tequila: A platform for rapid development of quantum algorithms.

Jakob S. Kottmann,1, 2, ∗ Sumner Alperin-Lea,1, † Teresa Tamayo-Mendoza,3, 1, 2 Alba Cervera-Lierta,1, 2

Cyrille Lavigne,1, 2 Tzu-Ching Yen,1 Vladyslav Verteletskyi,1 Philipp Schleich,4 Abhinav Anand,1 MatthiasDegroote,5, 1, 2 Skylar Chaney,1, 6 Maha Kesibi,1, 2 Naomi Grace Curnow,7 Brandon Solo,8 Georgios

Tsilimigkounakis,8 Claudia Zendejas-Morales,8 Artur F. Izmaylov,1, 9 and Alán Aspuru-Guzik1, 2, 10, 11, ‡

1Chemical Physics Theory Group, Department of Chemistry, University of Toronto, Canada.2Department of Computer Science, University of Toronto, Canada.

3Department of Chemistry and Chemical Biology, Harvard University4Center for Computational Engineering Science, RWTH Aachen University, Aachen, Germany.

5Current address: Boehringer Ingelheim, Amsterdam, Netherlands6Department of Physics, Williams College, Williamstown, MA, USA.

7University of Cambridge, United Kingdom8Quantum Open Source Foundation QC Mentorship Program

9Department of Physical and Environmental Sciences, University of Toronto Scarborough, Canada.10Vector Institute for Artificial Intelligence, Toronto, Canada.

11Canadian Institute for Advanced Research (CIFAR) Lebovic Fellow, Toronto, Canada.(Dated: February 26, 2021)

Variational quantum algorithms are currently the most promising class of algorithms for deploy-ment on near-term quantum computers. In contrast to classical algorithms, there are almost nostandardized methods in quantum algorithmic development yet, and the field continues to evolverapidly. As in classical computing, heuristics play a crucial role in the development of new quantumalgorithms, resulting in a high demand for flexible and reliable ways to implement, test, and sharenew ideas. Inspired by this demand, we introduce tequila, a development package for quantumalgorithms in python, designed for fast and flexible implementation, prototyping and deploymentof novel quantum algorithms in electronic structure and other fields. tequila operates with ab-stract expectation values which can be combined, transformed, differentiated, and optimized. Onevaluation, the abstract data structures are compiled to run on state of the art quantum simulatorsor interfaces.

I. INTRODUCTION

Quantum computing is currently in the NoisyIntermediate-Scale Quantum (NISQ) [1, 2] era, in whichdevices composed of a few qubits can execute non-trivial quantum computation while grappling with non-negligible noise and a lack of error correction. Theso-called hybrid quantum-classical algorithms, and, inparticular, the variational quantum algorithms (VQAs),constitute one of the most relevant NISQ algorithmssubset. Such algorithms leverage classical coprocessorsto iteratively improve the performance of parametrizedquantum circuits with respect to a variety of objec-tives. VQAs exhibit generally shallow depth and arepostulated to have greater resistance to noise, both ofwhich are necessary to extract the potential of nearterm devices. Algorithms like the Variational Quan-tum Eigensolver (VQE) [3] and the Quantum Approxi-mate Optimization Algorithm (QAOA) [4] have showngreat promise in solving difficult problems in a variety of

∗ E-mail: jakob.kottmann@utoronto.ca† E-mail: sumner.alperin@mail.utoronto.ca‡ E-mail: aspuru@utoronto.ca

fields, including quantum chemistry, materials science,finance and quantum machine learning [5].

The rapid expansion of VQAs and QML algo-rithms has been accompanied and assisted by an ever-expanding market of quantum information and sim-ulation packages. To date, over 100 different quan-tum simulation packages have been released. [6]. Manyof the full stack simulation packages are written inpython, and some of them are backed by leadingcompanies in the quantum industry, such as cirq [7],qiskit [8],q# [9], pyquil [10] and StrawberryFields [11] respectively developed by or affiliated withGoogle, IBM, Microsoft, Rigetti, and Xanadu. Whereapplicable, these industrially developed packages alsointegrate the cloud quantum computing services madeavailable by their respective organization.

However, the large variety of simulation softwareposes a challenge both for the validation of experimentsand the adoption of new algorithms. As the NISQ eracontinues to progress, quantum scientists stand to ben-efit markedly from a unified development framework inwhich the strengths and resources of different softwarepackages – both quantum and classical – can be easilycoordinated, with minimal constriction of algorithmicdesign choices, to further accelerate the pace of devel-opment. To this end, we introduce tequila.

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We note that a number of software packageshave been created to meet similar needs, such asPennylane [12], which made a pioneering stepin introducing automatic differentiation to varia-tional quantum algorithms, as well as classical quan-tum simulators with an extended interface, includ-ing Xacc [13], ProjectQ [14], Qibo [15], Yao [16],QuaSiMo/Qcor [17], and Qulacs [18], alongside sev-eral previously mentioned packages (see a recent re-view [2] for an overview). tequila differs from theaforementioned packages either by a difference in func-tionality or through its application programming inter-face (API). The benefits and strengths of individualpackages largely depend on the intended applicationsand individual preferences of the user.

tequila is an open-source python 3 software pack-age, which integrates diverse simulation software, clas-sical optimization routines, and powerful tools for themanipulation and combination of quantum circuits andvariational objectives thereof. Additionally, tequilahas a native interface for electronic structure packageslike psi4 [19], pyscf [20, 21] and madness [22]. Focusedon variational algorithms, whose objectives may requireclassical transformations on the output of expectationvalues or circuit measurements, tequila implementsconvenient tools for arithmetical operations on thosestructures, and interfaces the powerful autodifferentia-tion libraries jax [23] and autograd to allow hassle-free analytic differentiation of user-defined objectivefunctions. tequila maintains a firmly object-orienteduser interface, in which circuits, Hamiltonians, expec-tation values, and user-defined objectives can be con-veniently combined arithmetically, using code that rep-resents the underlying mathematics in a "blackboard"fashion.

In this article, we detail examples of how construc-tion, compilation, manipulation, differentiation, andoptimization of variational quantum objectives can beperformed in tequila to illustrate the core of the ap-plication programming interface. Further tutorials anddocumentation are available on the tequila Githubrepository [24]. We first present tequilas API and howit deals with the abstract data structures in the form ofobjectives, Hamiltonians and quantum circuits, and thesimulation and execution of these structures. We thenproceed to show particular usage examples that includequantum chemistry simulation and a quantum machinelearning application. Finally, we close this paper withsome conclusions and the further development of thisquantum software package.

II. HOW TEQUILA WORKS

The core intention of tequila, is to provide anopen-source environment for the rapid development and

L = <H>2

dLd0-- = 2<H>

d<H>d0--

Quantum

Computer

send instructions

sample

expectation values

Figure 1. Abstract illustration of black-box and explicit us-age of quantum computer simulation or execution backendsin tequila. (Top) Abstract algorithm development, treat-ing the quantum backend as a black-box sampling devicefor expectation values. (Bottom) Illustration of the under-lying modularized machinery that handles communicationwith the quantum backend.

demonstration of new ideas in quantum computation,with a focus on variational algorithms. tequila is de-signed through a high level of abstraction, resemblingthe chalk-and-blackboard mathematics underlying thealgorithm and inspired by the API of the madness [22]package. The user is offered a choice, illustrated inFig. 1, between treating quantum computers as blackbox samplers of abstract expectation values, or control-ling the use of the quantum computer more directly. Inthe following, we will describe how tequila may beused in both fashions.

A. Abstract data structures

tequila allows users to treat quantum backends,which serve as interfaces to real hardware or simula-tors, as abstract sampling devices, requiring little tono knowledge about the underlying technical detailsof simulation or execution. The user merely needsto be familiar with the physical principles of quan-tum computation, and have an idea about how thespecific problems of interest could be solved with ac-cess to quantum computers. The core functionality oftequila is provided by abstract data structures de-picted in Fig. 2 where the user only deals directly with

3

Figure 2. Abstract data structures in tequila. QubitHamiltonians (hermitian operators) define measurements inan expectation value, or generate a (parametrized) unitaryoperation within a quantum circuit. Objectives can beformed by arbitrarily transformed expectation values whichcan be automatically evaluated through several supportedquantum backends.

Objectives, Hamiltonians and Circuits. These lasttwo objects define abstract Hamiltonians and unitaries(quantum circuits) which can be combined to createabstract expectation values. tequila bundles theseabstract expectation values into objectives which canthen be transformed, combined or differentiated in ablackboard-style fashion.

1. Objectives

Objectives are callable data structures that hold alist containing abstract expectation values and variablesalongside a transformation that defines how those struc-tures shall be processed after evaluation. Formally, atequila objective can be written as

O = f (E0, E1, . . . , a0, a1, . . . ) , (1)

with variables ak and expectation values Ek. Theseexpectation values also depend on the variables akthrough the unitary circuits Uk,

Ek = 〈0|U†k (g(a, b, . . . ))HkUk (g(a, b, . . . )) |0〉 (2)

where the variables can be potentially transformed byarbitrary transformations g.

Operations on objectives return objects of the sametype, as illustrated in Fig. 5, rendering the combinationand extension of objectives more convenient. As exam-ple, consider the addition of the two objectives O1 = E2

1

and the more complicated O2 = e−E22 + c. In this ex-

ample, each of these objectives only carries one expec-tation value for simplicity and c can be a constant, avariable or a scalar function. The corresponding trans-formations in the data structures are f1(x) = x2 andf2(x) = e−x

2

+ c. If both objectives are added togetheras O3 = O1 + O2, the resulting data structure carriestwo abstract expectation values, and the transforma-tion becomes f3(x, y) = x2 + e−y

2

+ c. See Fig. 5 for anexplicit illustration of a similar example.

tequila objectives are fully differentiable. We makeuse of the shift-rule technique developed by Schuldet. al. [25], first implemented within pennylane [12],alongside automatic differentiation of the transforma-tion of the objective. For objectives whose expecta-tion values contain parametrized quantum gates thatdo not fulfill the requirements of being directly differ-entiable, we employ a variety of decomposition tech-niques, similar in spirit to those proposed in Ref. [26].Differentiation of an Objective again gives back anObjective, enabling convenient access to arbitrary or-der derivatives. An explicit illustration using automaticdecomposition of controlled rotations and the resultingObjective data structure is shown in Figs. 6 and 3 ,where we illustrate blackboard-style tequila code fora small toy model.

tequila objectives are abstract data structures thatcan be translated into various quantum backends whichare interfaces to existing quantum hardware or simula-tors. The compile function allows the translation of anabstract Objective into an Objective that is tied to aspecific backend. After compilation, the objective canbe used as an abstract function with respect to its pa-rameters. In the following code snipped, we presenta small example of how the second derivative of anObjective can be obtained as an abstract objective,which can then be translated to a quantum backend andlater on be used as an abstract function:

1 import tequila as tq2 import numpy as np3

4 # a trivial example5 objective=tq.Variable("a").apply(np.cos)6

7 # first derivative8 dO = tq.grad(objective , "a")9 # second derivative

10 ddO = tq.grad(dO , "a")11

12 # compiled second derivative13 compiled = tq.compile(ddO)14

15 # use like a function16 evaluated = compiled ({"a": 1.0})

By not setting the samples keyword in the evalua-tion, the result will become the exact simulation of allexpectation values. The original objective could be, for

4

example, the objective in Fig. 3. Note that the quantumbackend was not specified in the above code example.If this is the case, tequila will detect all supportedand installed backends automatically and choose theone most appropriate for the given task based on an in-trinsically defined hierarchy of efficiency for that task.The compile function takes additional keywords likebackend, samples, and noise in order to specify whichbackend to use, if finite samples are simulated or if anoise model is used for a simulation. If finite samplesare desired, those can be passed to the compile func-tion, influencing the automatic selection of the availablequantum backends. After compilation, the sample sizecan still be changed when calling the Objective.

2. Hamiltonians

Hamiltonians in tequila are represented as linearcombinations of tensor products of Pauli matrices –so called Pauli strings – and by default use open-fermion[27] as a backend for algebraic manipulations.Transformations into different data formats like thesymplectic binary form are also possible for more in-depth tasks (see the next section). In principle, everyHamiltonian class that is able to provide a list of ab-stract Pauli strings can be used within the tequilaarchitecture.

Within the abstract expectation values, the Hamil-tonians represent averaged measurements. A measure-ment of a single qubit in the computational basis can,for example, be represented by the two projectors

Q+ = |0〉 〈0| = 1

2(1 + σz) (3)

Q− = |1〉 〈1| = 1

2(1− σz) . (4)

Hamiltonians can be initialized directly, throughstrings, or from openfermion operators. Take for ex-ample the following Hamiltonian acting on three qubits(specified in parenthesis),

H = σx(0)σy(1) + 3σy(3). (5)

It can be initialized by combining primitives as

1 H = tq.paulis.X(0)*tq.paulis.Y(1)2 H += 3.0*tq.paulis.Y(3)

or from strings,

1 H = tq.QubitHamiltonian.from_string("1.0*X(0)*Y↪→ (1) + 3.0*Y(3)")

More sophisticated Hamiltonians, such as that of the

H = -X(0)X(1) + Z(0) + Y(1)

2_ e Y1-

e-iG=

-a2

12_

____ __ ___G_| |_

+O|.o...

L = <H>U(a) + e <H>U(a)

2dda_( )-

1 import tequila as tq2 from numpy import exp , pi3 a = tq.Variable("a")4 U = tq.gates.Ry(angle=(-a**2).apply(exp)*pi,

↪→ target =0)5 U += tq.gates.X(target=1, control =0)6 H = tq.QubitHamiltonian.from_string(" -1.0*X(0)X

↪→ (1) +0.5Z(0)+Y(1)")7 E = tq.ExpectationValue(H=H, U=U)8 dE = tq.grad(E, "a")9 objective = E + (-dE**2).apply(exp)

10 result = tq.minimize(method="phoenics",↪→ objective=objective)

4 2 0 2 4

0.5

0.0

0.5

1.0

1.5

phoenics

Figure 3. An illustrative toy model implemented and opti-mized within tequila. The top panel illustrates the modelsystem used, where defined loss function L was minimized.The middle panel depicts the implementation with tequila.The lower panel depicts an example of optimization resultsusing the phoenics optimizer.

5

Heisenberg model,

H =∑k

Jxσx(k)σx(k + 1)

+∑k

Jyσy(k)σy(k + 1)

+∑k

Jzσz(k)σz(k + 1)

+∑k

hσz(k), (6)

can be initialized in a similar way:1 H = tq.paulis.Zero()2 for k in range(n_qubits):3 H += Jx*tq.paulis.X([k,k+1])4 H += Jx*tq.paulis.Y([k,k+1])5 H += Jx*tq.paulis.Z([k,k+1])6 H += h*tq.paulis.Z([k])

where Jx,Jy,Jz and h are floating point numbers andthe integer n_qubits determines the number of sites.

Hamiltonians can also be defined more indirectly bydefining abstract qubit wave functions and forming op-erations on them in Dirac notation. Take for examplethe projector on a predefined wave function

H = |Ψ〉 〈Ψ| (7)

which can be defined within tequila as1 wfn = tq.QubitWaveFunction("1.0|00 > + 1.0|11 >")2 wfn = wfn.normalize ()3 H = tq.paulis.Projector(wfn)

where wfn is the |Ψ〉 wave function that can be initial-ized from strings or arrays of coefficients ci that cor-responds to the computational basis state |i〉 in binarynotation. The internal representation of the projectorafter initialization is decomposed into Pauli strings sothat we end up with the same data structure as before.As example, consider the Bell state wavefunction∣∣Ψ+

⟩=

1√2

(|00〉+ |11〉) , (8)

from the code snipped above. The projector takes theform ∣∣Ψ+

⟩ ⟨Ψ+∣∣ =

1

2

(|00〉 〈00|+ |11〉 〈00|

+ |00〉 〈11|+ |11〉 〈11|), (9)

and each of the four individual terms is decomposed intoPauli strings, similarly to Ref. [28], by using Eqs. (3)and (4) and

σ+ ≡ |0〉 〈1| = 1

2(σx + iσy) , (10)

σ− ≡ |1〉 〈0| = 1

2(σx − iσy) . (11)

Similar to Projector, the KetBra function oftequila decomposes the operator |Ψ〉 〈Φ| into a non-hermitian combination of Pauli strings. An arbitrarymatrix can then be decomposed into a tequila Hamil-tonian with the help of the KetBra function. By ap-plying the split function, the anti-hermitian and her-mitian parts can be separated for individual treatmentwithin abstract expectation values, as shown in thiscode example for a given N ×M matrix

1 n_qubits = np.ceil(np.log2(max(matrix.shape)))2 n_qubits = int(n_qubits)3

4 H = tq.paulis.Zero()5 for i in range(matrix.shape [0]):6 for j in range(matrix.shape [1]):7 H += matrix[i,j]*tq.paulis.KetBra(ket=i

↪→ , bra=j, n_qubits=n_qubits)8

9 hermitian , anti = H.split()

where the matrix is encoded in binary intodlog2 (max (M,N))e qubits.

The decompositions above use wavefunction syntaxfor convenience and are only feasible for problems thatcan be encoded into small analytical wave functions.Projectors on larger, potentially unknown, wavefunc-tions can be encoded as well, as long as the unitary cir-cuit that prepares them is known. The projector thenlooks like

|Ψ〉 〈Ψ| = UΨ |0〉 〈0|U†Ψ, (12)

where the unitary UΨ is a tequila quantum circuit.In Fig. 4 we illustrate both techniques applied for theevaluation of fidelities F = |〈Ψ|Φ〉|2.

3. Quantum Circuits

Abstract quantum circuits can be defined over prim-itive quantum gates, either from those in the tequilagate set, or by defining them over a generator which it-self is given by a tequila Hamiltonian. As an example,consider the y-rotation on qubit 0,

Ry(a) = e−a2 σy(0), (13)

which can be initialized in the following equivalent ways:1 U = tq.gates.Ry(angle =1.0, target =0)2 U = tq.gates.ExpPauli(angle =1.0,3 paulistring="Y(0)")4 g = tq.paulis.Y(0)5 U = tq.gates.Trotterized(angles =[1.0] ,6 generators =[g], steps =1)

The Trotterized function accepts arbitrary Hermitiangenerators and follows the same conventions as the one-qubit rotations with the exponent being −ia2 . Note thatthere is no approximation used in this example, since

6

1 # Fidelity with a known state2 # Bell state3 wfn = tq.QubitWaveFunction.from_string(4 "1.0*|00 > + 1.0*|11 >")5 wfn = wfn.normalize ()6

7 # compile as objective8 P = tq.paulis.Projector(wfn)9 O = tq.ExpectationValue(H=P, U=U)

10 F = tq.simulate(O)11

12 # Fidelity with the Bell state13 # encoded in circuit U214 U2 = tq.gates.H(0)15 U2 += tq.gates.CNOT (0,1)16 P0 = tq.paulis.Unit()17 for n in range(U2.n_qubits):18 P *= tq.paulis.Qp(n)19 O = tq.ExpectationValue(H=P0, U=U+U2.dagger ())20 F = tq.simulate(O)

Figure 4. Example portraying how to obtain the fidelitybetween two states using tequila and different strategies.One for states that are analytically known and one for statesencoded in quantum circuits. We use the same Bell state forboth illustrations. The fidelities are computed with respectto a state encoded in circuit U which is assumed to be al-ready initialized.

a single step in the formal trotterization is exact here;however this is not the case for all generators. In gen-eral tequila allows the definition of arbitrary gatesover Hermitian generators G =

∑k ckσk, represented

by a weighted sum over k Pauli strings σk, as a formalTrotter expansion with N steps,

U(a) = e−ia2G =

N∏n=1

K∏k=1

e−ia2

ckN σk . (14)

Parametrized gates like rotations can also be initial-ized by variables or transformations of variables. Takefor example the parametrized rotation:

Ry(f(a)) = e−if(a)2 σy , f(a) = e−

a2

4 , (15)

which can be initialized as

1 a = tq.Variable("a")2 fa = ( -0.25*a**2).apply(exp)3 U = tq.gates.Ry(angle=fa, target =0)

Here exp is the exponential function, for example fromnumpy, and the transformation of abstract variablesuses the same data structures as the abstract objec-tives. The actual values of the variables are not tiedto the circuit structures but are passed down when theobjectives are evaluated (see previous sections). In thisway, the objectives can be used like abstract functions.

Quantum circuits usually consist of more than onegate, and their construction can be achieved by sim-ply adding individual gates together. The arithmeticis here implemented according to the quantum circuitmodel where addition is interpreted as concatenatingtwo circuits, and the leftmost circuit is the one thatacts first. Take for example, the following unitary op-eration consisting of four individual unitary operationswritten in the opposite order using the language of uni-tary matrices

U = UdUcUbUa

= Ua + Ub + Uc + Ud

= Ua Ub Uc Ud . (16)

At the top level, tequila does not distinguish betweengates and circuits. The rotations above are initializedas circuits containing a single gate. See, for example,Fig. 3, and the applications illustrated later.

Circuits can be evaluated in the same way as objec-tives if one is interested in simulating the wave functionor sampling from its distribution and similarly with thecompile function:

1 wfn = tq.simulate(U, variables ={"a":1.0})

The returned data type is the same for finite sam-pling (samples=finite_value) and explicit simulation(samples=None), where the former contains the countsfor the corresponding measurements and the latter thesimulated amplitudes.

B. Simulation and execution

Some improvements in algorithmic quantum comput-ing concern more technical details in its implementationsuch as efficient unitary gate compilation, error miti-gation or optimizing measurement protocols. In thesecases, it is not sufficient to treat the quantum com-puter as a black box, and more details about the de-vice are necessary. tequila, due to its modular de-sign, offers the platform to implement and incorporatethese technical improvements quickly and easily, mak-ing them available, accessible, and easy to use for a

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Objective: ExpectationValues: List transformation: Callable

Objective: ExpectationValues: List transformation: Callable

+, - , *, /, ^

d/dx, f(...)

1 # initialize expectation values2 E0 = tq.ExpectationValue(H=H0 , U=U0)3 E1 = tq.ExpectationValue(H=H1 , U=U1)4 E2 = tq.ExpectationValue(H=H2 , U=U2)5

6 # combine them7 O1 = E0 + E18 O2 = 0.5*E2**29 O3 = O1**O2

Figure 5. Top: Abstract data structure for objectives intequila. Arithmetic operations, transformations or deriva-tives give back objects of the same type. Bottom: Exampleof arithmetic operations on tequila objectives and their re-sulting data structures. The right hand side shows the fullobjective defined as O3 = (E0 + E1)

12E2

2

1 U = tq.gates.H(0)2 U += tq.gates.Ry("a", target=1, control =0)3 O = tq.ExpectationValue(U=U, H=H)4

5 dO = tq.grad(O, "a")

H

Ry(a)

H

Rx(π2 ) Rz(g(a)) Rz(g(a)) R†

x(π2 )

Figure 6. Gradient compiling in tequila: (Top) tequilaTop level code. (Middle) The abstract circuit and the in-ternally compiled circuit. (Bottom) The objective structurerepresenting the gradient dO in the code. Ei

± denote the leftand right shifted expectation values with respect to the ithoccurence of the variable a in the compiled circuit. In thisspecific example the transformations g and g are rescalingby ± 1

2.

broad user base. For instance, a feature already in-cluded in tequila is the automatic compilation of ab-stract multi-qubit gates (like multi-Pauli rotations andcontrolled-rotations) into primitive quantum gates andthe option to optimize measurement protocols by group-ing the Hamiltonian into commuting cliques.

1. Optimized Measurement Protocols

The protocol for grouping a Hamiltionian into com-muting and qubit-wise commuting cliques are describedin detail in Refs. [29] and [30], respectively. The im-plementation thereof uses the binary-symplectic repre-sentation of the Hamiltonian, where each Pauli stringis represented by two integer arrays (see [29] for moredetails). tequila can convert the standard Hamilto-nians into binary-symplectic form and vice versa. Thegrouping algorithm transforms each expectation valueof an objective into multiple expectation values of theform

〈H〉U =∑i

⟨Hi

⟩UiU

(17)

where the individual Hamiltonians Hi are built up solelyfrom Pauli-Z and unit operators and the Ui correspondto Clifford gates added to the original unitary U . Opti-mizing the measurement protocol for an Objective willreturn an Objective as well. After the optimization,all non-trivial operators in the Hamiltonians are Pauli-Z operators, necessitating only a single type of mea-surement to be performed for each of them. This typeis automatically detected and applied if an evaluationwith finite sample sizes is requested. tequila will thenrequest measurements of the qubits supported in theHamiltonian in the computational basis from the corre-sponding quantum backend. The measurement countsp0(i), p1(i), of qubit i in the computational basis states|0〉 and |1〉, are then used to estimate the individualexpectation values in the objective. Take for example

E = 〈a Z(1) + c Z(1)Z(2)〉U= a 〈Z(1)〉U + c 〈Z(1)Z(2)〉U= a (p0(1)− p1(1))

+ c ((p0(1)− p1(1))(p0(2)− p1(2))) (18)

where Z = |0〉 〈0| − |1〉 〈1|. Note that strict equalityholds only for an infinite number of samples where px =| 〈x|U |0〉 |2 is the exact measurement probability.

Comprehension of the details of the implementa-tion of the measurement optimization requires in-depthtechnical knowledge about measurement on quantumcomputers, knowledge which the sole user of such adevice does usually not require. This optimization isavailable to all tequila users by the inclusion of a sim-ple optimize_measurements=true keyword statement

8

when initializing an expectation value (see the onlinetutorials [24] for explicit examples).

2. Gate Compilation and Translation

In order to simulate or execute abstract circuits andobjectives, the structures must first be translated intothe appropriate backend at the gate level. In tequilathis can be accomplished either at time of execution,or by compilation in advance through the tq.compilefunction. In its base version, tequila offers compi-lation for gradients and backends for (controlled) ro-tations, (controlled) exponentiated Pauli gates (powergates) and arbitrary quantum gates defined over gen-erators as in Eq. (14). For the future, we anticipatefurther improvement by the integration of specializedcompiler packages such as tket [31] or pyzx [32] forwhich basic functionality is already supported throughcorresponding interfaces within tequila.

Evaluation of a quantum circuit requires two proto-cols: compilation, followed by translation. In compi-lation, abstract tequila gates are mapped to a morerestricted set of gates by a tequila compiler object.This object is generally deployed within a pre-definedtequila function, although compilers may also be con-structed by the user. The aforementioned compiler se-quentially performs a number of compilations accord-ing to a series of Boolean arguments received upon ini-tialization. These compilations can be translation ofmulti-control or multi-target gates into a sequence ofsingle control or single target gates, compilation of con-trolled rotations into CNOT and single qubit gates, thecompilation of power gates into rotation gates, etc. Thiscompiler is also required when automatic differentiationof objectives is performed, and is handled automaticallyas a subroutine of tq.grad.

After an abstract circuit (or Objective contain-ing several abstract circuits) is compiled to a re-duced gate set, it may then be translated into thelanguage of a specific quantum backend like qulacs,qibo, qiskit, cirq or pyquil. This is accomplishedthrough the BackendCircuit object, more specificallythrough backend-specific inheritors of this class, such asBackendCircuitCirq or BackendCircuitQulacs, etc.These classes create the respective circuits in their tar-get packages from tequila QCircuit objects upon ini-tialization, and handle all tasks of simulation, sam-pling, updating of variables, etc. for the target quan-tum backend. Because each backend supports onlya subset of the available tequila abstract gates di-rectly, each BackendCircuit inheritor class containsthe hard-coded list of compilation instructions requiredto map an arbitrary tequila QCircuit into one con-taining only operations which are individually trans-latable into operations supported in the target back-

end. After the necessary compilation is performed, theBackendCircuit inheritor then translates each oper-ation, differentiating between parametrized gates, un-parametrized gates, and measurement operations asit does so in order to map tequila parameter argu-ments (variables and objectives thereof) into the vari-able placeholders used by the quantum backend.

Note that compiled objectives can still be combinedin the same fashion as before. This allows having objec-tives with expectation values that are themselves evalu-ated on different quantum backends. Therefore applica-tions where different parts of a variational algorithm areexecuted on different hardware or where part of an al-gorithm is simulated classically are naturally realizablewithin tequila.

3. Optimizers

Iterative classical optimization is a core subroutine ofany VQA, and tequila is purposefully tailored towardsimplifying this task. Any parametrized tequila objec-tive can be optimized, either through the use of thebuilt-in tequila gradient descent optimizer, or througha number of optimizer objects that provide an interfacebetween tequila and powerful optimization packageslike SciPy [33], GPyOpt [34], or Phoenics [35]. Alloptimizers in tequila inherit from a shared base class,designed for ease of extension. Additionally, tequilaimplements a class called History, which allows for easymanipulation and plotting of the trajectories followedby objectives and their parameters over the course ofsome optimization run. All the optimizers are callables,taking an Objective alongside a variety of keyword ar-guments, and return a NamedTuple specific to that op-timizer.

All supported methods are conveniently accessiblethrough the minimize function,

1 result = tq.minmize(objective , method="bfgs")

which can take the same keywords as compile andsimulate in order to control the quantum backend andadditional optimizer specific keywords like gradient,initial_values, or maxiter. Partial optimization ofa specific set of variables can be achieved by passingthem as a list with the variables keyword. We referto Fig. 3, the application section below and the tuto-rials provided on Github [24] for explicit use cases andillustrations.

tequila has its own gradient descent optimizer, ca-pable of optimizing by a variety of popular optimiza-tion routines, such as Adam, RMS-prop, Nesterov Mo-mentum, and more. Those methods can be selectedby the method keyword in the minimize function, inthe same way as presented in the code snipped above.

9

A detailed overview can be found in the online tutori-als. [24] In addition to the usual use through a func-tion call, the tequila gradient descent optimizer canbe used as a step-wise optimizer, to give users a morefine-grained control over its use. Both the tequilaGDOptimizer and the SciPyOptimizer can accept cus-tom gradients provided by the user over the gradientkeyword. These optimizers can combine their classicalupdate routines with the Quantum Natural Gradient(QNG) [36] a method of transforming the gradients ofexpectation values based on the Fubini-Study metricof said expectation value. Numerical gradients are alsoavailable for all optimizers by setting the gradient key-word accordingly within the minimize function:

1 # use SciPy ’s 2-point scheme2 result = tq.minmize(objective ,3 method="bfgs",4 gradient="2-point"5 method_options ={"

↪→ finite_diff_rel_step":1.e-4})6 # use tequila ’s 2-point scheme7 result = tq.minmize(objective ,8 method="bfgs",9 gradient ={"method":"2-point",

10 "stepsize":1.e-4})

At present, tequila has plugins to two Bayesianoptimization packages: GPyOpt [34], and Phoen-ics [35]. These packages allow for robust global op-timization and may serve well for objectives that con-tain a small or intermediate number of parameters butwhose gradients require a large array of expectation val-ues. Bayesian optimization has shown promise in theoptimization of quantum circuits. [37] These optimizerscan be accessed identically to the gradient descent opti-mizers, chiefly through the tq.minimize function. See,for example, Fig. 3.

4. Noise

Because of the noise-prone nature of near term quan-tum devices, the exploration of how a VQA behavesin the presence of quantum noise is crucial to evaluatethe performance of the algorithm at hand. Because theformalism for the simulation of quantum noise variesconsiderably among quantum backends, tequila at-tempts to abstract away more pain-staking details ofnoisy simulation so as to allow comparison between mul-tiple backends. A few assumptions are made in orderto accomplish this task:

1. All operations can be affected by noise.

2. Individual noise sources are independent of eachother.

3. Each noise source affects all k -qubit gates equally.

4. Each noise source affects k -qubit gates indepen-dently from 1,. . . k-1, k+1, . . .n-qubit gates (i.e,1-qubit gates are noised seperately from 2-qubitgates, etc).

5. The effects of noise on a given operation are inde-pendent from its position in the circuit.

These assumptions are currently necessary in orderto make the quantum noise in tequila backend-independent. Future releases of tequila may permitmore fine-grained, backend-restricted control over quan-tum noise; only universally-supported operations havebeen incorporated in this first release.

In tequila, noise is represented by a NoiseModelobject, itself a container for various QuantumNoise ob-jects. Six types of quantum noise are supported intequila, these being: bit flips, phase flips, ampli-tude damps, phase damps, simultaneous amplitude-phase damps, and symmetric depolarizations. EachQuantumNoise object holds a probability (or listthereof), and a level, designating the number of qubitsin the operations it should act upon. The NoiseModelobject contains and wraps over these QuantumNoises,and is passed to a quantum backend upon translationof a QCircuit or Objective. NoiseModel objects maybe combined with each other through simple addition.

The manner in which a NoiseModel object is trans-lated into the application of noise depends entirely onthe quantum backend in question. NoiseModel objectsmay be passed to simulate, compile, and minimizethrough the noise keyword argument of each function.Note that, because noise is probabilistic, the applica-tion of noise requires sampling; it can currently not becombined with wavefunction simulation.

Shown below is a simple example of the constructionof a NoiseModel and its application to the simulationof a simple quantum expectation value

1 U = tq.gates.X(0) + tq.gates.CNOT (0,1)2 H = tq.Paulis.Qm(1)3 O = tq.ExpectationValue(U,H)4 bf_1 = tq.circuit.noise.BitFlip(p=0.1, level =1)5 bf_2 = tq.circuit.noise.BitFlip(p=0.3, level =2)6 my_noise_model = bf_1 + bf_27 E_noisy=tq.simulate(O,samples =5000, noise=

↪→ my_noise_model)

where the noise model instructs the quantum backendto apply BitFlip noise with probability p = 0.1 to allsingle-qubit gates and p = 0.3 to all two-qubit gates.

5. Real devices

tequila is capable of executing and emulating cir-cuits and objectives on real quantum devices throughtheir corresponding interfaces: The quantum backends.To access or emulate a device, the user only needs

10

to supply a keyword argument, device, to compile,simulate, or minimize. The value of this keywordcan be any of several types, depending on the quan-tum backend through which the device is accessed; ingeneral, this may be a string, a dictionary, or somebackend-specific instantiation of the device as an objectin itself.

In the case of device emulation, the user can includethe specific properties and gate set of a real device in thesimulation by setting the device-specific noise model innoise=’device’, provided that the quantum backendin question provides some means of access to this noisemodeling, as do, for example, Qiskit and PyQuil.

Similar to noise modeling, the use of a device requiressampling. Shown below is a small example of how atequila user with an IBMQ account may use tequilato run a circuit on the IBMQ Vigo device

1 from qiskit import IBMQ2 IBMQ.load_account ()3 U = tq.gates.Rx(’a’ ,0)4 H = tq.Paulis.Y(0)5 O = tq.ExpectationValue(U,H)6 E = tq.simulate(O,backend=’qiskit ’,7 device=’ibmq_vigo ’,8 samples =1000 ,9 variables ={’a’:np.pi/2})

where more fine-grained device specifications, e.g. in-cluding IBMQ providers, can be specified by passingeither a dictionary or an already initialized IBMQ de-vice.

III. EXAMPLES AND APPLICATIONS

tequila is a general purpose library and aims tosimplify initial prototyping, testing, and deploymentfor quantum algorithm development. It currently pro-vides extended features for quantum chemistry appli-cations [38] and has also been used in the context ofquantum optics [39] and VQE extensions [40]. In thefollowing, we will illustrate how tequila can be appliedthrough explicit application examples. Further, moredetailed illustrations can be found in tutorials providedon Github [24].

A. Quantum Chemistry

One of the proposed killer applications [41] for quan-tum computers, and the original application proposedfor the quantum variational eigensolver [3, 42] is theelectronic structure problem of quantum chemistry.The goal of this application is to find well-behaved ap-proximations to the eigenvalues of the electronic Hamil-tonian which describes the electronic energy of molecu-

lar systems within the Born-Oppenheimer approxima-tion.

The electronic Hamiltonian for a molecule with Neelectrons can be written as

H (~r1, . . . , ~rNe) =

Ne∑k

h (~rk) +1

2

Ne∑k 6=l

g (~rk, ~rl) , (19)

where the one-electron potential h = T +V is the com-bination of the one-electron kinetic energy operator Tand the summed Coulomb potentials V between a sin-gle electron and the nuclear point charges, and g (~rk, ~rl)is the electron-electron Coulomb potential between twoelectrons at positions ~rk and ~rl . Given a set of or-thonormal one-electron basis-functions (spin-orbitals)the electronic Hamiltonian can be written in second-quantized form as

H =∑kl

hkla†kal +

1

2

∑klmn

gklmna†ka†l anam, (20)

where a†k (ak) are anti-commuting operators that cre-ate (annihilate) electrons in the spin-orbital k andh,g denote the integrals of the corresponding opera-tors and spin-orbitals in Dirac notation. If the setof spin-orbitals forms a complete orthonormal system,the second quantized Hamiltonian (20) is equivalent tothe electronic Hamiltonian (19) restricted to the anti-symmetric wave functions obeyed by fermionic systems.The fermionic anti-symmetry of the electronic states isconveniently guaranteed through the anti-commutationrelations of the second-quantized operators. Detailedintroductions can be found in textbooks [43–46], and inrecent reviews [2, 47–49].

The second-quantized Hamiltonian can be encodedinto a qubit Hamiltonian by application of the Jordan-Wigner or Bravyi-Kitaev transformations [50, 51], ormore recently developed encodings such as the Bravyi-Kitaev Superfast transformation [52]. tequila pro-vides a convenient interface to initialize qubit en-coded electronic Hamiltonians by deploying the trans-formations implemented in openfermion [27] and al-lows convenient integration of user-defined transforma-tions.. The molecular integrals can either be supplied asnumpy arrays or may be calculated by interfacing elec-tronic structure packages such as psi4 [53]. tequilainitializes a Molecule object which can then initializequbit encoded electronic Hamiltonians and qubit en-coded excitation generators, and can serve as an inter-face to classical methods of psi4. The Molecule objectensures that the use of encodings, active-spaces, basis-sets and molecular parameters stays consistent for allfurther initialization.

11

1. Molecules and Hamiltonians

Using only their high-level functionality, a Moleculecan be initialized by providing the molecular geometryand the one- and two-electron integrals h and g. Theconstant nuclear-repulsion can optionally be providedto be included in the results. If psi4 is installed, themolecular parameters and integrals can be computedautomatically by providing a Gaussian basis set (see forexample Ref. [49]). Molecular structure data can be ini-tialized explicitly or by passing a string that addressesa file in xyz format.

The following short example instructs one upon howto initialize a Molecule with psi4 or by providing theintegrals as numpy arrays h and g, which are assumedto be already initialized here. In order to comply withother quantum computing packages, the electron re-pulsion integrals g are expected in the openfermionconvention. In the example below we illustrate how toconstruct a tequila molecule from the psi4 interfaceor manually from numpy arrays of molecular integrals:

1 # Can be filename.xyz or explicit string2 geomstring = "He 0.0 0.0 0.0"3

4 # Molecule construction with Psi45 molecule = tq.chemistry.Molecule(6 geometry=geomstring ,7 basis_set="6-31G",8 transformation="bravy_kitaev")

1 # Manual molecule construction2 # resort g integrals (given as numpy array)3 # from Mulliken to openfermion convention4 g = tq.chemistry.NBodyTensor(elems=g,

↪→ scheme=’mulliken ’)5 g.reorder(to=’openfermion ’)6 g = g.elems7 molecule = tq.chemistry.Molecule(8 backend="base",9 geometry=geomstring ,

10 one_body_integrals=h,11 two_body_integrals=g,12 nuclear_repulsion =0.0,13 transformation="bravy_kitaev")

The two-body integrals g were assumed to be in Mul-liken notation (gMulliken

prqs ≡ (pq|g|rs) ≡ 〈pr| g |qs〉 ≡gDiracprqs ), used for example, by psi4. If installed, psi4is automatically detected by tequila and used as a de-fault. The backend keyword allows to demand specificbackends if multiple supported backends are installed.Most functionalities are implemented in a backend inde-pendent manner, allowing for convenient introductionof novel ways to represent Hamiltonians, such as forexample a basis-set-free approach [38] representing theHamiltonian with directly determined pair-natural or-bitals [54].

The transformation keyword specifies the qubit en-coding that will be used for further operations, i.e

Jordan-Wigner, etc. Currently, all transformationswithin openfermion are supported. Alternatively, acallable object can be passed as transformation al-lowing easy integration of user-defined transformations,providing a convenient interface for the future inte-gration of recently developed fermion-to-qubit transfor-mations. [52, 55, 56] Additional parameters for qubitencodings, as for exampl needed within the qubit-tapered [57] form of the Bravyi-Kitaev transforma-tion can be passed down as keyword arguments usingtransformation__ as prefix. If not provided tequilawill try to guess them. We refer to the online tutori-als [24] for explicit examples.

2. Active Spaces

If the psi4 interface is used, active spaces can be setthrough the active_orbitals keyword by selecting or-bitals which are labeled by their irreducible represen-tation for the underlying point group of the molecule.Additionally, the occupation of the reference de-terminant can be defined, where the default is theHartree-Fock reference computed by psi4 (the deter-minant constructed by the first Ne

2 spatial orbitals).Custom reference orbitals can be chosen with thereference keyword using the same input format as forthe active_orbitals. Without psi4 as a backend, ir-reducible representations are not considered and activespaces are set by an array of indices representing theactive orbitals.

Active spaces are tied to the Molecule objects andshouldn’t be changed after initialization. When the ac-tive space is set, all Hamiltonians, excitation generators,classical amplitudes and energies are computed withinthat active space. The following example illustrates theinitialization of a benzene molecule, restricted to the ac-tive space of its 6 conjugated π orbitals (three occupiedand three unoccupied):

1 active = {2 "B1u": [0,1],3 "B3g": [0,1],4 "B2g": [0],5 "Au": [0],6 }7 molecule = tq.chemistry.Molecule(8 geometry="benzene.xyz",9 basis_set=’sto -3g’,

10 active_orbitals=active11 )

Note that psi4 does not support the full D6h point-group and uses D2h instead, leading to different irre-ducible representations for the degenerate π orbitals(A2u → B1u, E1g → B2g,B3g, E2u → B1u,Au, B2g →B3g).

12

3. Unitary Coupled-Cluster

Unitary coupled-cluster (UCC) has become a promis-ing model for quantum chemistry on quantum comput-ers [3, 42] and several promising extensions thereof havebeen developed in recent years. Examples include ex-tended strategies [58–61] pair-excitation based [62] andadaptive strategies in the qubit [63–65] or fermionic [66,67] representation. tequila allows the user to combineunitary operators in the UCC framework to develop newapproaches.

The basic building blocks are unitary operators,

Upq (a) = e−ia2Gpq , (21)

generated by the hermitian fermionic n-body excitationgenerators

Gpq = i(a†paq − a†qap) (22)

Gpqrs = i(a†paqa†ras − h.c.) (23)

Gpq = i(∏n

a†pnaqn − h.c.), (24)

where p, q, r, s are arbitrary spin-orbital indices. Qubitencoded generators of this form can be created from theMolecule object by passing a list of (p0, q1) . . . tuples tothe function make_excitation_generator which willreturn a tequila Hamiltonian representing the qubitencoded hermitian generators of Eq. (24).

The generators can be used to define a Trotterizedunitary quantum gate,

1 idx = [(p0,q0),(p1,q1)]2

3 # definition over generators4 G = molecule.make_excitation_generator(idx)5 U = tq.gates.Trotterized(6 generators =[G],7 angles =["theta"]8 steps =19 )

10

11 # same unitary12 U = molecule.make_excitation_gate(idx)

where a single Trotter step suffices in this case due tothe commutativity of the Pauli strings originating froma single excitation generator. [58] Both ways of initial-izing an elementary fermionic excitation unitary resultin the same quantum circuits, initialization over themake_excitation_gate will, however, provide neces-sary information for efficient gradient compiling. Heretequila exploits certain properties of the fermionicgenerators in order to reduce the runtime of their gradi-ents significantly. Explicit details on this technique canbe found in Ref. [68].

tequila uses alternating enumeration for spin-orbitals, meaning that the spin-up orbital p↑ of spatialorbital p is enumerated with 2p and the corresponding

spin-down orbital p↓ is enumerated as 2p+1. Note thatthis enumeration is independent of the chosen qubit en-coding.

4. Interface to Classical Methods

tequila offers a convenient interface to psi4’s vari-ous classical methods by calling the compute_energymethod of the Molecule. Within unitary coupled-cluster the amplitudes of canonical coupled-cluster of-ten come in handy as potential starting points for fur-ther optimization, or for screening purposes. These canbe computed with the method compute_amplitudes.We refer to the online tutorials for explicit examples.

5. Example: 1-UpCCGSD

As an explicit example we illustrate how to im-plement the 1-UpCCGSD ansatz of Ref. [62] for aHydrogen-Flouride (HF) molecule in an active spacewith tequila. Other molecules and active spaces maybe explored in the same fashion by simply replacing thecorresponding lines of code.

The UpCCGSD ansatz is built up from the single anddouble excitation generators of equations (22) and (23)where the doubles are restricted to pairs of doubly-occupied orbitals Gp↑q↑p↓q↓ . This example employs asingle Trotter step, and orders the fermionic operatorsby orbital number, but note that other orderings arealso possible. [69, 70] The ansatz herein is constructedexplicitly, but note that the molecule structure alreadyoffers a convenient initialization of the k-UpCCGSDunitary through the make_upccgsd_ansatz function.The full code to optimize the UpCCGSD expectationvalue is as follows:

1 import tequila as tq2 # define the active orbitals3 active = active = {"A1":[1,2,3,4,5,6],4 "B1":[0],5 "B2":[0]}6 geomstring = "F 0.0 0.0 0.0\nH 0.0 0.0 1.0"7 # initialize the tequila molecule8 mol = tq.chemistry.Molecule(9 geometry=geomstring ,

10 basis_set=’sto -3g’,11 active_orbitals=active)12

13 # get some classical reference values14 EFCI = mol.compute_energy("fci")15 # initialize the hamiltonian16 H = mol.make_hamiltonian ()17

18 # initialize the circuit for the initial state19 U = mol.prepare_reference ()20 # add the 1-UpCCGSD primitive excitations21 for i in range(mol.n_orbitals):22 for a in range(i+1,mol.n_orbitals):

13

23 # electronic pair excitation24 U += mol.make_excitation_gate(25 [(2*i, 2*a) ,(2*i+1, 2*a+1)])26 # single electron excitation: spin -up27 U += mol.make_excitation_gate(28 [(2*i, 2*a)])29 # single electron excitation: spin -down30 U += mol.make_excitation_gate(31 [(2*i+1, 2*a+1)])32

33 # form the abstract expectation value34 E = tq.ExpectationValue(H=H, U=U)35

36 # bfgs with numerical gradients37 result = tq.minimize(objective=E,method="bfgs")38

39 print("Final VQE Energy:", result.energy)40 print("FCI Energy :", EFCI)

where the scipy implementation of the BFGS optimizeris chosen, and the gradient objectives are automaticallycompiled according to Ref. [68].

6. Example: Sequential Excited State Solver

One way for VQAs to optimize bound excited statesof a given Hamiltonian is to solve for the ground stateand project them out of the Hamiltonian [62, 71] by re-peating this procedure sequentially. After solving for nstates |Ψi〉, generated by the unitaries Ui, the expecta-tion value of the ansatz U and projected Hamiltonianis given by

⟨PHP

⟩U

=

n∑i=1

〈0|U† (1− |Ψi〉 〈Ψi|)HU |0〉

= 〈H〉 −n∑i

Ei 〈0|U†Ui |0〉 〈0|U†i U |0〉

= 〈H〉 −n∑i

Ei⟨Q+

⟩U†

i U(25)

where we used the idempotency of the projector P =

1 − |Ψi〉 〈Ψi| and assumed[H, P

]= 0 which is true

if the |Ψi〉 are true eigenstates of H and will there-fore only hold approximately within most VQAs. [62]The Q+ =

∏kQ+(k) operator denotes the projector

onto the all-zero qubit state |0 . . . 0〉 〈0 . . . 0| making theexpectation value

⟨Q+

⟩U†

i Uequivalent to the squared

overlap between the current ansatz U and the previ-ously found state generated by Ui.

In Fig. 7 we illustrate how such a sequential strategyfor excited states can be applied with tequila. In thisexample we use a customized designed quantum circuitU which could be replaced or combined with

1 U = mol.make_upgccsd_ansatz(label=i)

to solve with 1-UpCCGSD as done in Ref. [62]. Notethat the label is added to the variables of the circuit inorder to keep the different runs in the sequential solverdistinguishable.

B. Variational Quantum Classifier

As an example of a quantum machine learning ap-plication, consider a Variational Quantum Classifier(VQC). Herein is presented a tequila tutorial demon-strating the implementation a single-qubit classifierwith data re-uploading [72]. This VQC model encodesdata points into single-qubit rotational gates multipletimes within the circuit. Each layer is defined as

L(~x; ~θi

)= Rz

(x1 + θ1

i

)Ry(x0 + θ0

i

), (26)

where ~x = (x0, x1) are data points and ~θi are the op-timization parameters of layer i. The circuit is a con-catenation of layers, similar to other VQAs:

Uclass

(~x; ~θ1, ..., ~θl

)= L

(~x; ~θ1

)· · ·L

(~x; ~θl

). (27)

The single-qubit classifier defines a target state on theBloch sphere for each class. For a binary classification,the target states are simply the |0〉 and |1〉 states.

This circuit definition can be constructed in tequilausing the following function, which depends on the num-ber of layers and the data points:

1 def UClass(x, l)2 U = tq.QCircuit ()3 for i in range(l):4 th0 = tq.Variable ((0,i))5 th1 = tq.Variable ((1,i))6 U += tq.gates.Rx(x[0]+th0 , 0)7 U += tq.gates.Ry(x[1]+th1 , 1)8 return

where abstract variables θ and numerical values x canbe conveniently combined in the initialization of theparametrized gates.

We define the cost function as an objective to be min-imized using tequila optimizers. This Objective isconstructed from a training set containing points ~xi andcorresponding target states |yi〉 ∈ {|0〉 , |1〉} as

L =∑

(1− F (~xi, yi))2, (28)

with the fidelities between training points defined as

F (~xi, yi) = | 〈yi|Uclass(~xi; ~θ

)|0〉 |2

= 〈Pyi〉Uclass(~xi;~θ), (29)

and Pyi as |yi〉 〈yi|. This is similar to the illustration inFig. 4. The cost function for this VQC can be initializedas

14

0B1u

1B1u __

|

|__ ^]

_ ___ _______ __ ___________________

+O|.o...

+O|.o...

+O|.o...R (a)_| |_y

2_0-

e-i y

R =y

E = <H>U - EGS S2

1 import tequila as tq2 # define the geometry directly3 geomstring="be 0.0 0.0 0.0\nh 0.0 0.0 {R}\nh

↪→ 0.0 0.0 -{R}"4 R = 1.05 # define the |00><00|=Qp(0)Qp(1) qubit

↪→ projector6 Qp = tq.paulis.Qp([0 ,1])7 # define an active space8 active = {"b1u": [0], "b2u": [0]}9 # initialize the current molecule

10 mol = tq.chemistry.Molecule(geometry=geomstring↪→ .format(R=R), basis_set="6-31g",↪→ active_orbitals=active)

11 # get the hamiltonian12 H = mol.make_hamiltonian ()13 # collect results14 results = []15 # loop over ground and16 # first excited state17 for i in range (2):18 # toy circuit19 U = tq.gates.Ry((i, "a"), 0)20 U += tq.gates.CNOT(0, 1)21 U += tq.gates.CNOT(0, 2)22 U += tq.gates.CNOT(1, 3)23 U += tq.gates.X([2, 3])24 # initialize an expectation value25 E = tq.ExpectationValue(U, H)26 # get the active variables27 # (keep the old variables static later)28 active_vars = E.extract_variables ()29 # initialize the new variables30 angles = {angle: 0.0 for angle in

↪→ active_vars}31 # Define and add the32 # overlap expectation values33 # from the previous optimizations34 for data , U2 in results:35 S2 = tq.ExpectationValue(H=Qp ,36 U=U2.dagger () + U)37 E -= data.energy * S238 angles = {** angles , **data.angles}39 # optimize the objective40 result = tq.minimize(E,41 method="bfgs",42 variables=active_vars ,43 initial_values=angles)44 # keep the result of the45 # current optimization46 results.append ((result , U))

Figure 7. Toy example of an excited state calculation usingthe BeH2 molecule in an active space. The top panel illus-trates the toy model and the circuit deployed. The bottompanel shows the full implementation with tequila.

Figure 8. Results after running a 3-layer single-qubit classi-fier using tequila. The optimizer used is RMS-prop with400 and 1000 training and test points respectively. The ac-curacy achieved is 90.5%.

1 def cost(x, y, l):2 loss = 0.03 for i in range(len(y)):4 U = UClass(x[i], l)5 ystate ="|{}>".format(y[i])6 P = tq.paulis.Projector(ystate)7 F = tq.ExpectationValue(H=P, U=U)8 loss = loss + (1 - F)**29 return loss / len(x)

and, after initialization, can be optimized with one ofthe optimizers provided by tequila.

The results of a 2-D circle classification problem areshown in Fig. 8. Using a 3-layer single-qubit classifier,400 training points and the RMS-prop optimizationalgorithm, the accuracy achieved is in this case 90.5%.

IV. CONCLUSION

Herein we have introduced tequila, a full-stackopen-source python package for the rapid developmentand deployment of variational quantum algorithms.Through the deployment of novel callable structures,the incorporation of automatic differentiation, and theinclusion of extensible plugins for numerical optimiza-tion, quantum chemistry, and more, tequila is primedfor the convenient and intuitive transformation of ideasinto code. We seek to continuously forge tequila intoa wide and robust platform, permitting the quantumcommunity to work in a shared and accessible frame-work to further embolden the ever-accelerating pace ofquantum information science and quantum learning, inthe hope of enhancing collective mastery of the toolsset to emerge in the coming era.

15

ACKNOWLEDGEMENTS

We thank Shumpei Kobayashi for improvements onthe optimizer callbacks and Olga Orkut, Hermanni Hei-monen, Paul Nation, and Michał Stechły for valuablefeedback. This work was supported by the U.S. De-partment of Energy under Award No. DE-SC0019374as well as DE-AC02-05CH11231 through LBNL sub-grant 505736. A.A.-G. acknowledges the generous sup-port from Google, Inc. in the form of a Google Fo-cused Award. A.A.-G. also acknowledges support fromthe Canada Industrial Research Chairs Program andthe Canada 150 Research Chairs Program. A.A-G.also acknowledges the Vannevar Bush Faculty Fellow-ship under contract ONR N00014-16-1-200. P.S. ac-

knowledges support by a fellowship within the IFI pro-gramme of the German Academic Exchange Service(DAAD). A.F.I. acknowledges financial support fromZapata Computing Inc. and the Google Quantum Re-search Program. Computations were performed onthe niagara supercomputer at the SciNet HPC Consor-tium. [73, 74] SciNet is funded by: the Canada Founda-tion for Innovation; the Government of Ontario; OntarioResearch Fund - Research Excellence; and the Univer-sity of Toronto. S.C. acknowledges support from theMITACS globalink program. Contributions from B.S.,G.T, and C.Z.-M. were made within the QC MentorshipProgram of the Quantum Open Source Foundation. Wethank the generous support of Anders G. Frøseth.

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