On October 15, NTT and Ritsumeikan University announced
that they have developed a new calculation method
for designing distributed quantum computers that link multiple quantum chips.
They say the method can calculate the limit on the success probability
of quantum operations across chips
with more than 10 times the accuracy and more than 100 times the speed
of conventional methods.
The improvement is in the evaluation calculations used for design,
not in the processing speed of the quantum computer itself.
Background: Linking Multiple Chips to Scale Up
According to the announcement, typical quantum chips
require the entire circuit to be housed in a cryogenic refrigerator
to maintain the state of the qubits,
which limits the number of qubits that can be placed on a single chip.
A distributed architecture that links multiple chips
is seen as a promising way to overcome this constraint.
Operations between chips use quantum entanglement,
in which distant qubits share strong correlations.
However, when noise degrades the quality of entanglement,
the success probability of operations also falls.
Design requires knowing how far the success rate can be raised
for a given quality of entanglement.
What Changes: Breaking Down Candidates to Find the Limit on Success Probability
The new method uses algebraic geometry, a field of mathematics,
to organize candidate ways of carrying out operations,
then breaks the problem into smaller parts and examines each one individually.
Conventional methods either took a long time to calculate
or had to be redesigned for each target problem.
The new method is said to handle different quantum operations,
chip configurations, and types of noise.
The researchers also showed theoretically that,
for a wide range of operations across chips,
achieving a 100% success probability
requires entanglement of the highest quality.
This extends a result previously known for certain quantum gates
to a broader range of quantum operations, including measurements.
NTT Built the Theory and Programs; Ritsumeikan Supported the Mathematics
NTT was responsible for overall coordination of the research,
building the theory, creating the calculation programs,
and analyzing the results.
Professor Hiroyuki Osaka of Ritsumeikan University’s
College of Science and Engineering
supported the mathematical foundations
and confirmed the theoretical validity of the method.
The results were published in the journal
Letters in Mathematical Physics.
Going forward, the researchers aim to verify
theoretically optimal procedures on actual quantum devices.
They also expect the method to be applied
to investigating the performance limits of photonic quantum computers.
LC Signal’s Perspective: Estimating Required Quality at the Design Stage
According to the announcement,
preparing high-quality entanglement requires many qubits.
Being able to estimate in advance the quality and chip configuration
needed for target performance
seems likely to make it easier to decide where to use limited resources.
We will watch how verification on actual hardware
makes the conditions usable for design more concrete.
Key Sources
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NTT and Ritsumeikan University official announcement (Japanese), October 15, 2026:
分散型量子コンピュータの設計指針となる新計算法を開発
～光量子方式を含む大規模化に向け、数学の力で量子もつれの潜在能力を見える化～
(New calculation method developed as a design guideline
for distributed quantum computers: Using mathematics to visualize
the potential of quantum entanglement for scaling up,
including photonic quantum approaches)
This article was prepared with AI assistance using publicly available information
as of October 16, 2026.
Announcement dates follow the dates shown on the official pages.
Improvements in calculation speed and accuracy are based on the announcement
concerning the method for evaluating limits on success probability.
Theoretical and computational results are distinguished
from planned verification on actual devices.
“LC Signal’s Perspective” reflects this publication’s interpretation
of the available information.