Coherent ising machine (CIM) realizes 100000 spin qubit scale

On December 3, Yoshihisa Yamamoto, founder of coherent quantum computing and professor of Stanford University, shared the latest academic progress of coherent ising machine (CIM) at the frontier forum of quantum physics and Quantum Information Science (qpqis2021) hosted by Beijing quantum Academy.

A recent paper published in the progress of science shows that the coherent ising machine (CIM) has made a major technical breakthrough in the optical quantum computing scheme, realized the CIM computing experiment of 100512 spin quantum bits, broke through the 100000 mark, and is far ahead of all quantum computing technology schemes. The paper was published by Japan Telegraph and Telephone Corporation (NTT) in cooperation with Japan's National Institute of Informatics (NII).

Since 2021, the light quantum computing route has made remarkable progress in technology R & D, financing scale and market-oriented application. Therefore, some people in the industry call 2021 the "first year of light quantum".

Coherent ising machine (CIM) is a kind of optical quantum computing scheme. Because it has many advantages and potentials, it is one of the popular schemes at present. Currently engaged in CIM R & D include NTT, NII, NASA, Stanford University, California Institute of technology, University of Maryland, University of Tokyo and other research institutes and universities, as well as Bose quantum technology company in China.

In short, CIM is a special quantum computer for solving combinatorial optimization problems at high speed through a laser pulse called "DOPO". It uses DOPO network to simulate the "Ising model" in statistical physics, because the photon energy in DOPO is much greater than that of thermal noise in the environment, which enables researchers to realize the low-temperature spin behavior of photons through a set of physical systems at room temperature.

Details of CIM experimental system. PZT: optical fiber phase controller with piezoelectric element, BPF: optical bandpass filter, PMF: polarization maintaining fiber, ADC: analog-to-digital converter, DAC: digital to analog converter. (picture source: Paper 2 of 100000 spin coherent ising machine)

So how to realize CIM on the scale of 100000 spin qubits?
In the CIM system in this paper, the time-division multiplexed DOPO pulses are injected into the nonlinear optical waveguide as compressed vacuum pulses and circulate in the 5km optical fiber cavity. These pulses undergo digital assisted interaction and nonlinear amplitude evolution at the same time. The quantum effects between DOPO pulses are used to find the ground state energy state of Ising model, Finally, through the collective phase transition mapping of more than 100000 DOPO pulses, the optimal result is solved.

As early as 2016, the research team of NTT basic research laboratory released a set of CIM system. The system uses measurement / feedback technology to fully couple in the remote optical fiber cavity and generate 2000 DOPO pulses, which can realize up to 4 million coupling. As the technical leader of the R & D team, Dr. Hiroki takesue, a senior outstanding researcher of NTT basic research laboratory, said that on this basis, the scale of CIM can be further expanded to 100000 spins. In this process, the biggest challenge lies in the optical part and the measurement feedback part.

For the optical part, the team increased the length of the optical fiber cavity from 1km to 5km, and the repetition frequency of the pump pulse from 1GHz to 5GHz, resulting in more than 100000 DOPO pulses in the optical fiber cavity. However, correspondingly, a large fiber cavity will lead to large optical loss, which means that the gain of nonlinear waveguide needs to be increased for phase sensitive amplification. By improving the manufacturing process and the design of optical fiber coupling module, they developed a more efficient nonlinear waveguide, that is, PPLN waveguide module.

Appearance of 100512 spin CIM (picture from ntt1)

As for the measurement feedback part, the team has developed a system that can perform the multiplication calculation of 100k x 100k matrix and 100k element carrier within 25 microseconds, which can meet the requirements of pulse round-trip time of 5km optical fiber cavity.

Wu juhongshu stressed: "the system requires more than 50 FPGA chips. If we want to expand it to 1 million spin system computing in the future, we may need to develop a special circuit (ASIC) system for matrix computing."

Based on the improvement of the above optical system and measurement / feedback system, Wuju team finally completed the largest CIM in the world, which can realize 100000 pulses and up to 10 billion coupled DOPO networks. For the large-scale combinatorial optimization problem with 100000 calculation parameters, the experimental test shows that the solution is more than 1000 times faster and more accurate than the simulated annealing algorithm (SA) implemented on the classical compute.

Link:
[1] https://group.ntt/jp/newsrelease/2021/09/30/210930a.html
[2]T.Honjo,T.Sonobe,K.Inaba,T.Inagaki,T.lkuta,Y.Yamada,T.Kazama,K.Enbutsu,T.Umeki,R.Kasahara,K.i.Kawarabayashi,H.Takesue,100,000-spin coherent lsing machine. Sci.Adv.7,eabh0952(2021).