# AssertQubitIsInStateWithinTolerance operation

Namespace: Microsoft.Quantum.Diagnostics

Asserts that a qubit in the expected state.

expected represents a complex vector, $\ket{\psi} = \begin{bmatrix}a & b\end{bmatrix}^{\mathrm{T}}$. The first element of the tuples representing each of $a$, $b$ is the real part of the complex number, while the second one is the imaginary part. The last argument defines the tolerance with which assertion is made.

operation AssertQubitIsInStateWithinTolerance (expected : (Microsoft.Quantum.Math.Complex, Microsoft.Quantum.Math.Complex), register : Qubit, tolerance : Double) : Unit is Adj + Ctl


## Input

### expected : (Complex,Complex)

Expected complex amplitudes for $\ket{0}$ and $\ket{1}$, respectively.

### register : Qubit

Qubit whose state is to be asserted. Note that this qubit is assumed to be separable from other allocated qubits, and not entangled.

### tolerance : Double

Additive tolerance by which actual amplitudes are allowed to deviate from expected. See remarks below for details.

## Example

using (qubits = Qubit[2]) {
// Both qubits are initialized as |0〉: a=(1 + 0*i), b=(0 + 0*i)
AssertQubitIsInStateWithinTolerance((Complex(1., 0.), Complex(0., 0.)), qubits[0], 1e-5);
AssertQubitIsInStateWithinTolerance((Complex(1., 0.), Complex(0., 0.)), qubits[1], 1e-5);
Y(qubits[1]);
// Y |0〉 = i |1〉: a=(0 + 0*i), b=(0 + 1*i)
AssertQubitIsInStateWithinTolerance((Complex(0., 0.), Complex(0., 1.)), qubits[1], 1e-5);
}


## Remarks

The following Mathematica code can be used to verify expressions for mi, mx, my, mz:

{Id, X, Y, Z} = Table[PauliMatrix[k], {k, 0, 3}];
st = {{ reA + I imA }, { reB + I imB} };
M = st . ConjugateTranspose[st];
mx = Tr[M.X] // ComplexExpand;
my = Tr[M.Y] // ComplexExpand;
mz = Tr[M.Z] // ComplexExpand;
mi = Tr[M.Id] // ComplexExpand;
2 m == Id mi + X mx + Z mz + Y my // ComplexExpand // Simplify


The tolerance is the $L_{\infty}$ distance between 3 dimensional real vector (x₂,x₃,x₄) defined by $\langle\psi|\psi\rangle = x_1 I + x_2 X + x_3 Y + x_4 Z$ and real vector (y₂,y₃,y₄) defined by ρ = y₁I + y₂X + y₃Y + y₄Z where ρ is the density matrix corresponding to the state of the register. This is only true under the assumption that Tr(ρ) and Tr(|ψ⟩⟨ψ|) are both 1 (e.g. x₁ = 1/2, y₁ = 1/2). If this is not the case, the function asserts that l∞ distance between (x₂-x₁,x₃-x₁,x₄-x₁,x₄+x₁) and (y₂-y₁,y₃-y₁,y₄-y₁,y₄+y₁) is less than the tolerance parameter.

Note that the Adjoint and Controlled versions of this operation will not check the condition.