Researchers at QuTech have demonstrated a key technology for linking quantum nodes over existing telecom fiber, by realizing quantum interference of photons emitted by quantum devices at telecom wavelengths. The technique also allows for compensating intrinsic differences between the quantum devices.
For a quantum network to function it uses a particular quantum effect between the nodes in the network called quantum entanglement. Practically entanglement means that the state of a second qubit (that can be ‘1’ or ‘up’) is dictated by the state of a first qubit (i.e. ‘0’ or ‘down’). This entanglement can be used for generating secret keys for private communication, or teleportation of a quantum state between far away parties. QuTech was the first to establish a multi-node network in the lab [link], by using optically active spins in solids.
Entanglement between network nodes is generated by first entangling the spins with flying photonic qubits and then interfering those photons in a connecting middle station. For scaling this approach to larger networks several major challenges arise.
The approach used to tackle the challenges is based on a technique called ‘quantum frequency conversion’. It uses a strong pump laser that interacts with the single photons inside a specially designed crystal, subtracting energy in the process. The exact frequency that the single photons end up with can be controlled by adjusting the pump laser frequency, giving the researcher the ability to precisely match different nodes over a network.
The team demonstrated this technique by building two remote quantum nodes based on the nitrogen-vacancy (NV) center in diamond. They showed that the resonant emission of the NV centers (around 637nm) can be faithfully converted very precisely to a wavelength in the telecom L-band (1588nm) by locking to a central reference laser, over a broad range of frequencies. This technique facilitates scaling by removing the difference in emission frequency between nodes that existed before the conversion.
The way the researchers verified their scheme is through an interference experiment. It is done by sending single photons from the remote nodes through their respective converters, towards a beam splitter, after which they are detected. For the photons that were expected to be indistinguishable from each other, a reduction in simultaneous clicks in the detectors was observed, in line with the theory describing the process.
Looking forward, the presented results are an important milestone towards achieving entanglement of NV center nodes over tens of kilometers of deployed telecom fibers. Great effort has already been taken to prepare the nodes for operation over such a long-range connection. Entanglement between solid state quantum processors over such distance would constitute a proof of principle for future entanglement-based quantum networks on a metropolitan/national scale.
The work has been published in PRX Quantum.