Skip to content
6CCVD Research

Perspective on witnessing entanglement in hybrid quantum systems

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2021-09-13
JournalApplied Physics Letters
AuthorsYingqiu Mao, Ming Gong, Kae Nemoto, William J. Munro, Johannes Majer
InstitutionsNTT Basic Research Laboratories, Hefei National Center for Physical Sciences at Nanoscale
Citations1
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This perspective reviews the theoretical and experimental progress toward witnessing entanglement in hybrid quantum systems, specifically focusing on Nitrogen-Vacancy (NV-) spin ensembles (NVEs) coupled via superconducting circuits.

  • Core System: Hybrid architecture combining long coherence times of NV- spins in diamond with the fast, tunable scalability of superconducting resonators and transmon qubits.
  • Primary Goal: Rigorous experimental verification of entanglement between two remote, macroscopic NVEs.
  • Technical Challenge: NVEs contain greater than 1012 spins, leading to inhomogeneous broadening and complexity in state preparation and measurement.
  • Proposed Solution: Use superconducting qubits (Q1, Q2) as intermediaries to generate the required non-classical photonic Fock states and to retrieve entanglement information from the NVEs.
  • Key Operations: Entanglement generation is achieved via sequential ViSWAP (entangling) and iSWAP (state transfer) operations between the qubit and the ensembles.
  • Verification Method: Quantum state tomography is performed on the auxiliary qubits (Q1, Q2) after mapping the NVE states onto them, allowing for calculation of fidelity and concurrence.
  • Feasibility: Existing experimental results show strong coupling (NVE-resonator coupling of 9.6 MHz) and high-fidelity qubit operations (Tcoh = 13 ”s, readout fidelity 99.2%), suggesting the proposed scheme is feasible.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
NVE Spin Count> 1012spinsPer ensemble (orders of magnitude larger than previous studies)
Qubit-Resonator Coupling (gtrm/2π)44MHzTransmon qubit to transmission line resonator coupling
NVE-Resonator Coupling (gNVE/2π)9.6MHzCollective coupling strength (Ref. 19)
Qubit Coherence Time (Tcoh)13”sAllows iSWAP fidelity greater than 99%
Collective Polariton Decay Rate (Γ/2π)1.5MHzEstimates NVE iSWAP fidelity of 85%
Qubit Readout Fidelity99.2%Achieved in 88 ns using single-shot dispersive readout
iSWAP Operation Time (Flux Qubit)~30nsAchieved via vacuum Rabi oscillations (Ref. 15)
iSWAP Operation Time (Tunable Resonator)~500nsAchieved via adiabatic pulses (Ref. 13)
Niobium Chip Dimensions4 x 12mmSuperconducting chip coupling two diamonds (Fig. 1)
Entanglement Distance (Demonstrated)1.3kmPrevious work on single NV- centers (Ref. 27)

Key Methodologies

Section titled “Key Methodologies”

The proposed experimental verification relies on a sequence of controlled quantum operations using superconducting qubits to mediate entanglement and measurement of the NVEs.

  1. Photonic State Preparation: A superconducting qubit is used to generate the necessary initial photonic Fock state (non-classical source), which is required for entanglement generation, replacing standard coherent state inputs.
  2. State Transfer (iSWAP): Efficient quantum state transfer is achieved between the superconducting qubit and the NVEs using the iSWAP operation, typically realized through resonant coupling or vacuum Rabi oscillations.
  3. Entanglement Generation (ViSWAP): The ViSWAP operation (square root of iSWAP) is utilized to create entanglement between the qubit and the first NVE. Subsequent iSWAP operations transfer the state to the second NVE, resulting in entanglement between the two ensembles.
  4. Coupling Configuration: Two primary setups are proposed:
    • Setup 1 (Single Resonator Bus): All NVEs and qubits couple to a single, long resonator, maximizing coupling strength but risking frequency crowding.
    • Setup 2 (Individual Resonators): Each NVE couples to a separate resonator, which are then connected by a central tunable qubit (Q0), avoiding frequency crowding.
  5. Entanglement Retrieval: The quantum states of the entangled NVEs are mapped onto auxiliary superconducting qubits (Q1 and Q2).
  6. Verification via Tomography: Quantum state tomography is performed on the auxiliary qubits (Q1 and Q2) by measuring combinations of X, Y, and Z directions, allowing the full density matrix to be reconstructed and entanglement metrics (fidelity, concurrence) to be calculated.
  7. High-Fidelity Readout: Dispersive readout, potentially enhanced by a Purcell filter or parametric amplifier, is used to achieve rapid, high-fidelity single-shot measurement of the qubit states.

Commercial Applications

Section titled “Commercial Applications”

The successful realization of entanglement verification in this hybrid system demonstrates several key technologies critical for the advancement of solid-state quantum devices.

  • Quantum Computing: Provides a scalable platform for realizing hybrid quantum gates and architectures, combining the strengths of fast superconducting circuits and long-lived spin memories.
  • Quantum Memories: Enables the controlled storage and retrieval of quantum information in macroscopic solid-state spin ensembles (e.g., NV- centers), essential for robust quantum data storage.
  • Quantum Networking: Demonstrates the ability to entangle two macroscopic objects (diamonds) separated by centimeter distances, supporting the development of distributed quantum communication networks.
  • Quantum Sensing and Metrology: The ability to manipulate and witness entanglement in large spin ensembles is crucial for achieving quantum advantages in metrology protocols.
  • Quantum Transducers: The hybrid system acts as a transducer, allowing quantum information to be coherently transferred between microwave photons (superconducting circuits) and solid-state spins.
View Original Abstract

Hybrid quantum systems aim at combining the advantages of different physical systems and producing innovative quantum devices. In particular, the hybrid combination of superconducting circuits and spins in solid-state crystals is a versatile platform to explore many quantum electrodynamics problems. Recently, the remote coupling of nitrogen-vacancy center spins in diamond via a superconducting bus was demonstrated. However, a rigorous experimental test of the quantum nature of this hybrid system and, in particular, entanglement is still missing. We review the theoretical ideas to generate and detect entanglement and present our own scheme to achieve this.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2019 - Quantum supremacy using a programmable superconducting processor [Crossref]
  2. 2021 - An integrated space-to-ground quantum communication network over 4,600 kilometres [Crossref]
  3. 2010 - Quantum computers [Crossref]
  4. 2006 - A coherent all-electrical interface between polar molecules and mesoscopic superconducting resonators [Crossref]
  5. 2013 - Hybrid quantum circuits: Superconducting circuits interacting with other quantum systems [Crossref]
  6. 2015 - Quantum technologies with hybrid systems [Crossref]
  7. 2020 - Hybrid quantum systems with circuit quantum electrodynamics [Crossref]
  8. 2009 - Cavity QED based on collective magnetic dipole coupling: Spin ensembles as hybrid two-level systems [Crossref]
  9. 2010 - High-cooperativity coupling of electron-spin ensembles to superconducting cavities [Crossref]
  10. 2010 - Strong coupling of a spin ensemble to a superconducting resonator [Crossref]

Reads Similar to This

Varied Magnetic Phases in a van der Waals Easy-Plane Antiferromagnet Revealed by Nitrogen-Vacancy Center Microscopy Experimental investigation of multiple industrial wastes for geochemical carbon dioxide removal strategies The Useful Quantum Computing Techniques for Artificial Intelligence Engineers