Currently, the most sophisticated quantum computers in the world have processors implemented with superconducting circuits or trapped ions. Due to the inherent fragility of quantum states, these physical systems must be fully isolated from the external environment and maintained at cryogenic temperatures.
Photonic systems — which do not require vacuum or cooling devices — comprise another approach to quantum computing. However, many physicists believe that realizing large-scale photonic quantum computers is impractical due to the intrinsic difficulties introduced by the unique features of photons.
A new perspective article by Shuntaro Takeda and Akira Furusawa outlines two promising routes to large-scale photonic quantum computing that have recently emerged. The hybrid qubit-continuous variable approach and the time-domain multiplexing technique have introduced a new era in photonic quantum computing, which demonstrate this scalable method is indeed a possibility.
Historically, two complementary paths to photonic quantum computing, qubits and continuous-variables, have been separately pursued, each exploiting either the particle or the wave nature of light. The hybrid qubit-continuous variable approach aims to take the best of both worlds by combining robust qubit encoding and deterministic continuous variable gates.
Time-domain multiplexing, on the other hand, tackles the problem of needing large-scale photonic circuits to sequentially perform several operations on numerous qubits. The technique avoids this issue by encoding qubits in a train of optical pulses propagating in a single or few optical paths. These qubits are individually accessible and easily controllable with a small number of optical components at different times.
By combining the hybrid qubit-continuous variable approach and the time-domain multiplexing technique, Takeda and Furusawa believe photonic quantum computing can overcome its previous limitations, ultimately enabling large-scale fault-tolerant universal photonic quantum computing.
Source: “Toward large-scale fault-tolerant universal photonic quantum computing,” by S. Takeda and A. Furusawa, APL Photonics (2019). The article can be accessed at https://doi.org/10.1063/1.5100160.
