Preventing signal loss and garbling are key goals in developing effective quantum networks and eventually a quantum Internet. Today, AWS and Harvard researchers reported a new optical interface method that can operate in a wide range of temperatures including the cryogenic temperatures required by many quantum devices. In this instance, the researchers interfaced optical fibers with a “diamond” chip that acts as quantum memory which could be the heart of a quantum repeater.
“We’re announcing a first-of-its-kind advancement in photonic interconnection that overcomes traditional fabrication constraints, demonstrating a fiber-device interface that can withstand multiple cycles of cooling from room temperature to cryogenic temperatures – and back – without introducing additional losses,” write AWS researchers Denis Sukachev, Chawina De-Eknamkul, and Beibei Zeng in an AWS blog. “This packaged photonic interface operates at cryogenic temperatures and has record a low insertion loss of -0.4 dB (10%).”
The new approach was developed under the AWS Center for Quantum Networking’s research alliance with the Harvard Quantum Initiative (HQI). Their work is being published today in Applied Physics Letters magazine (Appl. Phys Lett. 123, 161106 (2023)).
AWS says the advance is an important step towards “building a quantum repeater, which corrects for photon loss without disrupting the quantum nature of the information that it carries. It catches and stores (rather than measure) the encoded qubits to overcome photon losses in the communication channel.”
There are, of course, many challenges in efforts to develop repeaters for quantum networks. AWS has been working hard and making progress on developing Diamond-vacancy quantum memory chips for use in quantum repeaters. (See HPCwire coverage, Catch the Flying Qubit – AWS Center for Quantum Networking)
Broadly speaking, all telecommunication information traveling through fibers is encoded in optical pulses, which need to be generated, modulated, transmitted, and detected. At each step, light is transferred from a fiber to an electro-optical device which performs necessary operations, introducing losses. For long haul communication, we compensate for these losses using additional hardware – increasing the cost and the energy consumption of the network.
The bloggers note that in quantum communications, these losses become more detrimental due to the use of weak optical signals and fundamental physics constraints associated with traditional amplification. “These challenges are compounded by the extreme temperature requirements of many quantum devices, which generally operate at -270C or colder. To date, traditional fabrication techniques haven’t been able to withstand extreme environments while also preserving every photon in an optical interface,” write the researchers. (Figure from latest work below)
Here’s brief excerpt from the blog describing the advance:
“Light in optical fibers is tightly confined to a region with the diameter of only a few micrometers (a human hair is ~100 micrometer in diameter). As a result, low-loss interfaces require precise (e.g., one micrometer) alignment of components, which can easily be disrupted, or potentially destroy the device.
“This becomes especially challenging for low-temperature operations used by many quantum devices. As components cool down, materials shrink at different rates depending on their composition – meaning that optical interfaces between different materials are almost certain to become misaligned as they cool. This is similar to the way that different materials in a bridge can be damaged by thermal expansion. To prevent this, bridge pavements have expansion joints which allow the pavement to expand and contract without breaking apart.
“To bypass this issue, alignment of an optical device with an optical fiber often happens after components have been cooled down. We can accomplish this with micro motors that allow optical fibers to be moved with an accuracy better than 1 micrometer while at cryogenic temperatures. This approach is critical to many proof-of-concept demonstrations in academia, but it’s not feasible for large scale deployments due to steep hardware cost (and reliability) issues. Components can also easily get out of alignment because of minuscule external factors, like mechanical vibration from traffic on nearby roads.
“In the newly published paper, AWS and Harvard scientists demonstrated cryogenic-compatible packaging between photonic devices on diamond chips and optical fibers, using an adiabatic coupling between a device and a fiber. In this method, a tapered end of the optical fiber is put in physical contact with a tapered end of the optical device allowing light to gradually pass through the interface (see Figure) with insertion losses smaller than -1dB. More importantly, strong van der Waals forces between tapered ends give the interface immunity against small displacements of the components: both tapered ends bend slightly, preserving the low insertion losses – a key feature to overcome thermal expansion issues.”
The AWS/Harvard work demonstrates a promising approach to building interfaces for large scale deployment.
The researchers confirmed the temperature stability of the package by repeatedly cooling it down to the liquid nitrogen temperature (-200C) and warming it back to room temperature. Finally, the package was cooled down to close to near absolute zero (-273C) inside a dilution refrigerator. During these thermal cycles, the insertion loss did not change, ‘proving the cryo-compatibility of the package.” The demonstrated insertion loss of -0.4dB at -273C is on-par with what has been previously achieved using micro motors for active fiber alignment at cryogenic temperatures.
The latest advancement, reports AWS, will not only enable building fundamental blocks of quantum networks, like deployable quantum repeaters, but could make classical telecom networks faster and more efficient.