FrozenQubits: Boosting Fidelity of QAOA by Skipping Hotspot Nodes
Quantum Approximate Optimization Algorithm (QAOA) is one of the leading
candidates for demonstrating the quantum advantage using near-term quantum
computers. Unfortunately, high device error rates limit us from reliably
running QAOA circuits for problems with more than a few qubits. In QAOA, the
problem graph is translated into a quantum circuit such that every edge
corresponds to two 2-qubit CNOT operations in each layer of the circuit. As
CNOTs are extremely error-prone, the fidelity of QAOA circuits is dictated by
the number of edges in the problem graph.
  We observe that majority of graphs corresponding to real-world applications
follow the ``power-law`` distribution, where some hotspot nodes have
significantly higher number of connections. We leverage this insight and
propose ``FrozenQubits`` that freezes the hotspot nodes or qubits and
intelligently partitions the state-space of the given problem into several
smaller sub-spaces which are then solved independently. The corresponding QAOA
sub-circuits are significantly less vulnerable to gate and decoherence errors
due to the reduced number of CNOT operations in each sub-circuit. Unlike prior
circuit-cutting approaches, FrozenQubits does not require any exponentially
complex post-processing step. Our evaluations with 5,300 QAOA circuits on eight
different quantum computers from IBM shows that FrozenQubits can improve the
quality of solutions by 8.73x on average (and by up to 57x), albeit utilizing
2x more quantum resources.
Ramin Ayanzadeh Narges Alavisamani Poulami Das Moinuddin Qureshi