Measurement and Quantum Computation
Quantum computation has the potential to perform certain computational tasks more efficiently than classical computation. In the past decade, there has been impressive progress in building quantum devices with many qubits. I will discuss the role of measurement in several frameworks of quantum computation. First, it is needed to readout the outcome in the computation. In some implementations, such as in the superconducting qubits, readout error can be as large as the two-qubit gate error. I will show that the so-called detector tomography can be used to correct the readout distribution and illustrate this with experiments on cloud quantum computers. Next, I will describe a simple iterative scheme with quantum circuits to measure a system in its energy eigenstate basis. This can be used to implement a Zeno approach that achieves adiabatic quantum computation. In the last part, I will discuss how single-qubit measurements by themselves can drive universal quantum computation, exploiting entanglement as a resource. If these entangled resource states emerge as unique ground states of nearest-neighbor interacting Hamiltonians, then they may be created by cooling the engineered Hamiltonians. A nonzero gap that separates the ground state from all excited states is thus a desired feature. I will show that some of the Affleck-Kennedy-Lieb-Tasaki (AKLT) models indeed possess these properties that they are gapped and their ground states can be used for universal quantum computation by measuring spins locally.
To watch online go to the IQUIST youtube channel: https://www.youtube.com/channel/UCCzAySwQXF8J4kRolUzg2ww