Quantum computation has shown advantages in several problems over the corresponding classical algorithms. The noisy intermediate-size
quantum devices with dozens of qubits make it more promising in the
past decade, as the quantum advantage was demonstrated
experimentally. Before entering the era of scalable quantum
computation, one has to resolve the errors in a quantum many-body
system, which is inevitable due to the environment interaction during
quantum control processes. The goal of quantum error correction is to
reduce such errors and increase decoherence time.
The most successful candidates of quantum error correction codes are topological codes, especially surface codes, which were discovered by
Alexei Kitaev. The ordered phase of the 2D
Ising model on a torus ensures that toric codes are fault-tolerant
below a critical error probability. Other than the FT threshold,
the topological codes feature efficient encoding and
decoding, local measurements, but suffer from asymptotically zero
code rate.
To encode more logical qubits with finite resources, one can loosen the condition on locality and extend to a broader class of quantum
low-density parity-check (LDPC)codes. There are many known algebraic
constructions for such codes, but only a precious few of them have
finite code rates and meet the fault-tolerant condition:
the stabilizer weight is bounded and the distance scales at least
logarithmically with the code size. In this thesis, I construct the
higher-dimensional quantum hypergraph product (HQHP) codes, which
generalize quantum hypergraph product (QHP) codes and toric codes in
all dimensions. The HQHP codes projected into a single space gives
subsystem product codes, which can then be gauge fixed to
concatenated codes or homological product codes. Those include some
common CSS codes, like Shor’s codes, Bacon Shor’s codes, and
subsystem QHP codes. The HQHP codes can be mapped to the tensor
product of chain complexes, which provides an algebraic framework to
construct quantum LDPC codes with finite code rates, square root
distances, FT thresholds and single-shot properties with redundant
checks. Meanwhile, their rich connection to other codes are very
instructive and may lead to further optimizations. Regarding the
remained procedures towards fault-tolerant universal quantum
computation through quantum LDPC codes, I will discuss the
fault-tolerant condition for each code, including distance,
stabilizer weight, decoding, fault-tolerant gates and measurement
protocols.