A phononic interface between a superconducting quantum processor and quantum networked spin memories
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-08-04 |
| Journal | npj Quantum Information |
| Authors | Tomáš Neuman, Matt Eichenfield, Matthew E. Trusheim, Lisa Hackett, Prineha Narang |
| Institutions | Massachusetts Institute of Technology, Harvard University |
| Citations | 39 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research proposes and models a hybrid quantum architecture designed to overcome scalability and connectivity limitations in current superconducting (SC) quantum processors by integrating solid-state spin memories.
- Core Value Proposition: Achieves high-fidelity, high-bandwidth quantum state transduction between a Superconducting (SC) microwave qubit and a Silicon-Vacancy (SiV-) electron spin in diamond.
- Performance Achievement: Estimated quantum state transfer fidelity (F) exceeds 99% (infidelity < 1%) at a MHz-scale bandwidth, leveraging experimentally informed parameters.
- Transduction Mechanism: The transfer is mediated by a phononic (acoustic) bus, coupled to the SC qubit via a tunable piezoelectric transducer and to the SiV- spin via strong spin-strain coupling.
- Cavity Design: Optimized mechanical cavities (including a Silicon-Diamond hybrid design) were modeled, achieving bare phonon-spin coupling rates (gorb/2π) up to 5.4 MHz and high mechanical quality factors (Q ~ 107).
- Quantum Memory Integration: The SiV- electron spin is coupled to nearby 13C nuclear spins, providing long-lived quantum memory with coherence times exceeding one minute.
- Network Connectivity: The architecture enables high-fidelity state teleportation across optical quantum networks using the SiV- optical transitions for heralded entanglement.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Target Transduction Fidelity (F) | > 0.99 | (unitless) | SC Qubit to Electron Spin |
| Transduction Infidelity (1-F) | < 0.001 | (unitless) | Achieved with high-Q cavity (speculative) |
| Transduction Bandwidth | MHz-scale | Hz | Operational speed |
| SC Qubit Decay Rate (γsc/2π) | 10 | kHz | Conservative estimate |
| Phonon-SC Qubit Coupling (gsc-p/2π) | 10 | MHz | Maximum achievable rate |
| Effective Phonon-Spin Coupling (gpe/2π) | 1 | MHz | Rate used for high-fidelity simulation |
| Bare Phonon-Spin Coupling (gorb/2π) | 5.4 | MHz | Calculated max for Si-Diamond cavity |
| Mechanical Quality Factor (Q) | ~107 | (unitless) | Required for high-fidelity transfer (γp/2π = 10-7 GHz) |
| Electron Spin Dephasing (γe/2π) | 10-5 | GHz | Upper bound for >99% fidelity |
| Nuclear Spin Coherence Time | > 1 | Minute | 13C Quantum Memory |
| Si-Diamond Cavity Frequency (ωp/2π) | 2.0 | GHz | Mechanical resonance (Fig. 2) |
| All-Diamond Cavity Frequency (ωp/2π) | 17.2 | GHz | Mechanical resonance (Fig. 3) |
| Diamond Layer Thickness (Si-Di Hybrid) | 100 | nm | Heterogeneous integration layer |
| Optical Quality Factor (Qopt) | 106 | (unitless) | All-Diamond cavity optical mode |
Key Methodologies
Section titled “Key Methodologies”- Quantum Master Equation Modeling: The system dynamics (SC qubit, phonon, electron spin) were solved using the Liouville equation of motion for the density matrix (ρ), incorporating Lindblad superoperators to account for T1 decay and T2 dephasing rates (γsc, γp, γe).
- State Transfer Protocol Simulation: High-fidelity transfer was achieved by simulating a sequence of two time-dependent SWAP gates. The coupling rates (gsc-p and gp-e) were modulated using a smooth hyperbolic secant pulse profile to minimize coherent reflections and account for bandwidth limitations.
- Finite-Element Method (FEM) Simulation: COMSOL Multiphysics was used to perform elastodynamics simulations of the mechanical cavities. This determined the distribution of elastic energy density and the zero-point strain field (gorb) necessary for strong phonon-spin coupling.
- Cavity Architecture Design: Two primary designs were analyzed:
- Silicon-Diamond Hybrid: A mechanical resonator embedded in a silicon phononic crystal, capped with a 100 nm diamond layer, optimized for high Q and coupling to a microwave circuit via a waveguide.
- All-Diamond Optomechanical: A diamond beam structure with elliptical holes, designed to simultaneously support high-Q phononic and optical modes for efficient SiV- addressing and strain concentration.
- Fidelity Optimization: The state-transfer fidelity (F) was calculated as the trace overlap between the initial and final density matrices, and maximized by adjusting the time delays between the applied coupling pulses for various combinations of coupling rates (gpe) and decoherence rates (γe, γp).
Commercial Applications
Section titled “Commercial Applications”The proposed SC-AA hybrid architecture enables critical functionalities for next-generation quantum technologies:
- Fault-Tolerant Quantum Computing: Provides a pathway to scalable quantum processors by combining the high gate fidelity of SC circuits (QPU) with the long coherence times of solid-state memories (AA spins).
- Quantum Network Nodes: SiV- centers act as high-fidelity quantum network ports, enabling deterministic state and gate teleportation over long distances via optical fiber interconnects.
- Modular Quantum Memory: Development of dense, addressable quantum memory registers based on 13C nuclear spins, providing minutes-long storage capacity accessible by the SC QPU.
- Quantum Transduction Devices: Commercialization of high-efficiency, high-bandwidth quantum transducers capable of converting microwave quantum signals (SC domain) to optical signals (telecom domain) using the phononic intermediary.
- Advanced Nanophotonics and Nanophononics: Utilization of optimized Silicon/Diamond heterogeneous integration techniques for manufacturing ultra-high-Q mechanical and optomechanical resonators.