Madrid. An international team of researchers produced, stored and retrieved quantum information for the first time, a crucial step in networking in that area of physics.
The ability to share quantum information is crucial in order to develop such networks for distributed computing and secure communication. Quantum computing will be useful in solving some important problems, such as optimizing financial risk, deciphering data, designing molecules, and studying material properties.
However, this development has been delayed because quantum information can be lost when transmitted over long distances. One way to overcome this barrier is to divide the network into smaller segments and link them all with a shared quantum state.
In order to do the above, a means is required to store the quantum information and retrieve it again: that is, a quantum memory device. First of all, it must talk
with another device that allows the creation of that data.
For the first time, the researchers created a system that interconnects these two key components and uses regular optical fibers to transmit the quantum data.
The feat was achieved by researchers from Imperial College London, the universities of Southampton, Stuttgart and Wurzburg in Germany, and the results were published in Science Advances.
Lead author Sarah Thomas, from the Department of Physics at Imperial College London, said in a statement: Interconnecting two key devices is a crucial step forward in enabling quantum networking, and we are very excited to be the first team to demonstrate it.
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Critical task
Lukas Wagner, from the University of Stuttgart and one of the study’s co-authors, added: Enabling long-distance locations, and even quantum computers, to connect is a critical task for future such networks.
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In common telecommunications, such as the Internet or telephone lines, information can be lost over long distances. To combat this, these systems use repeaters
at regular points, which read and re-amplify the signal, ensuring that it reaches its destination intact.
Classical repeaters, however, cannot be used with quantum information, since any attempt to read and copy it would destroy it. This is an advantage in one sense, since quantum connections cannot be blow
without destroying information and alerting users. But it is a challenge that must be addressed for long-distance quantum networks.
One way to overcome this problem is to share quantum information in the form of entangled light particles or photons. These share properties in such a way that one cannot be understood without the other. In order to share entanglement over long distances through a quantum network, two devices are needed: one to create them and another to store them and allow them to be retrieved later.
There are several devices used to create quantum information in the form of entangled photons and store it, but generating those light particles on demand and having a compatible quantum memory in which to store them eluded researchers for a long time.
Photons have certain wavelengths (which, in visible light, create different colors), but the devices to create and store them are often tuned to operate at different wavelengths, preventing them from interacting.
same wavelength
To make the devices interact, the team created a system in which both used the same wavelength. A quantum dot
It produced (non-entangled) photons, which were then passed to a quantum memory system that gathered the photons within a cloud of rubidium atoms. A laser turned the memory on and off, allowing it to be stored and released on demand.
Not only did the wavelength of these two devices coincide, but it was the same as the telecommunications networks used today, which allows it to be transmitted with normal fiber optic cables familiar in everyday Internet connections.
The quantum dot light source was created by researchers at the University of Stuttgart with support from the University of Würzburg, and then brought to the UK to interface with the quantum memory device created by the Imperial and Southampton team. The system was assembled in a basement laboratory at Imperial College London.
While stand-alone quantum dots and memories have been created that are more efficient than the new system, this is the first proof that devices can be made to interconnect and at telecommunications wavelengths.
The team will now look to improve the system, including making sure all photons are produced at the same wavelength, improving the length of time photons can be stored, and making the entire system smaller.
However, as a proof of concept, this is an important step forward, says co-author Dr Patrick Ledingham of the University of Southampton: Members of the quantum community have been actively trying to make this connection for some time. This includes us, who have tried this experiment twice before with different memory devices and quantum dots, for more than five years, which shows how difficult it is to do
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The big breakthrough this time was to call on experts to develop and execute each part of the experiment with specialized equipment and work together to synchronize the devices.