Japanese researchers work out theoretical universal quantum computer that could scale to millions of qubits

Japanese scientists have invented an approach to quantum computing that renders a far larger number of calculations more efficiently than existing quantum computers.

They believe the technological building blocks exist and that the quantum loops can be experimentally realized with current technology. Their theoretical works suggests this can scale to millions of qubits for a universal quantum computer.

Above the Loop-based architecture for universal quantum computing is shown, featuring a homodyne detector (HD), displacement operation (Disp.), a phase shifter (PS) and a variable beam splitter (VBS).

Under the new method, many pulses of light, each carrying information, are allowed to go around in a loop circuit indefinitely. The circuit performs multiple tasks, switching from one task to another rapidly through instant manipulations of the pulses.

The invention was announced in an article by University of Tokyo professor Akira Furusawa and assistant professor Shuntaro Takeda that was posted on an electronic version of the U.S. journal Physical Review Letters.

Physical Review Letters – Universal Quantum Computing with Measurement-Induced Continuous-Variable Gate Sequence in a Loop-Based Architecture

Arxiv – Universal Quantum Computing with Measurement-Induced Continuous-Variable Gate Sequence in a Loop-Based Architecture (5 pages)

(a) Measurement-induced squeezing gate [5]. (b) Single-loop architecture for single-mode Clifford (Gaussian) gates. (c)–(e) Procedure for an arbitrary single-mode Clifford gate. We assume that the input pulse initially arrives at the VBS at time t=0. (f) Programmable control sequence of system parameters. (g) Decomposition of an arbitrary n-mode Clifford gate
“We’ll start work to develop the hardware, now that we’ve resolved all problems except how to make a scheme that automatically corrects a calculation error,” Furusawa said.

In 2013, Furusawa’s team developed a basic system for optical quantum computing. The system requires more than 500 mirrors and lenses and occupies space 4.2 meters long and 1.5 meters wide, while it can handle only one pulse.

To boost the capacities, many units need to be connected. But that is difficult, given the size and complicated structure of the system.

In the new approach, a single circuit plays the role of many such systems.

In other types of quantum computers, including those using superconducting circuits, some are capable of handling up to dozens of qubits, or quantum bits, the basic unit of information in quantum computing.

Furusawa’s new approach will allow a single circuit to process more than 1 million qubits theoretically, his team said in a press release, calling it an “ultimate” quantum computing method.

Researchers theoretically showed that their quantum computation scheme using measurement-induced CV gates in a loop-based architecture provides the universal gate set for both qubits and CVs. This architecture offers electrical programmability of gate sequence and higher scalability, and also enables fault-tolerant quantum computation with logical qubits redundantly encoded in a large Hilbert space.

Experimental viability. They consider the feasibility of their architecture with current technology. Measurement-induced squeezing gates in our architecture was previously demonstrated for pulsed input states.

Although the measurement-induced cubic phase gate has not been demonstrated yet, recent progress in ancilla preparation makes its implementation within reach of current technology. Conversion of these gates into the loop architecture requires long (10 meter) and low-loss optical delay lines. Such delay lines have been developed in free-space or by using optical fibers in several experiments. In addition, the viability of loop-based beam splitter operation has been recently demonstrated in several experiments, such as quantum walk and boson sampling. These experiments demonstrate that fast dynamic control of optical switches and beam splitter transmissivity is possible while preserving the coherence of quantum states. The demonstrations described above show that all of the basic building blocks of their architecture are already available.


We propose a scalable scheme for optical quantum computing using measurement-induced continuous-variable quantum gates in a loop-based architecture. Here, time-bin-encoded quantum information in a single spatial mode is deterministically processed in a nested loop by an electrically programmable gate sequence. This architecture can process any input state and an arbitrary number of modes with almost minimum resources, and offers a universal gate set for both qubits and continuous variables. Furthermore, quantum computing can be performed fault tolerantly by a known scheme for encoding a qubit in an infinite-dimensional Hilbert space of a single light mode.