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IET Information Security

Physical feasibility of QKD based on probabilistic quantum circuits

Amor Gueddana, Vasudevan Lakshminarayanan

Published May 3, 2018• 3 citations
Publisher

Abstract

The authors present a comparison between the basic BB84 Quantum Key Distribution (QKD) and its optimised version based on Quantum Dense Coding, in terms of theoretical modelling and feasibility of circuits implementations. These circuits are mainly composed of probabilistic quantum gates, single photon sources, beam splitters (BSs), polarising BS and single photon detectors. They assume that the photonic devices composing the circuits are not perfect. They assess the average success probabilities of the two protocols depending on the success probability of the photonic sources used and the distance between Alice and Bob. They demonstrate that typical real‐world photonic devices permit the implementation of secure communication with 0.65 of success probability over 120 km, while it could reach more than 0.9 over 148 km if just deterministic sources are provided.

DOI
10.1049/iet-ifs.2017.0375

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Quantum key distribution is one of the essential developments to reduce the vulnerability in the security of the systems due to the emergence of quantum computers. As long as large-scale quantum computers emerge, all the systems will be vulnerable to attack. To secure the communication, we can use quantum key distribution protocols in the near future. In this work, I have experimentally verified the BB84 protocol with an eavesdropper. We have used a classical channel to send polarization states of light. Our source is a readily available He-Ne laser (632.8nm). To send secret Information in a secure way, we use Quantum key distribution. Sender Alice sends randomly chosen polarization states of light to Bob. The states Alice sends are |H>,|V>,|D>, and |A>, and similarly, Bob also selects some random basis and detects this Information. In this experiment, |H>,|V> states belong to the '+' basis and |D>,|A> on the 'X' basis. Detecting |H>,|D> states at the detector is considered as '1' and detecting |V>,|A> represents the '0' bit as the output. Alice and Bob can choose the basis randomly. We got these | H>,|V>,|D>, and |A> states by rotating the half-wave plate at angles respectively 0°,90°,45°, and -45°. We used a motorized rotator to rotate the two half-wave plates, one at Alice's side and one at Bob's side, to select the basis and generate these states. The motorized rotator runs on a python code. Python code is designed in such a way that it can generate random bits and can run Alice and Bob parts simultaneously. A polarized beam splitter is used just before the detector to split the beam into | H> and |V> states with 100% intensity at one detector and 0% at the other and |D> and |A> states with 50:50 intensity. Therefore we used one half-wave plate just before the PBS, which can convert |H> and |V> states into |D> and |A> states when we rotate at these particular angles. Using this, we can detect |D> and |A> states with 100% intensity at one detector and 0% at the other. Based upon the basic choices of Alice and Bob, at the receiver part, Bob will receive the bits. At the end of the process, both parties will verify the basis. When the basis is matching, they will keep those bits, and which are not matching, they will discard them. The successful bits will become the encryption key. I did this experiment by sending 1000 bits, and 452 bits among them were successful. For the security analysis of this protocol, we implemented a third-person Eve who tries to steal the Information Alice sends to Bob. Eve used one Beam splitter to take half part of the light intensity and sent another half to Bob. Eve's part has one 50:50 beam splitter, which splits the beam into two. Then, one beam goes to a PBS, then to the detectors, and the other beam goes to a setup that has HWP and PBS, then to the detectors. In this experiment, Eve is successful in stealing the Information correctly. As we are using classical light, which has a huge number of photons, anyone can take some photons and execute a photon number splitting attack; the security of this process is very low. We need a single-photon source and a single photon detector to get total security from this protocol. Right now, I have done this experiment with the available pieces of equipment I have. When the needed types of equipment are available to me, I can implement the single-photon QKD experiment also. After the experiment, we realized that the superposition of states always holds in this experiment, but states do not collapse, as we are not performing the quantum measurement. Also, I have implemented the QKD protocol in the IBM Quantum lab, where Alice sends 1000 bits to Bob, and 474 bits among them were successful. Also, Eve’s interception was detected successfully.

•Jan 1, 2026•0 citations
The authors present a comparison between the basic BB84 Quantum Key Distribution (QKD) and its optimised version based on Quantum Dense Coding, in terms of theoretical modelling and feasibility of circuits implementations. These circuits are mainly composed of probabilistic quantum gates, single photon sources, beam splitters (BSs), polarising BS and single photon detectors. They assume that the photonic devices composing the circuits are not perfect. They assess the average success probabilities of the two protocols depending on the success probability of the photonic sources used and the distance between Alice and Bob. They demonstrate that typical real‐world photonic devices permit the implementation of secure communication with 0.65 of success probability over 120 km, while it could reach more than 0.9 over 148 km if just deterministic sources are provided.
In this work, we simulate a fiber-based Quantum Key Distribution Protocol (QKDP) BB84 working at the telecoms wavelength 1550 nm with taking into consideration an optimized attack strategy. We consider in our work a quantum channel composed by probabilistic Single Photon Source (SPS), single mode optical Fiber and quantum detector with high efficiency. We show the advantages of using the Quantum Dots (QD) embedded in micro-cavity compared to the Heralded Single Photon Sources (HSPS). Second, we show that Eve is always getting some information depending on the mean photon number per pulse of the used SPS and therefore, we propose an optimized version of the QKDP BB84 based on Quantum Dense Coding (QDC) that could be implemented by quantum CNOT gates. We evaluate the success probability of implementing the optimized QKDP BB84 when using nowadays probabilistic quantum optical devices for circuit realization. We use for our modeling an abstract probabilistic model of a CNOT gate based on linear optical components and having a success probability of sqrt (4/27), we take into consideration the best SPSs realizations, namely the QD and the HSPS, generating a single photon per pulse with a success probability of 0.73 and 0.37, respectively. We show that the protocol is totally secure against attacks but could be correctly implemented only with a success probability of few percent.
Quantum key distribution (QKD) allows for provably secure key distribution between two trusted parties. Because the security and performance of QKD protocols rely on devices that behave according to specific assumptions, idealized or inaccurate assumptions about device behavior can introduce security loopholes. Real devices can never be perfectly characterized, and their performance metrics are always subject to certain error margins, which must be accounted for in a rigorous theoretical analysis. Only recently have rigorous finite-size results allowed for imperfect characterizations of devices (where device parameter have uncertainty margins) - an advance yet to be considered in experimental implementations of the BB84 protocol. In this work, we prove the security and analyze the performance of an implementation of the BB84 protocol using single photons generated by a semiconductor quantum dot light source in combination with dynamic polarization-state encoding. We consider the presence of incompletely characterized devices by accounting for imperfections in the single-photon source (in terms of finite g(2)(0)) as well as the receiver (non-ideal beam-splitters, finite detector efficiencies, and dark counts), all with error margins. The resulting protocol implementation shows competitive performance, paving the way towards practical and loop-hole free implementations of QKD.
Quantum key distribution (QKD) permits information-theoretically secure transmission of digital encryption keys, assuming that the behaviour of the devices employed for the key exchange can be reliably modelled and predicted. Remarkably, no assumptions have to be made on the capabilities of an eavesdropper other than that she is bounded by the laws of Nature, thus making the security of QKD "unconditional". However, unconditional security is hard to achieve in practice. For example, any experimental realisation can only collect finite data samples, leading to vulnerabilities against coherent attacks, the most general class of attacks, and for some protocols the theoretical proof of robustness against these attacks is still missing. For these reasons, in the past many QKD experiments have fallen short of implementing an unconditionally secure protocol and have instead considered limited attacking capabilities by the eavesdropper. Here, we explore the security of QKD against coherent attacks in the most challenging environment: the long-distance transmission of keys. We demonstrate that the BB84 protocol can provide positive key rates for distances up to 240 km without multiplexing of conventional signals, and up to 200 km with multiplexing. Useful key rates can be achieved even for the longest distances, using practical thermo-electrically cooled single-photon detectors.
Abstract The BB84 quantum key distribution (QKD) protocol sets the foundation for achieving secure communication against quantum threats. Since its inception, significant advancements have aimed towards frugal experimental implementations with enhanced security. In this paper, we report a passive polarization encoded BB84 protocol. By passively and randomly encoding the polarization states, the setup avoids active modulation and simplifies design with security standards analogous to entanglement-based QKD. The heralding of a probabilistic single-photon source ensures an extremely low probability of multi-photon emissions evading photon number-splitting attacks. The quality of the single-photon source and thus, the QKD setup is certified by the second-order correlation function, g (2) (0) = 0.006 ± 0.0002. Our protocol provides an optimisation trade-off between the conventional BB84 and the BBM92 protocol. The advantage relies on using fewer detectors (compared to the BBM92 protocol) and no reliance on active quantum random number generators (compared to the standard BB84) to drive Alice’s encoding scheme. Our implementation achieves a quantum bit error rate of 7% and a secure key rate of 5 kbps.
Quantum key distribution (QKD) has emerged as a revolutionary technique that can ensure unconditionally secure communication between two distant parties, referred to as Alice and Bob, by exploiting the fundamental principles of quantum physics. Here, we report experimental demonstration of a fully automated QKD system based on the BB84 protocol. A weak coherent pulse source is developed to generate stream of polarization encoded single photons. Our QKD system demonstrates a low QBER of ∼1.2% with sifted key generation rate of ∼70 Kbps at an average photon number per pulse ofμ=0.11\mu=0.11μ=0.11. The final secure key rate of ∼33 Kbps is achieved after performing post processing procedures viz. error correction and privacy amplification. The quantum communication link is established continuously for several hours and the performance is evaluated iteratively. The developed QKD system is shown to have the capability of distributing cryptographic keys securely and seamlessly, under practical operating conditions. Furthermore, we have successfully demonstrated unconditionally secure transmission and reception of an image between Alice and Bob connected over an unsecured public channel by utilizing quantum cryptography.
Quantum key distribution is one of the essential developments to reduce the vulnerability in the security of the systems due to the emergence of quantum computers. As long as large-scale quantum computers emerge, all the systems will be vulnerable to attack. To secure the communication, we can use quantum key distribution protocols in the near future. In this work, I have experimentally verified the BB84 protocol with an eavesdropper. We have used a classical channel to send polarization states of light. Our source is a readily available He-Ne laser (632.8nm). To send secret Information in a secure way, we use Quantum key distribution. Sender Alice sends randomly chosen polarization states of light to Bob. The states Alice sends are |H>,|V>,|D>, and |A>, and similarly, Bob also selects some random basis and detects this Information. In this experiment, |H>,|V> states belong to the '+' basis and |D>,|A> on the 'X' basis. Detecting |H>,|D> states at the detector is considered as '1' and detecting |V>,|A> represents the '0' bit as the output. Alice and Bob can choose the basis randomly. We got these | H>,|V>,|D>, and |A> states by rotating the half-wave plate at angles respectively 0°,90°,45°, and -45°. We used a motorized rotator to rotate the two half-wave plates, one at Alice's side and one at Bob's side, to select the basis and generate these states. The motorized rotator runs on a python code. Python code is designed in such a way that it can generate random bits and can run Alice and Bob parts simultaneously. A polarized beam splitter is used just before the detector to split the beam into | H> and |V> states with 100% intensity at one detector and 0% at the other and |D> and |A> states with 50:50 intensity. Therefore we used one half-wave plate just before the PBS, which can convert |H> and |V> states into |D> and |A> states when we rotate at these particular angles. Using this, we can detect |D> and |A> states with 100% intensity at one detector and 0% at the other. Based upon the basic choices of Alice and Bob, at the receiver part, Bob will receive the bits. At the end of the process, both parties will verify the basis. When the basis is matching, they will keep those bits, and which are not matching, they will discard them. The successful bits will become the encryption key. I did this experiment by sending 1000 bits, and 452 bits among them were successful. For the security analysis of this protocol, we implemented a third-person Eve who tries to steal the Information Alice sends to Bob. Eve used one Beam splitter to take half part of the light intensity and sent another half to Bob. Eve's part has one 50:50 beam splitter, which splits the beam into two. Then, one beam goes to a PBS, then to the detectors, and the other beam goes to a setup that has HWP and PBS, then to the detectors. In this experiment, Eve is successful in stealing the Information correctly. As we are using classical light, which has a huge number of photons, anyone can take some photons and execute a photon number splitting attack; the security of this process is very low. We need a single-photon source and a single photon detector to get total security from this protocol. Right now, I have done this experiment with the available pieces of equipment I have. When the needed types of equipment are available to me, I can implement the single-photon QKD experiment also. After the experiment, we realized that the superposition of states always holds in this experiment, but states do not collapse, as we are not performing the quantum measurement. Also, I have implemented the QKD protocol in the IBM Quantum lab, where Alice sends 1000 bits to Bob, and 474 bits among them were successful. Also, Eve’s interception was detected successfully.