June 28, 2023

Quantum computing is attracting attention as a technology with the power to significantly change the world. NEC is also devoting many resources to quantum computing as a focus area and accelerating its research. However, what exactly is a quantum computer? What unique characteristics does NEC's technology possess? We spoke with a researcher who is working in the field.

Front lines of quantum computing research

― What exactly is a quantum computer?

It is a computer which performs calculations by using "quantum superposition" and other characteristics of quantum mechanics. In the computers which we currently use (classical computers), each bit is either a "0" or "1" and these are used to perform calculations in binary notation. For example, with 8 bits, a value such as "10001111" may be used to perform a calculation. During calculation, the number may take one state in the range from "00000000" to "11111111" out of the 256 states (2 to the 8th power) available.

In contrast, bits in a quantum system can take a superposition state that is both "0" and "1." This characteristic is called "quantum superposition," and 2 to the 8th power states can be expressed simultaneously if this principle is utilized. In other words, 2 to the nth power calculations can be handled with one quantum state. It cannot simply retrieve all calculation results, but it has the potential to calculate more efficiently than classical computers by using algorithms which use interference in the superposition state. As a result, it will have a tremendous impact on our society as the number of bits increases. It is hoped that calculations which were impossible due to limitations on the computing performance of classical computers will be solvable within a realistic amount of time.

What is a quantum computer?

― Currently, how close are quantum computers to becoming a reality?

First, broadly speaking there are two types of quantum computers. One type is called a gate-based quantum computer, so it converts the previous computer bits into qubits. As discussed a moment ago, it is capable of fundamentally changing computer performance, but it is still being researched worldwide. Machines in the 400-bit class are currently the largest scale gate-based quantum computers, but it is still difficult to accurately control qubits, and reductions in the error rate and significant redundancy are needed to realize "fault-tolerant quantum computers."

The other type of a quantum computer uses quantum annealing. This is a computer which specializes in the problem called the "combinatorial optimization problem," D-wave, which is in collaboration with NEC, has announced a 7,000-bit class machine. The combinatorial optimization problem searches for the optimal solution while combining options and satisfying various constraints. For example, there is the "traveling salesman problem" which solves for the minimum cost to visit multiple cities and the "knapsack problem" which packs items into a knapsack with a fixed volume so as to maximize the total value.

The combinatorial optimization problem can be reduced to the model called the "Ising model" where the optimal solution corresponds to the minimum energy state of the Ising model. In quantum annealing, you search for the minimum energy state to solve for the optimal solution by representing and running the Ising model with qubits. However, the method called "simulated annealing," which performs this type of annealing operation using a classical computer, is already broadly used, and NEC is also currently deploying a service using the "SX-Aurora TSUBASA" vector machine. It is hoped that if this simulated annealing were to be replaced by quantum annealing, it would become possible to provide more precise solutions at a faster pace.

At NEC, we built the "NEC-AIST Quantum Technology Cooperative Research Laboratory" where we are researching both the gate-based quantum computer and quantum annealing.

NEC research with future potential

― What is NEC's current standing in quantum computer research?

NEC was the first company in the world to successfully demonstrate qubit operation using a superconducting solid-state element back in 1999. However, in recent years various companies and research institutions have started researching quantum computers and produced remarkable results. In 2014, NEC was the first in the world to successfully achieve a highly sensitive readout of qubits using a superconducting parametron circuit, and the company continues to accelerate its research.

For example, we are participating in the MOONSHOT research and development project for gate-based quantum computers led by the Cabinet Office. In order to carry out an error-free quantum calculation on a quantum computer which is vulnerable to disturbances in the environment and accumulates errors during the calculation, it is essential to realize "fault tolerance." The stated goal of this project is to achieve a fault-tolerant quantum computer by 2050. In the project using the superconducting quantum circuit method, NEC is directing a joint research project joined by many research institutions as the project manager.

We are also participating in the NEDO Project led by the Ministry of Economy, Trade and Industry to advance research and development of quantum annealing. In June of this year, we started joint research with Tohoku University on computation using quantum annealing machines via the cloud. This is an initiative focused on practical services. Currently we are in the validation phase at 8 bits, but our goal is to provide a service which utilizes the features of a hybrid operation combined with simulated annealing using the vector machines which have been provided thus far. Quantum annealing and vector machines each have their strengths and weaknesses, so we are aiming to build applications and platforms that can select those differences to solve problems. This can be said to be a unique strength of NEC which possesses both quantum and classical computers.

NEC’s position for quantum computer

― What would you say are NEC's unique qualities and strengths?

With respect to the quantum annealing which I am in charge of, it has the ability to maintain the "coherence time (time during which the quantum superposition state can be maintained)" for a longer time than conventional quantum annealing machines through a unique approach which is close to the gate-based quantum computer using superconducting parametron or Josephson Parametric Oscillator (JPO) circuits. Because a longer coherence time leads to more accurate calculations, quantum annealing will have a significant impact if we can realize multi-qubit system. In addition, it can solve combinatorial optimization problems with fewer bits than previous methods. Due to the adoption of an easy-to-scale structure for the architecture, it has sufficient future potential.

NEC's Uniqueness and Strengths

Aiming for multi-qubit support for practical application

― Tell us about your future prospects.

First there is the realization of multi-qubit system. In quantum annealing, we are still repeating basic experiments at 8 bits, so our goal is to achieve multi-qubit system and complete a practical machine as soon as possible. However, as I mentioned a moment ago, the method that NEC is advancing is unique for its long coherence time and the adoption of an easy-to-scale architecture. If we can effectively increase the number of bits, then I think it can be said that it has the potential to produce an extremely large effect.


― What points are preventing the development of multi-qubit support?

This is not limited to NEC, but the wiring and measuring instruments which increase in proportion to the number of bits are a bottleneck to the realization of multi-qubit system. Moreover, in a quantum computer, the quantum devices must be placed in a dilution refrigerator which can be chilled to about 10mK (-273.14°C, see note). The cost required to build the measurement system and the capacity and cooling capability of the refrigerator are the primary barriers to multi-qubit support.

That being said, it is not the case that you could just spend unlimited amounts of money to build a supermassive device. Ideas and innovation that can effectively streamline the quantum computer configuration are needed. New peripheral technologies which place control units inside the refrigerator are being sought through research and development conducted around the world including at NEC.

Note: 0K (Kelvin) is the temperature at which the temperature cannot go any lower (absolute zero).


― Please tell us about your current goals.

Because the number of bits is insufficient to advance practical application, we wish to expand our current 8-bit system to 100 bits or more. It may take some time, but we would like to continue advancing our research with a focus on practical application.

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