corresponds to a point, and the power operator in the parametric Hilbert space is
describes the dynamical equations of the parameters. If we define a general parametric force of the energy field and velocity in the parameter space as
is the unit vector, then the power of Eq. (
which is the same as that defined in Ref. [
]. Alternatively, the above equation gives
. Clearly, if the field satisfies
is conservative and has no singular point in the parameter space.
Therefore, if the parameters are controlled by the external driving, the bound Eq. (24) means that the non-adiabatic transition rates of the instantaneous eigenstates are limited by the fluctuations of “work” done by force
per second when the state moves along the parametric driving path C. Basically, the force field of
is determined by the energy structure of the system and the path C depends on the solutions of the parametric equations
due to the driving schemes.
. In most cases, the number of independent driving parameters satisfies
. Therefore, the driving parameters
are piecewise continuous real functions defined on the control time interval. Surely, equation (
) is not unique and different parameterized form can be obtained by different Lie algebras defined on the diving system. Generally, the solutions of Eq. (
) can be obtained by the Lie transformation method.
can be solved from Eq. (
which can be used to discuss the adiabatic control by Lewis–Riesenfeld invariants method.
Compare Eq. (
) with Eq. (
), we can see a similar form of adiabatic state on this basis because the force field disappears in
-parameter space along the parametric path of Eq. (
) (the path is on a constant-energy surface). Therefore, a proper parameter space chosen to reveal the topological properties of Hamiltonian becomes important for an adiabatic control.
3.1. Bound functions for nonadiabatic transition in an avoided level crossing system with one driving parameterSpecifically, we consider a two-state system as an example, such as in a scheme of Landau–Zener transition shown in Fig. 1,[27] with only one driving parameter
,
where
and
are the instantaneous energies of two bare states of
and
, and
is the coupling strength between them. Then the instantaneous eigenstates of Eq. (
33) satisfy
where the instantaneous energies are
and the instantaneous states are
with
Here, we also introduce an instantaneous energy gap
and we define
. Therefore, the nonadiabatic transition rates from states
have a limit of
where the power operator of the parametric Hamiltonian is defined by
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3.1.1. Landau–Zener transition by linear sweeping drivingIf the parameter λ(t) enables a linear sweeping scheme as
where
β is the chirp-rate of the driving field. The power operator
and its variance in state
is
As shown in Fig. 1, the maximum nonadiabatic transition happens at the energy-level crossing point by
(at λ = 0) where the energy gap in this case is
and the minimum gap is
. Then the bound function in this case for state
is
The above bound function reveals that the instantaneous transition rate is closely dependent on the energy gap and reaches the maximum value of
at the energy-level crossing point (
), which is often called the non-adiabatic point.
As the two-state system is closed, then
and the non-adiabatic bound
can be written as
which, actually, is similar to the Fermiʼs golden rule. Therefore, the bound function gives Eq. (
23) as
and which can be used to estimate the transition probability when
. As the maximum transition rate from ground state
to upper state
is
, the probability remaining in the state
after the parametric sweeping from right to left satisfies
which gives a lower probability in the state
after a time interval of
across the energy avoided crossing point. In Fig.
2, we exactly calculate the probabilities of
(thick solid lines) as well as
(thin solid lines), where the bound probability of
determined by Eq.(
39) and the Landau–Zener formula,
, for
are also shown by the horizonal dashed lines. If
, the system will keep an adiabatic evolution with a weak transition probability from the instantaneous state
to
. As this model can be exactly solved, the upper bound function seems to be a bad estimation compared to the exact Landau–Zener formula,
[28] but in a high dimensional parameter space, this bound will be a very useful tool for the real-time adiabatic control if an exact transition formula is absent.
3.1.2. Landau–Zener transition by back and forth sweeping drivingIf the driving parameter gives a back and forth sweeping scheme as
where
is the level sweeping amplitude around
. The power operator in this case is
and its variance is
Therefore, the real-time bound function of the transition rate is
and when
or
, the bound function will be
Figure 3 demonstrates the transition bound
and reveals that the transition rate is closely dependent on the energy gap of the system and, for all the above cases, the bound function reveals a closer condition than the conventional adiabatic approximation of
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
when the system starts from the ground state for
. Surely, this probability is a very bad prediction for a longtime evolution, but the bound function Γ(
t) can provide a better real-time estimation of the non-adiabatic transition rate for a robust feed-back adiabatic control during transitionless quantum driving for time-dependent quantum systems.
3.2. Adiabatic control of spin by the sweeping magnetic fieldsNow, let us calculate the upper bound function with more driving parameters in the two-state system with a general form of
The Hamiltonian can be expressed by Pauli operators
in a form of Eq. (
29)
where
and the parameters
and
can be any time-dependent functions for different driving schemes in a practical control process.
A typical application of the above model is for spin qubit control with a varying magnetic field shown in Fig. 4. The driving magnetic field is usually constructed by superposition of two varying magnetic fields, one is
along z direction and the other
is perpendicular to
and rotates with a frequency of ω.[16] The combined magnetic field
can be generally described by
where the magnitude of
is
B0(
t) and
α (
t) is the instaneous angle of the sweeping cone. The spin Hamiltonian
reads
where the Lamor frequency of electron spin is
As the sweeping magnetic field
can be denoted by a spherical coordinate:
, we can introduce three spherical driving parameters as
then the Hamiltonian of Eq. (
42) is put into a form of
which is defined in
λ-parameter space. Alternatively, another popular parameterized form,
is often used to define a generalized Bloch sphere where the new parameters are defined by
and we can easily find that the Bloch parameters
u(
t),
v(
t), and
w(
t) satisfy
It is well known that the spin operators
construct a SU(2) Lie algebra and the instantaneous spin eigenstates and eigenvalues are
which gives an energy gap of
.
The instantaneous eigenstates
and
represent the spin-up and spin-down states with respect to the transient direction of the magnetic field
, respectively. As spin states
follow the direction of
controlled by the driving parameters
,
, and
, the physical conditions for a practical spin control satisfy
(or
) because the internal Lamor frequency of electronic spin (
) is much larger than the varying speed of the classical driving parameters.
The general spin state Ψ(t) can then be expanded by
where the dynamical phases are
In this case, the dynamical equations for the transition rates are
and the transient power operator
of Hamiltonian Eq. (
42) with parameters
reads
We can easily verify that equation (50) satisfies Eq. (26) and the parametric path is determined by the driving scheme of Eq. (44). Therefore, the upper bound for non-adiabatic transition rate in this case can be estimated by Eq. (24) in details if the time-dependent functions of the driving parameters are given. In the following sections, the bound functions of adiabatic control with two different driving schemes are calculated and the geometric phases for the adiabatic spin dynamics are investigated.
3.2.1. Magnetic field sweeping in the azimuthal directionA 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
,
, and
(see Fig. 4), and the transition dynamics for this control scheme will be
where the detuning
. Consequently, the power operator reduces to
and the upper bound for non-adiabatic transition rates calculated by Eq. (
24) are
which can also be directly obtained from Eqs. (
51) and (
52), leading to a convectional adiabatic condition of adiabatic control for
. When the spin is initially in spin-down state, the probability estimation of Eq. (
23) on spin-up state at a later time
will be checked by the strict solution of
where the effective flip frequency is
and equation (
23) gives
which is exactly the expansion of Eq. (
53) at a very short time
. As the rigorous solution of this case is well studied,
[15] the geometric phase in this case is omitted.
3.2.2. Magnetic field sweeping in both azimuthal and polar directionsNow we would like to consider a more general controlling magnetic field for
,
, and
, then the driving power operator of the system is
and the transition dynamics for this control scheme are
The upper bound function of this control scheme becomes
which sets the modulus bound for the changing rates of both driving parameters as shown in Fig.
5. When both the changing rates
and
keep small, the dynamics will conduct an adiabatic evolution which leads to an adiabatic geometric phases for
as
Specifically, if the polar angle of
is controlled by a periodic function of
the upper bound function for the adiabatic control reads
The dynamics of transition rates
and the bound function are both shown in Fig. 5 for different driving parameters. The bound function exhibits a good real-time constraint on the transition rate and reveals that low sweeping frequencies (ω and ν) and small sweeping amplitude α1 are critical for an adiabatic state evolution.
For an adiabatic case of
, the adiabatic geometric phase along a parametric path will be strictly calculated by
where
is the Bessel function of the first kind,
is the initial polar angle,
and
ν are the sway amplitude and frequency of the driving magnetic filed
along the polar direction. Figure
6 compares the dynamics of strict phases with the geometric phases of
for the instantaneous spin-down state
under different frequency ratios
and
. The strict calculations on Eqs. (
54) and (
55) (the thin blue lines) verify that equation (
58) is a very good result when the bound function keeps small. However, if the parametric driving breaks the requirement of a small bound function, equation (
58) becomes inaccurate and the real phase of
will deviate from Eq. (
58) with irregular oscillations due to the non-adiabatic state transitions.
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. 6(b) and 6(c)) and the nonlinear part which induced by the parametric oscillation in the polar direction. The adiabatic geometric phases related to two periods of
and
are
which are displayed in Fig.
6 by the grey horizontal grid lines just for references. Clearly, in a conventional case of
, equation (
58) gives the familiar Berry phase in a cyclic period of
with a solid angle of
, deriving only from the linear part of Eq. (
58) (see Fig.
6(a)). However, for a control period of
T = 2
π/
ν along a closed parametric path for
, the invariant Berry phase will be
As we have two control frequencies, ω and ν, along the azimuthal and polar directions, respectively, there exist two evolutionary periods of T for
that will lead to rich geometric structures of the Lissajous parametric paths on Bloch sphere determined by the frequency ratio of
and the amplitude ratio of
(see Fig. 7). Therefore, the Berry phase in this case should be determined by a common period covering a closed path in the parameter space.
For the adiabatic dynamics, when the driving ratio
is a rational number, that is
, where
, the common period is
while the adiabatic geometric phase is
. For
, the path has a simple form and does not intersect with itself. But for
, the orbit intersects with itself and the region S covered by the path on the Bloch sphere will be divided into different overlapped areas and each area enclosed by a loop contributes its respective part because the total covering area is the sum of them. This is due to
where
is the azimuthal angle of the path
C on the Bloch sphere and
is the area element of the region
S covered by the path. Equation (
61) reveals a
-weighted area contribution to the geometric phase. That means when the polar angle
α is constant (
), the state
evolves along a circle path on the Bloch sphere and its geometric phase increases linearly with a slope proportional to
as shown in Fig.
6(a). Figure
6(b) demonstrates that, for a case of
and
, the path has a knot shape and the region
S is divided into two separated areas by one intersection point (one vertex). Therefore the phases exhibit a two step-like dynamics during the command period of
. While, for
and
, Figure
6(c) shows that the region
S consists of two overlapped areas, which leads to a different phase dynamics with only one step during the command period of
. The areas enclosed by more loops formed by the complicated periodic paths are shown in Fig.
7 and the parametric path has a close relation to a Eulerian path in a view of graph theory. However, when the ratio
is an irrational number, no closed path can be found in the parameter space to fulfill
and no invariant Berry phase exists in this case because the parametric path does not enclose an area (no enclosed boundary).
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.