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Exploring topological states with cold atoms and photons
$$ NSF, AFOSR MURI, DARPA OLE, MURI ATOMTRONICS
Harvard-MIT
Theory: Takuya Kitagawa, Dima Abanin, Erez Berg, Mark Rudner, Liang Fu, Takashi Oka, Immanuel Bloch, Eugene Demler
Experiments: I. Bloch’s group (MPQ/LMU) A. White’s group (Queensland)
Universality in condensed matter physicsSpontaneous symmetry breaking and order
keV MeV GeV TeVfeV peV µeV meV eV
pK nK µK mK K
neV
roomtemperature
LHCCold atoms experiments10‐11 ‐ 10‐10 K
Higgs mode in ultracold atoms, 2012Higgs mode of the standard model, 2012
Universality ofphysics
Order beyond symmetry breaking
What is the “quantum protectorate” of such precise quantization?
Current along x, measure voltage along y.On a plateau
In 1980 the first ordered phase beyond symmetry breaking was discovered
Integer Quantum Hall Effect: 2D electron gas in strong magnetic field show plateaus in Hall conductance
with an accuracy of 10-9
Topological orderIn a topologically ordered state some physical quantity is given by a discreet “topological invariant”. Some physical response function is determined by this quantized invariant.
Topological invariant: quantity that does not change under continuous deformations
Example of topological invariant in geometry
Gaussian curvature at every point on a surface
Gauss-Bonnet theorem for closed surfaces
g – integer genus of a surface g=0 g=1
How to define topological invariant for electrons in solids?
What kind of curvature can exist for electrons in solids?
Bloch’s theorem and Brillouin zoneOne electron wavefunction in a crystal (periodic) potential can be written as
As k changes, we map an “energy band”. Set of all bands is a “band structure”.
But … lattice momentum is periodic
k is “crystal momentum” restricted to Brillouin zone,a region of k-space with periodic boundaries.Function is periodic (same in every unit cell)
The Brillouin zone can playthe role of the surface. Important property of quantummechanics, the Berry phase, gives us the “curvature”.
Berry phaseConsider a quantum-mechanical system in a nondegenerate ground state, e.g. spin ½ particlein a magnetic field. The adiabatic theoremsays that if the Hamiltonian is changed slowly, thesystem remains in its instantaneous ground state.
Berry phase: when the Hamiltonian goes around a closed loop in parameter space, the system acquires a geometrical phase relative to initial state (in addition to the usual dynamical phase).
“Gauge transformation” of the Berry phase
Gauge invariant quantities are Berry curvature and closed loop integrals
From Berry phase to Chern numberThe change in the electron wavefunction within the Brillouin zone leads to a Berry connection and Berry curvature
Brillouinzone
Kx
Ky
Integral of F is quantized to be integer: first Chern number. It is like Gauss-Bonnet theorem for the Brillouin zone.
TKNN quantization of Hall conductivity for IQHEThouless et al., PRL 1982
Topological order and edge states
TKNN quantization exists only forinsulators with completely filled bands.
Conductance goes through gapless edge states.
Existence of topological invariant requires edge states
Topological invariant cannot change without closing of the insulating gap
Topology in one dimension:Berry phase and electric polarization
Polarization as Berry phase
Vanderbilt, King-SmithPRB 1993
Su-Schrieffer-Heeger Model
B A B BA
When dz(k)=0, states with t>0 and t<0 are topologically distinct.
Domain wall states in SSH ModelAn interface between topologically different states has protected midgap states
Absorption spectra onneutral and doped trans‐(CH)x
Topological states of matter
Integer and FractionalQuantum Hall effects
Exotic properties:quantized conductance (Quantum Hall systems, Quantum Spin Hall Sysytems)fractional charges (Fractional Quantum Hall systems, Polyethethylene)
This talk:How to explore topology of band structures with synthetic matter: cold atoms and photons
Extend to dynamics. Unique topological properties of dynamics
Quantum Spin Hall effect
3D topological insulators
Magnetization ‐ order parameter in ferromagnets
Nematic order parameter in liquid crystals
Order parameters can be measured
Outline
Exploring edge states in topological phases with photonsT. Kitagawa et al., PRA 82:33429 (2010)
Phys. Rev. B 82, 235114 (2010) Nature Comm. 3:882 (2012)
Theory + Experiments by MPQ groupPhys. Rev. Lett. 110:165304 (2013)
Nature Physics 9, 795 (2013)
Zak/Berry phase measurements as a probe of band topology in OLBloch+Ramsey interferenceexperiments with cold atoms
Probing band topology with Ramsey/Bloch interference
C. Salomon et al., PRL (1996)
Tools of atomic physics:Bloch oscillations
/2 pulse
Evolution
Tools of atomic physics:Ramsey interference
Used for atomic clocks, gravitometers, accelerometers, magnetic field measurements
/2 pulse + measurement ot Szgives relative phase accumulated by the two spin components
EvolutionEvolution
Zak phase probe of band topology in 1d
One dimensional superlatticesSu‐Schrieffer‐Heeger model
Experiments Marcos Atala, Monika Aidelsburger,Julio Barreiro, Immanuel Bloch (LMU/MPQ)
Theory: Takuya Kitagawa (Harvard), Dima Abanin(Harvard/Perimeter), Eugene Demler (Harvard)
Phys. Rev. Lett. 110:165304 (2013) Nature Physics 9, 795 (2013)
SSH model of polyacetylene
Analogous to bichromatic optical lattice potential
I. Bloch et al.,LMU/MPQ
B A B BA
Su, Schrieffer, Heeger, 1979
Dimerized model
A B A AB
Characterizing SSH model using Zak phase Two hyperfine spin states experience the same optical potential
/2a/2a
a
Zak phase is equal to 0
Problem: experimentally difficult to control Zeeman phase shift
Dynamic phases due todispersion and magnetic field fluctuations cancel.Interference measuresthe difference of Zakphases of the two bands in two dimerizations.Expect phase
Spin echo protocol for measuring Zak phase
Bloch oscillations measurements in LMU/MPQWith -pulse but no swapping of dimerization
Bloch oscillations measurements in LMU/MPQWith p-pulse and with swapping of dimerization
Zak phase measurements in LMU/MPQ
Zak phase measurements can be used to probetopological propertiesof Bloch bands in 2D and 3D
D. Abanin, T. Kitagawa, I. Bloch, E. DemlerPhys. Rev. Lett. 110:165304 (2013)
F. Grusdt, D. Abanin, E. DemlerPhys. Rev. A 89, 043621 (2014)
Integral of the Berry phase around the Dirac point
Measuring Berry curvature in 2d and Chern num
Manifestation of Berry phaseof Dirac points in grapheme:IQHE plateaus are shifted by 1/2
Interferometric probe of Berry curvature and Chern number in 2d systems
Extension to more exotic states: Quantum Spin Hall Effect states and Topological Insulators in 3D. Grusdt et al., Phys. Rev. A 89, 043621 (2014)
Discreet time quantum walk with photons
Observing edge states on topological domain boundaries
Topological properties of dynamics
Theory: T. Kitagawa et al., Phys. Rev. A 82:33429 (2010)Phys. Rev. B 82, 235114 (2010)
Experiments: T. Kitagawa et al., Nature Comm. 3:882 (2012)
Definition of 1D discrete Quantum Walk
1D lattice, particle starts at the origin
Analogue of classical random walk.
Introduced in quantum information:
Q Search, Q computations
Spin rotation
Spin-dependent Translation
Quantum walk with photons
Rotation is implemented by half-wave platesTranslation by bi-refringentcalcite crystals that displace only horizontally polarized light
Earlier realization of QW with photons: A. Schrieber et al., PRL (2010)
A. White’s group in QueenslandT. Kitagawa et al., Nature Comm. 3:882 (2012)
From discreet timequantum walks to
Topological Hamiltonians
T. Kitagawa et al., Phys. Rev. A 82, 033429 (2010)
Discrete quantum walk
One stepEvolution operator
Spin rotation around y axis
Translation
Effective Hamiltonian of Quantum WalkInterpret evolution operator of one step
as resulting from Hamiltonian.
Stroboscopic implementation of Heff
Spin-orbit coupling in effective Hamiltonian
From Quantum Walk to Spin-orbit Hamiltonian in 1d
Winding Number Z on the plane defines the topology!
Winding number takes integer values.Can we have topologically distinct quantum walks?
k-dependent“Zeeman” field
Split-step DTQW
Phase Diagram
Split-step DTQW
Detection of Topological phases:localized states at domain boundaries
Phase boundary of distinct topological phases has bound states
Bulks are insulators Topologically distinct, so the “gap” has to close
near the boundary
a localized state is expected
Apply site-dependent spin rotation for
Split-step DTQW with site dependent rotations
Experimental demonstration of topological quantum walk with photonsKitagawa et al., Nature Comm. 2012
Rotation is implemented by half-wave platesTranslation by birefringentcalcite crystals that displace only horizontally polarized light
Topological Hamiltonians in 2D with quantum walk
Schnyder et al., PRB (2008)Kitaev (2009)
What we discussed so far
Split time quantum walks provide stroboscopic implementationof different types of single particle Hamiltonians
By changing parameters of the quantum walk protocolwe can obtain effective Hamiltonians which correspond to different topological classes
Topological properties unique to dynamics
Floquet operator Uk(T) gives a map from a circle to the space of unitary matrices. It is characterized by the topological invariant
This can be understood as energy winding.This is unique to periodic dynamics. Energy defined up to 2/T
Topological properties of evolution operator
Floquet operator
Time dependent periodic Hamiltonian
Example of topologically non-trivial evolution operator
and relation to Thouless topological pumpingSpin ½ particle in 1d lattice. Spin down particles do not move. Spin up particles move by one lattice site per period
n1 describes average displacement per period.Quantization of 1 describes topological pumping of particles. This is another way to understand Thouless quantized pumping
group velocity
Experimental demonstration of topological quantum walk with photons
Kitagawa et al., Nature Comm. 3:882 (2012)
Boundary with topologically different evolution operators
Boundary with topologically similar evolution operators
Floquet states beyond in 2dbeyond quantum walk
Topological Floquet states in 2d
Kitagawa et al., PRB (2010)
Topological Floquet states in 2d Oka and Aoki, PRB (2009)Kitagawa et al., PRB (2010)Lindner, Refael, Galiski, Nat. Phys. (2011)
Observation of Floquet-Bloch states on the surface of a topological insulator
Gedik et al., Science (2013)
Summary
Observation of edge states in topological phases realized with photons
First direct measurementof Zak phase of a 1d band
Prospect of measuringtopological propertiesof 2d bands
How to measure Berry phase of Bloch states Naïve approach:
Move atom on a closed trajectoryaround Dirac pointMeasure accumulated phase
Problems with this approach: Need to move atom on a complicated curved trajectoryNeed to separate dynamical phase
C
Integral of the Berry phase is only well defined on a closed trajectory
Brillouin zone is a torus. There are two types of closed trajectories
is not gauge invariantC
gauge invariantintegral ofBerry curvature
Zak
From Berry phase to Zak phase
Zak phase: integral of Berry phase overreciprocal lattice vector
Zak phase measurements in LMU/MPQ
Universality of collective modes
keV MeV GeV TeVfeV peV µeV meV eV
pK nK µK mK K
neV
roomtemperature
LHC
He N
current experiments10-11 - 10-10 K
first BECof alkali atoms
M. Enders et al.,Observation of Higgs modein 2D superfluid in ultracoldatomsNature 2012