The Prelude

Primitive Operations and Functions

The primitive operations defined in the standard library roughly fall into one of several categories:

• Essential classical functions.
• Operations representing unitaries composed of Clifford and $T$ gates.
• Operations representing rotations about various operators.
• Operations implementing measurements.

Since the Clifford + $T$ gate set is universal for quantum computing, these operations suffice to approximately implement any quantum algorithm within negligibly small error. By providing rotations as well, Q# allows the programmer to work within the single qubit unitary and CNOT gate library. This library is much easier to think about because it does not require the programmer to directly express the Clifford + $T$ decomposition and because highly efficient methods exist for compiling single qubit unitaries into Clifford and $T$ gates (see here for more information).

Where possible, the operations defined in the prelude which act on qubits allow for applying the Controlled variant, such that the target machine will perform the appropriate decomposition.

All of the functions and operations defined in this portion of the prelude are in the Microsoft.Quantum.Primitive namespace, such that most Q# source files will have an open Microsoft.Quantum.Primitive; directive immediately following the initial namespace declaration.

Essential Classical Functions

These functions are primarily used to work with the Q# built-in data types Int, Double, and Range.

The Random operation has signature (Double[] => Int). It takes an array of doubles as input, and returns a randomly-selected index into the array as an Int. The probability of selecting a specific index is proportional to the value of the array element at that index. Array elements that are equal to zero are ignored and their indices are never returned. If any array element is less than zero, or if no array element is greater than zero, then the operation fails.

Common Single-Qubit Unitary Operations

The prelude also defines many common single-qubit operations. All of these operations allow both the Controlled and Adjoint functors.

Pauli Operators

The X operation implements the Pauli $X$ operator. This is sometimes also known as the NOT gate. It has signature (Qubit => Unit : Adjoint, Controlled). It corresponds to the single-qubit unitary:

$$\begin{bmatrix} 0 & 1 \\ % FIXME: this currently uses the quadwhack hack. 1 & 0 \end{bmatrix}$$

The Y operation implements the Pauli $Y$ operator. It has signature (Qubit => Unit : Adjoint, Controlled). It corresponds to the single-qubit unitary:

$$\begin{bmatrix} 0 & -i \\ % FIXME: this currently uses the quadwhack hack. i & 0 \end{bmatrix}$$

The Z operation implements the Pauli $Z$ operator. It has signature (Qubit => Unit : Adjoint, Controlled). It corresponds to the single-qubit unitary:

$$\begin{bmatrix} 1 & 0 \\ % FIXME: this currently uses the quadwhack hack. 0 & -1 \end{bmatrix}$$

Below we see these transformations mapped to the Bloch sphere (the rotation axis in each case is highlighted red):

It is important to note that applying the same Pauli gate twice to the same qubit cancels out the operation (because you have now performed a full rotation of 2π (360°) over the surface to the Bloch Sphere, thus arriving back at the starting point). This brings us to the following identity:

$$X^2=Y^2=Z^2=\boldone$$

This can be visualised on the Bloch sphere:

Other Single-Qubit Cliffords

The H operation implements the Hadamard gate. This interchanges the Pauli $X$ and $Z$ axes of the target qubit, such that $H\ket{0} = \ket{+} \mathrel{:=} (\ket{0} + \ket{1}) / \sqrt{2}$ and $H\ket{+} = \ket{0}$. It has signature (Qubit => Unit : Adjoint, Controlled), and corresponds to the single-qubit unitary:

$$\frac{1}{\sqrt{2}} \begin{bmatrix} 1 & 1 \\ % FIXME: this currently uses the quadwhack hack. 1 & -1 \end{bmatrix}$$

The Hadamard gate is particularly important as it can be used to create a superposition of the $\ket{0}$ and $\ket{1}$ states. In the Bloch sphere representation, it is easiest to think of this as a rotation of $\ket{\psi}$ around the x-axis by $\pi$ radians ($180^\circ$) followed by a (clockwise) rotation around the y-axis by $\pi/2$ radians ($90^\circ$):

The S operation implements the phase gate $S$. This is the matrix square root of the Pauli $Z$ operation. That is, $S^2 = Z$. It has signature (Qubit => Unit : Adjoint, Controlled), and corresponds to the single-qubit unitary:

$$\begin{bmatrix} 1 & 0 \\ % FIXME: this currently uses the quadwhack hack. 0 & i \end{bmatrix}$$

Rotations

In addition to the Pauli and Clifford operations above, the Q# prelude provides a variety of ways of expressing rotations. As described in The qubit | Microsoft Docs, the ability to rotate is critical to quantum algorithms.

We start by recalling that we can express any single-qubit operation using the $H$ and $T$ gates, where $H$ is the Hadamard operation, and where $$T \mathrel{:=} \begin{bmatrix} 1 & 0 \\ % FIXME: this currently uses the quad back whack hack. 0 & e^{i \pi / 4} \end{bmatrix}$$ This is the square root of the S operation, such that $T^2 = S$. The $T$ gate is in turn implemented by the T operation, and has signature (Qubit => Unit : Adjoint, Controlled), indicating that it is a unitary operation on a single-qubit.

Even though this is in principle sufficient to describe any arbitrary single-qubit operation, different target machines may have more efficient representations for rotations about Pauli operators, such that the prelude includes a variety of ways to convienently express such rotations. The most basic of these is the R operation, which implements a rotation around a specified Pauli axis, $$R(\sigma, \phi) \mathrel{:=} \exp(-i \phi \sigma / 2),$$ where $\sigma$ is a Pauli operator, $\phi$ is an angle, and where $\exp$ represents the matrix exponential. It has signature ((Pauli, Double, Qubit) => Unit : Adjoint, Controlled), where the first two parts of the input represent the classical arguments $\sigma$ and $\phi$ needed to specify the unitary operator $R(\sigma, \phi)$. We can partially apply $\sigma$ and $\phi$ to obtain an operation whose type is that of a single-qubit unitary. For example, R(PauliZ, PI() / 4, _) has type (Qubit => Unit : Adjoint, Controlled).

Note

The R operation divides the input angle by 2 and multiplies it by -1. For $Z$ rotations, this means that the $\ket{0}$ eigenstate is rotated by $-\phi / 2$ and the $\ket{1}$ eigenstate is rotated by $\phi / 2$, so that the $\ket{1}$ eigenstate is rotated by $\phi$ relative to the $\ket{0}$ eigenstate.

In particular, this means that T and R(PauliZ, PI() / 8, _) differ only by an irrelevant global phase. For this reason, $T$ is sometimes known as the $\frac{\pi}{8}$-gate.

Note also that rotating around PauliI simply applies a global phase of $\phi / 2$. While such phases are irrelevant, as argued in the conceptual documents, they are relevant for controlled PauliI rotations.

Within quantum algorithms, it is often useful to express rotations as dyadic fractions, such that $\phi = \pi k / 2^n$ for some $k \in \mathbb{Z}$ and $n \in \mathbb{N}$. The RFrac operation implements a rotation around a specified Pauli axis using this convention. It differs from R in that the rotation angle is specified as two inputs of type Int, interpreted as a dyadic fraction. Thus, RFrac has signature ((Pauli, Int, Int, Qubit) => Unit : Adjoint, Controlled). It implements the single-qubit unitary $\exp(i \pi k \sigma / 2^n)$, where $\sigma$ is the Pauli matrix corresponding to the first argument, $k$ is the second argument, and $n$ is the third argument. RFrac(_,k,n,_) is the same as R(_,-πk/2^n,_); note that the angle is the negative of the fraction.

The Rx operation implements a rotation around the Pauli $X$ axis. It has signature ((Double, Qubit) => Unit : Adjoint, Controlled). Rx(_, _) is the same as R(PauliX, _, _).

The Ry operation implements a rotation around the Pauli $Y$ axis. It has signature ((Double, Qubit) => Unit : Adjoint, Controlled). Ry(_, _) is the same as R(PauliY,_ , _).

The Rz operation implements a rotation around the Pauli $Z$ axis. It has signature ((Double, Qubit) => Unit : Adjoint, Controlled). Rz(_, _) is the same as R(PauliZ, _, _).

The R1 operation implements a rotation by the given amount around $\ket{1}$, the $-1$ eigenstate of $Z$. It has signature ((Double, Qubit) => Unit : Adjoint, Controlled). R1(phi,_) is the same as R(PauliZ,phi,_) followed by R(PauliI,-phi,_).

The R1Frac operation implements a fractional rotation by the given amount around the Z=1 eigenstate. It has signature ((Int,Int, Qubit) => Unit : Adjoint, Controlled). R1Frac(k,n,_) is the same as RFrac(PauliZ,-k.n+1,_) followed by RFrac(PauliI,k,n+1,_).

An example of a rotation operation (around the Pauli $Z$ axis, in this instance) mapped onto the Bloch sphere is shown below:

Multi-Qubit Operations

In addition to the single-qubit operations above, the prelude also defines several multi-qubit operations.

First, the CNOT operation performs a standard controlled-NOT gate, $$\operatorname{CNOT} \mathrel{:=} \begin{bmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 1 \\ 0 & 0 & 1 & 0 \end{bmatrix}.$$ It has signature ((Qubit, Qubit) => Unit : Adjoint, Controlled), representing that $\operatorname{CNOT}$ acts unitarily on two individual qubits. CNOT(q1, q2) is the same as (Controlled X)([q1], q2). Since the Controlled functor allows for controlling on a register, we use the array literal [q1] to indicate that we want only the one control.

The CCNOT operation performs doubly-controlled NOT gate, sometimes also known as the Toffoli gate. It has signature ((Qubit, Qubit, Qubit) => Unit : Adjoint, Controlled). CCNOT(q1, q2, q3) is the same as (Controlled X)([q1, q2], q3).

The SWAP operation swaps the quantum states of two qubits. That is, it implements the unitary matrix $$\operatorname{SWAP} \mathrel{:=} \begin{bmatrix} 1 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 1 \end{bmatrix}.$$ It has signature ((Qubit, Qubit) => Unit : Adjoint, Controlled). SWAP(q1,q2) is equivalent to CNOT(q1, q2) followed by CNOT(q2, q1) and then CNOT(q1, q2).

Note

The SWAP gate is not the same as rearranging the elements of a variable with type Qubit[]. Applying SWAP(q1, q2) causes a change to the state of the qubits referred to by q1 and q2, while let swappedRegister = [q2, q1]; only affects how we refer to those qubits. Moreover, (Controlled SWAP)([q0], q1, q2) allows for SWAP to be conditioned on the state of a third qubit, which we cannot represent by rearranging elements. The controlled-SWAP gate, also known as the Fredikin gate, is powerful enough to include all classical computation.

The MultiX operation performs a tensor product of Pauli $X$ gates to an array of qubits. It has signature (Qubit[] => Unit : Adjoint, Controlled). MultiX(qs) is equivalent to:

for (index in 0..Length(qs)-1)
{
X(qs[index]);
}


Tip

Later, in the canon, we will see how to express MultiX as ApplyToEach(X, _), allowing for generalizing to other operations.

Finally, the prelude provides two operations for representing exponentials of multi-qubit Pauli operators. The Exp operation performs a rotation based on a tensor product of Pauli matrices, as represented by the multi-qubit unitary $$\operatorname{Exp}(\vec{\sigma}, \phi) \mathrel{:=} \exp\left(i \phi \sigma_0 \otimes \sigma_1 \otimes \cdots \otimes \sigma_n \right),$$ where $\vec{\sigma} = (\sigma_0, \sigma_1, \dots, \sigma_n)$ is a sequence of single-qubit Pauli operators, and where $\phi$ is an angle. The Exp rotation represents $\vec{\sigma}$ as an array of Pauli elements, such that it has signature ((Pauli[], Double, Qubit[]) => Unit : Adjoint, Controlled).

The ExpFrac operation performs the same rotation, using the dyadic fraction notation discussed above. It has signature ((Pauli[], Int, Int, Qubit[]) => Unit : Adjoint, Controlled).

Warning

Exponentials of the tensor product of Pauli operators are not the same as tensor products of the exponentials of Pauli operators. That is, $e^{i (Z \otimes Z) \phi} \ne e^{i Z \phi} \otimes e^{i Z \phi}$.

Measurements

When measuring, the +1 eigenvalue of the operator being measured corresponds to a Zero result, and the -1 eigenvalue to a One result.

Note

While this convention might seem odd, it has two very nice advantages. First, observing the outcome $\ket{0}$ is represented by the Result value Zero, while observing $\ket{1}$ corresponds to One. Second, we can write out that the eigenvalue $\lambda$ corresponding to a result $r$ is $\lambda = (-1)^r$.

Measurement operations support neither the Adjoint nor the Controlled functor.

The Measure operation performs a joint measurement of one or more qubits in the specified product of Pauli operators. If the Pauli array and qubit array are different lengths, then the operation fails. Measure has signature ((Pauli[], Qubit[]) => Result).

Note that a joint measurement is not the same as measuring each qubit individually. For example, consider the state $\ket{11} = \ket{1} \otimes \ket{1} = X\otimes X \ket{00}$. Measuring $Z_0$ and $Z_1$ each individually, we get the results $r_0 = 1$ and $r_1 = 1$. Measuring $Z_0 Z_1$, however, we get the single result $r_{\textrm{joint}} = 0$, representing that the pairity of $\ket{11}$ is positive. Put differently, $(-1)^{r_0 + r_1} = (-1)^r_{\textrm{joint}})$. Critically, since we only learn the parity from this measurement, any quantum information represented in the superposition between the two two-qubit states of positive parity, $\ket{00}$ and $\ket{11}$, is preserved. This property will be essential later, as we discuss error correction.

For convenience, the prelude also provides two other operations for measuring qubits. First, since performing single-qubit measurements is quite common, the prelude defines a shorthand for this case. The M operation measures the Pauli $Z$ operator on a single qubit, and has signature (Qubit => Result). M(q) is equivalent to Measure([PauliZ], [q]).

The MultiM measures the Pauli $Z$ operator separately on each of an array of qubits, returning the array of Result values obtained for each qubit. In some cases this can be optimized. It has signature (Qubit[] => Result[]). MultiM(qs) is equivalent to:

mutable rs = new Result[Length(qs)];
for (index in 0..Length(qs)-1)
{
set rs[index] = M(qs[index]);
}
return rs;


Debugging

The final functions defined by the prelude provide useful tools for debugging and testing quantum programs. These will later be the basis for higher-level correctness testing in the canon.

First, the Assert operation asserts that measuring the given qubits in the given Pauli basis will always have the given result. If the assertion fails, the execution ends by calling fail with the given message. By default, this operation is not implemented; simulators that can support it should provide an implementation that performs runtime checking. Assert has signature ((Pauli[], Qubit[], Result, String) -> ()). Since Assert is a function with an empty tuple as its output type, no effects from having called Assert are observable within a Q# program.

The AssertProb operation function asserts that measuring the given qubits in the given Pauli basis will have the given result with the given probability, within some tolerance. Tolerance is additive (e.g. abs(expected-actual) < tol). If the assertion fails, the execution ends by calling fail with the given message. By default, this operation is not implemented; simulators that can support it should provide an implementation that performs runtime checking. AssertProb has signature ((Pauli[], Qubit[], Result, Double, String, Double) -> Unit). The first of Double parameters gives the desired probability of the result, and the second one the tolerance.

Finally, the Message function logs a message in a machine-dependent way. By default, this writes the string to the console. Message has signature ((String) -> Unit), again representing that emitting a debug log message cannot be observed from within Q#.

Extension Functions and Operations

In addition, the prelude defines a rich set of mathematical and type conversion functions at the .NET level for use within Q# code. For instance, the Microsoft.Quantum.Extensions.Math namespace defines useful operations such as Sin and Log. The implementation provided by the Quantum Development Kit uses the classical .NET base class library, and thus may involve an additional communicaions round trip between quantum programms and their classical drivers. While this does not present a problem for a local simulator, this can be a performance issue when using a remote simulator or actual hardware as a target machine. That said, an individual target machine may mitigate this performance impact by overriding these operations with versions that are more efficient for that particular system.

Math

The Microsoft.Quantum.Extensions.Math namespace provides many useful functions from the .NET base class library's System.Math class. These functions can be used in the same manner as any other Q# functions:

open Microsoft.Quantum.Extensions.Math;
// ...
let y = Sin(theta);


Where a .NET static method has been overloaded based on the type of its arguments, the corresponding Q# function is annotated with a suffix indicating the type of its input:

let x = AbsI(-3); // x : Int = 3
let y = AbsD(-PI()); // y : Double = 3.1415...


Type Conversions

The Microsoft.Quantum.Extensions.Convert namespace provides functions from the .NET base class library's System.Convert class that are relevant to Q# types. For example, the functions ToStringD and ToStringI convert inputs of type Double and Int, respectively, to String.

Bitwise Operations

Finally, the Microsoft.Quantum.Extensions.Bitwise namespace provides several useful functions for manipulating integers through bitwise operators. For instance, Parity returns the bitwise parity of an integer as another integer.