Nanotube double quantum dot spin transducer for scalable quantum information processing

At a Glance

Metadata	Details
Publication Date	2020-04-28
Journal	New Journal of Physics
Authors	Wanlu Song, Tianyi Du, Haibin Liu, Ralf Betzholz, Jianming CAI
Citations	2
Analysis	Full AI Review Included

Technical Documentation: Hybrid NV-Center/DQD Spin Transducer

This documentation analyzes the research proposing a scalable solid-state quantum architecture utilizing Nitrogen-Vacancy (NV) centers in diamond coupled to carbon-nanotube Double Quantum Dots (DQDs). This hybrid platform achieves steady-state entanglement between distant NV-center electron spins, a critical step toward scalable quantum computing based on nuclear spin cluster states.

Executive Summary

The following points summarize the core technical achievements and material requirements of the proposed scalable quantum architecture:

Scalable Entanglement: The research proposes a hybrid platform to achieve steady-state entanglement between NV-center electron spins separated by micrometer distances, overcoming the rapid decay of direct coupling.
Transducer Mechanism: Entanglement is mediated by the leakage current of a carbon-nanotube DQD operating in a Pauli-blockade regime, driving the NV spins into a maximally entangled state ($\vert\Phi^{-}\rangle$).
Material Requirement: The scheme relies fundamentally on high-quality diamond containing precisely positioned NV centers, ideally using isotopically purified diamond (low $^{13}$C) to minimize magnetic field noise and maximize coherence.
Operational Control: The system requires only feasible voltage control of the nanotubes (DQD gates) and microwave driving of the NV-center electron spins.
Performance Metrics: Optimized parameters achieve the maximally entangled state in a rapid timescale ($t_c = 45$ µs) and demonstrate tolerance to electric potential fluctuations (up to ±1 µeV).
Quantum Computing Application: The steady-state electron spin entanglement is exploited via hyperfine coupling to realize controlled-phase gates between associated $^{15}$N nuclear spins, enabling the generation of 1D and 2D cluster states for universal measurement-based quantum computation.
Fabrication Challenge: Successful implementation requires advanced fabrication techniques, specifically the use of diamond nanopillars and high-precision NV center positioning (e.g., via scanning probes).

Technical Specifications

Parameter	Value	Unit	Context
NV Zero-Field Splitting ($D/2\pi$)	2.87	GHz	NV-center electron spin ground state
Optimized Entanglement Time ($t_c$)	45	µs	Time to prepare maximally entangled state ($\vert\Phi^{-}\rangle$)
Optimized Tunneling Rate ($J/2\pi$)	24	MHz	Tunneling rate between DQD singlet states
Optimized Rabi Frequency ($\Omega/2\pi$)	0.6	MHz	Effective Rabi frequency of NV driving field
Electron Transport Rate ($\Gamma_{in}, \Gamma_{out}$)	0.5	GHz	Ideal case transport rate
Noise Tolerant Transport Rate ($\Gamma_{in}, \Gamma_{out}$)	2	GHz	Compensates for energy shift ($\Delta$) up to 1 µeV
External Magnetic Field ($B_z$)	5	mT	Used to lift degeneracy of NV sublevels
NV-QD Distance ($r_L, r_R$)	6	nm	Distance between NV center and quantum dot
Hyperfine Coupling ($A_{		}/2\pi$)	3.03
Electric Noise Tolerance ($\Delta$)	±1	µeV	Tolerance range for energy difference between DQD singlet states

Key Methodologies

The proposed scheme relies on precise material engineering and specific operational parameters to leverage the Pauli exclusion principle for entanglement generation.

Hybrid Platform Construction:
- Diamond pillars (nanopillars) containing single NV centers are fabricated.
- These pillars are positioned above carbon nanotubes bridging source and drain contacts.
- Gate voltages are applied below the nanotube to confine electrons and form a Double Quantum Dot (DQD).
NV Center Specification:
- NV centers are embedded in the diamond pillars, consisting of an electron spin (register) and a nuclear spin (memory).
- The use of $^{15}$N nuclear spins is proposed for cluster state generation due to its spin-1/2 nature.
- Critical Material Requirement: Isotopically purified diamond is necessary to reduce the influence of $^{13}$C nuclear spins, which cause magnetic field noise and decoherence.
Operational Regime (Pauli Blockade):
- A large bias voltage drives electron transport through the DQD via the cycle (0,1) $\leftrightarrow$ (1,1) $\leftrightarrow$ (0,2) $\rightarrow$ (0,1).
- The (1,1) $\rightarrow$ (0,2) transition is forbidden when the two electrons occupy specific triplet-like states ($\vert T_{0}\rangle$ or $\vert T_{+}\rangle$), enforcing the Pauli-blockade regime.
Entanglement Generation:
- Magnetic dipole-dipole coupling links the NV electron spins to the DQD valley-spin qubits.
- Microwave driving fields ($\Omega/2\pi = 0.6$ MHz) are applied to the NV electron spins.
- The system dynamically evolves toward the unique decoupled state, $\vert T_{0}\rangle \otimes \vert\Phi^{-}\rangle$, resulting in the steady-state maximal entanglement of the NV electron spins ($\vert\Phi^{-}\rangle$).
Scalability Implementation:
- The resulting electron spin entanglement is transferred to the $^{15}$N nuclear spins via hyperfine coupling ($A_{||}/2\pi \approx 3.03$ MHz) to realize a controlled-phase gate.
- Scalable 1D and 2D cluster states are generated by sequentially shifting the array of diamond nanopillars relative to the carbon nanotubes to implement controlled-phase gates on alternate nuclear spin pairs.

6CCVD Solutions & Capabilities

This research highlights the critical need for high-purity, precisely engineered diamond substrates. 6CCVD is uniquely positioned to supply the foundational materials necessary to replicate and scale this hybrid quantum architecture.

Applicable Materials for Quantum Hybrid Systems

Research Requirement	6CCVD Solution	Technical Justification
High Purity Substrate	Optical Grade Single Crystal Diamond (SCD)	Essential for minimizing intrinsic defects and ensuring long electron spin coherence times ($T_2$).
Noise Reduction	Isotopically Purified SCD	Required to reduce the natural abundance of $^{13}$C (nuclear spin 1/2), which is the primary source of magnetic field noise (decoherence) for NV centers.
Nanopillar Fabrication	Custom Thickness SCD Wafers	We offer SCD thickness from 0.1 µm up to 500 µm, ideal for subsequent etching processes (RIE/ICP) required to form the diamond nanopillars.
Scalable Architecture	Large-Area SCD Plates	We provide custom dimensions up to 125 mm (PCD) and large-area SCD plates, supporting the fabrication of 1D and 2D arrays (Fig. 4, 7).

Customization Potential for Hybrid Integration

The proposed architecture requires integrating diamond structures with carbon nanotubes and complex gate electrodes. 6CCVD provides the necessary material preparation and integration services:

High-Precision Polishing: The interface between the diamond nanopillars and the carbon nanotubes is critical. We offer ultra-low surface roughness (Ra < 1 nm for SCD) to ensure optimal coupling and minimize surface charge noise.
Custom Metalization Services: While the paper focuses on DQD voltage gates, future integration with superconducting circuits or complex gate structures will require contacts. 6CCVD offers in-house deposition of standard quantum metals, including Au, Pt, Pd, Ti, W, and Cu, tailored to specific lithography requirements.
NV Center Control: Although the paper discusses advanced scanning probe techniques for NV placement, 6CCVD can supply the high-quality, low-strain SCD substrates that are prerequisites for successful deterministic NV creation and positioning.

Engineering Support

6CCVD’s in-house team of PhD material scientists specializes in MPCVD growth parameters, defect engineering, and surface preparation for quantum applications. We offer consultation services to assist researchers in:

Material Selection: Determining the optimal isotopic purity and crystal orientation for maximizing NV center performance in hybrid systems.
Doping Strategy: Advising on controlled nitrogen incorporation (e.g., using $^{15}$N precursors) to ensure the desired NV concentration and nuclear spin properties for cluster state generation.
Interface Optimization: Consulting on surface termination and polishing requirements to facilitate robust integration with carbon nanotubes and gate electrodes.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Abstract One of the key challenges for the implementation of scalable quantum information processing is the design of scalable architectures that support coherent interaction and entanglement generation between distant quantum systems. We propose a nanotube double quantum dot spin transducer that allows to achieve steady-state entanglement between nitrogen-vacancy center spins in diamond with spatial separations up to micrometers. The distant spin entanglement further enables us to design a scalable architecture for solid-state quantum information processing based on a hybrid platform consisting of nitrogen-vacancy centers and carbon-nanotube double quantum dots.

Tech Support

Original Source

DOI: https://doi.org/10.1088/1367-2630/ab8e16

References

2010 - Quantum computers [Crossref]
2011 - Natural and artificial atoms for quantum computation [Crossref]
2007 - Linear optical quantum computing with photonic qubits [Crossref]
2008 - Quantum coherence and entanglement with ultracold atoms in optical lattices [Crossref]
2008 - Entangled states of trapped atomic ions [Crossref]
2008 - Superconducting quantum bits [Crossref]
1998 - Quantum computation with quantum dots [Crossref]
2007 - Spins in few-electron quantum dots [Crossref]
2008 - Coherent manipulation of single spins in semiconductors [Crossref]
2009 - Hybrid quantum devices and quantum engineering [Crossref]