Ingrid Fadelli is a writer for the website Phys.org.

A scalable quantum memory with a lifetime over 2 seconds and integrated error detection
A scanning electron microscope image (courtesy of the AWS Center for Quantum Networking) of an array of nanophotonic quantum memories on a diamond chip. The photonic devices are millionths of an inch wide. A combined team of Harvard and AWS scientists improved quantum memories by raising the operating temperatures of their system. The AWS-Harvard collaboration examined a leading quantum memory platform, silicon-vacancies in diamond crystals. For the silicon-vacancy in diamond, decoherence is driven by the interaction between a strain-sensitive transition and phonons which start to appear in the diamond at a temperature of 1 Kelvin. Using strain in the diamond lattice, researchers were able to increase the energy of the transition so that only higher energy phonons (which appear at 20 Kelvin)—ensuring that no thermal phonons were able to drive this transition even at 4 Kelvin. Operation at 4 Kelvin is a commercially critical milestone because it enables a transition away from specialized ultra-low-temperature cryogenics to cheaper, more reliable technologies. In the same paper, researchers also demonstrated carefully quantum gates between the silicon-vacancy electron qubit (which interacts easily with its environment—including light) and a more insensitive qubit encoded in the spin of the silicon nucleus. Utilizing these interactions, the researchers were able to demonstrate high fidelity information exchange between light, the electron qubit, and the nuclear qubit. These results show that the silicon-vacancy has deterministic access to multiple memory qubits within a single node—one of the key requirements for a scalable quantum network. Credit: Stas et al.

Classical computer memories can be stored as quantum states. There are a number of challenges that need to be overcome before quantum memory can be implemented on a big scale.

A promising quantum memory capable of error detection and with a lifetime or coherence time greater than 2 seconds has been developed by researchers at the center. This memory could be used to create quantum networks.

Users who are in different geographic locations can receive entangled quantum bits via a quantum network. qubits are usually used as single particles of light.

"These qubits need to be routed and processed, both to distribute them to different users and to overcome the distance limitations imposed by the transmission loss in fiber optics," David Levonian said. A quantum memory is a small quantum computer that can catch and store quantum bits that are not measured, as measuring them would destroy anyentanglement that they have. If needed, the stored quantum bits can be re-encoded.

Physicists and engineers have come up with a number of different systems that could act as quantum memory and allow the implementation of quantum networks, such as rare earth ion embedded in glass. Levonian and his colleagues used SiVs to create the system. These are quantum bits that are made up of electrons and are embedded in diamonds.

A scalable quantum memory with a lifetime over 2 seconds and integrated error detection
Figure from the Science paper: A) The quantum levels of the silicon-vacancy center in diamond. Electrical control pulses "MW" and "RF" can flip the nuclear and electron magnetic spins between up and down. B) and C) An electron microscope image of the device, surrounded by gold control wires. The silicon-vacancy is embedded in a patterned diamond wire that channels photons to it. Depending on the quantum state of the electron, the photons are reflected in different ways, enabling quantum information to be stored in the electron spin. These advances pave the way for widely deployed, reliable quantum repeaters that will enable eavesdropper-proof communications and private access to quantum computers. Researchers at Harvard and AWS continue to work towards improving availability and quality of qubit host materials, improving the scalability of repeater hardware, and developing theoretical understanding of networks and different qubits that will be required to make this technology publicly available. Credit: Stas et al. (Science, 2022)

Levonian said that they build guides that can concentrate the light near the SiVs. Our system is similar to the optical modulators that transmit most internet traffic. Our quantum memories can either transmit or reflect light depending on whether they are on or off. Ours can be switched on and off by a single electron and can be in a quantum superposition.

The objectives of the study were twofold. The idea of using the magnetic fields of individual nuclei as quantum memories was the first to be explored. The second goal was to make a quantum memory that could work at higher temperatures.

Levonian said that the study's first objective is a common one in the sub field of solid-state quantum memories. electron spins are sensitive to magnetic and electric fields." Researchers are looking at ways to transfer quantum information from electrons to more stable nuclear spins because of this sensitivity. We looked at how to transfer quantum information to the spin of the SiV.

The researchers had to come up with strategies that would reduce their SiVs' sensitivity to phonons in order to operate their quantum memory at higher temperatures. A SiV in a squeezed crystal should be less sensitive to phonons, according to previous studies.

The prediction was also confirmed by previous experiments. Levonian and his colleagues wanted to reproduce the effect that had been reported.

We were able to achieve both of our objectives, and we were able to take the information we had stored on the electron and swap it for something else. Our technology is a happy medium on many axes.

Although not the longest reported in the literature, the quantum memory created by this team of researchers can hold on to a quantum bit for a few milliseconds. It can process local quantum information 99% of the time, which is better than many platforms but worse than the very best.

Levonian said that the memory's efficiency at gathering up and storing photons is currently around 50%. It is a "heralded" quantum memory, which gives a signal when it catches a photon. It's crucial for designing efficient quantum networks.

It might not seem like a big deal, but the new quantum memory's ability to operate at 4 degrees kelvin instead of 0.1 degrees kelvin could have significant consequences for the future implementation of quantum networks. Cryogenic refrigerators that can bring the temperature down to 4Kelvin are about 5 times cheaper and 10 times smaller than a refrigerator that cools objects down to 0.1K.

The 'golden chandelier' that readers might have seen in articles on quantum computing is needed to get to 0.1 Kelvin. Our success in this study was due to luck, as we discovered that one of our devices had become strained during our manufacturing process, which allowed us to test the temperature resilience this impart to the SiVs. We need to find a way to make strained SiVs 100% of the time in order to build commercial devices.

P.-J. Stas and his team describe a multi-qubit quantum network. There is a science.add9771

Journal information: Science

There is a science network.

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