Standard library

The standard library for OpenQASM 3 consists of a single include file, called stdgates.inc. This file can be included in an OpenQASM 3 program with the statement

include "stdgates.inc";

Versioning

The standard library is versioned directly with the language itself. This documentation will note the version that any given gate in the standard library became available, and, if applicable, the version it was removed in.

Notes for implementations

An implementation is not required to make the standard library available in situations where the OpenQASM 3 program is being interpreted in terms of the instruction set architecture of a particular QPU. The rest of this section concerns only situations where the library is considered available.

Implementations should consider the stdgates.inc file to be locatable, regardless of any other settings that they might apply around include search paths.

Upon interpreting an include "stdgates.inc"; statement, implementations should make available exactly the set of gates defined in the standard library for the language version that matches the version-string statement, if present. If the version statement is not present, the implementation should make available the gates from the language version it is parsing the rest of the program under.

Implementations must not define gates that are not present in the chosen OpenQASM 3 version, where doing so could cause a user program to be ill formed (for example due to a user attempting to define their own gate with a clashing name).

An implementation is not required to supply a literal stdgates.inc file to the user.

An implementation must make all the gates available as if they were defined with gate statements with contents that match the mathematical descriptions given here. In addition, it is permissible for implementations to also provide implementations of the gates equivalent to a series of defcal statements for all qubits used in the program.

API Documentation

The content of the standard library is documented below, including the OpenQASM version that each gate was introduced in.

Where given, subscripts on mathematical gate symbols, qubit states and qubit operators are used to disambiguate which qubit is being referred to. This documentation uses a convention where the \(n\)th qubit argument in the code form is \(n\) places from the right in the description of the qubit state.

Single-qubit gates

gate p(λ) a

The phase gate \(P(\lambda)\). Defined by the mapping:

\[\begin{split}\texttt{p(λ) a;} \mapsto P(\lambda)\colon\ \left\{\begin{alignedat}[c]2 \lvert0\rangle &{}\to{}& &\lvert0\rangle\\ \lvert1\rangle &{}\to{}& e^{i\lambda} &\lvert1\rangle. \end{alignedat}\right.\end{split}\]

Equivalent to ctrl @ gphase(λ) a (see gphase).

See also

rz

The same operation up to a global phase.

phase

An alternative name for backwards compatibility with OpenQASM 2.

Added in version 3.0.

gate x a

The Pauli \(X\) gate. Defined by the mapping:

\[\begin{split}\texttt{x a;} \mapsto X\colon\ \left\{\begin{alignedat}[c]2 \lvert0\rangle &{}\to{}& &\lvert1\rangle\\ \lvert1\rangle &{}\to{}& &\lvert0\rangle. \end{alignedat}\right.\end{split}\]

Added in version 3.0.

gate y a

The Pauli \(Y\) gate. Defined by the mapping:

\[\begin{split}\texttt{y a;} \mapsto Y\colon\ \left\{\begin{alignedat}[c]2 \lvert0\rangle &{}\to{}& i &\lvert1\rangle\\ \lvert1\rangle &{}\to{}& {-}i &\lvert0\rangle. \end{alignedat}\right.\end{split}\]

Added in version 3.0.

gate z a

The Pauli \(Z\) gate. Defined by the mapping:

\[\begin{split}\texttt{z a;} \mapsto Z\colon\ \left\{\begin{alignedat}[c]2 \lvert0\rangle &{}\to{}& &\lvert0\rangle\\ \lvert1\rangle &{}\to{}& {-}&\lvert1\rangle. \end{alignedat}\right.\end{split}\]

Equivalent to p(pi) a in terms of p.

Added in version 3.0.

gate h a

The Hadamard gate \(H\). Defined by the mapping:

\[\begin{split}\texttt{h a;} \mapsto H\colon\ \left\{\begin{aligned}[c] \lvert0\rangle &\to \bigl(\lvert0 + \lvert1\rangle\bigr)/\sqrt2\\ \lvert1\rangle &\to \bigl(\lvert0 - \lvert1\rangle\bigr)/\sqrt2. \end{aligned}\right.\end{split}\]

Added in version 3.0.

gate s a

The \(S = \sqrt Z\) gate (see z). The square root is chosen conventionally, that is the gate is equivalent to \(P(\pi/2)\), in terms of p:

\[\begin{split}\texttt{s a;} \mapsto S\colon\ \left\{\begin{alignedat}[c]2 \lvert0\rangle &{}\to{}& &\lvert0\rangle\\ \lvert1\rangle &{}\to{}& i&\lvert1\rangle. \end{alignedat}\right.\end{split}\]

Added in version 3.0.

gate sdg a

Adjoint of s, \(S^\dagger\). Equivalent to \(P(-\pi/2)\), in terms of p:

\[\begin{split}\texttt{sdg a;} \mapsto S^\dagger\colon\ \left\{\begin{alignedat}[c]2 \lvert0\rangle &{}\to{}& &\lvert0\rangle\\ \lvert1\rangle &{}\to{}& i&\lvert1\rangle. \end{alignedat}\right.\end{split}\]

Added in version 3.0.

gate t

The \(T = \sqrt S\) gate (see s). The square root is chosen conventionally, that is the gate is equivalent to \(P(\pi/4)\), in terms of p:

\[\begin{split}\texttt{t a;} \mapsto T\colon\ \left\{\begin{alignedat}[c]2 \lvert0\rangle &{}\to{}& &\lvert0\rangle\\ \lvert1\rangle &{}\to{}& e^{i\pi/4}&\lvert1\rangle. \end{alignedat}\right.\end{split}\]

Added in version 3.0.

gate tdg a

Adjoint of t, \(T^\dagger\). Equivalent to \(P(-\pi/4)\), in terms of p:

\[\begin{split}\texttt{tdg a;} \mapsto \left\{T^\dagger\colon\ \begin{alignedat}[c]2 \lvert0\rangle &{}\to{}& &\lvert0\rangle\\ \lvert1\rangle &{}\to{}& e^{-i\pi/4}&\lvert1\rangle. \end{alignedat}\right.\end{split}\]

Added in version 3.0.

gate sx a

The \(\mathit{SX} = \sqrt X\) gate (see x). Explicitly, this has the action:

\[\begin{split}\texttt{sx a;} \mapsto \mathit{SX}\colon\ \left\{\begin{aligned}[c] \lvert0\rangle &\to \bigl(e^{i\pi/4}\lvert0\rangle + e^{-i\pi/4}\lvert1\rangle\bigr)/\sqrt2\\ \lvert1\rangle &\to \bigl(e^{-i\pi/4}\lvert0\rangle + e^{i\pi/4}\lvert1\rangle\bigr)/\sqrt2. \end{aligned}\right.\end{split}\]

Added in version 3.0.

gate rx(θ) a

Rotation about the \(X\) axis: \(\mathit{RX}(\theta) = \exp(-i\theta X)\):

\[\begin{split}\texttt{rx(θ) a;} \mapsto \mathit{RX}(\theta)\colon\ \left\{\begin{aligned}[c] \lvert0\rangle &\to \cos\theta\lvert0\rangle - i\sin\theta\lvert1\rangle\\ \lvert1\rangle &\to \cos\theta\lvert1\rangle - i\sin\theta\lvert0\rangle. \end{aligned}\right.\end{split}\]

Added in version 3.0.

gate ry(θ) a

Rotation about the \(Y\) axis: \(\mathit{RY}(\theta) = \exp(-i\theta Y)\):

\[\begin{split}\texttt{ry(θ) a;} \mapsto \mathit{RY}(\theta)\colon\ \left\{\begin{aligned}[c] \lvert0\rangle &\to \cos\theta\lvert0\rangle + \sin\theta\lvert1\rangle\\ \lvert1\rangle &\to \cos\theta\lvert1\rangle - \sin\theta\lvert0\rangle. \end{aligned}\right.\end{split}\]

Added in version 3.0.

gate rz(θ) a

Rotation about the \(Z\) axis: \(\mathit{RZ}(\theta) = \exp(-i\theta Z)\). Note that this differs from p by a global phase of half the rotation angle:

\[\begin{split}\texttt{rz(θ) a;} \mapsto \mathit{RZ}(\theta)\colon\ \left\{\begin{aligned}[c] \lvert0\rangle &\to \cos\theta\lvert0\rangle + \sin\theta\lvert1\rangle\\ \lvert1\rangle &\to \sin\theta\lvert0\rangle - \cos\theta\lvert1\rangle. \end{aligned}\right.\end{split}\]

See also

p

The same gate but with a different global-phase convention.

Added in version 3.0.

Two-qubit gates

All of the controlled gates defined in the standard library follow the same conventions of the ctrl modifier. Explicitly, the first qubit is the control and the second the target. The controlled gates are equivalent to applying the ctrl modifier to the relevant single-qubit gate.

gate cx a, b

Controlled \(X\) gate (see x). Its mapping is defined by \(\mathit{CX}_{ba} = I_b{\lvert0\rangle\!\langle0\rvert}_a + X_b{\lvert1\rangle\!\langle1\rvert}_a\), or explicitly:

\[\begin{split}\texttt{cx a, b;} \mapsto \mathit{CX}_{ba}\colon\ \left\{\begin{aligned}[c] {\lvert00\rangle}_{ba} &\to {\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &\to {\lvert11\rangle}_{ba}\\ {\lvert10\rangle}_{ba} &\to {\lvert10\rangle}_{ba}\\ {\lvert11\rangle}_{ba} &\to {\lvert01\rangle}_{ba}. \end{aligned}\right.\end{split}\]

See also

CX

An all-caps alias for backwards compatibility with OpenQASM 2.0.

Added in version 3.0.

gate cy a, b

Controlled \(Y\) gate (see y). Its mapping is defined by \(\mathit{CY}_{ba} = I_b{\lvert0\rangle\!\langle0\rvert}_a + Y_b{\lvert1\rangle\!\langle1\rvert}_a\), or explicitly:

\[\begin{split}\texttt{cy a, b;} \mapsto \mathit{CY}_{ba}\colon\ \left\{\begin{alignedat}[c]2 {\lvert00\rangle}_{ba} &{}\to{}& &{\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &{}\to{}& i &{\lvert11\rangle}_{ba}\\ {\lvert10\rangle}_{ba} &{}\to{}& &{\lvert10\rangle}_{ba}\\ {\lvert11\rangle}_{ba} &{}\to{}& {-}i &{\lvert01\rangle}_{ba}. \end{alignedat}\right.\end{split}\]

Added in version 3.0.

gate cz a, b

Controlled \(Z\) gate (see z). Its mapping is symmetrical in qubit argument, and defined by \(\mathit{CZ}_{ba} = I_b{\lvert0\rangle\!\langle0\rvert}_a + Z_b{\lvert1\rangle\!\langle1\rvert}_a\), or explicitly:

\[\begin{split}\texttt{cz a, b;} \mapsto \mathit{CZ}_{ba}\colon\ \left\{\begin{alignedat}[c]2 {\lvert00\rangle}_{ba} &{}\to{}& &{\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &{}\to{}& &{\lvert01\rangle}_{ba}\\ {\lvert10\rangle}_{ba} &{}\to{}& &{\lvert10\rangle}_{ba}\\ {\lvert11\rangle}_{ba} &{}\to{}& {-}&{\lvert11\rangle}_{ba}. \end{alignedat}\right.\end{split}\]

Added in version 3.0.

gate cp(λ) a, b

Controlled \(P(\lambda)\) gate (see p). Its mapping is defined by \(\mathit{CP}_{ba}(\lambda) = I_b{\lvert0\rangle\!\langle0\rvert}_a + P_b(\lambda){\lvert1\rangle\!\langle1\rvert}_a\), or explicitly:

\[\begin{split}\texttt{cp(λ) a, b} \mapsto \mathit{CP}_{ba}(\lambda)\colon\ \left\{\begin{alignedat}[c]2 {\lvert00\rangle}_{ba} &{}\to{}& &{\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &{}\to{}& &{\lvert01\rangle}_{ba}\\ {\lvert10\rangle}_{ba} &{}\to{}& &{\lvert10\rangle}_{ba}\\ {\lvert11\rangle}_{ba} &{}\to{}& e^{i\lambda}&{\lvert11\rangle}_{ba}. \end{alignedat}\right.\end{split}\]

The difference in global phase between p and rz makes cp and crz distinct in their action.

Added in version 3.0.

gate crx(θ) a, b

Controlled \(X\) rotation with an angle \(\theta\) (see rx). Its mapping is defined by \(\mathit{CRX}_{ba}(\theta) = I_b{\lvert0\rangle\!\langle0\rvert}_a + \mathit{RX}_b(\theta){\lvert1\rangle\!\langle1\rvert}_a\), or explicitly:

\[\begin{split}\texttt{crx(θ) a, b;} \mapsto \mathit{CRX}_{ba}(\theta)\colon\ \left\{\begin{aligned}[c] {\lvert00\rangle}_{ba} &\to {\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &\to {\bigl(\cos\theta\lvert0\rangle - i\sin\theta\lvert1\rangle\bigr)}_b{\lvert1\rangle}_a\\ {\lvert10\rangle}_{ba} &\to {\lvert10\rangle}_{ba}\\ {\lvert11\rangle}_{ba} &\to {\bigl(\cos\theta\lvert1\rangle - i\sin\theta\lvert0\rangle\bigr)}_b{\lvert1\rangle}_a\\ \end{aligned}\right.\end{split}\]

Added in version 3.0.

gate cry(θ) a, b

Controlled \(Y\) rotation with an angle \(\theta\) (see ry). Its mapping is defined by \(\mathit{CRY}_{ba}(\theta) = I_b{\lvert0\rangle\!\langle0\rvert}_a + \mathit{RY}_b(\theta){\lvert1\rangle\!\langle1\rvert}_a\), or explicitly:

\[\begin{split}\texttt{cry(θ) a, b;} \mathit{CRY}_{ba}(\theta)\colon\ \left\{\begin{aligned}[c] {\lvert00\rangle}_{ba} &\to {\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &\to {\bigl(\cos\theta\lvert0\rangle + \sin\theta\lvert1\rangle\bigr)}_b{\lvert1\rangle}_a\\ {\lvert10\rangle}_{ba} &\to {\lvert10\rangle}_{ba}&\\ {\lvert11\rangle}_{ba} &\to {\bigl(\cos\theta\lvert1\rangle - \sin\theta\lvert0\rangle\bigr)}_b{\lvert1\rangle}_a\\ \end{aligned}\right.\end{split}\]

Added in version 3.0.

gate crz(θ) a, b

Controlled \(Z\) rotation with an angle \(\theta\) (see rz). Its mapping is defined by \(\mathit{CRZ}_{ba}(\theta) = I_b{\lvert0\rangle\!\langle0\rvert}_a + \mathit{RZ}_b(\theta){\lvert1\rangle\!\langle1\rvert}_a\), or explicitly:

\[\begin{split}\texttt{crz(θ) a, b;} \mathit{CRZ}_{ba}(\theta)\colon\ \left\{\begin{alignedat}[c]2 {\lvert00\rangle}_{ba} &{}\to{}& &{\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &{}\to{}& e^{-i\theta/2} &{\lvert01\rangle}_{ba}\\ {\lvert10\rangle}_{ba} &{}\to{}& &{\lvert10\rangle}_{ba} \vphantom{e^{i\theta/2}}\\ {\lvert11\rangle}_{ba} &{}\to{}& e^{i\theta/2} &{\lvert11\rangle}_{ba}\\ \end{alignedat}\right.\end{split}\]

The difference in global phase between p and rz makes cp and crz distinct in their action.

Added in version 3.0.

gate ch a, b

Controlled Hadamard gate (see h). Its mapping is defined by \(\mathit{CH}_{ba} = I_b{\lvert0\rangle\!\langle0\rvert}_a + H_b{\lvert1\rangle\!\langle1\rvert}_a\), or explicitly:

\[\begin{split}\texttt{ch a, b;} \mapsto \mathit{CH}_{ba}\colon\ \left\{\begin{aligned}[c] {\lvert00\rangle}_{ba} &\to {\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &\to {\bigl(\lvert0\rangle + \lvert1\rangle\bigr)}_b{\lvert0\rangle}_a / \sqrt2\\ {\lvert10\rangle}_{ba} &\to {\lvert10\rangle}_{ba}\\ {\lvert11\rangle}_{ba} &\to {\bigl(\lvert0\rangle - \lvert1\rangle\bigr)}_b{\lvert0\rangle}_a / \sqrt2\\ \end{aligned}\right.\end{split}\]

Added in version 3.0.

gate cu(θ, φ, λ, γ) a, b

A four-parameter version the controlled-\(U\) gate. In contrast to other standard-library controll gates, this gate as an additional parameter over its base u gate. The fourth parameter, \(\gamma\), controls the relative phase of the controlled operation.

Explicitly, the action in terms of \(U\) is

\[\texttt{cu(θ, φ, λ, γ) a, b;} \mapsto \mathit{CU}_{ba}(\theta, \phi, \lambda, \gamma) = I_b{\lvert0\rangle\langle0\rvert}_a + e^{i\gamma} U_b(\theta, \phi, \lambda){\lvert1\rangle\langle1\rvert}_a.\]

Added in version 3.0.

gate swap a, b

Swap the states of qubits a and b. Defined by the symmetrical action:

\[\begin{split}\texttt{swap a, b;} \mapsto \mathit{SWAP}_{ba}\colon\ \begin{aligned}[c] {\lvert00\rangle}_{ba} &\to {\lvert00\rangle}_{ba}\\ {\lvert01\rangle}_{ba} &\to {\lvert10\rangle}_{ba}\\ {\lvert10\rangle}_{ba} &\to {\lvert01\rangle}_{ba}\\ {\lvert11\rangle}_{ba} &\to {\lvert11\rangle}_{ba}. \end{aligned}\end{split}\]

Added in version 3.0.

Three-qubit gates

gate ccx a, b, c

The double-controlled \(X\) gate (see x and cx). Also known as the Toffoli gate. The first two qubits are the controls and the last is the target. Its explicit action in terms of \(X\) is:

\[\texttt{ccx a, b, c;} \mapsto \mathit{CCX} = I_c {\bigl(I - \lvert11\rangle\!\langle11\rvert\bigr)}_{ba} + X_c{\lvert11\rangle\!\langle11\rvert}_{ba},\]

or in fully explicit mapping terms:

\[\]

Added in version 3.0.

gate cswap a, b, c

The controlled swap (see swap). The first qubit is the control, and the last two are the swap targets, so its action is:

\[\texttt{cswap a, b, c;} \mapsto \mathit{CSWAP}_{cba} = I_{cb}{\lvert0\rangle\!\langle0\rvert}_a + \mathit{SWAP}_{cb}{\lvert1\rangle\!\langle1\rvert}_a\]

Added in version 3.0.

OpenQASM 2.0 compatibility

Both OpenQASM 2.0 and OpenQASM 3 define the builtin U gate (though note that OpenQASM 3 differs from OpenQASM 2 by a phase; u3 is identical to the U of OpenQASM 2). In addition, OpenQASM 2.0 had a CX builtin, which in OpenQASM 3.0 is provided as an alias convenience only by stdgates.inc, since the ctrl modifier made it unnecessary as a builtin.

gate CX a, b

A convenience alias for cx.

Added in version 2.0.

Changed in version 3.0: In OpenQASM 2.0, CX was a built-in gate, so was automatically defined. From OpenQASM 3.0 onwards, it is part of the standard library.

While OpenQASM 2.0 had no formal standard library, the content of the original IBM Quantum Experience include file qelib1.inc was described in the paper, and this became an informal, de facto standard library of the language.

Most of the standard gates in it are described above. In addition, qelib1.inc included some aliases for other gates, and \(ZYZ\) Euler-rotation gates u1, u2 and u3. These are reproduced in stdgates.inc to ease the transition.

gate phase(λ) a

Alias for p.

Added in version 3.0.

gate cphase(λ) a, b

Alias for cp.

Added in version 3.0.

gate id a

Single-qubit identity gate. This gate is an explicit no-op in idealized mathematical terms, but an implementation is free to assign a duration to it (as with any gate), if desired.

Added in version 3.0.

gate u1(λ) a

Single-argument form of the OpenQASM 2.0 U gate. Equivalent to p(λ).

Added in version 3.0.

gate u2(φ, λ) a

Two-argument form of the OpenQASM 2.0 U gate. Equivalent to u3(π/2, φ, λ) (see u3).

Added in version 3.0.

gate u3(θ, φ, λ) a

Three-argument form of the OpenQASM 2.0 U gate. Note that this differs from the OpenQASM 3 definition of U by an additional factor of \(e^{-i(\theta + \phi + \lambda)/2)}\), i.e. in OpenQASM 3 the mathematical equivalence is:

\[\texttt{u3(θ, φ, λ) a;} \mapsto \mathit{U3}(\theta, \phi, \lambda) = e^{-i(\theta + \phi + \lambda)/2)}U(\theta, \phi, \lambda).\]

Added in version 3.0.