An efficient quantum algorithm for preparation of uniform quantum superposition states

Abstract

Quantum state preparation involving a uniform superposition over a non-empty subset of n-qubit computational basis states is an important and challenging step in many quantum computation algorithms and applications. In this work, we address the problem of preparation of a uniform superposition state of the form $\left| {\Psi }\right\rangle = \frac{1}{\sqrt{M}}\sum _{j = 0}^{M - 1} \left| {j}\right\rangle$, where M denotes the number of distinct states in the superposition state and $2 \le M \le 2^n$. We show that the superposition state $\left| {\Psi }\right\rangle$ can be efficiently prepared, using a deterministic approach, with a gate complexity and circuit depth of only $O(\log _2~M)$ for all M. This demonstrates an exponential reduction in gate complexity in comparison with other existing deterministic approaches in the literature for the general case of this problem. Another advantage of the proposed approach is that it requires only $n=\lceil \log _2~M\rceil$ qubits. Furthermore, neither ancilla qubits nor any quantum gates with multiple controls are needed in our approach for creating the uniform superposition state $\left| {\Psi }\right\rangle$. It is also shown that a broad class of nonuniform superposition states that involve a mixture of uniform superposition states can also be efficiently created with the same circuit configuration that is used for creating the uniform superposition state $\left| {\Psi }\right\rangle$ described earlier, but with modified parameters.

Appendix

A sample source code for implementation of our proposed Algorithm 1 for creation of the uniform superposition state $\frac{1}{\sqrt{M}} \, \sum _{j=0}^{M-1} \, \left| {j}\right\rangle$ on Qiskit platform is presented below.