A Phononic Bus for Coherent Interfaces Between a Superconducting Quantum Processor, Spin Memory, and Photonic Quantum Networks
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-03-18 |
| Journal | arXiv (Cornell University) |
| Authors | Tomas Neuman, Matt Eichenfield, Matthew E. Trusheim, Lisa Hackett, Prineha Narang |
| Citations | 4 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research introduces a novel hybrid quantum architecture designed to bridge superconducting (SC) quantum processors (QPUs) with solid-state spin quantum memories (QMs) and photonic networks, mediated by a phononic (acoustic) bus.
- Core Value Proposition: The architecture combines the high speed and fidelity of SC gates with the long coherence times of solid-state artificial atom (AA) spin qubits (SiV- in diamond) and their associated 13C nuclear spin memories.
- Performance Achievement: Numerical simulations predict quantum state transduction fidelity exceeding 99% (infidelity < 1%) at a MHz-scale bandwidth, sufficient for integration into fault-tolerant systems.
- Transduction Mechanism: State transfer is achieved via a cascaded process: SC Qubit ↔ Phonon Bus (piezoelectric coupling) ↔ AA Electron Spin (spin-strain coupling).
- Key Component Design: High-quality (Q > 107) mechanical cavities were designed (e.g., Silicon/Diamond hybrid) to maximize zero-point strain fluctuations, enabling strong coupling (gorb/2π up to 5.4 MHz) between a single phonon and the electron spin.
- Memory and Networking: The electron spin acts as a network hub, coupling to long-lived 13C nuclear spin memory (T2 > 1 second) and providing a high-fidelity optical interface for long-range quantum networking via heralded entanglement.
- Scalability: The proposed architecture supports a large quantum memory capacity, estimated to reach kqubits (thousands of qubits) by multiplexing SC QPUs to multiple AA/nuclear spin registers.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Transduction Fidelity (F) | > 0.99 | Dimensionless | SC Qubit to Electron Spin (Target) |
| Transduction Infidelity (1 - F) | < 0.1 | % | Achievable with optimized parameters |
| Transduction Bandwidth | MHz-scale | Hz | Overall system speed |
| SC Qubit Coupling (gsc-p/2π) | 50 | MHz | Tunable electro-mechanical coupling (Assumed) |
| SC Qubit Decoherence (γsc/2π) | 10-5 (10) | GHz (kHz) | Conservative estimate (T1 ~ 100 µs) |
| Electron Spin Decoherence (γe/2π) | 10-5 (10) | GHz (kHz) | Required for >99% fidelity |
| Nuclear Spin Decoherence (γn/2π) | 1 | Hz | 13C memory (T2 ~ 1 second) |
| Effective Phonon-Spin Coupling (gp-e/2π) | 1 | MHz | Rate used in high-fidelity simulation |
| Bare Phonon-Spin Coupling (gorb/2π) | 5.4 | MHz | Calculated maximum for Si/Diamond cavity |
| Mechanical Quality Factor (Qp) | ~ 107 | Dimensionless | Required for low phonon decay (γp/2π = 10-7 GHz) |
| Si/Diamond Cavity Frequency (ωp/2π) | 2.0 | GHz | Mechanical mode resonance |
| All-Diamond Cavity Frequency (ωp/2π) | 17.2 | GHz | Mechanical mode resonance |
| Diamond Layer Thickness | 100 | nm | Heterogeneous integration onto Silicon |
| Zero-Point Strain (Required) | 10-9 to 10-8 | Dimensionless | Necessary for strong coupling |
| Optical Quality Factor (Qopt) | 106 | Dimensionless | All-diamond optomechanical cavity |
| Optical Wavelength (λopt) | 732 | nm | SiV- optical addressing |
| Operating Temperature | ~mK | Kelvin | Required for low thermal occupation of modes |
Key Methodologies
Section titled “Key Methodologies”The transduction scheme relies on integrating and controlling four distinct quantum interfaces (QI1-QI4) using advanced material design and control pulses:
- Phononic Cavity Design and Simulation: High-Q mechanical resonators were designed using finite-element numerical simulations (Comsol Multiphysics) based on the continuum description of elasticity. Two primary designs were analyzed: a Silicon phononic crystal cavity capped with a 100 nm diamond layer, and an all-diamond optomechanical cavity.
- SC Qubit to Phonon Coupling (QI1): Tunable electro-mechanical coupling (gsc-p) is achieved via piezoelectric transduction. The coupling is modulated in time using a smooth, time-symmetric pulse (sech-shaped) to implement a “pitch-and-catch” scheme for high-fidelity state transfer into the mechanical mode.
- Strain Concentration: Cavity geometries (e.g., thin diamond constriction) were optimized to concentrate the elastic energy density of the mechanical mode, maximizing the zero-point strain (εxx, εyy) at the SiV- defect location.
- Phonon to Electron Spin Coupling (QI2): An effective controllable Jaynes-Cummings interaction (gp-e) is engineered between the mechanical mode strain and the SiV- electron spin fine-structure states. This control relies on applying external static or oscillating magnetic fields, or using microwave/optical Raman drives.
- State Transfer Protocol: The overall SC-to-Spin transduction is achieved by applying two sequential, time-delayed SWAP-like pulses (gsc-p(t) followed by gp-e(t)) to transfer the quantum state coherently through the transient phonon mode.
- Electron Spin to Nuclear Spin Memory (QI3): The electron spin state is transferred to a nearby 13C nuclear spin (QM) via the hyperfine interaction. This is achieved using a sequence of controlled and uncontrolled rotations (SWAP gate) driven by external microwave fields and dynamical decoupling sequences.
Commercial Applications
Section titled “Commercial Applications”The proposed hybrid architecture addresses critical bottlenecks in current quantum technologies, enabling several high-impact applications:
- Fault-Tolerant Quantum Computing (FTQC): Provides the essential components for a scalable QPU: fast, high-fidelity processing (SC circuits) combined with long-lived, high-fidelity memory (13C nuclear spins).
- Quantum Internet and Networking: The SiV- optical interface enables the device to serve as a quantum network node, facilitating long-distance quantum communication via heralded entanglement and quantum state teleportation.
- Distributed Quantum Architectures: The phononic bus and multiplexing capabilities allow a central SC QPU to access a large, distributed quantum memory register (estimated kqubits capacity), overcoming limitations in SC qubit number and connectivity.
- Quantum Transduction: Provides a high-bandwidth, high-fidelity method for converting quantum information between microwave (SC) and optical (photonic network) domains, a key requirement for heterogeneous quantum systems.
- Advanced Quantum Sensing: The ability to prepare highly entangled Greenberger-Horne-Zeilinger (GHZ) states of multiple electron spins coupled to the phononic cavity can be used to boost phase sensitivity to strain, potentially enabling ultra-precise mechanical or inertial sensors.
- Solid-State Qubit Integration: Drives the development of advanced material integration techniques, specifically the heterogeneous integration of high-quality diamond nanomembranes hosting SiV- defects onto silicon substrates.
View Original Abstract
We introduce a method for high-fidelity quantum state transduction between a superconducting microwave qubit and the ground state spin system of a solid-state artificial atom, mediated via an acoustic bus connected by piezoelectric transducers. Applied to present-day experimental parameters for superconducting circuit qubits and diamond silicon vacancy centers in an optimized phononic cavity, we estimate quantum state transduction with fidelity exceeding 99% at a MHz-scale bandwidth. By combining the complementary strengths of superconducting circuit quantum computing and artificial atoms, the hybrid architecture provides high-fidelity qubit gates with long-lived quantum memory, high-fidelity measurement, large qubit number, reconfigurable qubit connectivity, and high-fidelity state and gate teleportation through optical quantum networks.