# The Prelude

The Q# compiler and the target machines included with the Quantum Development Kit provide a set of intrinsic functions and operations that can be used when writing quantum programs in Q#.

## Intrinsic Operations and Functions

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

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.

Many of the functions and operations defined in this portion of the prelude are in the @"microsoft.quantum.intrinsic" namespace, such that most Q# source files will have an open Microsoft.Quantum.Intrinsic; directive immediately following the initial namespace declaration. The Microsoft.Quantum.Core namespace namespace is automatically opened, so that functions such as Length function can be used without an open statement at all.

### 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 operation implements the Pauli $X$ operator. This is sometimes also known as the NOT gate. It has signature (Qubit => Unit is Adj + Ctl). 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 operation implements the Pauli $Y$ operator. It has signature (Qubit => Unit is Adj + Ctl). 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 operation implements the Pauli $Z$ operator. It has signature (Qubit => Unit is Adj + Ctl). 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 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 is Adj + Ctl), 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 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 is Adj + Ctl), 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 single-qubit operations, 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 operation, such that $T^2 = S$. The $T$ gate is in turn implemented by the T operation operation, and has signature (Qubit => Unit is Adj + Ctl), 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 <xref:Microsoft.Quantum.Intrinsic.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 is Adj + Ctl), 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 is Adj + Ctl).

Note

The <xref:Microsoft.Quantum.Intrinsic.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 operation implements a rotation around a specified Pauli axis using this convention. It differs from R operation 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 is Adj + Ctl). 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 operation implements a rotation around the Pauli $X$ axis. It has signature ((Double, Qubit) => Unit is Adj + Ctl). Rx(_, _) is the same as R(PauliX, _, _).

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

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

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

The R1Frac operation operation implements a fractional rotation by the given amount around the Z=1 eigenstate. It has signature ((Int,Int, Qubit) => Unit is Adj + Ctl). 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 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 is Adj + Ctl), 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 operation performs doubly-controlled NOT gate, sometimes also known as the Toffoli gate. It has signature ((Qubit, Qubit, Qubit) => Unit is Adj + Ctl). CCNOT(q1, q2, q3) is the same as (Controlled X)([q1, q2], q3).

The SWAP operation 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 is Adj + Ctl). 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 Fredkin gate, is powerful enough to include all classical computation.

Finally, the prelude provides two operations for representing exponentials of multi-qubit Pauli operators. The Exp operation 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 is Adj + Ctl).

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

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 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 operation measures the Pauli $Z$ operator on a single qubit, and has signature (Qubit => Result). M(q) is equivalent to Measure([PauliZ], [q]).

The <xref:microsoft.quantum.measurement.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;


## 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.Math namespace namespace defines useful operations such as Sin function and Log function. The implementation provided by the Quantum Development Kit uses the classical .NET base class library, and thus may involve an additional communications round trip between quantum programs 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.Math namespace 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.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...


### Bitwise Operations

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