Fix edge-case comparisons in VerifyOp (#586)

Co-authored-by: Sai Rohan Reddy Kondlapudi <rohan@mac.lan>
Co-authored-by: Sai Rohan Reddy Kondlapudi <rohan@1x-nat-vl932-172-30-250-57.wireless.umass.edu>
Co-authored-by: Sai Rohan Reddy Kondlapudi <rohan@umass-ssid-vl931-172-30-187-195.wireless.umass.edu>

Author: 4 people

src/misc_ops.jl

@@ -128,21 +128,25 @@ end
 struct VerifyOp <: AbstractOperation
     good_state::Stabilizer
     indices::AbstractVector{Int}
-    VerifyOp(s,indices) = new(canonicalize_rref!(copy(stabilizerview(s)))[1],indices)
+    function VerifyOp(s, indices)
+        gs = canonicalize_rref!(copy(stabilizerview(s)))[1]
+        r,n = size(gs)
+        if(r!=n)
+            throw(ArgumentError(lazy"""The argument you have provided for good_state is not a logical state within the codespace. Expected a pure $n - qubit stabilizer state (i.e. $n independent stabilizer generators on $n qubits), but good_state has only $r independent stabilizer generators."""))
+        end
+        return new(gs, indices)
+    end
 end
 
 # TODO this one needs more testing
 function applywstatus!(s::AbstractQCState, v::VerifyOp) # XXX It assumes the other qubits are measured or traced out
     # TODO QuantumClifford should implement some submatrix comparison
-    r,n = size(v.good_state)
-    if(r!=n)
-        throw(ArgumentError("""The argument you have provided for good_state is not a logical state within the codespace. Expected a pure $n - qubit stabilizer state (i.e. $n independent stabilizer generators on $n qubits), but good_state has only $r independent stabilizer generators."""))
-    end
-    canonicalize_rref!(quantumstate(s),v.indices) # Document why rref is used
-    sv = tab(s)
+    sv = traceout!(copy(quantumstate(s)), setdiff(1:nqubits(s),v.indices))
+    sv = stabilizerview(sv)
+    canonicalize_rref!(sv, v.indices)
+    sv = tab(sv)
     good_state = tab(v.good_state)
 
-    
     for i in eachindex(good_state)
         (sv.phases[end-i+1]==good_state.phases[end-i+1]) || return s, false_success_stat
         for (j,q) in zip(eachindex(good_state),v.indices)

test/test_noisycircuits.jl

@@ -333,15 +333,11 @@
 
     @testset "VerifyOp" begin
         using QuantumClifford.ECC: Steane7, parity_checks, naive_encoding_circuit
+        using QuantumClifford: tensor
         @testset "Stabilizer passed as good_state is not a logical state" begin
             good_state = parity_checks(Steane7()) #passing in a code instead of a state within codespace
-            verify = VerifyOp(good_state, 1:7)
-            reg = Register(one(MixedDestabilizer,7),6) #dummy register to pass into applywstatus!
-        
+            @test_throws ArgumentError VerifyOp(good_state, 1:7) #should throw an error since good_state isn't a logical state i.e. has only 6 stabilizer generators
 
-            @test_throws ArgumentError applywstatus!(reg, verify) #should throw an error since good_state isn't a logical state
-            
-    
         end
 
         @testset "Accepts pure good_state argument" begin
@@ -358,6 +354,19 @@
             _, status = applywstatus!(reg, verify)
             @test status == true_success_stat
         end
+
+        @testset "handles sub-tableau with ancillas correctly " begin
+            s = S"+ XXX__
+            + ZZ___
+            + Z_Z__
+            - ZZ_Z_
+            - _ZZ_Z"
+            good_state = S"XXX ZZI IZZ"
+            verify = VerifyOp(good_state, 1:3)
+            _, status = applywstatus!(s, verify) 
+            @test status == true_success_stat #this test would have given a false_success_stat in the previous implementation of applywstatus! for VerifyOp
+
+        end
 
 
     end