Partial Trace is not implemented in density package

[itsubaki]

q/pkg/quantum/density/matrix.go

Lines 67 to 70 in de7ab9d

	func (m *Matrix) PartialTrace() complex128 {
	// TODO
	return complex(0, 0)
	}

[itsubaki]

q/pkg/quantum/density/matrix.go

Lines 71 to 112 in 42e44a4

	func (m *Matrix) PartialTrace(index int) *Matrix {
	n := m.NumberOfBit()
	f := fmt.Sprintf("%s%s%s", "%0", strconv.Itoa(n), "s")
	out := matrix.Zero(number.Pow(2, n-1))
	d := m.Dimension()
	for i := 0; i < d; i++ {
	ibin := fmt.Sprintf(f, strconv.FormatInt(int64(i), 2))
	k, kk := fmt.Sprintf("%s%s", ibin[:index], ibin[index+1:]), string(ibin[index])
	for j := 0; j < d; j++ {
	jbin := fmt.Sprintf(f, strconv.FormatInt(int64(j), 2))
	l, ll := fmt.Sprintf("%s%s", jbin[:index], jbin[index+1:]), string(jbin[index])
	if kk != ll {
	continue
	}
	r := number.Must(strconv.ParseInt(k, 2, 0))
	c := number.Must(strconv.ParseInt(l, 2, 0))
	out[r][c] = out[r][c] + m.m[i][j]
	// fmt.Printf("[%v][%v] = [%v][%v] + [%v][%v]\n", r, c, r, c, i, j)
	//
	// 4x4 explicit
	// index -> 0
	// out[0][0] = m.m[0][0] + m.m[2][2]
	// out[0][1] = m.m[0][1] + m.m[2][3]
	// out[1][0] = m.m[1][0] + m.m[3][2]
	// out[1][1] = m.m[1][1] + m.m[3][3]
	//
	// index -> 1
	// out[0][0] = m.m[0][0] + m.m[1][1]
	// out[0][1] = m.m[0][2] + m.m[1][3]
	// out[1][0] = m.m[2][0] + m.m[3][1]
	// out[1][1] = m.m[2][2] + m.m[3][3]
	}
	}
	return &Matrix{m: out}
	}