Phase portraits (p vs. x) of the classical kicked rotor at different kicking strengths. The top row shows, from left to right, K = 0.5, 0.971635, 1.3. The bottom row shows, from left to right, K = 2.1, 5.0, 10.0. The phase portrait at the chaotic boundary is the upper middle plot, with KC = 0.971635. At and above KC, regions of uniform, grainy-coloured, quasi-random trajectories appear and eventually consume the entire plot, indicating chaos.

The kicked rotator, also spelled as kicked rotor, is a paradigmatic model for both Hamiltonian chaos (the study of chaos in Hamiltonian systems) and quantum chaos. It describes a free rotating stick (with moment of inertia ${\displaystyle I}$) in an inhomogeneous "gravitation like" field that is periodically switched on in short pulses. The model is described by the Hamiltonian

${\displaystyle {\mathcal {H))(\theta ,p,t)={\frac {p^{2)){2I))+K\cos \theta \sum _{n=-\infty }^{\infty }\delta \left({\frac {t}{T))-n\right)}$,

where ${\displaystyle \theta \in [0,2\pi [}$ is the angular position of the stick (${\displaystyle \theta =\pi }$ corresponds to the position of the rotator at rest), ${\displaystyle p}$ is the conjugated momentum of ${\displaystyle \theta }$, ${\displaystyle \textstyle K}$ is the kicking strength, ${\displaystyle T}$ is the kicking period and ${\displaystyle \textstyle \delta }$ is the Dirac delta function.

## Classical properties

### Stroboscopic dynamics

The equations of motion of the kicked rotator write

${\displaystyle {\frac {\mathrm {d} \theta }{\mathrm {d} t))={\frac {\partial {\mathcal {H))}{\partial p))={\frac {p}{I))\quad {\text{and))\quad {\frac {\mathrm {d} p}{\mathrm {d} t))=-{\frac {\partial {\mathcal {H))}{\partial \theta ))=K\sin \theta \sum _{n=-\infty }^{\infty }\delta \left({\frac {t}{T))-n\right)}$
Theses equations show that between two consecutive kicks, the rotator simply moves freely: the momentum ${\displaystyle p}$ is conserved and the angular position growths linearly in time. On the other hand, during each kick the momentum abruptly jumps by a quantity ${\displaystyle KT\sin \theta }$, where ${\displaystyle \theta }$ is the angular position near the kick. The kicked rotator dynamics can thus be described by the discrete map[1]
${\displaystyle p_{n+1}=p_{n}+KT\sin \theta _{n}\quad {\text{and))\quad \theta _{n+1}=\theta _{n}+{\frac {T}{I))p_{n+1))$
where ${\displaystyle \theta _{n))$ and ${\displaystyle p_{n))$ are the canonical coordinates at time ${\displaystyle t=nT^{-))$, just before the ${\displaystyle n}$-th kick. It is usually more convenient to introduce dimensionless momentum ${\textstyle p\rightarrow p/{\frac {I}{T))}$, time ${\textstyle t\rightarrow t/T}$ and kicking strength ${\textstyle K\rightarrow K/{\frac {I}{T^{2))))$ to reduce the dynamics to the single parameter map
${\displaystyle p_{n+1}=p_{n}+K\sin \theta _{n}\quad {\text{and))\quad \theta _{n+1}=\theta _{n}+p_{n+1))$
known as Chirikov standard map, with the caveat that ${\displaystyle p_{n))$is not periodic as in the standard map. However, one can directly see that two rotators with same initial angular position ${\displaystyle \theta _{0))$ but shifted dimensionless momentum ${\displaystyle p_{0))$ and ${\textstyle p_{0}+2\pi l}$ (with ${\displaystyle l}$ an arbitrary integer) will have the same exact stroboscopic dynamics, but with dimensionless momentum shifted at any time by ${\textstyle 2\pi l}$ (this is why stroboscopic phase portraits of the kicked rotator are usually displayed in a single momentum cell ${\textstyle p\in [-\pi ,\pi ]}$).

### Transition from integrability to chaos

The kicked rotator is a prototype model to illustrate the transition from integrability to chaos in Hamiltonian systems and in particular the Kolmogorov–Arnold–Moser theorem. In the limit ${\displaystyle K=0}$, the system describes the free motion of the rotator, the momentum is conserved (the system is integrable) and the corresponding trajectories are straight lines in the ${\displaystyle (\theta ,p)}$ plane (phase space), that is tori. For small, but non-vanishing perturbation ${\displaystyle K}$, instabilities and chaos starts to develop. Only quasi-periodic orbits (represented by invariant tori in phase space) remain stables, while other orbits become unstables. For larger ${\displaystyle K}$, invariant tori are eventually destroyed by the perturbation. For the value ${\displaystyle K=K_{c}\approx 0.971635\dots }$, the last invariant tori connecting ${\displaystyle \theta =-\pi }$ and ${\displaystyle \theta =\pi }$ in phase space is destroyed.

Kicker Rotor Phase Portrait Animation

### Diffusion in momentum direction

For ${\displaystyle K>K_{c))$, chaotic unstable orbits are no longer constraints by invariant tori in the momentum direction and can explore the full phase space. For ${\displaystyle K\gg K_{c))$, the particle after each kicks typically moved over a large distance, which strongly modifies the amplitude and sign of the following kick. At long time enough, the particle as thus been submitted to a series of kicks with quasi-random amplitudes. This quasi-random walk is responsible for a diffusion process in the momentum direction ${\displaystyle \langle (\Delta p_{n})^{2}\rangle =2D_{\text{cl))n}$ (where the average runs over different initial conditions).

More precisely, after ${\displaystyle n}$ kicks, the momentum ${\displaystyle p_{n))$ of a particle with initial momentum ${\displaystyle p_{0))$ writes ${\textstyle p_{n}=p_{0}+K\sum _{i=0}^{n-1}\sin \theta _{i))$[2] (obtained by iterating ${\displaystyle n}$ times the standard map). Assuming that kicks are randoms and uncorrelated in time, the spreading of the momentum distribution writes

${\displaystyle \left\langle {(\Delta p)}^{2}\right\rangle =\left\langle {(p_{n}-p_{0})}^{2}\right\rangle =K^{2}\sum _{i=0}^{n-1}\left\langle {\sin }^{2}\theta _{i}\right\rangle +K^{2}\sum _{i\neq j}^{}\left\langle \sin \theta _{i}\sin \theta _{j}\right\rangle \approx K^{2}\sum _{i=0}^{n-1}\left\langle {\sin }^{2}\theta _{i}\right\rangle ={\frac {1}{2))Kn}$
The classical diffusion coefficient in momentum direction is then given in first approximation by ${\textstyle D_{\text{cl))={\frac {K^{2)){4))}$. Corrections coming from neglected correlation terms can actually be taken into account, leading to the improved expression[3]
${\displaystyle D_{\text{cl))={\frac {K^{2)){4))[1-2J_{2}(K)+2J_{2}^{2}(K)]}$
where ${\textstyle J_{2))$ is the Bessel function of first kind.

## The quantum kicked rotator

### Stroboscopic dynamics

The dynamics of the quantum kicked rotator (with wave function ${\displaystyle |\psi (t)\rangle }$) is governed by the time dependent Schrödinger's equation

${\displaystyle i\hbar {\frac {\partial }{\partial t))|\psi (t)\rangle =\left[{\frac ((\hat {p))^{2)){2I))+K\cos {\hat {\theta ))\sum _{n=-\infty }^{\infty }\delta \left({\frac {t}{T))-n\right)\right]|\psi (t)\rangle }$

with ${\displaystyle [{\hat {\theta )),{\hat {p))]=i\hbar }$ (or equivalently ${\textstyle \langle \theta |{\hat {p))|\psi \rangle =i\hbar {\frac {\partial \psi }{\partial \theta ))}$).

As for classical dynamics, a stroboscopic point of view can be adopted by introducing the time propagator over a kicking period ${\displaystyle {\hat {U))}$ (that is the Floquet operator) so that ${\displaystyle |\psi (t+T)\rangle ={\hat {U))|\psi (t)\rangle }$. After a careful integration of the time-dependent Schrödinger's equation, one finds that ${\displaystyle {\hat {U))}$ can be written as the product of two operators

${\displaystyle {\hat {U))=\exp \left[-i{\frac ((\hat {p))^{2}T}{2I\hbar ))\right]\exp \left[-i{\frac {KT}{\hbar ))\cos {\hat {\theta ))\right]}$
We recover the classical interpretation: the dynamics of the quantum kicked rotor between two kicks is the succession of a free propagation during a time ${\displaystyle T}$ followed by a short kick. This simple expression of the Floquet operator ${\displaystyle {\hat {U))}$ (a product of two operators, one diagonal in momentum basis, the other one diagonal in angular position basis) allows to easily numerically solve the evolution of a given wave function using split-step method.

Because of the periodic boundary conditions at ${\displaystyle \theta =\pm \pi }$, any wave function ${\displaystyle |\psi \rangle }$ can be expanded in a discrete momentum basis ${\textstyle |l\rangle }$ (with ${\displaystyle p=l\hbar }$, ${\displaystyle l}$ integer) see Bloch theorem), so that

${\displaystyle \langle \theta |\psi \rangle =\sum _{l=-\infty }^{\infty }\langle l|\psi \rangle \mathrm {e} ^{il\theta }\Leftrightarrow \langle l|\psi \rangle =\int _{-\pi }^{\pi }{\frac {\mathrm {d} x}{2\pi ))\langle \theta |\psi \rangle \mathrm {e} ^{-il\theta ))$

Using this relation with the above expression of ${\displaystyle {\hat {U))}$, we find the recursion relation[4]

${\displaystyle \langle l|\psi (t+T)\rangle =\exp \left(-i{\frac {l^{2}\hbar T}{2))\right)\sum _{m=-\infty }^{\infty }(-i)^{m-l}J_{m-l}\left({\frac {KT}{\hbar ))\right)\langle m|\psi (t)\rangle }$
where ${\displaystyle \textstyle {J}_{n))$ is a Bessel function of first kind.

Demonstration
Indeed, we have
${\displaystyle \langle l|\psi (t+T)\rangle =\langle l|{\hat {U))|\psi (t)\rangle =\exp \left(-i{\frac {l^{2}\hbar T}{2I))\right)\langle l|\exp \left(-i{\frac {KT\cos {\hat {\theta ))}{\hbar ))\right)|\psi (t)\rangle }$
${\displaystyle \langle l|\psi (t+T)\rangle =\exp \left(-i{\frac {l^{2}\hbar T}{2I))\right)\int _{-\pi }^{\pi }{\frac {\mathrm {d} \theta }{2\pi ))\mathrm {e} ^{-il\theta }\exp \left(-i{\frac {KT\cos {\hat {\theta ))}{\hbar ))\right)\langle \theta |\psi (t)\rangle }$
${\displaystyle \langle l|\psi (t+T)\rangle =\exp \left(-i{\frac {l^{2}\hbar T}{2I))\right)\int _{-\pi }^{\pi }{\frac {\mathrm {d} \theta }{2\pi ))\mathrm {e} ^{-il\theta }\sum _{n=-\infty }^{\infty }(-i)^{n}J_{n}\left({\frac {KT}{\hbar ))\right)\mathrm {e} ^{-in\theta }\sum _{m=-\infty }^{\infty }\mathrm {e} ^{im\theta }\langle m|\psi (t)\rangle }$
${\displaystyle \langle l|\psi (t+T)\rangle =\exp \left(-i{\frac {l^{2}\hbar T}{2I))\right)\sum _{n,m=-\infty }^{\infty }(-i)^{n}J_{n}\left({\frac {KT}{\hbar ))\right)\left[\int _{-\pi }^{\pi }{\frac {\mathrm {d} \theta }{2\pi ))\mathrm {e} ^{i(m-l-n)\theta }\right]\langle m|\psi (t)\rangle }$
${\displaystyle \langle l|\psi (t+T)\rangle =\exp \left(-i{\frac {l^{2}\hbar T}{2I))\right)\sum _{n,m=-\infty }^{\infty }(-i)^{n}J_{n}\left({\frac {KT}{\hbar ))\right){\frac {\sin([m-l-n])\pi }{(m-l-n)\pi ))\langle m|\psi (t)\rangle }$
So that we recover the result, keeping only non vanishing terms ${\displaystyle m-l-n=0}$ in the double sum.

### Dynamical localization

It has been discovered[1] that the classical diffusion is suppressed in the quantum kicked rotator. It was later understood[5][6][7][8] that this is a manifestation of a quantum dynamical localization effect that parallels Anderson localization. There is a general argument[9][10] that leads to the following estimate for the breaktime of the diffusive behavior

${\displaystyle t^{*}\ \approx \ D_{cl}/\hbar ^{2))$

Where ${\displaystyle D_{cl))$ is the classical diffusion coefficient. The associated localization scale in momentum is therefore ${\displaystyle \textstyle {\sqrt {D_{cl}t^{*))))$.

#### Link with Anderson tight-binding model

The quantum kicked rotor can actually formally be related to the Anderson tight-binding model a celebrated Hamiltonian that describes electrons in a disordered lattice with lattice site state ${\displaystyle |n\rangle }$, where Anderson localization takes place (in one dimension)

${\displaystyle {\hat {H))=\sum _{n}\varepsilon _{n}|n\rangle \langle n|+\sum _{n\neq m}t_{n-m}|n\rangle \langle m|}$
where the ${\displaystyle \varepsilon _{n))$ are random on-site energies, and the ${\displaystyle t_{n-m))$ are the hoping amplitudes between sites ${\displaystyle n}$ and ${\displaystyle m}$.

In the quantum kicked rotator it can be shown,[11] that the plane wave ${\displaystyle |p\rangle }$ with quantized momentum ${\displaystyle p=n\hbar }$ play the role of the lattice sites states. The full mapping to the Anderson tight-binding model goes as follow (for a given eigenstates of the Floquet operator, with quasi-energy ${\displaystyle \omega }$)

${\displaystyle t_{n}=-\int _{-\pi }^{\pi }{\frac {\mathrm {d} x}{2\pi ))\tan[K\cos(x)/2]\mathrm {e} ^{-ixn}\quad {\text{and))\quad \varepsilon _{n}=\tan(\omega /2-n^{2}/4)}$
Dynamical localization in the quantum kicked rotator then actually takes place in the momentum basis.

#### The effect of noise and dissipation

If noise is added to the system, the dynamical localization is destroyed, and diffusion is induced.[12][13][14] This is somewhat similar to hopping conductance. The proper analysis requires to figure out how the dynamical correlations that are responsible for the localization effect are diminished.

Recall that the diffusion coefficient is ${\displaystyle D_{cl}\approx K^{2}/2}$, because the change ${\displaystyle (p(t)-p(0))}$ in the momentum is the sum of quasi-random kicks ${\displaystyle K\sin(x(n))}$. An exact expression for ${\displaystyle D_{cl))$ is obtained by calculating the "area" of the correlation function ${\displaystyle C(n)=\langle \sin(x(n))\sin(x(0))\rangle }$, namely the sum ${\displaystyle D=K^{2}\sum C(n)}$. Note that ${\displaystyle C(0)=1/2}$. The same calculation recipe holds also in the quantum mechanical case, and also if noise is added.

In the quantum case, without the noise, the area under ${\displaystyle C(n)}$ is zero (due to long negative tails), while with the noise a practical approximation is ${\displaystyle C(n)\mapsto C(n)e^{-t/t_{c))}$ where the coherence time ${\displaystyle t_{c))$ is inversely proportional to the intensity of the noise. Consequently, the noise induced diffusion coefficient is

${\displaystyle D\approx D_{cl}t^{*}/t_{c}\quad [{\text{assuming ))t_{c}\gg t^{*}]}$

Also the problem of quantum kicked rotator with dissipation (due to coupling to a thermal bath) has been considered. There is an issue here how to introduce an interaction that respects the angle periodicity of the position ${\displaystyle x}$ coordinate, and is still spatially homogeneous. In the first works [15][16] a quantum-optic type interaction has been assumed that involves a momentum dependent coupling. Later[17] a way to formulate a purely position dependent coupling, as in the Calderia-Leggett model, has been figured out, which can be regarded as the earlier version of the DLD model.

### Experimental realization with cold atoms

The first experimental realizations of the quantum kicked rotator have been achieved by Mark G. Raizen group[18][19] in 1995, later followed by the Auckland group,[20] and have encouraged a renewed interest in the theoretical analysis. In this kind of experiment, a sample of cold atoms provided by a magneto-optical trap interacts with a pulsed standing wave of light. The light being detuned with respect to the atomic transitions, atoms undergo a space-periodic conservative force. Hence, the angular dependence is replaced by a dependence on position in the experimental approach. Sub-milliKelvin cooling is necessary to obtain quantum effects: because of the Heisenberg uncertainty principle, the de Broglie wavelength, i.e. the atomic wavelength, can become comparable to the light wavelength. For further information, see.[21] Thanks to this technique, several phenomena have been investigated, including the noticeable:

• quantum Ratchets;[22]
• the Anderson transition in 3D.[23]

## References

1. ^ a b G. Casati, B.V. Chirikov, F.M. Izrailev and J. Ford, in Stochastic Behaviour in classical and Quantum Hamiltonian Systems, Vol. 93 of Lecture Notes in Physics, edited by G. Casati and J. Ford (Springer, N.Y. 1979), p. 334
2. ^ Zheng, Yindong; Kobe, Donald H. (2006). "Anomalous momentum diffusion in the classical kicked rotor". Chaos, Solitons & Fractals. 28 (2): 395–402. Bibcode:2006CSF....28..395Z. doi:10.1016/j.chaos.2005.05.053. ISSN 0960-0779.
3. ^ Ott, Edward (2008). Chaos in dynamical systems. Cambridge Univ. Press. ISBN 978-0-521-81196-5. OCLC 316041428.
4. ^ Zheng, Yindong; Kobe, Donald H. (2007). "Momentum diffusion of the quantum kicked rotor: Comparison of Bohmian and standard quantum mechanics". Chaos, Solitons & Fractals. 34 (4): 1105–1113. Bibcode:2007CSF....34.1105Z. doi:10.1016/j.chaos.2006.04.065. ISSN 0960-0779.
5. ^ Fishman, Shmuel; Grempel, D. R.; Prange, R. E. (1982). "Chaos, Quantum Recurrences, and Anderson Localization". Physical Review Letters. 49 (8): 509–512. Bibcode:1982PhRvL..49..509F. doi:10.1103/PhysRevLett.49.509. ISSN 0031-9007.
6. ^ Grempel, D. R.; Prange, R. E.; Fishman, Shmuel (1984). "Quantum dynamics of a nonintegrable system". Physical Review A. 29 (4): 1639–1647. Bibcode:1984PhRvA..29.1639G. doi:10.1103/PhysRevA.29.1639. ISSN 0556-2791.
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8. ^ Fishman, Shmuel; Grempel, D. R.; Prange, R. E. (1987). "Temporal crossover from classical to quantal behavior near dynamical critical points". Physical Review A. 36 (1): 289–305. Bibcode:1987PhRvA..36..289F. doi:10.1103/PhysRevA.36.289. ISSN 0556-2791. PMID 9898683.
9. ^ B.V. Chirikov, F.M. Izrailev and D.L. Shepelyansky, Sov. Sci. Rev. 2C, 209 (1981).
10. ^ Shepelyansky, D.L. (1987). "Localization of diffusive excitation in multi-level systems". Physica D: Nonlinear Phenomena. 28 (1–2): 103–114. Bibcode:1987PhyD...28..103S. doi:10.1016/0167-2789(87)90123-0. ISSN 0167-2789.
11. ^ Fishman, Shmuel; Grempel, D. R.; Prange, R. E. (1982-08-23). "Chaos, Quantum Recurrences, and Anderson Localization". Physical Review Letters. 49 (8): 509–512. Bibcode:1982PhRvL..49..509F. doi:10.1103/PhysRevLett.49.509.
12. ^ Ott, E.; Antonsen, T. M.; Hanson, J. D. (1984). "Effect of Noise on Time-Dependent Quantum Chaos". Physical Review Letters. 53 (23): 2187–2190. Bibcode:1984PhRvL..53.2187O. doi:10.1103/PhysRevLett.53.2187. ISSN 0031-9007.
13. ^ Cohen, Doron (1991). "Quantum chaos, dynamical correlations, and the effect of noise on localization". Physical Review A. 44 (4): 2292–2313. Bibcode:1991PhRvA..44.2292C. doi:10.1103/PhysRevA.44.2292. ISSN 1050-2947. PMID 9906211.
14. ^ Cohen, Doron (1991). "Localization, dynamical correlations, and the effect of colored noise on coherence". Physical Review Letters. 67 (15): 1945–1948. arXiv:chao-dyn/9909016. Bibcode:1991PhRvL..67.1945C. doi:10.1103/PhysRevLett.67.1945. ISSN 0031-9007. PMID 10044295.
15. ^ Dittrich, T.; Graham, R. (1986). "Quantization of the kicked rotator with dissipation". Zeitschrift für Physik B. 62 (4): 515–529. Bibcode:1986ZPhyB..62..515D. doi:10.1007/BF01303584. ISSN 0722-3277. S2CID 189792730.
16. ^ Dittrich, T; Graham, R (1990). "Long time behavior in the quantized standard map with dissipation". Annals of Physics. 200 (2): 363–421. Bibcode:1990AnPhy.200..363D. doi:10.1016/0003-4916(90)90279-W. ISSN 0003-4916.
17. ^ Cohen, D (1994). "Noise, dissipation and the classical limit in the quantum kicked-rotator problem". Journal of Physics A: Mathematical and General. 27 (14): 4805–4829. Bibcode:1994JPhA...27.4805C. doi:10.1088/0305-4470/27/14/011. ISSN 0305-4470.
18. ^ Moore, F. L.; Robinson, J. C.; Bharucha, C. F.; Sundaram, Bala; Raizen, M. G. (1995-12-18). "Atom Optics Realization of the Quantum $\ensuremath{\delta}$-Kicked Rotor". Physical Review Letters. 75 (25): 4598–4601. doi:10.1103/PhysRevLett.75.4598. PMID 10059950.
19. ^ Klappauf, B. G.; Oskay, W. H.; Steck, D. A.; Raizen, M. G. (1998). "Observation of Noise and Dissipation Effects on Dynamical Localization". Physical Review Letters. 81 (6): 1203–1206. Bibcode:1998PhRvL..81.1203K. doi:10.1103/PhysRevLett.81.1203. ISSN 0031-9007.
20. ^ Ammann, H.; Gray, R.; Shvarchuck, I.; Christensen, N. (1998). "Quantum Delta-Kicked Rotor: Experimental Observation of Decoherence". Physical Review Letters. 80 (19): 4111–4115. Bibcode:1998PhRvL..80.4111A. doi:10.1103/PhysRevLett.80.4111. ISSN 0031-9007.
21. ^ M. Raizen in New directions in quantum chaos, Proceedings of the International School of Physics Enrico Fermi, Course CXLIII, Edited by G. Casati, I. Guarneri and U. Smilansky (IOS Press, Amsterdam 2000).
22. ^ Gommers, R.; Denisov, S.; Renzoni, F. (2006). "Quasiperiodically Driven Ratchets for Cold Atoms". Physical Review Letters. 96 (24): 240604. arXiv:cond-mat/0610262. Bibcode:2006PhRvL..96x0604G. doi:10.1103/PhysRevLett.96.240604. ISSN 0031-9007. PMID 16907228. S2CID 36630433.
23. ^ Chabé, Julien; Lemarié, Gabriel; Grémaud, Benoît; Delande, Dominique; Szriftgiser, Pascal; Garreau, Jean Claude (2008). "Experimental Observation of the Anderson Metal-Insulator Transition with Atomic Matter Waves". Physical Review Letters. 101 (25): 255702. arXiv:0709.4320. Bibcode:2008PhRvL.101y5702C. doi:10.1103/PhysRevLett.101.255702. ISSN 0031-9007. PMID 19113725. S2CID 773761.