Harvard SiV two-node Boston-metro loop

Harvard SiV two-node Boston-metro loop

Two silicon-vacancy (SiV) quantum-memory nodes — diamond chips with engineered colour-centre defects coupled to nanophotonic cavities — entangled across 35 km of installed Boston-area telecom fibre. The first entanglement distribution between solid-state nanophotonic memory nodes through an urban deployed-fibre link.

Operator
Lukin / Lončar / Park groups, Harvard; fibre route through the Boston urban environment (collaboration framing varies in coverage; the deployment ran on commercially-installed dark fibre, not lab spools)
Location
Boston–Cambridge metro, Massachusetts, USA
Year
Published May 2024 (work conducted prior)
Technology
Entanglement-based; SiV colour-centre memories in diamond nanophotonic cavities; telecom-wavelength conversion to the C-band for fibre transport
Scale
Two nodes; 35 km deployed fibre loop (one continuous urban route)
Status
Research demonstration
Commercial model
Academic / sponsored research; not open to outside customers

What it is

Each node is a diamond chip with a silicon-vacancy centre embedded in a nanophotonic cavity. The SiV's electronic spin is the stationary qubit; the cavity boosts the optical interaction so a single photon can carry the SiV's state out of the diamond and into a fibre. Photons from each node are converted to the telecom C-band and routed through 35 km of installed Boston-metro fibre to a midpoint station that performs the heralded-entanglement Bell-state measurement. A successful heralding signal projects the two distant SiV memories into an entangled state. Knaut et al. 2024

The deployed-fibre context is the key result. Earlier nanophotonic-memory entanglement demonstrations ran on spools of fibre in a single lab; the Knaut et al. work moves the same architecture onto fibre with installed splices, real temperature drift, and urban-environment phase noise. The 35 km figure is the fibre route length, not a straight-line distance.

Verified claims

  • Two-node entanglement on a 35 km deployed Boston-metro fibre loop — published in Nature 629.573 (2024). Knaut et al. 2024
  • SiV nanophotonic-cavity memory nodes — silicon-vacancy colour centres in diamond, evanescently coupled to a sculpted nanophotonic cavity. Knaut et al. 2024
  • Heralded entanglement protocol at a midpoint Bell-state measurement — the architecture is the standard MeetInTheMiddle pattern, not direct transmission of one half of a Bell pair. Knaut et al. 2024
  • Telecom-wavelength conversion is part of the stack — the SiV emission is converted to the C-band so that the photons can survive 35 km of fibre with manageable loss. Knaut et al. 2024

Things to note

  • Two-node research demonstration. The work entangles two nanophotonic memory nodes through field fibre and characterises the rates and fidelities; there is no outside-user service or commercial offering.
  • 35 km is the deployed-fibre route length. The fibre loops through the Boston metropolitan area; the two nodes are co-sited at Harvard, with the long fibre serving as the channel.
  • The 35 km figure is the deployed-fibre route length, not a free-space distance record across all platforms. Earlier all-photonic and trapped-ion demonstrations reach further. The contribution is the combination of solid-state nanophotonic memories with a deployed urban fibre route.
  • Coverage describing this as a "quantum internet between buildings" refers to the fibre topology, not multiple end-user sites — both nodes run in a single laboratory.