src/ops/timeevolution.js
/*
* Copyright (c) 2018 Isaac Phoenix (tearsofphoenix@icloud.com).
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import assert from 'assert'
import math from 'mathjs'
import {BasicGate} from './basics'
import QubitOperator, {stringToArray} from './qubitoperator'
import {setEqual, isComplex, isNumeric} from '../libs/polyfill'
import {Ph} from './gates'
import {NotMergeable} from "../meta/error";
/**
* @class TimeEvolution
* @desc
Gate for time evolution under a Hamiltonian (QubitOperator object).
This gate is the unitary time evolution propagator:
exp(-i * H * t),
where H is the Hamiltonian of the system and t is the time. Note that -i
factor is stored implicitely.
@example
wavefunction = eng.allocateQureg(5)
hamiltonian = 0.5 * QubitOperator("X0 Z1 Y5")
# Apply exp(-i * H * t) to the wavefunction:
TimeEvolution(time=2.0, hamiltonian=hamiltonian) | wavefunction
Attributes:
time(float, int): time t
hamiltonian(QubitOperator): hamiltonaian H
*/
export default class TimeEvolution extends BasicGate {
/**
* @constructor
Note:
The hamiltonian must be hermitian and therefore only terms with
real coefficients are allowed.
Coefficients are internally converted to float.
@param {number} time time to evolve under (can be negative).
@param {QubitOperator} hamiltonian hamiltonian to evolve under.
@throws {Error} If time is not a numeric type and hamiltonian is not a QubitOperator.
@throws {NotHermitianOperatorError} If the input hamiltonian is not hermitian (only real coefficients).
*/
constructor(time, hamiltonian) {
super()
if (typeof time !== 'number') {
throw new Error('time needs to be a (real) numeric type.')
}
if (!(hamiltonian instanceof QubitOperator)) {
throw new Error('hamiltonian needs to be QubitOperator object.')
}
this.time = time
this.hamiltonian = hamiltonian.copy()
Object.keys(hamiltonian.terms).forEach((term) => {
const item = this.hamiltonian.terms[term]
if (isNumeric(item)) {
if (isComplex(item)) {
if (math.im(item) === 0) {
this.hamiltonian.terms[term] = math.re(item)
} else {
throw new Error('hamiltonian must be '
+ 'hermitian and hence only '
+ 'have real coefficients.')
}
} else {
this.hamiltonian.terms[term] = item
}
} else {
throw new Error('hamiltonian must be '
+ 'hermitian and hence only '
+ 'have real coefficients.')
}
})
}
getInverse() {
return new TimeEvolution(-this.time, this.hamiltonian)
}
/**
Return self merged with another TimeEvolution gate if possible.
Two TimeEvolution gates are merged if:
1) both have the same terms
2) the proportionality factor for each of the terms
must have relative error <= 1e-9 compared to the
proportionality factors of the other terms.
Note:
While one could merge gates for which both hamiltonians commute,
we are not doing this as in general the resulting gate would have
to be decomposed again.
Note:
We are not comparing if terms are proportional to each other with
an absolute tolerance. It is up to the user to remove terms close
to zero because we cannot choose a suitable absolute error which
works for everyone. Use, e.g., a decomposition rule for that.
@param {TimeEvolution} other TimeEvolution gate
@throws {NotMergeable} If the other gate is not a TimeEvolution gate or
hamiltonians are not suitable for merging.
@return {TimeEvolution} New TimeEvolution gate equivalent to the two merged gates.
*/
getMerged(other) {
const rel_tol = 1e-9
if (!(other instanceof TimeEvolution)) {
throw new NotMergeable('Cannot merge these two gates.')
}
const k1 = Object.keys(this.hamiltonian.terms)
const k2 = Object.keys(other.hamiltonian.terms)
if (setEqual(new Set(k1), new Set(k2))) {
let factor
Object.keys(this.hamiltonian.terms).forEach((term) => {
const v1 = this.hamiltonian.terms[term]
const v2 = other.hamiltonian.terms[term]
if (typeof factor === 'undefined') {
factor = math.divide(v1, v2)
} else {
const tmp = math.divide(v1, v2)
if (math.abs(math.subtract(factor, tmp)) > rel_tol * math.max(math.abs(factor), math.abs(tmp))) {
throw new NotMergeable('Cannot merge these two gates.')
}
}
})
const newTime = this.time + other.time / factor
return new TimeEvolution(newTime, this.hamiltonian)
} else {
throw new NotMergeable('Cannot merge these two gates.')
}
}
/**
Operator| overload which enables the following syntax:
@example
TimeEvolution(...) | qureg
TimeEvolution(...) | (qureg,)
TimeEvolution(...) | qubit
TimeEvolution(...) | (qubit,)
Unlike other gates, this gate is only allowed to be applied to one
quantum register or one qubit.
@example
wavefunction = eng.allocateQureg(5)
hamiltonian = QubitOperator("X1 Y3", 0.5)
TimeEvolution(time=2.0, hamiltonian=hamiltonian) | wavefunction
While in the above example the TimeEvolution gate is applied to 5
qubits, the hamiltonian of this TimeEvolution gate acts only
non-trivially on the two qubits wavefunction[1] and wavefunction[3].
Therefore, the operator| will rescale the indices in the hamiltonian
and sends the equivalent of the following new gate to the MainEngine:
@example
h = QubitOperator("X0 Y1", 0.5)
TimeEvolution(2.0, h) | [wavefunction[1], wavefunction[3]]
which is only a two qubit gate.
@param {Array.<Qubit>|Qureg|Qubit} qubits one Qubit object, one list of Qubit objects, one Qureg
object, or a tuple of the former three cases.
*/
or(qubits) {
// Check that input is only one qureg or one qubit
qubits = BasicGate.makeTupleOfQureg(qubits)
if (qubits.length !== 1) {
throw new Error('Only one qubit or qureg allowed.')
}
// Check that if hamiltonian has only an identity term,
// apply a global phase
const keys = Object.keys(this.hamiltonian.terms)
const v = this.hamiltonian.terms[[]]
if (keys.length === 1 && typeof v !== 'undefined') {
new Ph(math.multiply(-this.time, v)).or(qubits[0][0])
return
}
const num_qubits = qubits[0].length
let non_trivial_qubits = new Set()
keys.forEach(key => {
const term = stringToArray(key)
term.forEach(([index, action]) => {
non_trivial_qubits.add(index)
})
})
if (Math.max(...non_trivial_qubits) >= num_qubits) {
throw new Error('hamiltonian acts on more qubits than the gate is applied to.')
}
// create new TimeEvolution gate with rescaled qubit indices in
// this.hamiltonian which are ordered from
// 0,...,len(non_trivial_qubits) - 1
const new_index = {}
non_trivial_qubits = Array.from(non_trivial_qubits).sort()
non_trivial_qubits.forEach((looper, i) => {
new_index[looper] = i
})
const new_hamiltonian = new QubitOperator()
assert(Object.keys(new_hamiltonian.terms).length === 0, '')
Object.keys(this.hamiltonian.terms).forEach((term) => {
const parts = stringToArray(term)
const newTerm = parts.map(([index, action]) => [new_index[index], action])
new_hamiltonian.terms[newTerm] = this.hamiltonian.terms[term]
})
const new_gate = new TimeEvolution(this.time, new_hamiltonian)
const new_qubits = non_trivial_qubits.map((looper) => qubits[0][looper])
// Apply new gate
const cmd = new_gate.generateCommand(new_qubits)
cmd.apply()
}
equal() {
throw new Error('Not implemented')
}
toString() {
return `exp(${-this.time}j * (${this.hamiltonian}))`
}
}
