Functions for using Trotterisation to approximate the action of exponentials of weighted sums of Pauli tensors upon Quregs. More...

Functions
void	applyTrotterizedControlledPauliStrSumGadget (Qureg qureg, int control, PauliStrSum sum, qreal angle, int order, int reps, bool permuteTerms)

void	applyTrotterizedMultiControlledPauliStrSumGadget (Qureg qureg, int *controls, int numControls, PauliStrSum sum, qreal angle, int order, int reps, bool permuteTerms)

void	applyTrotterizedMultiStateControlledPauliStrSumGadget (Qureg qureg, int controls, int states, int numControls, PauliStrSum sum, qreal angle, int order, int reps, bool permuteTerms)

void	applyTrotterizedNonUnitaryPauliStrSumGadget (Qureg qureg, PauliStrSum sum, qcomp angle, int order, int reps, bool permuteTerms)

void	applyTrotterizedPauliStrSumGadget (Qureg qureg, PauliStrSum sum, qreal angle, int order, int reps, bool permuteTerms)

Detailed Description

Functions for using Trotterisation to approximate the action of exponentials of weighted sums of Pauli tensors upon Quregs.

Function Documentation

◆ applyTrotterizedControlledPauliStrSumGadget()

void applyTrotterizedControlledPauliStrSumGadget	(	Qureg	qureg,
		int	control,
		PauliStrSum	sum,
		qreal	angle,
		int	order,
		int	reps,
		bool	permuteTerms )

Note: Documentation for this function or struct is under construction!

Warning: This function has not yet been unit tested and may contain bugs. Please use with caution!

Author: Tyson Jones; Vasco Ferreira (randomisation)

See also

Definition at line 190 of file trotterisation.cpp.

  {
    validate_quregFields(qureg, __func__);
    validate_pauliStrSumFields(sum, __func__);
    validate_pauliStrSumIsHermitian(sum, __func__);
    validate_controlAndPauliStrSumTargets(qureg, control, sum, __func__);
    validate_trotterParams(order, reps, __func__);
    
    bool onlyLeftApply = false;
    internal_applyAllTrotterRepetitions(qureg, &control, nullptr, 1, sum, angle, order, reps, onlyLeftApply, permuteTerms, __func__);
}

◆ applyTrotterizedMultiControlledPauliStrSumGadget()

void applyTrotterizedMultiControlledPauliStrSumGadget	(	Qureg	qureg,
		int *	controls,
		int	numControls,
		PauliStrSum	sum,
		qreal	angle,
		int	order,
		int	reps,
		bool	permuteTerms )

Note: Documentation for this function or struct is under construction!

Warning: This function has not yet been unit tested and may contain bugs. Please use with caution!

Author: Tyson Jones; Vasco Ferreira (randomisation)

See also

Definition at line 204 of file trotterisation.cpp.

  {
    validate_quregFields(qureg, __func__);
    validate_pauliStrSumFields(sum, __func__);
    validate_pauliStrSumIsHermitian(sum, __func__);
    validate_controlsAndPauliStrSumTargets(qureg, controls, numControls, sum, __func__);
    validate_trotterParams(order, reps, __func__);
 
    bool onlyLeftApply = false;
    internal_applyAllTrotterRepetitions(qureg, controls, nullptr, numControls, sum, angle, order, reps, onlyLeftApply, permuteTerms, __func__);
}

◆ applyTrotterizedMultiStateControlledPauliStrSumGadget()

void applyTrotterizedMultiStateControlledPauliStrSumGadget	(	Qureg	qureg,
		int *	controls,
		int *	states,
		int	numControls,
		PauliStrSum	sum,
		qreal	angle,
		int	order,
		int	reps,
		bool	permuteTerms )

Note: Documentation for this function or struct is under construction!

Warning: This function has not yet been unit tested and may contain bugs. Please use with caution!

Author: Tyson Jones; Vasco Ferreira (randomisation)

See also

Definition at line 218 of file trotterisation.cpp.

  {
    validate_quregFields(qureg, __func__);
    validate_pauliStrSumFields(sum, __func__);
    validate_pauliStrSumIsHermitian(sum, __func__);
    validate_controlsAndPauliStrSumTargets(qureg, controls, numControls, sum, __func__);
    validate_controlStates(states, numControls, __func__); // permits states==nullptr
    validate_trotterParams(order, reps, __func__);
 
    bool onlyLeftApply = false;
    internal_applyAllTrotterRepetitions(qureg, controls, states, numControls, sum, angle, order, reps, onlyLeftApply, permuteTerms, __func__);
}

◆ applyTrotterizedNonUnitaryPauliStrSumGadget()

void applyTrotterizedNonUnitaryPauliStrSumGadget	(	Qureg	qureg,
		PauliStrSum	sum,
		qcomp	angle,
		int	order,
		int	reps,
		bool	permuteTerms )

Warning: This function has not yet been unit tested and may contain bugs. Please use with caution!

A generalisation of applyTrotterizedPauliStrSumGadget() which accepts a complex angle and permits sum to be non-Hermitian, thereby effecting a potentially non-unitary and non-CPTP operation.

Formulae

Let \( \hat{H} = \) sum and \( \theta = \) angle \( \in \mathbb{C} \). This function approximates the action of

\[ \exp \left(\iu \, \theta \, \hat{H} \right) \]

via a Trotter-Suzuki decomposition of the specified order and number of repetitions (reps).

‍See applyTrotterizedPauliStrSumGadget() for more information about the decomposition, and other parameters.

Equivalences

When angle is set to \( \theta = \iu \, \Delta \tau \) and sum = \( \hat{H} \) is Hermitian, this function (approximately) evolves qureg in imaginary-time for duration \( \Delta \tau \), effecting non-unitary propagator
\[ \exp(- \Delta \tau \hat{H}) \]
as utilised by applyTrotterizedImaginaryTimeEvolution().
When angle is real and sum is Hermitian (i.e. has approximately real coefficients), the effected operation is unitary and this function becomes equivalent to applyTrotterizedPauliStrSumGadget().

Constraints

This function only ever effects \( \exp \left(\iu \, \theta \, \hat{H} \right) \) exactly when all PauliStr in sum = \( \hat{H} \) commute.

Parameters

[in,out]	qureg	the state to modify.
[in]	sum	a weighted sum of Pauli strings to approximately exponentiate.
[in]	angle	an effective prefactor of `sum` in the exponent.
[in]	order	the order of the Trotter-Suzuki decomposition (e.g. `1`, `2`, `4`, ...).
[in]	reps	the number of Trotter repetitions.
[in]	permuteTerms	whether to randomly reorder Pauli terms at each repetition.

Exceptions

error

if qureg or sum are uninitialised.
if sum contains non-identities on qubits beyond the size of qureg.
if order is not 1 nor a positive, even integer.
if reps is not a positive integer.
if internal allocation needed for term permutation fails.

Author: Tyson Jones; Vasco Ferreira (randomisation)

Definition at line 167 of file trotterisation.cpp.

                                                                                                                                    {
    validate_quregFields(qureg, __func__);
    validate_pauliStrSumFields(sum, __func__);
    validate_pauliStrSumTargets(sum, qureg, __func__);
    validate_trotterParams(order, reps, __func__);
    // sum is permitted to be non-Hermitian
 
    // |psi> -> U |psi>, rho -> U rho U^dagger
    bool onlyLeftApply = false;
    internal_applyAllTrotterRepetitions(qureg, nullptr, nullptr, 0, sum, angle, order, reps, onlyLeftApply, permuteTerms, __func__);
}

◆ applyTrotterizedPauliStrSumGadget()

void applyTrotterizedPauliStrSumGadget	(	Qureg	qureg,
		PauliStrSum	sum,
		qreal	angle,
		int	order,
		int	reps,
		bool	permuteTerms )

Warning: This function has not yet been unit tested and may contain bugs. Please use with caution!

Effects an approximation to the exponential of sum, weighted by angle times \( i \), upon qureg, via the symmetrized Trotter-Suzuki decomposition (arXiv).

Increasing reps (the number of Trotter repetitions) or order (an even, positive integer or one) improves the accuracy of the approximation by reducing the "Trotter error" due to non-commuting terms of sum, though increases the runtime linearly and exponentially respectively. Using permuteTerms the ordering of terms in the sum can also be randomised, which generally improves the accuracy of the approximation for low order decompositions (arXiv).

Formulae

Let \( \hat{H} = \) sum and \( \theta = \) angle \( \in \mathbb{R} \). This function approximates the action of

\[ \exp \left(\iu \, \theta \, \hat{H} \right) \]

via a Trotter-Suzuki decomposition of the specified order and number of repetitions (reps). Simulation is exact, regardless of order or reps, only when all terms in sum commute.

Important: Observe that \( \theta \) lacks the \( -\frac{1}{2} \) prefactor present in other functions like applyPauliGadget().

To be precise, let \( r = \) reps and assume sum is composed of \( T \)-many terms of the form

\[ \hat{H} = \sum\limits_j^T c_j \, \hat{\sigma}_j \]

where \( c_j \) is the coefficient of the \( j \)-th PauliStr \( \hat{\sigma}_j \).

When order=1, this function performs first-order Trotterisation, where the terms of sum are effected in a repeated, arbitrary but fixed order.
\[ \exp(\iu \, \theta \, \hat{H} ) \approx \prod\limits^{r} \prod\limits_{j=1}^{T} \exp \left( \iu \, \frac{\theta \, c_j}{r} \, \hat\sigma_j \right). \]
When order=2, this function performs the lowest order "symmetrized" Suzuki decomposition, whereby each repetition effects the terms of sum forward then in reverse.
\[ \exp(\iu \, \theta \, \hat{H} ) \approx \prod\limits^{r} \left[ \prod\limits_{j=1}^{T} \exp \left( \iu \frac{\theta \, c_j}{2 \, r} \hat\sigma_j \right) \prod\limits_{j=T}^{1} \exp \left( \iu \frac{\theta \, c_j}{2 \, r} \hat\sigma_j \right) \right]. \]
Greater, even values of order (denoted by symbol \( n \)) invoke higher-order symmetrized decompositions \( S[\theta,n,r] \). These see the lower order Trotter circuits repeated twice forward, then reversed, then twice forward again, recursively. To be precise, letting \( p = \left( 4 - 4^{1/(n-1)} \right)^{-1} \), these satisfy
\begin{align*} S[\theta, n, 1] &= \left( \prod\limits^2 S[p \, \theta, n-2, 1] \right) S[ (1-4p)\,\theta, n-2, 1] \left( \prod\limits^2 S[p \, \theta, n-2, 1] \right), \\ S[\theta, n, r] &= \prod\limits^{r} S\left[\frac{\theta}{r}, n, 1\right]. \end{align*}

‍These formulations are taken from 'Finding Exponential Product Formulas of Higher Orders', Naomichi Hatano and Masuo Suzuki (2005) (arXiv).

When permuteTerms=true, the terms of sum are effected in a random order at each repetition. That is, each repetition of the Trotter-Suzuki decomposition is evaluated with the sum

\[ \hat{H} = \sum\limits_j^T c_{\pi(j)} \, \hat{\sigma}_{\pi(j)} \]

where \( \pi \) is a randomly selected permutation.

Equivalences

By passing \( \theta = - \Delta t / \hbar \), this function approximates unitary time evolution of a closed system under the time-independent Hamiltonian sum = \( \hat{H} \) over a duration of \( \Delta t \), as described by propagator
\[ \hat{U}(\Delta t) = \exp(- \iu \, \Delta t \,\hat{H} \, / \, \hbar), \]
as utilised by the function applyTrotterizedUnitaryTimeEvolution().
This function is equivalent to applyTrotterizedNonUnitaryPauliStrSumGadget() when passing a qcomp instance with a zero imaginary component as the angle parameter. This latter function is useful for generalising dynamical simulation to imaginary-time evolution.

Constraints

Unitarity of the prescribed exponential(s) requires that sum is Hermitian, ergo containing only real coefficients. Validation will check that sum is approximately Hermitian, permitting coefficients with imaginary components smaller (in magnitude) than epsilon.
\[ \max\limits_{i} |c_i| \le \valeps \]
where the validation epsilon \( \valeps \) can be adjusted with setQuESTValidationEpsilon(). Otherwise, use applyTrotterizedNonUnitaryPauliStrSumGadget() to permit non-Hermitian sum and ergo effect a non-unitary exponential(s).
The angle parameter is necessarily real to retain unitarity, but can be relaxed to an arbitrary complex scalar (i.e. a qcomp) using applyTrotterizedNonUnitaryPauliStrSumGadget(). This permits cancelling the complex unit \( i \) to effect non-unitary \( \exp(\theta \, \hat{H}) \) as is useful for imaginary-time evolution.
This function only ever effects \( \exp \left(\iu \, \theta \, \hat{H} \right) \) exactly when all PauliStr in sum = \( \hat{H} \) commute, or reps \( \rightarrow \infty \).

Parameters

[in,out]	qureg	the state to modify.
[in]	sum	a weighted sum of Pauli strings to approximately exponentiate.
[in]	angle	the prefactor of `sum` times \( i \) in the exponent.
[in]	order	the order of the Trotter-Suzuki decomposition (e.g. `1`, `2`, `4`, ...).
[in]	reps	the number of Trotter repetitions.
[in]	permuteTerms	whether to randomly reorder Pauli terms at each repetition.

Exceptions

error

if qureg or sum are uninitialised.
if sum is not approximately Hermitian.
if sum contains non-identities on qubits beyond the size of qureg.
if order is not 1 nor a positive, even integer.
if reps is not a positive integer.
if internal allocation needed for term permutation fails.

See also

Author: Tyson Jones; Vasco Ferreira (randomisation)

Definition at line 179 of file trotterisation.cpp.

                                                                                                                          {
    validate_quregFields(qureg, __func__);
    validate_pauliStrSumFields(sum, __func__);
    validate_pauliStrSumTargets(sum, qureg, __func__);
    validate_pauliStrSumIsHermitian(sum, __func__);
    validate_trotterParams(order, reps, __func__);
 
    bool onlyLeftApply = false;
    internal_applyAllTrotterRepetitions(qureg, nullptr, nullptr, 0, sum, angle, order, reps, onlyLeftApply, permuteTerms, __func__);
}