Other Optimization Methods¶

In the following, we compare Krotov’s method to other numerical optimization methods that have been used widely in quantum control, with an emphasis on methods that have been implemented as open source software.

Iterative schemes from variational calculus¶

Gradient-based optimal control methods derive the condition for the optimal control field from the application of the variational principle to the optimization functional in Eq. (1). Since the functional depends both on the states and the control field, it is necessary to include the equation of motion (Schrödinger or Liouville-von-Neumann) as a constraint. That is, the states \(\{\ket{\phi_k}\}\) must be compatible with the equation of motion under the control fields \(\{\epsilon_l(t)\}\). In order to convert the constrained optimization problem into an unconstrained one, the equation of motion is included in the functional with the co-states \(\ket{\chi_k(t)}\) as Lagrange multipliers [61, 62, 63, 64].

The necessary condition for an extremum becomes \(\delta J = 0\) for this extended functional. Evaluation of the extremum condition results in [64]

(20)¶\[\Delta \epsilon_l(t) \propto \frac{\delta J}{\delta \epsilon_l} \propto \Im \big\langle \chi_k(t) \big\vert \Op{\mu} \big\vert \phi_k(t) \big\rangle\,,\]

where \(\Op{\mu} = \partial \Op{H} / \partial \epsilon_l(t)\) is the operator coupling to the field \(\epsilon_l(t)\). Equation (20) is both continuous in time and implicit in \(\epsilon_l(t)\) since the states \(\ket{\phi_k(t)}\), \(\ket{\chi_k(t)}\) also depend on \(\epsilon_l(t)\). Numerical solution of Eq. (20) thus requires an iterative scheme and a choice of time discretization.

The most intuitive time-discretization yields a concurrent update scheme [5, 44, 64],

(21)¶\[\Delta \epsilon_l^{(i)}(t) \propto \Im \big\langle \chi_k^{(i-1)}(t) \big\vert \Op{\mu} \big\vert \phi_k^{(i-1)}(t) \big\rangle\,.\]

Here, at iterative step \((i)\), the backward-propagated co-states \(\{\ket{\chi_k(t)}\}\) and the forward-propagated states \(\{\ket{\phi_k(t)}\}\) both evolve under the ‘guess’ controls \(\epsilon^{(i-1)}_l(t)\) of that iteration. Thus, the update is determined entirely by information from the previous iteration and can be evaluated at each point \(t\) independently. However, this scheme does not guarantee monotonic convergence, and requires a line search to determine the appropriate magnitude of the pulse update [64].

A further ad-hoc modification of the functional [65] allows to formulate a family of update schemes that do guarantee monotonic convergence [66, 67]. These schemes introduce separate fields \(\{\epsilon_l(t)\}\) and \(\{\tilde\epsilon_l(t)\}\) for the forward and backward propagation, respectively, and use the update scheme [68]

(22)¶\[\begin{split}\begin{aligned} \epsilon_l^{(i)}(t) & = (1-\delta) \tilde\epsilon_l^{(i-1)}(t) - \frac{\delta}{\alpha} \Im \big\langle \chi_k^{(i-1)}(t) \big\vert \Op{\mu} \big\vert \phi_k^{(i)}(t) \big\rangle \\ \tilde\epsilon_l^{(i)}(t) & = (1-\eta) \epsilon_l^{(i-1)}(t) - \frac{\eta}{\alpha} \Im \big\langle \chi_k^{(i)}(t) \big\vert \Op{\mu} \big\vert \phi_k^{(i)}(t) \big\rangle\,, \end{aligned}\end{split}\]

with \(\delta, \eta \in [0, 2]\) and an arbitrary step width \(\alpha\). For the control of wavepacket dynamics, an implementation of this generalized class of algorithms is available in the WavePacket Matlab package [69].

Krotov’s method¶

The method developed by Krotov [40, 41, 42, 43] and later translated to the language of quantum control by Tannor and coworkers [5, 22, 44, 45, 46] takes a somewhat unintuitive approach to disentangle the interdependence of field and states by adding a zero to the functional. This allows to construct an updated control field that is guaranteed to lower the value of the functional, resulting in monotonic convergence. The full method is described in Krotov’s Method, but its essence can be boiled down to the update in each iteration \((i)\), Eq. (3), taking the form

(23)¶\[\Delta \epsilon_l^{(i)}(t) \propto \Im \big\langle \chi_k^{(i-1)}(t) \big\vert \Op{\mu} \big\vert \phi_k^{(i)}(t) \big\rangle\,,\]

with co-states \(\ket{\chi_k(t)^{(i-1)}}\) backward-propagated under the guess controls \(\{\epsilon_l^{(i-1)}(t)\}\) and the states \(\ket{\phi^{(i)}_k(t)}\) forward-propagated under the optimized controls \(\{\epsilon_l^{(i)}(t)\}\). Compared to the concurrent form of Eq. (21), the Krotov update scheme is sequential: The update at time \(t\) depends on the states forward-propagated using the updated controls at all previous times, see Time discretization for details.

It is worth noting that the sequential update can be recovered as a limiting case of the monotonically convergent class of algorithms in Eq. (22), for \(\delta = 1\), \(\eta = 0\). This may explain why parts of the quantum control community consider any sequential update scheme as “Krotov’s method” [70, 71]. However, following Krotov’s construction [40, 41, 42, 43] requires no ad-hoc modification of the functional and can thus be applied more generally. In particular, as discussed in Second order update, a second-order construction can address non-convex functionals.

In all its variants [5, 22, 44, 45, 46], Krotov’s method is a first-order gradient with respect to the control fields (even in the second-order construction which is second order only with respect to the states). As the optimization approaches the optimum, this gradient can become very small, resulting in slow convergence. It is possible to extend Krotov’s method to take into account information from the quasi-Hessian [23]. However, this “K-BFGS” variant of Krotov’s method is a substantial extension to the procedure as described in Krotov’s Method, and is currently not supported by the krotov package.

The update Eq. (23) is specific to the running cost in Eq. (2). In most of the Iterative schemes from variational calculus, a constraint on the pulse fluence is used instead. Formally, this is also compatible with Krotov’s method, by choosing \(\epsilon_{l, \text{ref}}^{(i)}(t) \equiv 0\) in Eq. (2) [72]. It turns the update equations (23), (21) into replacement equations, with \(\epsilon_l^{(i)}(t)\) on the left-hand side instead of \(\Delta\epsilon_l^{(i)}(t)\), cf. Eq. (22) for \(\delta = 1\), \(\eta = 0\). In our experience, this leads to numerical instability and should be avoided. A mixture of update and replacement is possible when a penalty of the pulse fluence is necessary [73].

GRadient Ascent Pulse Engineering (GRAPE)¶

While the monotonically convergent methods based on variational calculus must “guess” the appropriate time discretization, and Krotov’s method finds the sequential time discretization by a clever construction, the GRAPE method sidesteps the problem by discretizing the functional first, before applying the variational calculus.

Specifically, we consider the piecewise-constant discretization of the dynamics onto a time grid, where the final time states \(\{\ket{\phi_k^{(i-1)}(T)}\}\) resulting from the time evolution of the initial states \(\{\ket{\phi_k}\}\) under the guess controls \(\epsilon^{(i-1)}_n\) in iteration \((i)\) of the optimization are obtained as

(24)¶\[\ket{\phi^{(i-1)}_k(T)} = \Op{U}^{(i-1)}_{N_T} \dots \Op{U}^{(i-1)}_{n} \dots \Op{U}^{(i-1)}_{1} \big\vert \phi_k \big\rangle\,,\]

where \(\Op{U}^{(i-1)}_{n}\) is the time evolution operator on the time interval \(n\) in Hilbert space,

\[\Op{U}^{(i-1)}_{n} = \exp\Bigg[ -\frac{\mathrm{i}}{\hbar} \Op{H}\big( \underbrace{\epsilon^{(i-1)}(\tilde t_{n-1})}_{\epsilon^{(i-1)}_n} \big) \dd t\Bigg];\qquad \tilde{t}_n \equiv t_n + \dd t / 2\,.\]

The independent control parameters are now the scalar values \(\epsilon_n\), respectively \(\epsilon_{ln}\) if there are multiple control fields indexed by \(l\).

The GRAPE method looks at the direct gradient \(\partial J/\partial \epsilon_n\) and updates each control parameter in the direction of that gradient [21]. The step width must be determined by a line search.

Typically, only the final time functional \(J_T\) has a nontrivial gradient. For simplicity, we assume that \(J_T\) can be expressed in terms of the complex overlaps \(\{\tau_k\}\) between the target states \(\{\ket{\phi_k^{\tgt}}\}\) and the propagated states \(\{\ket{\phi_k(T)}\}\), as e.g. in Eqs. (5), (7). Using Eq. (24) leads to

(25)¶\[\begin{split}\begin{split} \frac{\partial \tau_k}{\partial \epsilon_n} &= \frac{\partial}{\partial \epsilon_n} \big\langle \phi_k^{\tgt} \big\vert \Op{U}^{(i-1)}_{N_T} \dots \Op{U}^{(i-1)}_{n} \dots \Op{U}^{(i-1)}_{1} \big\vert \phi_k \big\rangle \\ &= \underbrace{% \big\langle \phi_k^{\tgt} \big\vert \Op{U}^{(i-1)}_{N_T} \dots \Op{U}^{(i-1)}_{n+1}}_{% \bra{\chi^{(i-1)}_k(t_{n+1})} } \, \frac{\partial\Op{U}^{(i-1)}_{n}}{\partial\epsilon_n} \, \underbrace{% \Op{U}^{(i-1)}_{n-1} \dots \Op{U}^{(i-1)}_{1} \big\vert \phi_k \big\rangle}_{% \ket{\phi^{(i-1)}_k(t_n)} }\ \end{split}\end{split}\]

as the gradient of these overlaps. The gradient for \(J_T\), respectively \(J\) if there are additional running costs then follows from the chain rule. The numerical evaluation of Eq. (25) involves the backward-propagated states \(\ket{\chi_k(t_{n+1})}\) and the forward-propagated states \(\ket{\phi_k(t_n)}\). As only states from iteration \((i-1)\) enter in the gradient, GRAPE is a concurrent scheme.

The comparison of the sequential update equation (23) of Krotov’s method and the concurrent update equation (21) has inspired a sequential evaluation of the “gradient”, modifying the right-hand side of Eq. (25) to \(\langle \chi_k^{(i-1)}(t_{n+1}) \vert \partial_\epsilon U_n^{(i-1)} \vert \phi_k^{(i)}(t_n)\rangle\). That is, the states \(\{\ket{\phi_k(t)}\}\) are forward-propagated under the optimized field [74]. This can be generalized to “hybrid” schemes that interleave concurrent and sequential calculation of the gradient [71]. An implementation of the concurrent/sequential/hybrid gradient is available in the DYNAMO Matlab package [71]. The sequential gradient scheme is sometimes referred to as “Krotov-type” [71, 75]. To avoid confusion with the specific method defined in Krotov’s Method, we prefer the name “sequential GRAPE”.

GRAPE does not give a guarantee of monotonic convergence. As the optimization approaches the minimum of the functional, the first order gradient is generally insufficient to drive the optimization further [23]. To remedy this, a numerical estimate of the Hessian \(\partial^2 J_T/\partial \epsilon_j \partial \epsilon_{j^\prime}\) should also be included in the calculation of the update. The L-BFGS-B quasi-Newton method [76, 77] is most commonly used for this purpose, resulting in the “Second-order GRAPE” [78] or “GRAPE-LBFGS” method. L-BFGS-B is implemented as a Fortran library [77] and widely available, e.g. wrapped in optimization toolboxes like SciPy [79]. This means that it can be easily added as a “black box” to an existing gradient optimization. As a result, augmenting GRAPE with a quasi-Hessian is essentially “for free”. Thus, we always mean GRAPE to refer to GRAPE-LBFGS. Empirically, GRAPE-LBFGS usually converges monotonically.

Thus, for (discretized) time-continuous controls, both GRAPE and Krotov’s method can generally be used interchangeably. Historically, Krotov’s method has been used primarily in the control of molecular dynamics, while GRAPE has been popular in the NMR community. Some potential benefits of Krotov’s method compared to GRAPE are [23]:

Krotov’s method mathematically guarantees monotonic convergence in the continuous-time limit. There is no line-search required for the step width \(1 / \lambda_{a, l}\).
The sequential nature of Krotov’s update scheme, with information from earlier times entering the update at later times within the same iteration, results in faster convergence than the concurrent update in GRAPE [71, 80]. This advantage disappears as the optimization approaches the optimum [23].
The choice of functional \(J_T\) in Krotov’s method only enters in the boundary condition for the backward-propagated states, Eq. (15), while the update equation stays the same otherwise. In contrast, for functionals \(J_T\) that do not depend trivially on the overlaps [81, 82, 83, 84, 85], the evaluation of the gradient in GRAPE may deviate significantly from its usual form, requiring a problem-specific implementation from scratch. This may be mitigated by the use of automatic differentiation in future implementations [86, 87].

GRAPE has a significant advantage if the controls are not time-continuous, but are physically piecewise constant (“bang-bang control”). The calculation of the GRAPE-gradient is unaffected by this, whereas Krotov’s method can break down when the controls are not approximately continuous. QuTiP contains an implementation of GRAPE limited to this use case.

Variants of gradient-ascent can be used to address pulse parametrizations. That is, the control parameters may be arbitrary parameters of the control field (e.g., spectral coefficients) instead of the field amplitude \(\epsilon_n\) in a particular time interval. This is often relevant to design control fields that meet experimental constraints. One possible realization is to calculate the gradients for the control parameters from the gradients of the time-discrete control amplitudes via the chain rule [88, 89, 90, 91]. This approach has recently been named “GRadient Optimization Using Parametrization” (GROUP) [92]. An implementation of several variants of GROUP is available in the QEngine C++ library [93]. An alternative for a moderate number of control parameters is “gradient-optimization of analytic controls” (GOAT) [94]. GOAT evaluates the relevant gradient with forward-mode differentiation; that is, \({\partial \tau_k}/{\partial \epsilon_n}\) is directly evaluated alongside \(\tau_k\). For \(N = \Abs{\{\epsilon_m\}}\) control parameters, this implies \(N\) forward propagations of the state-gradient pair per iteration. Alternatively, the \(N\) propagations can be concatenated into a single propagation in a Hilbert space enlarged by a factor \(N\) (the original state paired with \(N\) gradients).

A benefit of GOAT over the more general GROUP is that it does not piggy-back on the piecewise-constant discretization of the control field, and thus may avoid the associated numerical error. This allows to optimize to extremely high fidelities as required for some error correction protocols [94].

GRAPE in QuTiP¶

An implementation of GRAPE is included in QuTiP, see the section on Quantum Optimal Control in the QuTiP docs. It is used via the qutip.control.pulseoptim.optimize_pulse() function. However, some of the design choices in QuTiP’s GRAPE effectively limit the routine to applications with physically piecewise-constant pulses (where GRAPE has an advantage over Krotov’s method, as discussed in the previous section).

For discretized time-continuous pulses, the implementation of Krotov’s method in optimize_pulses() has the following advantages over qutip.control.pulseoptim.optimize_pulse():

Krotov’s method can optimize for more than one control field at the same time (hence the name of the routine optimize_pulses() compared to optimize_pulse()).
Krotov’s method optimizes a list of Objective instances simultaneously. The optimization for multiple simultaneous objectives in QuTiP’s GRAPE implementation is limited to optimizing a quantum gate. Other uses of simultaneous objectives, such as optimizing for robustness, are not available.
Krotov’s method can start from an arbitrary set of guess controls. In the GRAPE implementation, guess pulses can only be chosen from a specific set of options (including “random”). Again, this makes sense for a control field that is piecewise constant with relatively few switching points, but is very disadvantageous for time-continuous controls.
Krotov’s method has complete flexibility in which propagation method is used (via the propagator argument to optimize_pulses()), while QuTiP’s GRAPE only allows to choose between fixed number of methods for time-propagation. Supplying a problem-specific propagator is not possible.

Thus, QuTiP’s GRAPE implementation and the implementation of Krotov’s method in this package complement each other, but will not compare directly.

Gradient-free optimization¶

In situations where the problem can be reduced to a relatively small number of control parameters (typically less than \(\approx20\), although this number may be pushed to \(\approx50\) by sequential increase of the number of parameters and re-parametrization [95, 96]), gradient-free optimization becomes feasible. The most straightforward use case are controls with an analytic shape (e.g. due to the constraints of an experimental setup), with just a few free parameters. As an example, consider control pulses that are restricted to a Gaussian shape, so that the only free parameters are peak amplitude, pulse width and delay. The control parameters are not required to be parameters of a time-dependent control, but may also be static parameters in the Hamiltonian, e.g. the polarization of the laser beams utilized in an experiment [97].

A special case of gradient-free optimization is the Chopped RAndom Basis (CRAB) method [98, 99]. The essence of CRAB is in the specific choice of the parametrization in terms of a low-dimensional random basis, as the name implies. Thus, it can be used when the parametrization is not pre-defined as in the case of direct free parameters in the pulse shape discussed above. The optimization itself is normally performed by Nelder-Mead simplex based on this parametrization, although any other gradient-free method could be used as well. An implementation of CRAB is available in QuTiP, see QuTiP’s documentation of CRAB, and uses the same qutip.control.pulseoptim.optimize_pulse() interface as the GRAPE method discussed above (GRAPE in QuTiP) with the same limitations. CRAB is prone to getting stuck in local minima of the optimization landscape. To remedy this, a variant of CRAB, “dressed CRAB” (DCRAB) has been developed [95] that re-parametrizes the controls when this happens.

Gradient-free optimization does not require backward propagation, only forward propagation of the initial states and evaluation of the optimization functional \(J\). The functional is not required to be analytic. It may be of a form that does not allow calculation of the gradients \(\partial J_T / \partial \bra{\phi_k}\) (Krotov’s method) or \(\partial J / \partial \epsilon_j\) (GRAPE). The optimization also does not require any storage of states. However, the number of iterations can grow extremely large, especially with an increasing number of control parameters. Thus, an optimization with a gradient-free method is not necessarily more efficient overall compared to a gradient-based optimization with much faster convergence. For only a few parameters, however, it can be highly efficient. This makes gradient-free optimization useful for “pre-optimization”, that is, for finding guess controls that are then further optimized with a gradient-based method [32].

Generally, gradient-free optimization can be easily realized directly in QuTiP or any other software package for the simulation of quantum dynamics:

Write a function that takes an array of optimization parameters as input and returns a figure of merit. This function would, e.g., construct a numerical control pulse from the control parameters, simulate the dynamics using qutip.mesolve.mesolve, and evaluate a figure of merit (like the overlap with a target state).
Pass the function to scipy.optimize.minimize for gradient-free optimization.

The implementation in scipy.optimize.minimize() allows to choose between different optimization methods, with Nelder-Mead simplex being the default. There exist also more advanced optimization methods available in packages like NLopt [100] or Nevergrad [101] that may be worth exploring for improvements in numerical efficiency and additional functionality such as support for non-linear constraints.

Choosing an optimization method¶

Fig. 2 Decision tree for the choice of a numerical open-loop optimization method. The choice of control method is most directly associated with the number of control parameters (\(n\)). For “piecewise-constant controls”, the control parameters are the values of the control field in each time interval. For “analytical” controls, we assume that the control fields are described by a fixed analytical formula parametrized by the control parameters. The “non-analytical” controls for CRAB refer to the random choice of a fixed number of spectral components, where the control parameters are the coefficients for those spectral components. Each method in the diagram is meant to include all its variants, a multitude of gradient-free methods and e.g. DCRAB for CRAB, GRAPE-LBFGS and sequential/hybrid gradient-descent for GRAPE, and K-BFGS for Krotov’s method, see text for detail.¶

In the following, we discuss some of the concerns in the choice of optimization methods. The discussion is limited to iterative open-loop methods, where the optimization is based on a numerical simulation of the dynamics. It excludes analytical control methods such as geometric control, closed-loop methods, or coherent feedback control; see Ref. [102] for an overview.

Whether to use a gradient-free optimization method, GRAPE, or Krotov’s method depends on the size of the problem, the requirements on the control fields, and the mathematical properties of the optimization functional. Gradient-free methods should be used if the number of independent control parameters is smaller than \(\approx 20\), or the functional is of a form that does not allow to calculate gradients easily. It is always a good idea to use a gradient-free method to obtain improved guess pulses for use with a gradient-based method [32].

GRAPE or its variants should be used if the control parameters are discrete, such as on a coarse-grained time grid, and the derivative of \(J\) with respect to each control parameter is easily computable. Note that the implementation provided in QuTiP is limited to state-to-state transitions and quantum gates, even though the method is generally applicable to a wider range of objectives.

When the control parameters are general analytic coefficients instead of time-discrete amplitudes, the GROUP [89, 90, 92] or GOAT [94] variant of gradient-ascent may be a suitable choice. GOAT in particular can avoid the numerical error associated with time discretization. However, as the method scales linearly in memory and/or CPU with the number of control parameters, this is best used when then number of parameters is below 100.

Krotov’s method should be used if the control is close to time-continuous, and if the derivative of \(J_T\) with respect to the states, Eq. (15), can be calculated. When these conditions are met, Krotov’s method gives excellent convergence. The general family of monotonically convergent iteration schemes [66] may also be used.

The decision tree in Fig. 2 can guide the choice of an optimization method. The key deciding factors are the number of control parameters (\(n\)) and whether the controls are time-discrete. Of course, the parametrization of the controls is itself a choice. Sometimes, experimental constraints only allow controls that depend on a small number of tunable parameters. However, this necessarily limits the exploration of the full physical optimization landscape. At the other end of the spectrum, arbitrary time-continuous controls such as those assumed in Krotov’s method have no inherent constraints and are especially useful for more fundamental tasks, such as mapping the design landscape of a particular system [103] or determining the quantum speed limit, i.e., the minimum time in which the system can reach a given target [15, 104, 105].