Quantum Networks with SiV Centers

Quantum networks are an exciting complement to quantum computing, as they allow us to send quantum states and share entanglement over long distances, enabling applications from quantum key distribution to connecting future quantum computers and quantum sensing of astronomical sources. In this lab, we use the silicon-vacancy (SiV) defect in diamond as an ideal platform to realize deployed real-world quantum networks and explore quantum information applications of distributing entanglement via light.

Intrinsic Two-Qubit Solid-State Qubit Platform

SiVs are created when two carbon atoms in diamond are replaced with a silicon atom, allowing it to intrinsically contain two qubits in the form of an electron spin and a long-lived nuclear spin qubit (> 2 seconds) from the ²⁹Si isotope; both spins are fully controllable into entangled two-qubit states with high fidelity (>99%) using microwave pulses. Additionally, control of weakly coupled ¹³C isotopes in the diamond matrix could further expand this two-qubit platform into a three-qubit register with two network memory qubits (²⁹Si and ¹³C).

The SiV lives in a fiber-coupled nanophotonic cavity to enhance its interaction with light, allowing incoming light to be entangled with the electron spin qubit. Our platform enjoys the advantageous combination of being able to generate entanglement that is both high-fidelity and high-efficiency, as well as scalability from the ability to manufacture hundreds of cavities on a single chip.

Multi-Node Quantum Networks and Memory-Enhanced Communication

We have demonstrated several key milestones in quantum networking on our platform, including quantum frequency conversion to the low-loss telecom band (1350 nm), storing an encoded photon sent from MIT Lincoln Labs over 50 km of deployed fiber onto the SiV memory, and entangling two SiVs in separate labs over 35 km of fiber around the Boston area, storing the entanglement for over 1 second.

One of our main research goals is on pushing the SiV platform to realize other milestones useful for near-term quantum networks and communication.

Ongoing research directions include:

Can we build a deployable quantum repeater node to enable long distance quantum networks while avoiding exponential photon loss?
Can we fabricate strain-tunable SiVs to avoid frequency conversion between different nodes’ SiVs and enjoy faster entanglement rates?
Can we increase our number of network nodes into a three-node quantum network and perform network protocols such as distillation of quantum states
Can we incorporate other quantum computing platforms like atoms into our network and achieve a hybrid quantum network?

Quantum Network Applications

In addition to improving the capabilities of our quantum network, our other main goal is to utilize the SiV platform to realize exciting quantum network applications and demonstrate proof-of-concept experiments in other areas like network security and astronomy.

Ongoing research directions include:

Blind Quantum Computing

Can we use photons entangled with our SiV qubits to enable a client with only measurement capability to perform quantum computation without leaking the contents of the computation?

Non-Local Interferometry

Quantum Telescope - Can we use two entangled nodes at a distance to perform interferometry of faint astronomical sources more efficiently?

Quantum Camera - Can we perform non-local operations on different “pixels” of distant sources using multiple SiVs?

References:

Entanglement-assisted non-local optical interferometry in a quantum network, Nature (2026) (arXiv)

The sensitivity of non-local optical measurements at low light intensities, such as those involved in long baseline telescope arrays, can be improved by using remote entanglement. Here, we demonstrate the use of entangled quantum memories in a quantum network of SiV centers in diamond nano-cavities to experimentally perform such non-local phase measurements.

Universal distributed blind quantum computing with solid-state qubits, Science (2025) (arXiv)

A new type of quantum computing, "blind quantum computing", allows private users to access remote quantum computers without revealing any details. Here, we use electron and nuclear spins over different labs as computational resources at the servers, and the client uses photons to mediate gates applying on the spins. We demonstrated the universal gate set for blind quantum computing and explored a small-scale blind quantum algorithm leveraging distributed computational resources.

Entanglement of Nanophotonic Quantum Memory Nodes in a Telecom Network, Nature (2024) (arXiv)

We entangled two electron and nuclear spins located in different labs with entanglement lasting over 1 second. By performing frequency conversion to the low-loss telecom band, we extend the entanglement length to 40 km of spooled fiber and over 35 km of deployed fiber in the Boston area.

Telecom Networking with a Diamond Quantum Memory, PRX Quantum (2024) (arXiv)

We demonstrated bidirectional quantum frequency conversion of single photons from the SiV’s native wavelength of 737 nm to the low-loss 1350 nm telecom band and vice versa while maintaining low noise and high indistinguishability. We used this to store telecom pulses sent over a 50 km deployed fiber link from MIT Lincoln Lab onto an SiV at Harvard with a storage fidelity of 87%.

Robust multi-qubit quantum network node with integrated error detection, Science (2022) (arXiv)

We expanded the SiV system to a two-qubit register using the ²⁹Si nuclear spin as a memory qubit lasting over 2 seconds. With a highly-strained device, we entangled the electron spin with a photon at 1.5K and implemented the PHONE gate to perform photon-nucleus entanglement with integrated error detection.

Efficient Source of Shaped Single Photons Based on an Integrated Diamond Nanophotonic System, Phys. Rev. Lett. (2022) (arXiv)

We created a new cavity design optimized for photon outcoupling efficiency and used the SiV as a deterministic single-photon source. We showed arbitrarily-shaped photon pulses with high efficiency (14.9%) and high purity (g⁽²⁾(0) = 0.0168), enabling the generation of streams of up to 11 consecutive photons in-fiber.

Experimental demonstration of memory-enhanced quantum communication, Nature (2020) (arXiv)

We stored two time separated phase-encoded photon pulses onto the SiV, and then read out the combined parity state of the encoded bits. This implements an asynchronous Bell-state measurement, a key component of quantum repeaters, giving us quantum communication rates surpassing loss-equivalent direct transmission.