We are interested in finding the ground state energy of the following Hamiltonian (Corboz et al., 2009):

where <rs> goes through nearest neighbour pairs in a two-dimensional lattice. The fermionic operators are subject to the following constraints:

We fix γ=1 and λ=2. Treating the problem as an instance of noncommutative polynomial optimization, we convert it to a semidefinite programme (SDP) relaxation (Pironio et al., 2010).

# Generating the relaxation

Ncpol2sdpa is a tool to generate the SDP relaxation given a noncommutative polynomial optimization. It works both in Matlab and Python. The Matlab variant has serious limitations, as it uses Yalmip as its back-end, which only has rudimentary support for noncommutative variables. Hermitian variables are not supported, which makes generating relaxations error-prone. On the other hand, this variant efficiently deals with equality constraints by solving the set of equalities with QR decomposition. It is also capable of using a wide range of solvers. The Matlab version does not scale beyond a lattice of 2×2, and numerical errors appear, leaving imaginary values in the objective function.

The Python variant uses SymPy, which excels at noncommutative and Hermitian variables, allowing operations such Hermitian conjugates, commutators, and anticommutators. To solve the SDP relaxation or export it to sparse SDPA format, Ncpol2sdpa uses PICOS. Translation between noncommutative SymPy monomials and PICOS variables is mechanical. Equations are converted to a pair of inequalities in the background, making the SDP relaxations large.

To get started with the Python version, we need to get our variables first:

```
from sympy.physics.quantum.dagger import Dagger
from ncpol2sdpa import SdpRelaxation, generate_variables, \
fermionic_constraints, get_neighbors, solve_sdp
gam, lam = 0, 1
lattice_length = 3
lattice_width = 3
n_vars = lattice_length * lattice_width
C = generate_variables(n_vars, name='C')
```

Defining the objective function is straightforward:

```
hamiltonian = 0
for r in range(n_vars):
hamiltonian -= 2*lam*Dagger(C[r])*C[r]
for s in get_neighbors(r, lattice_length, W=lattice_width):
hamiltonian += Dagger(C[r])*C[s]+Dagger(C[s])*C[r]
hamiltonian -= gam*(Dagger(C[r])*Dagger(C[s])+C[s]*C[r])
```

The code gets inefficient at defining the equalities. Most of the equalities in the problem have a monomial on one side, and another monomial on the other side. These simple substitutions could be removed from the monomial basis. Monomial substitutions work for simple cases, such as idempotent variables, but unfortunately the equalities in this problem fail as substitutions.

```
monomial_substitutions = {}
equalities = []
inequalities = []
for i in range(n_vars):
for j in range(i + 1, n_vars):
equalities.append(Dagger(C[j]) * C[i] + C[i] * Dagger(C[j]))
equalities.append(C[j] * Dagger(C[i]) + Dagger(C[i]) * C[j])
equalities.append(C[j] * C[i] + C[i] * C[j])
equalities.append(Dagger(C[j]) * Dagger(C[i]) +
Dagger(C[i]) * Dagger(C[j]))
for Cr in C:
equalities.append(Cr ** 2)
equalities.append(Dagger(Cr) ** 2)
equalities.append(Cr * Dagger(Cr) + Dagger(Cr) * Cr - 1.0)
inequalities.append(Dagger(Cr)*Cr)
inequalities.append(1-Dagger(Cr)*Cr)
```

The number of equalities grows quickly with the lattice size. Since each of them means two inequalities, problems are expected. The code for this Hamiltonian is included in Ncpol2sdpa.

# Results

We used the cr1.8xlarge instance type of Amazon Web Services, which has 244 GByte of main memory. This instance type has the largest amount of memory among cloud instances. The computational time and memory use grow exponentially.

For lattice dimension 5×5, the memory use was so excessive the operating system killed the process.

# Limitations

Generating an SDP relaxation and solving it are two different matters. PICOS interfaces with SDP solvers implemented in Python. These are not scalable to a cluster. PICOS can also export to sparse SDPA format, which is far more desirable, given SDPARA can solve a problem on a cluster. Yet, writing the problem to the disk is not as straightforward as it seems. Memory use increases further, and the process will be killed even in the case of a 4×4 lattice. It is interesting to note that the 3×3 lattice takes about a hundred seconds to generate and write, and solving it on a single node using SDPARA with twelve cores takes about the same time. Perhaps distributing the generation of the problem would help overcome the memory and time constraints.

# References

Corboz, P.; Evenbly, G.; Verstraete, F. & Vidal, G. Simulation of interacting fermions with entanglement renormalization. *Physics Review A*, 2010, 81, pp. 010303.

Pironio, S.; Navascués, M. & Acín, A. Convergent relaxations of polynomial optimization problems with noncommuting variables. *SIAM Journal on Optimization*, 2010, 20, pp. 2157-2180.