Photonic Interconnect

What Is a Photonic Interconnect?

A photonic interconnect is a link that uses photons to carry quantum information, or to create entanglement, between separate quantum systems, such as two processors or two modules of one machine. Photons suit the job because they travel through optical fiber or free space with little interaction with their surroundings, even at room temperature.

The term also appears in classical computing, where optical links move data between chips and racks. Here it means the quantum sense: a channel that builds shared quantum correlations across a gap. It is a core ingredient of distributed quantum computing and of modular quantum computing, where several smaller processors are joined instead of building one large one.

How a Photonic Interconnect Creates Entanglement

Most designs use matter qubits as memory and photons as messengers. A typical heralded entanglement sequence runs as follows:

Each node emits a photon whose polarization, frequency or arrival time is entangled with the state of its matter qubit.
The two photons travel to a midpoint station and overlap on a beam splitter.
Single-photon detectors register the photons. Certain click patterns herald that the two matter qubits are now in an entangled Bell state.
If no heralding pattern appears, the attempt is discarded and repeated.

Because the herald announces success, photon loss lowers the rate of entanglement rather than corrupting the result. With linear optics alone, only two of the four Bell states can be told apart, so in common two-photon schemes the success probability per attempt is roughly

\[ P_{\text{success}} \lesssim \tfrac{1}{2}\,\eta^{2} \]

where \( \eta \) is the probability that a photon emitted by one node is detected at the midpoint. The \( \eta^{2} \) factor is why collection and transmission efficiency matter so much.

Once a remote pair exists, local operations and classical communication can use it for quantum teleportation of a state or a gate between the nodes. In 2025, Main and colleagues at Oxford used this approach to run a small instance of Grover's algorithm across two trapped-ion modules joined by an optical link.

What Limits Photonic Interconnects Today

Several physical effects set the limits:

Photon collection. An emitter radiates in many directions, so coupling its photon into a single fiber mode needs lenses, cavities or both.
Fiber loss. Telecom fiber attenuates light by roughly 0.2 dB per km near 1550 nm, but many atomic transitions emit at wavelengths with far higher loss, so frequency conversion is often needed.
Speed mismatch. Remote entanglement is typically much slower than local gates, so the link often sets the pace.
Waiting time. Matter qubits must stay coherent until the herald arrives, which ties link design to memory lifetimes.
No amplification. The no-cloning theorem forbids copying an unknown quantum state, so a lossy channel cannot be boosted like a classical signal. Longer distances call for a quantum repeater that chains shorter entangled links.

Photonic Interconnects Across Qubit Platforms

Each platform needs its own interface between a stored qubit and a photon.

Platform	Photon interface	Main challenge
Trapped ions	Ion emits a photon entangled with its internal state; remote ion entanglement shown by Moehring and colleagues in 2007	Low collection efficiency, modest rates
Diamond vacancy centers	Spin-photon entanglement; remote entanglement over 1.3 km (Hensen and colleagues, 2015)	Much emission falls outside the useful spectral line
Neutral atoms	Atoms in separate optical cavities entangled through a photon (Ritter and colleagues, 2012)	Combining cavities with large atom arrays
Superconducting qubits	Microwave photons natively; an optical link needs a microwave-to-optical transducer	Transducer efficiency and added noise

Where Neutral Atoms Fit, and Where They Do Not

Inside a single array, neutral-atom computers get connectivity by physically moving atoms with optical tweezers and by using Rydberg interactions for gates. Gates within an array therefore do not need a photonic link.

A photonic interconnect serves a different goal: joining separate arrays or vacuum systems. Photon-mediated entanglement of neutral atoms has been shown with atoms in cavities (Ritter and colleagues, 2012; Hofmann and colleagues, 2012, with atoms about 20 m apart). Integrating such cavities with large tweezer arrays remains a research problem. This entry describes the research field, not a capability of any QuEra product.

FAQ

Is a photonic interconnect the same as photonic quantum computing?

No. Photonic quantum computing encodes qubits in photons and computes with them. A photonic interconnect uses photons only as links between qubits stored in matter, such as ions, atoms or spins, which do the computing.

Does a photonic interconnect allow communication faster than light?

No. The heralding signal and the classical messages needed to complete teleportation travel no faster than light. Entanglement alone carries no usable message.

Why not send a qubit's state directly as a photon?

Direct state transfer was proposed by Cirac and colleagues in 1997, but a lost photon destroys the state. Heralded entanglement tolerates loss, since failed attempts are simply retried before any data is committed.

How fast are photonic links today?

Reported rates vary widely with platform and design, but remote entanglement is generally much slower than local gates. Raising rate and fidelity together is a main engineering goal.

Key Takeaways

A photonic interconnect links separate quantum modules by creating entanglement between matter qubits through photons.
Heralding makes loss cost speed rather than accuracy, but link rates remain well below local gate speeds.
Each platform needs its own qubit-to-photon interface, and superconducting qubits additionally need transducers.
Neutral-atom arrays handle connectivity inside an array by moving atoms; photonic links are a research route to joining modules.