By Sreraman Muralidharan and Vladimir S. Malinovsky
Author Bio: Dr. Sreraman Muralidharan and Dr. Vladmir S. Malinovsky are in the US Army Research Laboratory in Maryland.
The invention of internet has revolutionized the world and any information we need about a topic is only a click away from our home. Internet makes it possible for us to have information transfer between different ends of the globe with high communication speed. Have you ever wondered how it is possible to have so much information transferred between different continents? The answer lies in encoding information into light pulses and having them go through optical fibers. These optical fibers are sometimes buried under the land or laid on seabeds.
There is a major challenge that has to be overcome while transferring light through optical fibers. That is the exponential loss that arises because of the absorption or scattering of light in the optical fiber. This is overcome by using amplifiers at intermediate nodes and amplifying the signal at these nodes and retransmitting them to the next node, while making sure that the integrity of the signal is maintained. However, information transferred using this method is not so secure and can be hacked. So, in future, how are we going to make sure that our information is more secure against intruders? Quantum mechanics, provides the key.
Quantum mechanics is branch of science that deals with the fundamental study of nature at small (atomic) scales. Though the field emerged because of mankind curiosity to understand the nature, we are now planning on using them to create a more secure internet. They key is to encode the information into a quantum state of a single photon (particle of light) rather than bright light pulses. Due to the no-cloning theorem , which states that unknown quantum states cannot be cloned deterministically, any eavesdropping can be detected.
However, transferring a single photon through an optical fiber and receiving them at the other end is exponentially hard due to losses of the channel. In addition, single photons cannot be amplified like bright light pulses. So, we need alternative mechanisms to transfer a quantum state across different continents. To overcome these difficulties, devices known as “quantum repeaters” have been proposed . Depending on the methods used to correct loss and operation errors, quantum repeaters can be classified into three generations [2,3,4,5,6,7]. In this article, we will not get into the details of the three generations, but will provide a description of how the third generation works .
Third generation quantum repeaters provides the fastest communication rates compared to the first two generations. However, they are more technologically demanding. Here, the quantum state of a single photon is encoded into a block of photons in some error correcting code and transmitted through the fiber. When the photons travel through the fiber, some of them are lost, while others are transmitted successfully. In addition to loss, the photons can also undergo operation errors such as depolarization and dephasing. When the block of photons reach the other station with these errors, a quantum error correction procedure is carried out to correct these errors.
Owing to a fundamental theorem in quantum information theory, the loss error rates need to be less than 50% for quantum error correcting codes to correct so the coupling efficiency between the photon and the station needs to be sufficiently high. Here, the communication rates is solely dependent only on the time taken to do local operations for the quantum error correction procedure.
The repeater parameters such as repeater spacing, number of photons and communication rates can be optimized in a quantum repeater architecture. If the technological requirements for third generation quantum repeaters have been fulfilled, then it can give rise to the realization of spectacular applications such as quantum secret sharing , quantum clocks and quantum key distribution between different continents and can lead eventually to a fully secure quantum internet.
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