Quantum key distribution (QKD) allows to share symmetric encryption keys between two remote parties, in an unconditionally secure way. Most of the QKD protocols rely on the usage of single photons, which however are difficult to produce in a practical way. The introduction of the decoy-method [1] allowed the usage of attenuated laser pulses whilst demonstrating unconditionally security. Since each qubit needs to carry a random global phase, this protocol relies on phase-randomized coherent states [2] which are normally produced exploiting gain switching mode [3].
Quantum key distribution (QKD) allows to share symmetric encryption keys between two remote parties, in an unconditionally secure way. Most of the QKD protocols rely on the usage of single photons, which however are difficult to produce in a practical way. The introduction of the decoy-method [1] allowed the usage of attenuated laser pulses whilst demonstrating unconditionally security. Since each qubit needs to carry a random global phase, this protocol relies on phase-randomized coherent states [2] which are normally produced exploiting gain switching mode [3].
Abstract: The quantum key distribution (QKD) technique establishes secret keys shared between two communicating parties. Theoretically, unconditional security provided by QKD is guaranteed by the fundamental laws of quantum physics. in the real life, it is still possible to obtain unconditionally secure QKD, even with (phase randomized) attenuated laser pulses, as theoretically demonstrated by Gottesman-Lo-L¨utkenhaus-Preskill (GLLP). However, one must pay a steep price by placing severe limits on the distance and the key generation rate. These problems were solved using the decoy state method introduced by Hwang. In this paper, we have proposed a method to estimate parameters of the decoy state method based on two decoy state protocol for both BB84 and SARG04. The vacuum and weak decoy state protocol has been introduced as a special case of two decoy states protocol. This method has given different lower bound of the fraction of single-photon counts (y1), the fraction of two-photon counts (y2), the upper bound QBER of single-photon pulses (e1), the upper bound QBER of two-photon pulses (e2) and the lower bound of key generation rate for both BB84 and SARG04. The fiber based QKD systems also have been simulated using the proposed method for BB84 and SARG04. The numerical simulation has shown that the fiber based QKD systems using the proposed method for BB84 are able to achieve both a higher secret key rate and greater secure distance than that of SARG04. Key words: Quantum cryptography Quantum key distribution Decoy state protocol and optical communications
Quantum key distribution (QKD) allows two distant parties to share a secret key and realizes a communication with information-theoretic security by combining it with one-time-pad encryption.Since the Bennett-Brassard 1984 (BB84) protocol was proposed, a large number of researches on QKD have been conducted from both aspects of theory and implementations.For implementations of QKD, weak coherent pulses (WCP) are heavily used as optical signals because they are easily generated by typical lasers and attenuators.The security for QKD with WCP has been also studied along the development of the implementations.In this thesis, the security analysis of the QKD with WCP is considered and further developed from two aspects.First, the security of the differential-quadrature-phase-shift (DQPS) protocol is proved.The DQPS protocol has essentially the same set up as the phase-encoding BB84 (PE-BB84) protocol, which is one of the most frequently demonstrated protocols.Since the known proof techniques for the BB84 protocol is not directly applicable, a modified approach is developed which is suitable for the DQPS protocol.As a result, the advantage of the DQPS protocol in the key generation rate over the PE-BB84 protocol is shown in the asymptotic limit where the size of communication data is assumed to be infinite.Second, a new method for security analysis with finite-key size is proposed as a suitable method for QKD protocols using WCP.Differently from the current method based on simple random sampling, the proposed method relies on Bernoulli sampling, which is associated with binomial distribution.The security of the BB84 protocol is proved by using the Bernoulli-sampling method, enabling a simpler analysis with a smaller number of parameters to be estimated compared to the method with simple random sampling.The required number of detected signals to generate a secret key is shown to be smaller than 10 4 , which is drastic improvement from the number ∼ 10 7 obtained in the previous result.The proposed method is also applied to the DQPS protocol, and its advantage over the PE-BB84 protocol is certified even in the finite-key regime.i
Quantum key distribution (QKD) allows two users to communicate with theoretically provable secrecy [1]. This is vitally important to secure the confidential data of governments, businesses and individuals. As the technology is adopted by a wider audience, a quantum network will become necessary for multi-party communication, as in the classical communication networks in use today. Unfortunately, a number of phase-encoded QKD protocols based on weak coherent pulses have been developed. Whilst the first protocol, proposed by Bennett and Brassard in 1984 (BB84), is still commonly used, other protocols such as differential phase shift [2] or coherent one way QKD [3] are also adopted. Each protocol has its benefits; however all would require a different transmitter and receiver, increasing the complexity and cost of quantum networks. In this work we demonstrate a multi-protocol transmitter [4-6] that also has the benefits of small footprint, low power consumption and low complexity. We use this transmitter to give the first experimental demonstration of an improved version of the BB84 protocol, known as the differential quadrature phase shift protocol. We have achieved megabit per second secure key rates at short distances, and have shown secure key rates that are, on average, 2.71 times higher than the standard BB84 protocol. This enhanced performance over such a commonly adopted protocol, at no expense to experimental complexity, could lead to a widespread migration to the new protocol. The security of the BB84 protocol relies on each signal and reference pulse pair being globally phase randomised with respect to all other pulse pairs. In the DQPS protocol, blocks with a length L ≥ 2 are used and each block has a globally random phase with respect to all other blocks. Implementing this protocol would ordinarily require a high-speed random number generator and a phase modulator. As well as increasing device complexity, it would also require an unrealistic continuous source of electrical modulation signals for complete security. The transmitter we use injects light from a master laser diode into a 2 GHz gain-switched slave laser diode. The principal of optical injection locking means that the slave laser inherits the phase of the master laser. We apply modulations to the master laser current within a block to control the phase of the slave laser output pulses, and then drive the master laser below threshold for a short period of time when phase randomisation is required. This ensures the lasing comes from below threshold, thus the phase adopted by the slave laser pulse is completely random. We perform an autocorrelation measurement on the blocks to show their randomness. [1] N. Gisin et al. Rev. Mod. Phys. 74, 145 (2002). [2] K. Inoue et al. Phys. Rev. Lett. 89, 037902 (2002). [3] D. Stucki et al. Appl. Phys. Lett. 87 194108 (2005). [4] Z. Yuan et al. Phys. Rev. X. 6, 031044 (2016). [5] G. L. Roberts et al. Laser Phot. Rev. 11, 1700067 (2017). [6] G. L. Roberts et al. arXiv:1709.04214 [quant-ph] (2017).
Quantum key distribution (QKD), a technology that enables perfectly secure communication, has evolved to the stage where many different protocols are being used in real-world implementations. Each protocol has its own advantages, meaning that users can choose the one best-suited to their application, however each often requires different hardware. This complicates multi-user networks, in which users may need multiple transmitters to communicate with one another. Here, we demonstrate a direct-modulation based transmitter that can be used to implement most weak coherent pulse-based QKD protocols with simple changes to the driving signals. This also has the potential to extend to classical communications, providing a low chirp transmitter with simple driving requirements that combines phase shift keying with amplitude shift keying. We perform QKD with concurrent time-bin and phase modulation, alongside phase randomisation. The acquired data is used to evaluate secure key rates for time-bin encoded BB84 with decoy states and a finite key-size analysis, giving megabit per second secure key rates, 1.60 times higher than if purely phase-encoded BB84 was used.
Quantum Key Distribution (QKD) enables secure key exchange, based on fundamental laws of quantum mechanics [1] . There exist various protocols which are mathematically proven to be secure, however, for an actual implementation vulnerabilities, due to non-ideal components, must be considered. For example, the usage of faint lasers instead of single-photon sources undermines the security, as even for mean photon numbers per pulse µ ≪ 1 there is a non-vanishing probability of emitting multiple photons. The decoy protocol [2] , [3] offers a countermeasure. When randomly switching between two or more different pulse intensity levels, by exploiting their non-orthogonality, the so-called photon number splitting (PNS) attack can be revealed.
We suggest a protocol for quantum key distribution—a technology allowing two distant parties to create an unconditionally secure cryptographic key. For the creation of the key we suggest to use laser pulses weakened to the single-photon level of duration T , the pulse carrying the value “1” being shifted in time by T /2 compared to the pulse carrying the value “0”. The overlap of the pulses provides their non-orthogonality and, therefore, impossibility to discriminate between them with certainty. Besides the signal pulses the protocol uses coherent decoy pulses, having longer duration than the signal ones and providing a more effective protection from a wide class of attacks. Security of the protocol is based on interferometric control of the pulse coherence at the receiving station. We analyze the security of the protocol against a number of intercept-resend attacks and on the basis of this analysis substantiate the necessity of decoy state implementation.