† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Emergency Project, Grant Nos. 11447025 and 11847308).
The adiabatic control is a powerful technique for many practical applications in quantum state engineering, light-driven chemical reactions and geometrical quantum computations. This paper reveals a speed limit of nonadiabatic transition in a general time-dependent parametric quantum system that leads to an upper bound function which lays down an optimal criteria for the adiabatic controls. The upper bound function of transition rate between instantaneous eigenstates of a time-dependent system is determined by the power fluctuations of the system relative to the minimum gap between the instantaneous levels. In a parametric Hilbert space, the driving power corresponds to the quantum work done by the parametric force multiplying the parametric velocity along the parametric driving path. The general two-state time-dependent models are investigated as examples to calculate the bound functions in some general driving schemes with one and two driving parameters. The calculations show that the upper bound function provides a tighter real-time estimation of nonadiabatic transition and is closely dependent on the driving frequencies and the energy gap of the system. The deviations of the real phase from Berry phase on different closed paths are induced by the nonadiabatic transitions and can be efficiently controlled by the upper bound functions. When the upper bound is adiabatically controlled, the Berry phases of the electronic spin exhibit nonlinear step-like behaviors and it is closely related to topological structures of the complicated parametric paths on Bloch sphere.
Quantum mechanics concerns more about equilibrium properties of a system by solving stationary Schrödinger equations to find intrinsic energy structures. It is a time-independent problem and many numerical methods have been developed such as the first principle calculation based on the density functional theory. However, in recent years, more attentions turn to quantum state control in order to manipulate quantum systems in a desired manner, guiding a quantum system evolving from an initial ground state to a target state through purposely designed parametric driving.[1]
The quantum control or engineering is definitely a time-dependent problem and needs real-time feedback for a practical application, for example, in the intermediate-state guiding chemical reactions (quantum catalysis)[2] or in the adiabatic quantum computations.[3] The time-dependent problems are more complicated than stationary ones because the system is non-conservative and will evolve to nonequilibrium states far-from-equilibrium dynamic,[4] which even disables an efficient description of the system because no quantum state can survive for such a quickly driving on the system.[5] Generally, not as that in the time-independent Hilbert space, an external driving always prevents the quantum system from keeping a complete basis because the Hilbert space is generally open. Traditionally, the main analytical method to deal with time-dependent problems is the perturbation theory, which holds a complete perturbed Hilbert space because the perturbation is weak. However, in many practical situations, the weak-driving condition fails and the perturbation method is invalid even in a very short driving time. But, fortunately, in most cases, the quantum control happens between two systems with different time scales, one is a classical driving system with relatively slow characteristic frequencies, and the other is the driven quantum one holding a fast intrinsic frequency determined by its internal levels. In this situation the adiabatic theory becomes a powerful tool and the adiabatic approximation is an important condition for the adiabatic dynamics. However, the normal condition of adiabatic approximation sometimes brings controversial results which, mainly, are derived from a rough static criterion.[6] Therefore, the adiabatic theorem should be carefully used because it is significant to design a robust strategy for coherent adiabatic control. As a strict condition for adiabatic approximation requires a very slow evolution of the quantum sate, fast adiabatic evolution[7] or short-cut adiabaticity[8] in order to overcome it are also presented.
Due to the above studies on time-dependent problems, in this paper, we will consider a general state evolution in a time-dependent parametric driving system. A speed limit of the state transition and a tighter condition for adiabatic approximation are found. The existence of an upper bound function for nonadiabatic transition is related to the energy uncertainty on the speed of quantum evolution[9] and consistent with the trade-off between speed and cost in the shortcut to adiabaticity.[10] Our work reveals that the speed of non-adiabatic transition between instantaneous states is limited by the state power fluctuations relative to the minimum spectral gap of the system. Therefore, this real-time bound function can provide a necessary and sufficient condition for the adiabatic dynamics if the bound function is closely controlled by a feedback mechanism.[11] The calculations of this upper bound for adiabatic dynamics on Landau–Zener transition and electronic spin control are conducted in a general two-state time-dependent model. The adiabatic Berry phases along more complicated state loops in a high-dimensional driving parameter space are investigated.
Generally, a parametric driving quantum system can be described by the time-dependent Hamiltonian
If the Hamiltonian
However, the detailed evolution of R(t) and
As the Hilbert space of a real parametric driving system is closed (the parameters now are real functions for the Hermitian property of
By using Eq. (
Based on instantaneous basis, we can write Eq. (
Then, if a state evolution is controlled by Eq. (
Comparing the adiabatic energy
As a reliable condition for the adiabatic approximation is a key for precise control on geometric phase, the conventional condition of Eq. (
We can see that the conventional adiabatic condition of Eq. (
If we introduce a driving power operator of the system by the negative energy changing rate of the system
Based on Eq. (
Considering N independent driving parameters
Therefore, if the parameters are controlled by the external driving, the bound Eq. (
Usually, a parametric driving Hamiltonian
Further, if there exists a dynamical invariant for the above Hamiltonian
Specifically, we consider a two-state system as an example, such as in a scheme of Landau–Zener transition shown in Fig.
Here, we also introduce an instantaneous energy gap
If the parameter λ(t) enables a linear sweeping scheme as
As shown in Fig.
As the two-state system is closed, then
If the driving parameter gives a back and forth sweeping scheme as
Therefore, the real-time bound function of the transition rate is
Figure
As there is no explicit analytical formula for transition probability in this case, the bound functions can give a rough transient estimation of the probability remaining in the state
Now, let us calculate the upper bound function with more driving parameters in the two-state system with a general form of
A typical application of the above model is for spin qubit control with a varying magnetic field shown in Fig.
As the sweeping magnetic field
It is well known that the spin operators
The instantaneous eigenstates
The general spin state Ψ(t) can then be expanded by
In this case, the dynamical equations for the transition rates are
We can easily verify that equation (
A magnetic field sweeping in the azimuthal direction with a fixed polar angle α is a conventional scheme to drive electronic spins, which is realized by the driving parameters of
Now we would like to consider a more general controlling magnetic field for
Specifically, if the polar angle of
The dynamics of transition rates
For an adiabatic case of
The dynamics of the nonlinear geometric phase in an adiabatic process can be divided into two different parts: the linear part which increases linearly with time (the dotted lines in Figs.
As we have two control frequencies, ω and ν, along the azimuthal and polar directions, respectively, there exist two evolutionary periods of T for
For the adiabatic dynamics, when the driving ratio
Therefore, the above analysis indicates that the phase evolution of a quantum state is closely associated with the geometric structure of its evolutionary path in the parameter space. Above all, the topological property of the Berry phase can be perfectly controlled by the parametric paths through a designed parametric driving under the control of the time-dependent bound function proposed in this paper.
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