**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.

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).