Practical Quantum Cryptography Research Articles

The spontaneous emission performance of a quantum emitter coupled to a hybrid plasmonic waveguide with specified output polarization for on-chip plasmonic single-photon source

Nowadays, most of the on-chip plasmonic single-photon sources emit an unpolarized stream of single photons that demand a subsequent polarizer stage in a practical quantum cryptography system. In this paper, we numerically demonstrated the coupling of the light emitted from a quantum emitter (QE) at 700 nm wavelength to the propagation mode supported by an on-chip hybrid plasmonic waveguide (HPW) polarization rotator. Our results proved that the light emitted is linearly polarized at 0º, 45º/−45º, and 90º with propagation lengths of 5 μm, 3.3 μm, and 3.9 μm, respectively. Moreover, high power-conversion efficiency was obtained from an applied transverse magnetic (TM) mode (0º-polarization) to a transverse electric (TE) (90º-polarization) and a linearly polarized light at 45º/−45º of 97% and a 98%, respectively. Furthermore, we obtained almost a 3-fold enhancement of the total decay rate of the QE with high emission coupling efficiency (β-factor) of 88%, 80%, and 87% to the corresponding waveguide mode for 0º, 45º/−45º, and 90º, respectively. Our work paves the way towards more efficient, compact, and less complicated on-chip plasmonic single-photon sources with a specified output polarization.

Thermal states quantum cryptography with linear optics cloning machine

We improve the thermal states quantum cryptography by inserting the linear optics cloning machine (LOCM) into Bob’s side. This approach can enhance the effectiveness of the practical quantum cryptography under noisy resources and channel in terms of secret key rate and transmission distance. We show that the security of the thermal states protocol with LOCM has advantages over that of the original protocol at various wavelengths.

Quantum key distribution using basis encoding of Gaussian-modulated coherent states

The continuous-variable quantum key distribution (CVQKD) has been demonstrated to be available in practical secure quantum cryptography. However, its performance is restricted strongly by the channel excess noise and the reconciliation efficiency. In this paper, we present a quantum key distribution (QKD) protocol by encoding the secret keys on the random choices of two measurement bases: the conjugate quadratures $X$ and $P$. The employed encoding method can dramatically weaken the effects of channel excess noise and reconciliation efficiency on the performance of the QKD protocol. Subsequently, the proposed scheme exhibits the capability to tolerate much higher excess noise and enables us to reach a much longer secure transmission distance even at lower reconciliation efficiency. The proposal can work alternatively to strengthen significantly the performance of the known Gaussian-modulated CVQKD protocol and serve as a multiplier for practical secure quantum cryptography with continuous variables.

Practical quantum cryptography

A quantum cryptography system based on a 4-basis protocol with geometrically uniform states is tested in a series of experiments. Quantum states of light transmitted through real fiber optic communication channels to a distance of 32 km in the presence of uncontrolled external actions are prepared, transformed, and measured. It is shown that the chosen algorithms of processing quantum information are adequate and can be used as foundations of practical devices in protected communication lines.1

Time-polarization coding in quantum cryptography

We present a quantum key distribution (QKD) method that allows us to demonstrate practical quantum cryptography. It is used for obtaining the cryptographic key to encode the information communicated between to legitimate parties. It generally relates to a secured key sharing method and system between a plurality of transmitters and a plurality of receivers, more particularly to a system and method of QKD between transmitters and receivers. The aspect of the method is a QKD system capable of forming the first signals with different phases or polarization and the second signals with different delay time. The transmitter sends the two signals to share the key with receiver. The first signal is generated in different phases or polarization to protect the transmission channel and to be the reference time for second signal. The state of first signal and the delay time between the first signal and second signal is the key. The method leads to increase the length of the shared key. This method is used to implement the practical QKD system that can be used for switching between our protocol and BB84 protocol in optical fiber (one-way system) and free space setups with unconditionally security. Secure keys generated by the QKD network are managed in the key management layer by key management agents (KMAs), and used to encrypt the data and send it to receivers. The proposed schemes aimed to provide a secure end to end communication.

Security analysis of BB84 protocol in the collective-rotation noise channel

Most of quantum cryptography protocols are designed under the ideal conditions without considering the impact of noise in actual communication; thus they may result in that the confidential information cannot be transmitted to the receiver accurately or eavesdroppers can steal the confidential information by mixing in noise. Therefore, analyzing the security of quantum cryptography protocols under noise conditions is of great significance. For the purpose of analyzing the security of quantum BB84 protocol in collective-rotation noise, firstly this paper introduces the quantum BB84 protocol, and considers the influence of environmental noise on it. An explanation should be stated that in a noise environment, the effects of noise and eavesdropping cannot be distinguished between each other. So the mechanism for which the error bit is simply used as the criterion to judge whether there exists eavesdropping in the BB84 protocol, cannot be used in the noise environment. The mechanism to judge whether there exists eavesdropping in quantum noise channel needs to be modified and improved for protecting the information. An initial qubit error rate can be set according to the noisy quantum channel. If the qubit error rate of the quantum communication channel is larger than that, it can be determined that the quantum channel is not secure and exists eavesdropping, no matter what the reason is. And on this basis, the collective-rotation noise model will be established in quantum channel by using the particle deflection model and distinguish the noise from the eavesdropping in quantum channel quantitatively, and the relationship of the amount of information that eavesdroppers can steal, the quantum bits error rate and the noise level will be analyzed by using the von Neumann entropy. Finally, the noise critical point will be calculated by using the collective noise model and the relationship between the amount of information that eavesdroppers can steal, at the quantum bits error rate, and the noise level. Through the analysis, we can know that in the existing noise level, the most of the eavesdropping can steal 25% of the key from the communication. However, the Eve's eavesdropping behavior will be detected, so that Alice and Bob will give up the current consultation key, and restart the key negotiation. This result shows that the quantum BB84 protocol is safe and secure in the collective-rotation noise channel. The research results of this paper will enrich the theory of quantum cryptography, and the innovation of security detection methods in quantum cryptographic protocols will help promote the process of practical quantum cryptography.

Practical position-based quantum cryptography

We study a general family of quantum protocols for position verification and present a new class of attacks based on the Clifford hierarchy. These attacks outperform current strategies based on port-based teleportation for a large class of practical protocols. We then introduce the Interleaved Product protocol, a new scheme for position verification involving only the preparation and measurement of single-qubit states for which the best available attacks have a complexity exponential in the number of classical bits transmitted.

Implementation of Positive Operator-Valued Measure in Passive Faraday Mirror Attack* *Supported by National Natural Science Foundation of China under Grant Nos. 61472446, U1204602, and National High Technology Research and Development Program of China under Grant No. 2011AA010803, and the Open Project Program of the State Key Laboratory of Mathematical Engineering and Advanced Computing under Grant No. 2013A14

Passive Faraday-mirror (PFM) attack is based on imperfect Faraday mirrors in practical quantum cryptography systems and a set of three-dimensional Positive Operator-Valued Measure (POVM) operators plays an important role in this attack. In this paper, we propose a simple scheme to implement the POVM in PFM attack on an Faraday–Michelson quantum cryptography system. Since the POVM can not be implemented directly with previous methods, in this scheme it needs to expand the states sent by Alice and the POVM operators in the attack into four-dimensional Hilbert space first, without changing the attacking effect by calculation. Based on the methods proposed by Ahnert and Payne, the linear-optical setup for implementing the POVM operators is derived. At last, the complete setup for realizing the PFM attack is presented with all parameters. Furthermore, our scheme can also be applied to realize PFM attack on a plug-and-play system by changing the parameters in the setup.

Trojan-horse attacks threaten the security of practical quantum cryptography

A quantum key distribution (QKD) system may be probed by an eavesdropper Eve by sending in bright light from the quantum channel and analyzing the back-reflections. We propose and experimentally demonstrate a setup for mounting such a Trojan-horse attack. We show it in operation against the quantum cryptosystem Clavis2 from ID Quantique, as a proof-of-principle. With just a few back-reflected photons, Eve discerns Bobʼs (secret) basis choice, and thus the raw key bit in the Scarani–Acín–Ribordy–Gisin 2004 protocol, with higher than 90% probability. This would clearly breach the security of the cryptosystem. Unfortunately, Eveʼs bright pulses have a side effect of causing a high level of afterpulsing in Bobʼs single-photon detectors, resulting in a large quantum bit error rate that effectively protects this system from our attack. However, in a Clavis2-like system equipped with detectors with less-noisy but realistic characteristics, an attack strategy with positive leakage of the key would exist. We confirm this by a numerical simulation. Both the eavesdropping setup and strategy can be generalized to attack most of the current QKD systems, especially if they lack proper safeguards. We also propose countermeasures to prevent such attacks.

A Practical Quantum Cryptography Transmitter

The extend success of any communication system essentially depends on the strength of the secrecy of transporting the information between the partners. The classical cryptography for the wired and wireless communication systems can not withstand against the challenges of new technologies. So, it is necessary to invent a new method to encrypt the information. The solution is quantum cryptography in communication system. The absolute security can be achieve with quantum cryptography protocol via building an effective hardware for satisfying the single-photon source must requirement by controlling the value of mean photon number. This was approximately achieved in this work by building a driving circuit that provide very short pulses, less than 10 ns for Laser Diode with output maximum power of 0.99 mW and wavelength 650 nm. These short pulses enable getting faint laser pulses that were further attenuated to reach mean photon number equal to 0.08 or less.

Effect of imperfect Faraday mirrors on the security of a Faraday–Michelson quantum cryptography system

The one-way Faraday–Michelson system is a very useful practical quantum cryptography system where Faraday mirrors (FMs) play an important role. In this paper we analyze the security of this system against imperfect FMs. We consider the security loophole caused by imperfect FMs in Alice’s and Bob’s security zones. Then we implement a passive FM attack in this system. By changing the values of the imperfection parameters of Alice’s FMs, we calculate the quantum bit error rate between Alice and Bob induced by Eve and the probability that Eve obtains outcomes successfully. It is shown that the imperfection of one of Alice’s two FMs makes the system sensitive to an attack. Finally we give a modified key rate as a function of the FM imperfections. The security analysis indicates that both Alice’s and Bob’s imperfect FMs can compromise the secure key.

A grip on misbehaviour

Physicists have come up with a way to characterize and command untrusted quantum systems. Two experts discuss the significance of these findings for fundamental science and for practical quantum computation and cryptography. See Article p.456 Experimental interactions with quantum systems are necessarily limited, so is it possible to control and command such systems? Ben Reichardt et al. address this basic philosophical question about quantum mechanics with reference to quantum computation and cryptography, which involve imperfectly modelled or 'untrusted' systems. The authors describe a scheme that enables characterization of large quantum systems, providing a test of whether a claimed quantum computer is truly quantum. The results imply that it is possible to command an untrusted quantum system using classical interventions.

Quantum key distribution using quantum dot single-photon emitting diodes in the red and near infrared spectral range

We report on in-lab free space quantum key distribution (QKD) experiments over 40 cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range. In the case of infrared emitting devices, we achieve sifted key rates of 27.2 kbit s−1 (35.4 kbit s−1) at a quantum bit error rate (QBER) of 3.9% (3.8%) and a g(2)(0) value of 0.35 (0.49) at moderate (high) excitation. The red emitting diodes generate sifted keys at a rate of 95.0 kbit s−1 at a QBER of 4.1% and a g(2)(0) value of 0.49. This first successful proof of principle QKD experiment based on electrically operated semiconductor single-photon sources can be considered as a major step toward practical and efficient quantum cryptography scenarios.

Security proof of counterfactual quantum cryptography against general intercept-resend attacks and its vulnerability

Counterfactual quantum cryptography, recently proposed by Noh, is featured with no transmission of signal particles. This exhibits evident security advantages, such as its immunity to the well-known photon-number-splitting attack. In this paper, the theoretical security of counterfactual quantum cryptography protocol against the general intercept-resend attacks is proved by bounding the information of an eavesdropper Eve more tightly than in Yin's proposal [Phys. Rev. A 82 042335 (2010)]. It is also shown that practical counterfactual quantum cryptography implementations may be vulnerable when equipped with imperfect apparatuses, by proving that a negative key rate can be achieved when Eve launches a time-shift attack based on imperfect detector efficiency.

Development of Single-photon Detectors for Practical Quantum Cryptography

Development of Single-photon Detectors for Practical Quantum Cryptography

Practical quantum cryptography for secure free-space communications

Quantum Cryptography is an emerging technology in which two parties may simultaneously generate shared, secret cryptographic key material using the transmission of quantum states of light. The security of these transmissions is based on the inviolability of the laws of quantum mechanics and information – theoretically secure post- processing methods. An adversary can neither successfully tap the quantum transmissions, nor evade detection, owing to Heisenberg’s uncertainty principle. In this paper we describe the theory of quantum cryptography and the most recent results from our experimental free space system with which we have demonstrated for the first time the feasibility of quantum key generation over a point-to-point outdoor atmospheric path in daylight. We achieved a transmission distance of 0.5km which was limited only by the length of the test range. Our results provide strong evidence that cryptographic key material could be generated on the demand between a ground station and a satellite (or between two satellites), allowing a satellite to be securely re-keyed on orbit. We present a feasibility analysis of surface-to-satellite quantum key generation.

Cryptographic robustness of practical quantum cryptography: BB84 key distribution protocol

In real fiber-optic quantum cryptography systems, the avalanche photodiodes are not perfect, the source of quantum states is not a single-photon one, and the communication channel is lossy. For these reasons, key distribution is impossible under certain conditions for the system parameters. A simple analysis is performed to find relations between the parameters of real cryptography systems and the length of the quantum channel that guarantee secure quantum key distribution when the eavesdropper’s capabilities are limited only by fundamental laws of quantum mechanics while the devices employed by the legitimate users are based on current technologies. Critical values are determined for the rate of secure real-time key generation that can be reached under the current technology level. Calculations show that the upper bound on channel length can be as high as 300 km for imperfect photodetectors (avalanche photodiodes) with present-day quantum efficiency (η ≈ 20%) and dark count probability (p dark ∼ 10−7).

Beyond single-photon counting

The ability to distinguish how many photons comprise a particular state of light leads to significant benefits in practical quantum information processing and quantum cryptography. Superconducting nanostructures provide an effective solution at telecom wavelengths.

Extensible router for a quantum key distribution network

Building a quantum key distribution network is crucial for practical quantum cryptography. We present a scheme to build a star topology quantum key distribution network based on wavelength division multiplexing which, with current technology, can connect at least a hundred users. With the scheme, a 4-user demonstration network was built up and key exchanges were performed.

Error reconciliation for practical quantum cryptography

Error reconciliation is a necessary step for quantum key distribution process. In this paper, the correction ability of error reconciliation procedure which is called Winnow protocol is analyzed and estimated, moreover, it gives some expressions about it. The experiment results proved the capability of Winnow protocol are excellent. It is also indicates that the smallest N has the most correction efficiency where from the communication times between the legitimate parties and the data remaining efficiency consideration.