First, we consider a phase modulation pattern for the one-dimensional lattice, which was introduced by Martin Wimmer in [51]. This potential pattern appears as a static periodic array of alternating potential wells and barriers. Since the on-site acquired phase ϕ has to exchange every time step between the two pseudospin components l and r, the elementary unit cell comprises two time steps and two positions as illustrated in Fig.1.10. Note, that here odd and even modulation time steps are unambiguously linked to the respective sublattices
Figure 1.10: Static periodic potential pattern, applied via phase modulation of the wavefunction.
and therefore the effective evolution operator has to necessarily contain two time steps, unlike its unmodulated counterpart. In this regard, the evolution operator reads
ˆ
U = ˆΦ(−ϕ) ˆS ˆC ˆΦ(ϕ) ˆS ˆC, (1.57)
where the Shifter ˆS and the Coin ˆC had been introduced earlier and ˆ Φ(φ)==def e iϕ 0 0 e−iϕ = eiϕ ˆσz (1.58)
is the phase shift operator, acting homogeneously on internal pseudospin states. It is also expressed in terms of the Pauli matrix σz responsible for rotation of a pseudospin in its
associated Hilbert space. The phase operators ˆΦ(ϕ) and ˆΦ(−ϕ) are commuting with the Shifter and thus together with the Coin ˆC, they form the generalized Coin
ˆ C(ϕ) = ˆΦ(−ϕ) ˆC ˆΦ(ϕ) = √1 2 1 ie−2iϕ ie2iϕ 1 , (1.59)
where the coin of the pseudospin walk is chosen as a reference point.
Accounting for the doubled time period, we represent the fundamental states of the system in terms of extended Floquet-Bloch waves and diagonalize the evolution matrix in order to find the dispersion relations. Thereby, we come up with two bands for the Hadamard Walk E±= ± 1 2arccos cos 2k + cos 2ϕ 2 (1.60)
and for the pseudospin walk
E± = ±
1
2arccos
cos 2k − cos 2ϕ
2 , (1.61)
where the prefactor 1/2 is kept for conformity with the results in subsection 1.2.2. Both band structures are demonstrated in Fig.1.11 for different potential heights ϕ. It is easy to see that the periodic potential allows one to bridge these two walks, turning one walk into another at ϕ = π/2 via continuous deformation of the bands. Such a deformation, however, does not deliver the same eigenvectors for both walks even though their band structures are identical. This is because the generalized coin ˆC(ϕ) still preserves mirror symmetry of the
Figure 1.11: Various amplitudes ϕ of phase modulation lead to deformations of the band structure and change sizes of the band gaps. The upper and the lower row correspond to the Hadamard and the pseudospin walk, respectively.
beam splitter and consequently the eigenspectrum of pseudospins should obey this symmetry as well, while the polarization eigenmodes do not. So, the macroscopic dynamics of both walks can be made identical via the periodic potential despite that their microscopic features remain different.
Finally, we can approximate the generalized band structure around the center of the BZ k = 0. Thus, choosing the pseudospin walk as a reference point, we make the Tay- lor expansion for all ϕ except π/2 and come up with the nonrelativistic Schr¨odinger-like Hamiltonian E± ≈ ± 1 2arccos 1 − cos 2ϕ 2 ± k2 2 q 1 −(1−cos 2ϕ)4 2 + O(k4), (1.62)
while at the special point ϕ = π/2 we get relativistic energies of the Hadamard Walk for a massless particle E± = ± k √ 2 + O(k 3). (1.63)
The first constant term of the Schr¨odinger-like energy is associated with the rest energy of a quasiparticle and quasiantiparticle.
Er = ±
1
2arccos
1 − cos 2ϕ
2 , (1.64)
that the effective mass can be derived as m = ± r 1 −(1 − cos 2ϕ) 2 4 = sin 2Er. (1.65)
Interestingly, the mass-energy equivalence equation Er = mc2 holds only for small enough
rest energies |Er| π/4 with the constant effective speed of light c = 1/
√
2, but as the band gap 2|Er| becomes larger, the increase of the effective mass slows down until the mass reaches
its limit of ±1 at Er = ±π/4. Recalling that the mass is also the inverse of the group velocity
dispersion 1/E00(k), we conclude that dispersion-free localization of an immobile wavepacket is impossible in the lattice and thus a photon can not behave as a heavy classical immobile particle. However, in the relativistic Hadamard limit the dispersion is zero and thus the localized photon can move without spreading only as a relativistic massless particle. This is in agreement with the intrinsic relativistic nature of photons.
Further on, we will examine the topological charge of the pseudospin at the degeneracy point at k = 0 and ϕ = π/2. To do so, we shall consider the celebrated Berry phase [52], which is a phase difference acquired by a complex wavefunction of a quantum system under its adiabatic evolution along a closed loop in a parametric space of the Hamiltonian. This phase has a topological origin. In particular, the famous result derived by Berry himself in [52] and by Aharonov and Anandan in [53] is that the electron’s spin placed in a magnetic field and evolving adiabatically along a closed loop C around a degeneracy point of the Hamiltonian acquires the Berry phase γ(C) = ±sΩ(C), where Ω(C) is the solid angle that the loop C subtends at the degeneracy, while s = 1/2 is the spin angular momentum of the electron. A more relevant example, however, is the two-component local pseudospin of an electron in the graphene layer modeled as a hexagonal lattice with tight-binding approximation [54]. In this case, the pseudospin eigenvector |ψ(kx, ky)i of a fixed band circumventing a closed loop
around the Dirac two-fold degeneracy point in the two-dimensional reciprocal space (kx, ky)
will acquire the Berry phase γ = ±s2π = π, where the solid angle is now replaced by the planar angle of 2π and the quantum number s coincides with the fermionic spin-1/2 despite the wavefunction |ψ(kx, ky)i is not the genuine spin of the electron.
To find the Berry phase, we consider the Hamiltonian operator ˆH(k, ϕ) of the one- dimensional walk, which periodically spans the two-dimensional parametric space (k, ϕ) and which can be found via the generalized Euler’s formula
ˆ
H(k, ϕ) = E(k, ϕ)[(ex(k, ϕ) ˆσx+ ey(k, ϕ) ˆσy + ez(k, ϕ) ˆσz], (1.66)
ˆ
U (k, ϕ)== edef i ˆH(k,ϕ) = cos E(k, ϕ) ˆI + i sin E(k, ϕ)[ex(k, ϕ) ˆσx+ ey(k, ϕ) ˆσy + ez(k, ϕ) ˆσz],
(1.67) where ±E(k, ϕ) are quasienergies of the upper and the lower band given in eq. (1.61), ˆσ are Pauli matrices and ~e(k, ϕ) is a real-valued vector normalized to unity and representing the eigenvectors on the so-called Bloch or Poincare sphere. Straightforward derivation of the vector ~e(k, ϕ) yields:
~ e = 1 2 sin E(k, ϕ) cos 2ϕ + cos 2k sin 2ϕ − sin 2k sin 2ϕ + sin 2k . (1.68)
In order to describe a closed loop around the two-fold degeneracy point k = 0, ϕ = π/2, we use the parametrization k = R cos α and ϕ = π/2 + R sin α, where R is a fixed radius of the
loop and α runs from 0 to 2π. Substituting these parameters into the unit vector and taking the limit R → 0, we obtain
~e ≈ 0 − cos (α − π/4) − sin (α − π/4) , (1.69)
which remains a unit vector. Note that, as expected, the Hamiltonian takes the relativis- tic form ±(~e~ˆσ)k/√2 around the degeneracy point. Recalling the definitions of the Pauli matrices, we consider the eigenvalue problem ~e |ψi± = ± |ψi± and obtain the following eigenmodes: |ψi± = i cos (α+π/4)±1 sin (α+π/4) 1 ! . (1.70)
In order to resolve the uncertainty 0/0 at α − π/4 = ±π/2, we perform a series of standard trigonometric transformations and come up with the following expressions:
|ψi+ = i cosα 2 + π 8 sinα2 + π8 , |ψi− = −i sinα 2 + π 8 cosα2 + π8 . (1.71)
The half angle α/2 indicates that the eigenmodes acquire the Berry phase of π in the associ- ated Hilbert space upon the full cycle around the Dirac point in the parametric space. There- fore, the two-component pseudospin around that degeneracy point behaves as a fermionic spin-12. Further in section 1.7, it will become clear that this result is not generic, because time-reversal transformation of the pseudospin at another Dirac point (k = π/2, ϕ = 0) op- positely indicate an integer spin (bosonic) behaviour and thus one can anticipate the Berry phase of 0 at that point. One can also expect the same ambiguity for the helicity in the Hadamard Walk. Moreover, we presume that the fermion-boson duality of the pseudspin is essentially the reason, why correlated photons can exhibit both quantum statistics in the same system [37, 41]. Moreover, an intermediate behaviour with fractional exchange statis- tics is possible in continuous quantum walks [55]. The fact that even helicity does not behave as a genuine integer spin is related to the absence of photon’s mass and of a rest frame. In electromagnetism, this fact is translated to the absence of longitudinal electromagnetic waves in homogeneous media.