# PrepareArbitraryState operation

Warning

This documentation refers to the Classic QDK, which has been replaced by the Modern QDK.

Please see https://aka.ms/qdk.api for the API documentation for the Modern QDK.

Namespace: Microsoft.Quantum.Preparation

Package: Microsoft.Quantum.Standard

Warning

PrepareArbitraryState has been deprecated. Please use PrepareArbitraryStateCP operation instead.

Given a set of coefficients and a little-endian encoded quantum register, prepares an state on that register described by the given coefficients.

```
operation PrepareArbitraryState (coefficients : Microsoft.Quantum.Math.ComplexPolar[], qubits : Microsoft.Quantum.Arithmetic.LittleEndian) : Unit is Adj + Ctl
```

## Description

This operation prepares an arbitrary quantum state $\ket{\psi}$ with complex coefficients $r_j e^{i t_j}$ from the $n$-qubit computational basis state $\ket{0 \cdots 0}$. In particular, the action of this operation can be simulated by the a unitary transformation $U$ which acts on the all-zeros state as

$$ \begin{align} U\ket{0...0} & = \ket{\psi} \\ & = \frac{ \sum_{j=0}^{2^n-1} r_j e^{i t_j} \ket{j} }{ \sqrt{\sum_{j=0}^{2^n-1} |r_j|^2} }. \end{align} $$

## Input

### coefficients : ComplexPolar[]

Array of up to $2^n$ complex coefficients represented by their absolute value and phase $(r_j, t_j)$. The $j$th coefficient indexes the number state $\ket{j}$ encoded in little-endian format.

### qubits : LittleEndian

Qubit register encoding number states in little-endian format. This is expected to be initialized in the computational basis state $\ket{0...0}$.

## Output : Unit

## Remarks

Negative input coefficients $r_j < 0$ will be treated as though
positive with value $|r_j|$. `coefficients`

will be padded with
elements $(r_j, t_j) = (0.0, 0.0)$ if fewer than $2^n$ are
specified.

## References

- Synthesis of Quantum Logic Circuits Vivek V. Shende, Stephen S. Bullock, Igor L. Markov https://arxiv.org/abs/quant-ph/0406176

## See Also

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