Loop-based photonic quantum computer

A photonic quantum computer has the advantage of room-temperature operation and applicability to optical communication, and thus is a promising candidate to realize a universal quantum computer. Thus far, there have been proof-of-principle demonstrations of elementary quantum logic gates and quantum error correction with small-scale optical circuits. However, there still remain several obstacles for large-scale optical quantum computing. One is that large-scale quantum computing requires large-scale optical circuits, thus requiring a large number of optical components and space. Another problem is that different optical circuits are necessary for different quantum computation since one optical circuit can perform only one specific quantum computation.

 Recently, we proposed a new architecture for optical quantum computation which can efficiently perform arbitrarily large-scale quantum computation with minimal optical components [1]. In this architecture, a string of input and ancillary pulses in a single optical beam are sent to a nested loop circuit in Fig.1. All of these pulses are first stored in the outer loop, which acts as an optical memory to store many optical quantum states. In contrast, the inner loop is a quantum processor which sequentially processes the optical pulses and perform quantum computation. Here, quantum logic gates are programmably performed by dynamic control of system parameters, such as optical switches, beam splitter transmissivity, phase shifters, and amplifier gain.

This architecture can deal with any number of optical pulses without increase in optical circuit scale, and repeat an unlimited number of quantum operations by measuring each pulse within the coherence time of the light source, thus offering high scalability. Furthermore, it offers a universal gate set for both qubits and continuous variables once suitable ancillary pulses are provided. In addition, arbitrary quantum computation can be performed with the same optical circuit only by changing the program to control the system parameters. Our scheme will promote large-scale optical quantum computing and also greatly reduces resources and cost for the development of photonic quantum computers. We are now developing this loop-based photonic quantum computer while analyzing computational accuracy and implementation methods of quantum algorithms in this architecture.

Fig. 1. Loop-based architecture for quantum computing [1]. HD, Homodyne detector; Disp., Displacement operation; PS, Phase shifter; VBS, Variable Beam Splitter.

[1] S. Takeda and A. Furusawa, gUniversal quantum computing with measurement-induced continuous-variable gate sequence in a loop-based architecture,h Phys. Rev. Lett 119, 120504 (2017).