Non-dispersive wave packets in periodically driven quantum systems

More documents

Recommendations

Info

428 A. Buchleitner et al. / Physics Reports 368 (2002) 409–547 I2 = n2 + 2 4 ˝ ; (45) where n1;n2 are two non-negative integers, and 1; 2 the Maslov indices (counting the number of caustics encountered on the torus) along the 1; 2 directions. Once again, the semiclassical energy levels (which now depend on two quantum numbers) are obtained by substitution of these quantized values into the classical Hamilton function. An alternative formulation of the EBK criterium is possible using the original position-momentum coordinates. Indeed, Eq. (45) just expresses that, along any closed loop on the invariant torus, the phase accumulated by the wave function is an integer multiple of 2 (modulo the Maslov contribution). Using the canonical invariance of the total action [77], the EBK quantization conditions can be written as 1 ˜p:d˜r = ni + 2 closed path i i ˝ (46) 4 where the integral has to be taken along two topologically independent closed paths ( 1; 2) onthe invariant torus. Note that, as opposed to the WKB procedure in a 1D situation, the EBK quantization uses the invariant tori of the classical dynamics, not the trajectories themselves. When there is a stable periodic orbit, it is surrounded by invariant tori. The smallest quantized torus around the stable orbit is associated with a quantum number equal to zero for the motion transverse to the orbit: it de nes a narrow tube around the orbit, whose projection on con guration space will be localized in the immediate vicinity of the orbit. Thus, the corresponding wave function will also be localized close to this narrow tube, i.e., along the stable periodic orbit in con guration space. Transversely to the orbit, the wave function (or the Wigner function) will essentially look like the ground state of an harmonic oscillator, i.e., like a Gaussian. Finally, let us note that it is also possible to develop an analogous EBK scheme for periodically time-dependent Hamiltonians [80] using the notion of an extended phase space [3]. Such an approach will be extensively used in the next section, so it is discussed in detail there. 2.3. Scars When a periodic orbit is unstable, there is no torus closely surrounding it. However, it often happens that quantum eigenstates exhibit an increased probability density in the vicinity of unstable periodic orbits. This scarring phenomenon is nowadays relatively well understood, and the interested reader may consult Refs. [81,82]. Similarly, some quantum states have an increased probability density in the vicinity of an unstable equilibrium point [33,83,84]. This localization is only partial. Indeed, since a quantum eigenstate is a stationary structure, some probability density must localize along the unstable directions of the classical Hamiltonian ow [77], and the localization cannot be perfect. This is in sharp contrast with stable equilibrium points and stable periodic orbits which—see above—optimally support localized eigenstates. Note that there is, however, a big di erence between scarring and localization in the vicinity of an unstable xed point. The latter phenomenon is of purely classical origin. Indeed, close to an equilibrium point, the velocity goes to zero and the particle consequently spends more time close to
A. Buchleitner et al. / Physics Reports 368 (2002) 409–547 429 the equilibrium point than further away from it. The quantum eigenfunctions have the same property: the probability density is large near the equilibrium point. This trivial enhancement of the probability density is already well known for a one-dimensional system where the WKB wave function, Eq. (37), diverges when the momentum tends to zero. The localization e ect near an unstable point is just the quantum manifestation of the critical slowing down of the classical particle [60]. 3. Non-dispersive wave packets and their realization in various atomic systems 3.1. General model—non-linear resonances In this section, we present the general theory of non-dispersive wave packets. As explained in Section 1.4, the basic ingredients for building a non-dispersive wave packet are a non-linear dynamical system and an external periodic driving which is resonant with an internal frequency of the dynamical system. We present here a very general theory starting out from classical mechanics which provides us with the most suggestive approach to non-linear resonances. In a second step, we choose a pure quantum approach giving essentially the same physics. We use a one-dimensional model, which displays all the interesting features of non-linear resonances. While the direct link between classical non-linear resonances, the corresponding Floquet states, and non-dispersive wave packets has been identi ed only recently [43,32,33,49,64] some aspects of the developments presented below may be found in earlier studies [39,85,40,42]. Several complications not included in the simple one-dimensional model are important features of “real systems”. They are discussed at a later stage in this paper: • the e ect of additional degrees of freedom, in Sections 3.3.2–3.5; • higher non-linear resonances (where the driving frequency is a multiple of the internal frequency), in Section 5; • an unbounded phase space, leading to the decay of non-dispersive wave packets (as “open quantum systems”), in Section 7.1; • sources of “decoherence”, such as spontaneous emission of atomic wave packets, in Section 7.2; • deviations from temporal periodicity, in Section 8.3. In particular cases, an apparently simpler approach is also possible (such as the use of the rotating frame for a Rydberg atom exposed to a circularly polarized electromagnetic eld, see Section 3.4). Despite all its advantages, it may be quite speci c and too restricted to reveal non-linear resonances as the actual cause of the phenomenon. Here, we seek the most general description. 3.1.1. Classical dynamics Let us start from a time-independent, bounded, one-dimensional system described by the Hamilton function H0(p; z). Since energy is conserved, the motion is con ned to a one-dimensional manifold in two-dimensional phase space. Except for energies which de ne a xed point of the Hamiltonian dynamics (such that 9H0=9z = 0 and 9H0=9p = 0; these xed points generically only exist at some isolated values of energy, for example at E = 0 for the harmonic oscillator), the motion is periodic in time, and the phase space trajectory is a simple closed loop.
Page 1 and 2: Abstract Physics Reports 368 (2002)
Page 3 and 4: A. Buchleitner et al. / Physics Rep
Page 19: A. Buchleitner et al. / Physics Rep
Page 43 and 44: → ; → ; A. Buchleitner et al. /
Page 71 and 72:
A. Buchleitner et al. / Physics Rep
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
|ψ (z)| 2 |ψ (z)| 2 0.3 0.2 0.1 0
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
γ /∆ σ/∆ 10 -1 10 -2 10 -3 0.
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137 and 138:
Page 139:
show all

Non-dispersive wave packets in periodically driven quantum systems

Create successful ePaper yourself

Delete template?

Save as template?