Lemma 11. Let C = U
m
...U
1
be an n-qubit quantum circuit where m =poly(n) such that each qubit
is acted upon by at most three gates. Then there exists a (5, 4)-local Hamiltonian H =
P
m
0
i=1
H
i
acting on n
0
= poly(n, m) qubits such that the following holds:
1. If there exists a state |ψi such that C accepts |ψi with probability 1−, then hφ|H|φi ≤
poly(m)
,
where
|φi =
1
√
m + 1
m
X
i=0
U
i
...U
1
|ψi ⊗ |0i ⊗|ii, (5)
with i written in unary notation.
2. If C accepts any state |ψi with probability at most , then for all |ψi, hψ|H|ψi ≥
1−
poly(m)
.
By starting with an error-amplified QCMA verifier for which the parameter is exponentially
small, we can make the completeness and soundness parameters 2
−poly(n)
and
1
poly(n)
respectively
in Lemma 11. The second step of our construction consists in showing how the resulting instance of
the (5, 4)-local Hamiltonian can be converted into a 2-layered local Hamiltonian such that all terms
inside each of the layers commute, and are 15-local. The completeness and soundness parameters
will remain of order 2
−poly(n)
and
1
poly(n)
respectively.
We explain the intuition behind the construction. The first step is to individually modify each of
the local terms from the input Hamiltonian instance into local terms such that the new Hamiltonians
have orthogonal range. This results in a local Hamiltonian G that is trivially commuting. However,
it is now easy for a prover to specify a state in the simultaneous ground space of all the local terms,
irrespective of whether the initial instance was satisfiable or not.
To prevent this we create another Hamiltonian R that forces any ground state to have high over-
lap with the ground space of each Hamiltonian in G. Specifically, consider a qubit j ∈ {1, . . . , n},
and let H
i
1
(j)
, H
i
2
(j)
, H
i
3
(j)
, H
i
4
(j)
be the local terms acting on it. We first allocate an ancilla qudit
of dimension 4 (equivalently, two qubits) j
0
∈ {1, . . . , n} for j. We then create four new Hamilto-
nians H
0
i
t
(j)
= H
i
t
(j)
⊗ |t − 1iht − 1| for t ∈ {1, . . . , 4}, where the second projector acts on j
0
, and
G = H
0
i
1
(j)
+ H
0
i
2
(j)
+ H
0
i
3
(j)
+ H
0
i
4
(j)
. Note that a malicious prover can now easily create a state
in the simultaneous null eigenspace of H
0
i
2
(j)
, H
0
i
3
(j)
, H
0
i
4
(j)
by creating a state that is |00i on j
0
.
To prevent this, we introduce a penalty Hamiltonian R = I ⊗(I − |γihγ|), where |γi =
1
2
P
3
i=0
|ii.
Provided a large enough energy penalty is associated with R, any state in a low eigenspace of the
H
0
i
t
(j)
is forced to be close to the null eigenspace of R on j
0
, namely, by being in the state |γi,
effectively translating the commuting terms in G into a sum of the non-commuting original terms.
In this case, there is a constant overlap with the projectors of each of H
0
i
t
(j)
, t ∈ {1, . . . , 4} on the
Hilbert space of the qudit j
0
, and thus none of the Hamiltonians can be trivially annihilated.
Lemma 12. (Ground Space Preserving Layers) Let H =
P
m
i=1
H
i
be a (5, 4)-local Hamiltonian
acting on a system of n qubits numbered from 1 to n, where kH
i
k ≤ 1. For each qubit j ∈ {1, . . . , n},
introduce a 4-dimensional ancilla qudit j
0
. Let c and s be two reals such that s −c ≥ m
−b
(for some
b > 0).
For i ∈ {1, . . . , m}, let H
i
act on qubits {j
1
(i), . . . , j
5
(i)}. For t ∈ {1, . . . , 5}, let p
t
(i) ∈
{1, . . . , 4} be such that H
i
is the p
t
(i)-th Hamiltonian acting on j
t
(i). Define G
i
acting on qubits
{j
1
(i), ..., j
5
(i)} and {j
0
1
(1), . . . , j
0
5
(i)} as
G
i
= H
i
⊗ |p
1
(i) − 1ihp
1
(i) − 1| ⊗ ... ⊗ |p
5
(i) − 1ihp
5
(i) − 1|.
12