229 research outputs found
Optimal Transport with Cyclic Symmetry
We propose novel fast algorithms for optimal transport (OT) utilizing a
cyclic symmetry structure of input data. Such OT with cyclic symmetry appears
universally in various real-world examples: image processing, urban planning,
and graph processing. Our main idea is to reduce OT to a small optimization
problem that has significantly fewer variables by utilizing cyclic symmetry and
various optimization techniques. On the basis of this reduction, our algorithms
solve the small optimization problem instead of the original OT. As a result,
our algorithms obtain the optimal solution and the objective function value of
the original OT faster than solving the original OT directly. In this paper,
our focus is on two crucial OT formulations: the linear programming OT (LOT)
and the strongly convex-regularized OT, which includes the well-known
entropy-regularized OT (EROT). Experiments show the effectiveness of our
algorithms for LOT and EROT in synthetic/real-world data that has a
strict/approximate cyclic symmetry structure. Through theoretical and
experimental results, this paper successfully introduces the concept of
symmetry into the OT research field for the first time
Si/SiGe2重量子ドットにおけるスピン量子ビットの実験的研究
学位の種別: 課程博士審査委員会委員 : (主査)東京大学教授 樽茶 清悟, 大阪大学教授 大岩 顕, 東京大学教授 中村 泰信, 筑波大学教授 都倉 康弘, 東京大学准教授 長田 俊人University of Tokyo(東京大学
Feedback-based active reset of a spin qubit in silicon
Feedback control of qubits is a highly demanded technique for advanced
quantum information protocols such as quantum error correction. Here we
demonstrate active reset of a silicon spin qubit using feedback control. The
active reset is based on quantum non-demolition readout of the qubit and
feedback according to the readout results, which is enabled by hardware data
processing and sequencing. We incorporate a cumulative readout technique to the
active reset protocol, enhancing initialization fidelity above a limitation
imposed by accuracy of the single QND measurement fidelity. Based on an
analysis of the reset protocol, we suggest a way to achieve the initialization
fidelity sufficient for the fault-tolerant quantum computation
A shuttling-based two-qubit logic gate for linking distant silicon quantum processors
Control of entanglement between qubits at distant quantum processors using a
two-qubit gate is an essential function of a scalable, modular implementation
of quantum computation. Among the many qubit platforms, spin qubits in silicon
quantum dots are promising for large-scale integration along with their
nanofabrication capability. However, linking distant silicon quantum processors
is challenging as two-qubit gates in spin qubits typically utilize short-range
exchange coupling, which is only effective between nearest-neighbor quantum
dots. Here we demonstrate a two-qubit gate between spin qubits via coherent
spin shuttling, a key technology for linking distant silicon quantum
processors. Coherent shuttling of a spin qubit enables efficient switching of
the exchange coupling with an on/off ratio exceeding 1,000 , while preserving
the spin coherence by 99.6% for the single shuttling between neighboring dots.
With this shuttling-mode exchange control, we demonstrate a two-qubit
controlled-phase gate with a fidelity of 93%, assessed via randomized
benchmarking. Combination of our technique and a phase coherent shuttling of a
qubit across a large quantum dot array will provide feasible path toward a
quantum link between distant silicon quantum processors, a key requirement for
large-scale quantum computation
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