The Implementation of the Frequency-Time Encoded Decoy-State Protocol with the Slow-Light Effect for Quantum Memories

Abstract (Swedish)

Quantum key distribution (QKD) is a secure encryption key generation process to be used by two users in the presence of an eavesdropper. The no-cloning theorem allows the sender "Alice" to securely send qubits with single photons to the receiver "Bob". However, due to real-life imperfections, it is not always possible to have a single-photon source with its properties matching to the quantum memories based on the rare-earth ions. Besides, it might be resource demanding to build and use such a source on our quantum memories. In order to overcome this problem, one can use a special protocol called the decoy state protocol. In the decoy state protocol, it is possible to have a secure communication channel while having a multi-photon source that can send two different states with different photon number distributions.

In this project, the decoy state protocol has been implemented on our current setup to be used in the determination of the efficiencies of the quantum memories. The performance of the quantum memories developed in this group is polarization dependent. Besides, maintaining and detecting the polarization of the qubits is another challenge. Thus, the encoding type of the protocol has been selected to be Frequency-Time (FT), in which the security of the protocol is maintained by the time-frequency uncertainty. Moreover, in this project, a new technique has been introduced, which allows Bob to detect the frequencies of few-photon pulses in the single photon regime. This new technique is based on the slow-light effect, which can be achieved by using special materials, where the speed of light is reduced by 4 to 5 orders of magnitude compared to its speed in vacuum. The speed of light in this special material is frequency dependent, thus, photons with different frequencies will be distinguishable, since they will be separated in time domain. It has been determined that this method gives promising results for the measurements in the field of the QKD. Additionally, this thesis contains some discussions about possible developmental steps which can be used to improve the implemented protocol.

Popular Abstract

Since the dawn of the first civilizations, the security of information has proven vital to the success of any given community. History records the first attempts at using cryptography techniques in a 1900 BC to 600 BC time frame. In ancient times, the working mechanism of this technique involved the use of letters in the alphabet. To secure the information, these techniques shifted the letters’ number in the alphabet, in a way that all letters would be changed to the position determined by the amount of that number. This way, if the shift variable is equal to nine, then the word ‘quantum’ will be ‘zdjwcdv’. In ancient times, this was considered to be quite a feat for information security. However, due to the advancement of our computation power, these ancient ciphers would now most certainly be instantly solved. This explains that current encryption techniques, such as RSA, present more complex algorithms in order to secure data flows in digital communication networks. An RSA encryption secures the key by a mathematical process that uses prime factorization. Even though there is no practical way of factoring in two prime numbers using conventional computation techniques, it is not proven that it can not be practical. This notion would explain why the RSA cryptosystem is both and unproven and secure technique. In any case, it is clear that an unconventional quantum computer can easily reduce the processing time of any decryption without the knowledge of any secret variable. We could foresee, therefore, a scenario in which if someone collected all the data encrypted with RSA today, he would be able to brute force it to gain the information whenever quantum computers become available. However, there are some other ways to protect the information. For instance, when using a one-time pad encryption technique, the information has been proven to be secure if the encryption key is securely shared between users. However, the main issue remains, which is that the key should still be shared and securely distributed. Quantum key distribution (QKD) is an encryption key generation method in which the key is secured by the fundamental laws of physics. In this project, a QKD protocol has been implemented to be used in order to determine the efficiencies of quantum memories. The reason is that the quantum memories can also be used as quantum repeaters, which are necessary to increase the distance between users to metropolitan distances while allowing
users to communicate securely. Additionally, this project proposes a new frequency measurement technique for the sources that send few-photons in each pulse. This technique uses the slow-light effect where the speed of light is reduced by 4 to 5 orders of magnitude compared to its speed in the vacuum. The frequency measurements with this technique gives promising results for the usage of the slow-light effect in the key distribution process.

• author: Dinçer, Koray
• supervisor: Andreas Walther
• organization: Department of Physics; Atomic Physics
• course: FYSM60 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: QKD, Decoy-State, Frequency-Time, Slow-light, Quantum Repeater, Quantum Memory
• language: English
• id: 8919569
• date added to LUP: 2017-06-29 20:56:38
• date last changed: 2017-06-29 20:56:38