How do I implement the HHL with negative eigenvalues?
This paper (https://arxiv.org/abs/1803.01486) says that:
what if $\lambda<0$? This problem actually does not hard to solve, since we can choose $t$ small enough, such that $|\lambda t| < \pi$. So if $2 \pi y/N > \pi$, then we believe that $\lambda t = 2 \pi y/N − 2\pi=−2\pi (N−y)/N$.
However, doing so introduces a problem at $\lambda = 0$, where the solution diverges.
Note 1: In Cirq, the code only works where eigenvalues of the Hamiltonian is positive. How do I adjust this code so it also works with negative eigenvalues? https://github.com/quantumlib/Cirq/blob/master/examples/hhl.py
Note 2: in Qiskit,the quantum phase estimation EigsQPE has an input
negative_evals. It seems this adds one more ancilla qubit. What do that they do here?