Introducing a new temperature-resistant packaging technique for optical devices

This post was contributed by Denis Sukachev, Chawina De-Eknamkul, and Beibei Zeng – research scientists from the AWS Center for Quantum Networking.

Telecommunications is the backbone of our modern lives, supporting commerce, travel, and social life across cities, countries, and continents. All this activity is enabled by sharing information between computers connected with optical fibers. Information traveling through these 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.

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.

Today, 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. Published in the Applied Physics Letters magazine (Appl. Phys Lett. 123, 161106 (2023)), this packaged photonic interface operates at cryogenic temperatures and has record a low insertion loss of -0.4 dB (10%). It was developed under the AWS Center for Quantum Networking’s research alliance with the Harvard Quantum Initiative (HQI).

First, some background and terminology

Today’s announcement is an important step towards our center’s goal to build 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.

A central component of this device is a stationary qubit — a quantum memory which can communicate with photons and store information. The most promising stationary qubit systems are built on chips and need to interface efficiently with these fibers. This advancement will not only enable building fundamental blocks of quantum networks, like deployable quantum repeaters, but could make classical telecom networks faster and more efficient.

You’ll sometimes hear us refer to techniques that integrate different components together with optical fibers as “packaging” of optical devices. Packaging is a key part of the deployment of almost all optical devices and is an active area of research for telecommunication applications.

Finally, quantum networks are an emerging technology that distribute entangled quantum bits to geographically separated users. These networks can enable new communication tasks such as the generation of shared cryptographic keys whose security does not depend on algorithmic complexity. This security can supplement and extend the current state-of-the-art in public key cryptography and future-proof it against advances in computing hardware. Quantum networks can also provide private access to cloud-based third-party quantum computers, where customers can run computations without exposing their code and data. You can read more about this in our post introducing these ideas.

How it works

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.

After optical alignment is done, the components are fixed together with temperature-stable epoxy which is precisely dispersed at carefully-chosen locations along the tapered adiabatic interface (see Figure again), resulting in a permanent packaging of an optical fiber and diamond photonic crystal with an insertion loss of -0.4 dB.

Figure. (a) Illustration of the fiber-to-chip packaging concept. A free-standing diamond waveguide has a tapered end which is used to create an adiabatic coupling interface with a tapered fiber. The tapered optical fiber is aligned to the tapered waveguide end and secured it in place using UV epoxy droplets. The inset shows a microscope image of the packaged device (diamond chip is 4 mm x 4 mm). (b) Scanning electron microscope image of a packaged device. Out of three tapered diamond devices (red), the middle one is attached to a tapered optical fiber (green) and the grey area is the diamond surface. The white scale bar is 5 micrometers.

The team 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.

Potential for commercial application

The low-loss, cryo-compatible optical packaging technique developed by AWS and Harvard scientists is the foundation for a variety of telecom applications. For example, after first developing this packaging technique for diamond nanophotonic devices, AWS scientists applied the technique to package optical fibers with lithium niobate waveguides, which are the basis for high-speed optical modulators used in telecommunication networks. More importantly, highly efficient, and cryo-compatible interconnects will allow deployment of multiplexed quantum network nodes based on solid-state quantum memories by eliminating the costly and complex equipment, like micro motors, currently in use for building cryogenic optical interfaces.

Research reported in this publication was supported as part of the AWS Center for Quantum Networking’s research alliance with the Harvard Quantum Initiative (or HQI).