
z
y
x
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Degenerate state space ๐๐ , ๐ = 1,โฆ , ๐ then:
๐1๐2โฎ๐๐
๐โฒ1๐โฒ2โฎ๐โฒ๐
= ๐11 โฏ ๐1๐โฎ โฑ โฎ๐๐1 โฏ ๐๐๐
๐1๐2โฎ๐๐
Non-abelian anyons
Candidate space to store and process quantum information
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Physical realization of Non-abelian anyons:
1. Degenerate ground state
2. Finite energy gap ฮฮ for ground state
3. Adiabaticity
4. Anyons being far apart
5. All local operators have vanishing correlation functions apart from identity
Physical realization of Non-abelian anyons
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Trapped ๐โ gas
Conductance: ๐ = ๐๐2
โ where ๐ a fractional number
โFractional charge
Abelian anyons for ๐ =1
3
Laughlin states: Trial wavefunctions Expect โ fractional statistics = anyonic statistics
๐ต๐ง
๐ธ๐ฅ
Fractional Quantum Hall effect
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Time evolution of a state:
โฃ ๐ ฮค > = exp i๐พ๐ ๐ถ expโ๐2๐
โ ๐๐กฮค
0
๐ธ๐ ๐ ๐ก โฃ ๐ 0 >
where ๐พ๐(๐ถ) is Berryโs geometric phase:
๐พ๐ ๐ถ = ๐ < ๐, ๐ ๐ก โฃ ๐ป๐ ๐, ๐ ๐ก > ๐๐ ๐ถ= ๐ด๐๐๐
๐๐ถ
= 1
2๐น๐๐๐๐
๐ โง ๐๐ ๐๐
Geometric phase
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Vector potential gauge transformation: ๐ด๐ โ ฮ๐ โ ๐๐๐๐
Vector field:
(๐น๐๐)๐๐ โ (๐๐ฮ๐ โ ๐๐ฮ๐)
๐๐
โ invariant under gauge transformations
If (๐น๐๐)๐๐ โ 0 i.e. not diffeomorphic invariant as well:
โ Case 1: Non-degenerate state space โ Abelian geometric phase, U(1)
Example:
Geometric phase
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โ Case 2: Degenerate state space ((๐น๐๐)๐๐ a matrix):
Example: ๐ด๐ต๐ถ โ ๐ถ๐ต๐ด Transformations of the state space are elements of SU(2) For N degenerate state space transformations are elements of SU(N)
Geometric phase
Fibonacci anyons & Topological Quantum Computing
A
B
C 1
2
3 4
5
6
7
1 7
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In general: โฃ ๐ C > = ฮ๐ด ๐ถ โฃ ๐ 0 >
ฮ๐ด ๐ถ = exp i๐พ๐ ๐ถ
the following properties hold:
(A) ฮฮ ๐ถ2 โ ๐ถ1 = ฮฮ ๐ถ2 ฮฮ ๐ถ1 ๐ถ1, ๐ถ2 paths in parametric space
(B) ฮฮ ๐ถ0 = ๐ผ ๐ถ0 point
(C) ฮฮ ๐ถโ1 = ฮโ1๐ด(๐ถ) ๐ถ clockwise path,
๐ถโ1 anti-clockwise path
(D) ฮฮ ๐ถ โ ๐ = ฮฮ(๐ถ) ๐ is a function of time t
Geometric phase
(A)+(B)+(C) Forms a Group
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โข Relate parametric space to anyons coordinates
โข Assume vector field ๐น๐๐ is confined to anyons position
Hence:
non-abelian geometric evolutions in a system
phases evolutions of non-abelian anyons
Geometric phase
A B
M ๐ 2
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Quasiparticle worldlines forming braids carry out unitary transformations on a Hilbert space of n anyons. This Hilbert space is exponentially large and its states cannot be distinguished by local measurements
candidate model for fault-tolerant quantum computing
Topological Quantum Computer
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1997 A.Kitaev: System of non-abelian anyons with suitable properties can efficiently simulate a quantum circuit
2000 Freedman, Kitaev and Wang: system of anyons can be simulated by a quantum circuit
โEquivalence of the two views of the system, i.e. between an anyonic computational model (e.g. a Topological quantum computer) and a
quantum circuit
Is there an anyonic computational model that can simulate a quantum circuit that exhibits universality?
Topological Quantum Computer
Fibonacci anyons & Topological Quantum Computing Christos Charalambous 19/45 Fibonacci anyons & Topological Quantum Computers 13/05/13
Vu Pham
1. Quantum Computer Quantum Computing Decoherence
2. โAnyโ-ons Abelian anyons Non-abelian anyons Physical realization of non-abelian anyons Fractional Quantum Hall effect Geometric phase Topological Quantum Computer
3. Computing using anyons Braiding Fusion Recap: Fusion rules for minimal models Fusion space F & R matrices The unitary B-matrix Compatibility equations: Pentagon and Hexagon equations
4. Fibonacci model Fibonacci/Yang Lee model Chern-Simons theory Recap of WZW models
5. Summary
Contents
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World lines cannot cross
โ braids ๐๐ (particle histories) in
distinct topological classes
1:1 correspondence of the topological
classes with distinct elements of a Braid set
1. Any braid can be obtained by
multiplying elementary braids
2. The inverse of any braid exists
3. Existence of Vacuum
4. Associativity for disjoint ๐๐
Braiding
Braid group
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๐๐ โ ๐๐ฅ๐โ๐๐๐๐ ๐๐ ๐๐กโ ๐๐๐ ๐ + 1 ๐กโ ๐๐๐ฆ๐๐
๐(๐๐) representations of the Braid group = the unitary transformations
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Defining relations of Braid group for an anyonic model:
1. Exchanges of disjoint particles commute: ฯ๐ฯ๐ = ฯ๐ฯ๐ โฃ ๐ โ ๐ โฃโฅ 2
2. Yang-Baxter relation:
ฯ๐ฯ๐+1ฯ๐ = ฯ๐+1ฯ๐ฯ๐+1 ๐ = 1,2,โฆ , ๐ โ 2
=
Braiding
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Fusion: The process of bringing two particles together
A non abelian anyonic model is defined starting from the superselection sector:
Finite set of particles that are linked by the following fusion rules, and their charges are conserved under local operations
Fusion algebra: ๐ ร ๐ = ๐๐๐๐๐๐
where ๐๐๐๐ can be matrices
Abelian ๐๐๐๐=1
Non-abelian ๐๐๐๐๐ > 1
Associativity: ๐e๐๐๐ ๐๐๐๐ = ๐e๐๐๐ ๐๐๐๐
Commutativity: ๐๐๐๐= ๐๐๐๐
Fusion
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Same fusion algebra:
ฯ๐ ร ฯ๐ = ๐๐๐๐
๐
ฯ๐
Commutativity also holds:
๐๐๐๐ = ๐๐๐๐
Associativity :
๐๐๐๐๐
๐๐๐๐ = ๐๐๐๐
๐
๐๐๐๐
Recap: Fusion rules for minimal models
Minimal models can be mapped to anyonic models
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Fusion spaces ๐๐๐๐: are subspaces of the space of all possible fusion outcomes which are also Hilbert spaces:
๐๐โจ๐๐ = โจ๐๐๐๐๐๐๐๐๐
The logical states โฃ 0 > & โฃ 1 > will be encoded in one of these fusion spaces
Relation of dimensionality of fusion spaces and quantum dimension:
๐๐๐๐ = ๐๐๐๐๐๐๐
Simplest non-abelian example: fusion rule for Spin- 1
2 particles
1
2โจ1
2= 0โจ1
(i.e. 2x2=1+3)
Fusion space
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Fibonacci anyons fusion rule:
1 = vacuum
ฯ = non-abelian anyon
The Fusion (Anyonic) Hilbert space:
โข Fusion trees are orthogonal basis elements of a Hilbert space
โข If the initial and final states are fixed then the dimension of this space depends on the number of in-between outcomes. For the above example the dimension is 2.
Fusion space basis (fusion trees)
Fibonacci anyons & Topological Quantum Computing
ฯ ฯ
ฯ
ฯ
1
ฯ ฯ
ฯ
ฯ
ฯ
โฃ ๐๐ โ 1 ๐ โ ๐ > โฃ ๐๐ โ ๐ ๐ โ ๐ >
๐โจ๐ = 1โจ๐
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The order of the fusion should not be relevant (associativity):
Therefore there exists a matrix F that transforms one basis to the other:
Exchange of a & b before fusion= self-rotation of outcome c after fusion:
therefore just a phase factor is obtained. For many in between outcomes R = diagonal matrix:
F & R matrices
ฯ ฯ
ฯ
ฯ
1,ฯ
ฯ ฯ
ฯ
ฯ
1,ฯ โ
B A
D
C B A
D
C
F
The F-matrix The R-matrix
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Consider superposition of multiple fusion outcomes, and consider the braiding of two particles that do no have a direct fusion outcome
exchanges result in a non-diagonal matrix R By applying F matrices on the R matrices we can change to a basis where the anyons do have a direct fusion outcome:
Example:
F & R fully describe all the processes we can do in an anyonic model of computation
The unitary braiding matrix: B-matrix
๐ต = ๐น๐ ๐นโ1 = ๐(๐๐)
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Fusion is associative Associativity of fusion + allow pentagon equation must hold: braiding Hexagon equation
These two equations encode all the constraints we can impose on F & R
Compatibility equations: Pentagon & Hexagon equations
Fibonacci anyons & Topological Quantum Computing Christos Charalambous 29/45 Fibonacci anyons & Topological Quantum Computers 13/05/13
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1. Quantum Computer Quantum Computing Decoherence
2. โAnyโ-ons Abelian anyons Non-abelian anyons Physical realization of non-abelian anyons Fractional Quantum Hall effect Geometric phase Topological Quantum Computer
3. Computing using anyons Braiding Fusion Recap: Fusion rules for minimal models Fusion space F & R matrices The unitary B-matrix Compatibility equations: Pentagon and Hexagon equations
4. Fibonacci model Fibonacci/Yang Lee model Chern-Simons theory Recap of WZW models
5. Summary
Contents
Fibonacci anyons & Topological Quantum Computing Christos Charalambous 30/45 Fibonacci anyons & Topological Quantum Computers 13/05/13
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Simple and rich structure
Encoding of logical states (the naive way):
Fibonacci anyonic model
Fibonacci anyons & Topological Quantum Computing
๐โจ๐ = 1โจ๐
1
ฯ
โฃ >
โฃ >
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Simple and rich structure
Encoding of logical states:
The last state is not a problem as we will see right now:
Fibonacci anyonic model
Fibonacci anyons & Topological Quantum Computing
๐โจ๐ = 1โจ๐
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From fusion rules and pentagon equation for Fibonacci model:
๐น๐๐๐1 = ๐น1๐๐๐ = ๐น
๐1๐๐ = ๐น
๐๐1๐ = 1
๐น๐๐๐๐ =
1
๐
1
๐
1
๐โ1
๐
where ฯ is the golden ratio ฯ =(1+ 5)
2
From hexagon equation and Yang-baxter relation:
๐ ๐1๐ = ๐ 1๐๐ = 1
๐ ๐๐ = ๐๐4๐/5 00 โ๐๐2๐/5
Fibonacci anyonic model
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Braiding matrices obtained from F and R:
๐1 =
๐2 =
Fibonacci anyonic model
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Dimension of Hilbert space for Fibonacci model with the constraint that no two consecutive 1โs can appear
Bratelli diagram:
2 1 ฯ
ฯ 1 ฯ
1 ฯ ฯ 1 ฯ
3 5
Fibonacci anyonic model
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Dimension of the space is โ ฮฆ๐
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Universality
Solovay and Kitaev (version of brute force search algorithm):
Combine short braids โ can obtain a long braid that with arbitrary accuracy ฮต will simulate a desired single qubit unitary operation
Bonesteel, Hormozi: 2-qubit entangling gate CNOT
1st observation:
Fibonacci anyonic model
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ฯ ฯ ฯ
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2nd observation: Case 1: upper qubit is
Case 2: upper qubit is
Fibonacci anyonic model
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โ ๐(๐1) & ๐(๐2) acting on the logical states can perform any unitary evolution in SU(N)
โ universal computations
Conclusion:
Fibonacci anyonic model:
โข Can achieve universal computing
โข well-controlled accuracy
โข Requires 4n physical anyons for encoding n logical qubits (i.e. polynomial scaling)
Fibonacci anyonic model
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spin-1 particles: 1โจ1 = 0โจ1โจ2 Similarity to: ๐ โ ๐ = 0โ ๐ if spin-2 is cut off
๐๐(2)๐: โโ quantizedโโ version of SU(2) obtained by truncating the possible values of the angular momentum to
j = 0,1
2, 1,3
2โฆ ,๐
2
e.g. ๐๐(2)3 = {0,1
2, 1,3
2}
Consider only 0 & 1 particles of ๐๐(2)3
โsubgroup (โโevenโโ part) of ๐๐(2)3 โ superselection sector of Fibonacci model
A model described by such symmetry is the ๐บ๐ผ(๐)๐ WZW model coupled to a U(1) gauge field
Fibonacci anyons and ๐๐(2)3
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๐๐ถ๐ ๐ด =๐
4๐ ๐3๐ฅ๐๐๐๐
ฮ
๐ก๐ ๐ด๐๐๐๐ด๐ + ๐2
3๐ด๐๐ด๐๐ด๐ =
1
4๐ ๐3๐ฅ๐ฟ๐ถ๐(๐ด)ฮ
where k: coupling constant , ๐ = ฮฃ ร R (2+1D) , A is a gauge field
No metric โ invariant under diffeomorphisms
Non Abelian Gauge transformations:
๐ดโฒ๐ = ๐๐ด๐๐โ1 โ ๐๐๐๐๐
โ1 where g:๐ โ ๐บ
โ๐ฟ๐ถ๐ ๐ด
โฒ =
๐ฟ๐ถ๐(๐ด) โ ๐๐๐๐๐๐๐๐ก๐ ๐๐๐ ๐
โ1๐ด๐ โ๐
3๐๐๐๐๐ก๐ ๐โ1(๐๐๐)๐
โ1(๐๐๐)๐โ1(๐๐๐)
For suitable boundary conditions the 1st extra term vanishes in the action.
Non-abelian Chern-Simons Theory
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In the case of simple compact groups e.g. G=SU(2):
๐ ๐ =1
24๐2๐๐๐๐๐ก๐ ๐โ1(๐๐๐)๐
โ1(๐๐๐)๐โ1(๐๐๐)
where ฯ(g)=winding number and we realize that is proportional to the 2nd extra term in the Lagrangian. Hence the action becomes:
โ ๐๐ถ๐ ๐ดโฒ = ๐๐ถ๐ ๐ด + 2๐๐ ๐(๐)
โข If ฯ(g)=0 (small gauge transformations, low energies)
โ action is invariant
โข If ฯ(g)โ 0 (large gauge transformations, high energies)
โ failure of gauge invariance
Case 1: k integer โ OK
Case 2: k not an integer โ gapped pure gauge degrees of freedom for the high energy theory
Non-abelian Chern-Simons Theory
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๐ป =๐
4๐๐ก๐ ๐ด2๐0๐ด1 โ ๐ด1๐0๐ด2 โ ๐ฟ = 0
Easily seen if we choose gauge ๐ด0 = 0 where the momenta
canonically conjugated to: ๐ด1: โ๐
4๐๐ด2 , ๐ด2:
๐
4๐๐ด1
Introduce spatial boundaries ๐ = ๐ฮฃ ร R
Locally (bulk part): Gauge invariance BUT globally: topological obstruction in making the gauge field zero everywhere if ฮฃ topologically non-trivial
โ Chern-Simons gauge invariant up to a surface term โ physical topological degrees of freedom
โ Chern-Simons: theory of ground state of a 2D topologically ordered system in ฮฃ
Non-abelian Chern-Simons Theory
Fibonacci anyons & Topological Quantum Computing Christos Charalambous 42/45 Fibonacci anyons & Topological Quantum Computers 13/05/13
Vu Pham
Conclusion โข For low energies: โ Chern Simons theory describes non abelian anyons. โข For high energies: โ Difficult to disentangle physical topological degrees of freedom from unphysical local gauge degrees of freedom
โ hence have to consider Chern Simons as low energy effective field theory
What is the theory that describes the excitations of these
quasiparticles?
Non-abelian Chern-Simons Theory
Fibonacci anyons & Topological Quantum Computing Christos Charalambous 43/45 Fibonacci anyons & Topological Quantum Computers 13/05/13
Vu Pham
WZW models describe Symmetry Protected topological Phases (SPT) in 2D at an open boundary with symmetry SU(2).
Showed global and local SU(2) invariance of ๐ฝ+ (the WZW charge carrying covariant current density) coupled with an external field action
Showed that integrating action with an external field leads to an effective field theory:
WZW action coupled with external field at low energies
โ Chern Simon action
For WZW models:
โ k= number of anyon species in the theory โ integer
โ gapless WZW gauge degrees of freedom = CS pure gauge degrees of freedom
Recap of WZW models
Fibonacci anyons & Topological Quantum Computing Christos Charalambous 44/45 Fibonacci anyons & Topological Quantum Computers 13/05/13
โSolved problem of Chern-Simons in higher energies
Vu Pham
1. Defined Quantum Computer and identified problem of decoherence
2. Identified topological properties as a remedy for the problem
3. Identified anyons as systems that exhibit such topological properties and hence under specific conditions can accomodate a Topological quantum computer 4. Examined Fibonacci anyons as candidate particles for performing Universal Topological quantum computing
5. Showed that such particles can be theoretically described in the context of a CFT model
Summary
Fibonacci anyons & Topological Quantum Computers 13/05/13 Christos Charalambous 45/45