imag time qaoa and fix H2 training. (#13)

Author: GiggleLiu

examples/QAOA/QAOA.jl

@@ -1,3 +1,4 @@
+# required packages: NLopt, SCS and Convex
 using Yao, BitBasis
 using NLopt
 

examples/QAOA/imag_time_evolve.jl

@@ -0,0 +1,51 @@
+"""imaginary time evolution solution to maximum cut problem."""
+using Yao
+
+@const_gate ZZ = mat(kron(Z,Z))
+
+function maxcut_circuit(W::AbstractMatrix, τ)
+    nbit = size(W, 1)
+    ab = chain(nbit)
+    for i=1:nbit,j=i+1:nbit
+        if W[i,j] != 0
+            #push!(ab, put(nbit, (i,j)=>0.5*W[i,j]*ZZ))
+            push!(ab, put(nbit, (i,j)=>rot(ZZ, -im*W[i,j]*τ)))
+        end
+    end
+	return ab
+end
+
+function maxcut_h(W::AbstractMatrix)
+    nbit = size(W, 1)
+    ab = Add{nbit}()
+    for i=1:nbit,j=i+1:nbit
+        if W[i,j] != 0
+            #push!(ab, put(nbit, (i,j)=>0.5*W[i,j]*ZZ))
+            push!(ab, put(nbit, (i,j)=>0.5*W[i,j]*ZZ))
+        end
+    end
+	return ab
+end
+
+using Random, Test
+@testset "iqaoa circuit" begin
+	Random.seed!(2)
+	nbit = 5
+	# exact solution is [(1,3,4), (2,5)]
+	W = [0 5 2 1 0;
+		 5 0 3 2 0;
+		 2 3 0 0 0;
+		 1 2 0 0 4;
+		 0 0 0 4 0]
+
+	c = maxcut_circuit(W, 10)
+	h = maxcut_h(W)
+	reg = uniform_state(nbit) |> c
+
+	# check the correctness of result
+	opt_pl = reg |> probs
+	config = argmax(opt_pl)-1
+	@show BitStr64{nbit}(config), opt_pl[config+1]
+	@test config == bit"01101" || config == bit"10010"
+	@test isapprox(expect(h, reg |> normalize!), -5.5)
+end

examples/VQE/H2.jl

@@ -1,5 +1,4 @@
 using Yao
-
 using YaoExtensions
 
 # arXiv: 1704.05018, table S2
@@ -20,13 +19,13 @@ end
 
 using Flux: Optimise
 function train!(circ, hamiltonian; optimizer, niter::Int=100)
-     circ = circ |> autodiff(:BP)
      params = parameters(circ)
      dispatch!(circ, :random)
      for i=1:niter
-         Optimise.update!(optimizer, params, get_gradient(circ, hamiltonian))
+         _, grad = expect'(hamiltonian, zero_state(nqubits(circ)) => circ)
+         Optimise.update!(optimizer, params, grad)
          dispatch!(circ, params)
-         println("Energy = $(expect(hamiltonian, zero_state(nqubits(h)) |> circ))")
+         println("Energy = $(expect(hamiltonian, zero_state(nqubits(h)) |> circ) |> real)")
      end
      return expect(hamiltonian, zero_state(nqubits(h)) |> circ)
 end