Goal 6 TARUCHA Seigo PM Progress Report : [4] Development of electron wave-packet qubits with new principle

Progress Report

Last updated: September 15, 2026

Development of scalable Silicon quantum computer technology[4] Development of electron wave-packet qubits with new principle

Progress until FY2025

1. Outline of the project

The purpose of this project is to develop a new quantum computation method using qubits based on electron wavepackets. Uniquely, these wavepackets propagate stably through semiconductor circuits without deformation. Our future goal is a loop circuit system where numerous wavepacket qubits repeatedly traverse the same quantum circuits under varying conditions. This approach aims to efficiently control many qubits using minimal hardware.
To realize this, we are tackling key challenges: developing technologies for (1) high-density generation and high-fidelity manipulation of these wavepacket qubits, and (2) quantum mechanical control of their propagation and the readout of individual packets. We aim to establish the foundational technology for this novel quantum computing paradigm.

Fig.1 — Figure 1 A schematic diagram of a quantum logic circuit using electron wavepacket qubits. Electron wavepacket qubits are generated on demand from an electron source, and their quantum state is manipulated by a small number of logic gates incorporated into a loop-type circuit.

2. Outcome so far

In Objective 1, we conducted experiments on the transport of electron wavepackets in quantum interferometers and designed a quantum computation system using them. We injected DC and pulsed currents simultaneously into a quantum interferometer and evaluated the visibility of interference. It was found that the visibility of the interference strongly depends on the magnitude of the DC voltage, while for pulses of less than 1 mV, it is almost independent of their amplitude. This result suggests that the quantum state of electrons injected by DC voltages relaxes in short distances, whereas the state of short electron wavepackets injected by electric pulses is preserved over long distances. We also found that the electron wavepacket propagates independently of the DC current. On the other hand, one of the issues for conventional gate-defined interferometers is that we cannot fully suppress electron backscattering. We designed quantum circuits using quantum Hall edge channels that do not suffer from backscattering. We also designed a new medium-scale quantum computation system and fabricated a prototype device.

Fig.2 — Figure 2 Home-made electric pulse generator (left) and printed circuit board for high-frequency measurement used in the experiment (right).

Regarding Objective 2, we developed and demonstrated a new method to control the quantum states of electron wavepackets by using locally confined structures placed at appropriate intervals relative to the wavepacket length. This method can be applied to delay and synchronization control of electron wavepackets and helps prevent information leakage to surrounding circuits, enabling more reliable quantum-state control.

Fig.3 — Figure 3 A Speed control of wavepackets using local confinement. Velocity modulation is achieved for $L_{p} > L_{QPC}$

We have also worked on a detection scheme for electron wavepackets using spin singlet–triplet qubits as highly sensitive charge detectors. We established a high-frequency measurement environment at cryogenic temperatures, demonstrated the generation and control of electron wavepackets in the 10 GHz range, and observed Pauli spin blockade. These achievements provide a solid experimental foundation for single-electron wavepacket detection.

3. Future plans

Based on our achievements to date, we have established a solid technological foundation for realizing quantum computing based on electron wavepackets. Moving forward, we will work on developing picosecond-scale electron wavepackets, demonstrating high coherence over circuit lengths exceeding 1 mm, and achieving high-sensitivity single-electron wavepacket readout. Through these efforts, we aim to realize an electron wavepacket quantum computer capable of flexibly controlling many wavepacket qubits using only a small number of quantum circuits.

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