A quantum network link that runs at the speed of a quantum computer
Preprint, not yet peer-reviewed | Experiment
It reaches one heralded entangled pair per millisecond between two different qubit platforms, four times the best photonic rate between two trapped ions, and gives a full error budget for the link.
Our understanding
Future quantum computers could be built from many smaller modules that have to talk to each other. Here, two very different quantum parts were linked: a single charged atom held in a trap, and a tiny flaw inside a diamond chip. The atom sends out single particles of light. Most of them are lost, but each one that bounces off the diamond and is detected confirms a new link between the two. About 1,000 links per second were made, as fast as one computing step of the machines now being planned. Error-corrected computing between modules needs 10 to 100 times more. The links are 88 % accurate. That is not yet good enough, and the authors list where the errors come from.
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The details
Key number: 1.03 ± 0.01 kHz ion-SiV Bell-pair rate at 87.9 ± 0.7 % fidelity, counting generation time only (612 ± 4 Hz including tomography readout).
What they did
A large quantum computer could be built from many modules that have to share entanglement with each other, the way servers in a data centre share a network. The team connected two very different kinds of qubit: a single barium ion held in an electric trap, and a silicon-vacancy colour centre, a defect inside a diamond chip with a built-in nanophotonic cavity, cooled to about 100 millikelvin. The ion emits a photon. Its colour (493.5 nm to 737.45 nm) and encoding are converted so the diamond can accept it. The photon is reflected by the diamond cavity only if the defect’s spin is in one state, and it is then detected. Each detected photon announces that the ion and the diamond defect are now entangled. The link produced 1.03 ± 0.01 thousand entangled pairs per second (1.03 kHz) with a fidelity of 87.9 ± 0.7 %, from about 1.1 million attempts per second, with 731 attempts per pair on average. This rate counts generation time only. Including the readout for state tomography, which is not optimised for speed, 612 pairs per second were generated and measured.
Why it matters
One entangled pair per millisecond is comparable to the cycle time of the trapped-ion processors now being planned. It is four times the rate of the fastest photonic link between two trapped ions reported so far (250 pairs per second), although here the second node is a diamond memory, not a second ion processor. The trick is to let each platform do what it is good at: the ion is a well-controlled qubit that emits photons at a high rate, and the diamond defect is a quantum memory that interacts with photons efficiently inside its cavity. Because the memory reflects the photon, a single detected photon is enough to confirm entanglement, and no two-photon interference is needed.
What is still open
The paper itself gives the target: for continuous error-corrected operations between modules, estimates call for 10 to 100 kHz of entanglement at an error rate about ten times that of local two-qubit gates. This link is at 1 kHz and 12 % infidelity. The authors break the infidelity down by subsystem (ion 4.3 points, frequency conversion 2.9, memory 3.9, lost heralding photons 1.9), write that none of the dominant errors is fundamental, and expect better ion-to-fiber coupling, a more efficient signal chain and a phase gate on the memory to raise the rate above 5 kHz. How far can rate and fidelity be pushed together, given that fewer lost photons would improve both at once? (The last clause is our inference from the trade-off between rate and fidelity shown in the paper.)
The authors note: “For known algorithms and plausible near-term qubit fidelities and quantum error correction schemes, estimates of the entanglement rate needed to support continuous logical operations between modules range from 10 to 100 kHz” (arXiv:2610.10705v2, p. 1)
