Preparing Multipartite Entangled Spin Qubits via Pauli Spin Blockade
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-02-26 |
| Journal | Scientific Reports |
| Authors | Sinan Bugu, Fatih Ăzaydin, T. Ferrus, Tetsuo Kodera |
| Institutions | Tokyo Institute of Technology, Tokyo International University |
| Citations | 22 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Cavity-Free W State Preparation: The research proposes a novel, solid-state scheme for preparing large-scale multipartite entangled W states using electron spins in a Double Quantum Dot (DQD) system, eliminating the need for complex photon assistance or optical cavities.
- Pauli Spin Blockade (PSB) Fusion: The core mechanism uses PSB to verify the successful fusion of two smaller W states (Wn and Wm) into a larger state (Wn+m-2). Fusion occurs only when the two input electrons have anti-parallel spins, resulting in the absence of PSB.
- Detection Method: Successful fusion (absence of PSB) is detected rapidly via standard transport measurement or radio-frequency reflectometry (RF-SET), relying on well-known charge sensing techniques.
- Enhanced Success Rate: An improved setup incorporates a Toffoli gate and a CNOT gate, along with an ancillary spin-down electron, applied before the DQD. This enhancement converts the initial âfailureâ cases (parallel spin inputs) into successful fusion outcomes, significantly boosting the overall efficiency.
- Industry Compatibility: The scheme is based entirely on electron spin manipulation in semiconductor DQDs, recognized as the most reliable and industry-compatible platform for scalable quantum information processing.
- Scalability: The fusion approach allows for the progressive creation of arbitrarily large W states by iteratively fusing smaller components.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| DQD Spin Coherence Time | Tens of | ms | Achieved in optimized DQD systems (e.g., Silicon). |
| QD Spin Coherence Time | Few tens of | ”s | Typical coherence time for single quantum dots. |
| Electron Injection Time | Few hundreds of | ps | Time required for electrons to tunnel into the DQD. |
| PSB Sensing Time (RF Reflectometry) | Few | ns | Fast measurement technique for spin state determination. |
| PSB Sensing Time (Standard) | Few hundreds of | ns | Typical time for transport measurement methods. |
| Fusion Fidelity (Noisy Setup) | (1-p)2 + p2 | Dimensionless | Where p is the probability of an electron spin flip during the process. |
| Minimum Fusible W State Size (Basic) | 3 | Qubits | Required for the initial fusion setup (W3 + W3). |
| Minimum Fusible W State Size (Enhanced) | 2 | Qubits | Achieved using the Toffoli+CNOT enhancement (allows fusion of EPR pairs, W2). |
| Preferred Material Platform | Silicon (Si) | N/A | Preferred over III-V materials due to generally negligible Spin-Orbit Coupling (SOC) and non-zero nuclear spin moments. |
Key Methodologies
Section titled âKey Methodologiesâ- Initial State Preparation: Two W states (WA of n electrons and WB of m electrons) are prepared in quantum memories. The goal is to fuse these into Wn+m-2.
- Electron Injection: A single electron is extracted from WA and sent to Dot 1, and a single electron from WB is sent to Dot 2 via side injection contacts (A and B).
- DQD Tuning and PSB Regime: The DQD is electrostatically tuned using side gates (Vsg1, Vsg2) to operate in the Pauli Spin Blockade (PSB) regime. This regime ensures that inter-dot tunneling (current flow) is blocked if the two electrons have parallel spins (triplet state, T(1, 1)) but allowed if they have anti-parallel spins (singlet state, S(1, 1)).
- Spin State Determination via Charge Sensing: The presence or absence of PSB is measured using a charge sensor (e.g., radio-frequency single-electron transistor, RF-SET) or standard transport measurement, determining the spin configuration of the two electrons in the DQD.
- Fusion Outcome Logic:
- Success: Absence of PSB (anti-parallel spins, ââ or ââ) implies successful fusion, yielding Wn+m-2.
- Recycle: Presence of PSB (parallel spins, ââ) implies the states are kept intact but reduced in size (Wn-1, Wm-1).
- Failure: Presence of PSB (parallel spins, ââ) implies the W states collapse to separable spin-down states.
- Enhanced Fusion Mechanism (Optional): To improve success probability, the two input electrons and an ancillary spin-down electron are passed through a Toffoli gate followed by a CNOT gate before entering the DQD. This logic operation ensures that the initial âfailureâ case (ââ) is converted into a successful fusion outcome, increasing the final state size by one qubit (Wn+m-1).
Commercial Applications
Section titled âCommercial ApplicationsâThe proposed methodology directly supports the development of scalable quantum hardware, particularly within the solid-state domain:
- Semiconductor Quantum Processors: Provides a fundamental building block for generating complex entangled states necessary for universal quantum computation, leveraging established semiconductor fabrication techniques (e.g., Si/SiGe heterostructures).
- Quantum Repeaters and Networks: The fusion scheme is essential for creating long-distance entanglement by linking smaller entangled segments, forming the basis for robust quantum communication infrastructure.
- High-Fidelity Qubit Control Systems: The reliance on fast, high-fidelity gate operations (Toffoli, CNOT) and rapid charge sensing (ns timescale) drives requirements for advanced, low-noise cryogenic control electronics and pulse sequencing hardware.
- Quantum Memory Devices: Since the W states are encoded in electron spins within DQDs, which exhibit long coherence times, this preparation method is optimized for generating entangled states intended for long-term quantum information storage.
- Advanced Metrology and Sensing: The core technique of PSB sensing, often implemented using RF-SETs, is a highly sensitive method for detecting single-electron charge and spin states, applicable to high-precision quantum sensing applications.
View Original Abstract
Abstract Preparing large-scale multi-partite entangled states of quantum bits in each physical form such as photons, atoms or electrons for each specific application area is a fundamental issue in quantum science and technologies. Here, we propose a setup based on Pauli spin blockade (PSB) for the preparation of large-scale W states of electrons in a double quantum dot (DQD). Within the proposed scheme, two W states of n and m electrons respectively can be fused by allowing each W state to transfer a single electron to each quantum dot. The presence or absence of PSB then determines whether the two states have fused or not, leading to the creation of a W state of n + m â 2 electrons in the successful case. Contrary to previous works based on quantum dots or nitrogen-vacancy centers in diamond, our proposal does not require any photon assistance. Therefore the âcomplexâ integration and tuning of an optical cavity is not a necessary prerequisite. We also show how to improve the success rate in our setup. Because requirements are based on currently available technology and well-known sensing techniques, our scheme can directly contribute to the advances in quantum technologies and, in particular in solid state systems.