Researchers Demonstrate Missing Link For Quantum Internet

Mikhail Lukin, professor of physics named after George Fasmer Leverett (not pictured) and GSAS students, David Levonian (left) and Mihir Bhaskar, Harvard researchers who created the missing link to the ultra-secure quantum Internet at LISE. Courtesy: Chris Snibbe / Harvard Staff Photographer.

Quantum Internet can be used to send messages that cannot be hacked, increase GPS accuracy and enable cloud quantum computing. For more than twenty years, dreams of creating such a quantum network have remained largely unattainable due to the difficulty of sending quantum signals over long distances without loss.

Now, researchers at Harvard and the Massachusetts Institute of Technology have found a way to fix signal loss with a prototype quantum node that can capture, store, and confuse bits of quantum information. Research is the missing link to practical quantum internet and an important step forward in the development of distant quantum networks.

“This demonstration is a conceptual breakthrough that can expand the maximum possible range of quantum networks and potentially open many new applications in a way that is impossible using any existing technologies,” said Mikhail Lukin, professor of physics named after George Fasmer Leverett and co-director of the Harvard Quantum Initiative. “This is the realization of the goal that was achieved by our quantum science and the engineering community for over two decades. "

Study published in Nature,

All types of communication technologies – from the first telegraph to the modern fiber-optic Internet – should have taken into account the fact that signals deteriorate and are lost when transmitting over long distances. The first transponders that receive and amplify signals to correct this loss were designed to amplify telegraph signals with wire fading in the mid-1800s. Two hundred years later, repeaters are an integral part of our long-distance communications infrastructure.

In a classic network, if Alice in New York wants to send Bob a message in California, the message moves from coast to coast more or less in a straight line. Along the way, the signal passes through repeaters, where it is read, amplified, and corrected for errors. The whole process is vulnerable to attacks at any time.

However, if Alice wants to send a quantum message, the process will be different. Quantum networks use quantum particles of light – individual photons – to transmit quantum states of light over long distances. These networks have a trick that classical systems do not have: entanglement.

Obfuscation – what Einstein called "eerie action at a distance" – allows bits of information to be perfectly correlated at any distance. Since quantum systems cannot be observed without change, Alice could use entanglement to tell Bob without fear of eavesdroppers. This concept is the basis for applications such as quantum cryptography – security, which is guaranteed by the laws of quantum physics.

However, long-distance quantum communication is also affected by the usual loss of photons, which is one of the main obstacles to the implementation of large-scale quantum Internet. But the same physical principle that makes quantum communication super-safe also makes it impossible to use existing classical repeaters to eliminate information loss.

How can you amplify and correct a signal if you cannot read it? The solution to this seemingly impossible task involves the so-called quantum repeater. Unlike classical repeaters, which amplify a signal through an existing network, quantum repeaters create a network of entangled particles through which a message can be transmitted.

In essence, a quantum repeater is a small specialized quantum computer. At each stage of such a network, quantum repeaters should be able to capture and process the quantum bits of quantum information to correct errors and store them long enough for the rest of network be ready. Until now, this has been impossible for two reasons: firstly, single photons are very difficult to capture. Secondly, quantum information is notoriously fragile, which makes it very difficult to process and store for long periods of time.

Lukin Laboratory, in collaboration with Marco Loncar, Professor of Electrical Engineering Tiancai Lin at the Harvard School of Engineering and Applied Sciences John A. Paulson (SEAS),

Hongkun Park, Mark Hyman Jr., professor of chemistry at Harvard School of Arts and Sciences (FAS), and Dirk Englund, associate professor of electrical engineering and computer science at the Massachusetts Institute of Technology (MIT), are working to use a system that can both perform well. tasks are silicon vacancies of color centers in diamonds.

These centers are tiny defects in the atomic structure of diamond that can absorb and emit light, causing the brilliant colors of diamond.

“Over the past few years, our laboratories have worked to understand and control individual silicon color centers, especially how to use them as quantum memory devices for single photons,” said Mihir Bhaskar, a graduate student of the Lukina group.

Researchers have integrated a separate color center into the cavity of a diamond nanotube, which limits information photons and forces them to interact with a single color center. They then placed the device in a dilution refrigerator, which reaches a temperature close to absolute zero, and sent individual photons via fiber optic cables to the refrigerator, where they were effectively captured and caught by the center of color.

A device can store quantum information in milliseconds — long enough to transmit information over thousands of kilometers. Electrodes embedded around the cavity were used to supply control signals for processing and storing information stored in memory.

“This device combines the three most important elements of a quantum repeater – long memory, the ability to efficiently capture information from photons and the way it is processed locally,” said Bart Machielse, a graduate student at the Laboratory for Nanoscale Optics. "Each of these problems was resolved separately, but not one device combined all three."

“We are currently working on expanding this study by incorporating our quantum memories into real urban fiber optic channels,” said Ralph Reading, Ph.D. in Lukin’s group. “We plan to create large networks of entangled quantum memories and explore the first applications of quantum Internet.”

“This is the first demonstration at the system level, combining the main achievements in the field of nanotechnology, photonics and quantum control, which demonstrates a clear quantum advantage in the transmission of information using quantum repeater nodes. We look forward to exploring new unique applications using these methods, ”Lukin said.

First quantum orientation by enhancing quantum measurements

Additional Information:
Experimental memory demonstration of enhanced quantum communication, Nature (2020). DOI: 10.1038 / s41586-020-2103-5

Researchers Demonstrate Missing Link For Quantum Internet (2020, March 23)
restored March 23, 2020

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