Is there a state vector simulator written in pure python?

Question

For educational purposes, I am looking for a simulator written in pure python.

It may use scientific-python libraries such as numpy to exploit the data structures they provide, but the core algorithms should be written on top of these libraries.

Answer 1

There is code to apply single-qubit gates and controlled gates here:

Depending on endianess, you may have to adjust index. The type of self in this code is just a wrapper around numpy's ndarray class with added properies to compute the number of qubits (nbits)

  def apply1(self, gate, index) -> None:
    """Apply single-qubit gate to this state."""

    # To maintain qubit ordering in this infrastructure,
    # index needs to be reversed.
    #
    index = self.nbits - index - 1
    two_q = 1 << index
    g00 = gate[0, 0]
    g01 = gate[0, 1]
    g10 = gate[1, 0]
    g11 = gate[1, 1]
    for g in range(0, 1 << self.nbits, 1 << (index+1)):
      for i in range(g, g + two_q):
        t1 = g00 * self[i] + g01 * self[i + two_q]
        t2 = g10 * self[i] + g11 * self[i + two_q]
        self[i] = t1
        self[i + two_q] = t2

  def applyc(self, gate, control, target) -> None:
    """Apply a controlled 2-qubit gate via explicit indexing."""

    # To maintain qubit ordering in this infrastructure,
    # index needs to be reversed.
    qbit = self.nbits - target - 1
    two_q = 2**qbit
    control = self.nbits - control - 1
    g00 = gate[0, 0]
    g01 = gate[0, 1]
    g10 = gate[1, 0]
    g11 = gate[1, 1]
    for g in range(0, 1 << self.nbits, 1 << (qbit+1)):
      idx_base = g * (1 << self.nbits)
      for i in range(g, g + two_q):
        idx = idx_base + i
        if idx & (1 << control):
          t1 = g00 * self[i] + g01 * self[i + two_q]
          t2 = g10 * self[i] + g11 * self[i + two_q]
          self[i] = t1
          self[i + two_q] = t2
```