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Although quantum communication systems are being deployed on a global scale, their realistic security certification is not yet available. Here we present a security evaluation and improvement protocol for complete quantum communication systems. The protocol subdivides a system by defining seven system implementation sub-layers based on a hierarchical order of information flow; then it categorises the known system implementation imperfections by hardness of protection and practical risk. Next, an initial analysis report lists all potential loopholes in its quantum-optical part. It is followed by interactions with the system manufacturer, testing and patching most loopholes, and re-assessing their status. Our protocol has been applied on multiple commercial quantum key distribution systems to improve their security. A detailed description of our methodology is presented with the example of a subcarrier-wave system. Our protocol is a step towards future security evaluation and security certification standards., Comment: Improved readability and minor updates, Published in Scientific Reports volume 11, Article number: 5110 (2021), 17 pages, 3 figures, 3 tables

It is well-known that no local model - in theory - can simulate the outcome statistics of a Bell-type experiment as long as the detection efficiency is higher than a threshold value. For the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality this theoretical threshold value is $\eta_{\text{T}} = 2 (\sqrt{2}-1) \approx 0.8284$. On the other hand, Phys.\ Rev.\ Lett.\ 107, 170404 (2011) outlined an explicit practical model that can fake the CHSH inequality for a detection efficiency of up to $0.5$. In this work, we close this gap. More specifically, we propose a method to emulate a Bell inequality at the threshold detection efficiency using existing optical detector control techniques. For a Clauser-Horne-Shimony-Holt inequality, it emulates the CHSH violation predicted by quantum mechanics up to $\eta_{\text{T}}$. For the Garg-Mermin inequality - re-calibrated by incorporating non-detection events - our method emulates its exact local bound at any efficiency above the threshold. This confirms that attacks on secure quantum communication protocols based on Bell violation is a real threat if the detection efficiency loophole is not closed., Comment: 7 pages, 3 figures

[This corrects the article DOI: 10.1371/journal.pone.0236630.].

The ability of an eavesdropper (Eve) to perform an intercept-resend attack on a free-space quantum key distribution (QKD) receiver by precisely controlling the incidence angle of an attack laser has been previously demonstrated. However, such an attack could be ineffective in the presence of atmospheric turbulence due to beam wander and spatial mode aberrations induced by the air's varying index of refraction. We experimentally investigate the impact turbulence has on Eve's attack on a free-space polarization-encoding QKD receiver by emulating atmospheric turbulence with a spatial light modulator. Our results identify how well Eve would need to compensate for turbulence to perform a successful attack by either reducing her distance to the receiver, or using beam wavefront correction via adaptive optics. Furthermore, we use an entanglement-breaking scheme to find a theoretical limit on the turbulence strength that hinders Eve's attack., Comment: 10 pages, 5 figures

The ability of an eavesdropper to compromise the security of a quantum communication system by changing the angle of the incoming light is well-known. Randomizing the role of the detectors has been proposed to be an efficient countermeasure to this type of attack. Here we show that the proposed countermeasure can be bypassed if the attack is generalized by including more attack variables. Using the experimental data from existing literature, we show how randomization effectively prevents the initial attack but fails to do so when Eve generalizes her attack strategy. Our result and methodology could be used to scrutinize a free-space quantum communication receiver against detector-efficiency-mismatch type attacks.

In the last decade, efforts have been made to reconcile theoretical security with realistic imperfect implementations of quantum key distribution (QKD). Implementable countermeasures are proposed to patch the discovered loopholes. However, certain countermeasures are not as robust as would be expected. In this paper, we present a concrete example of ID Quantique's random-detector-efficiency countermeasure against detector blinding attacks. As a third-party tester, we have found that the first industrial implementation of this countermeasure is effective against the original blinding attack, but not immune to a modified blinding attack. Then, we implement and test a later full version of this countermeasure containing a security proof [C. C. W. Lim et al., IEEE Journal of Selected Topics in Quantum Electronics, 21, 6601305 (2015)]. We find that it is still vulnerable against the modified blinding attack, because an assumption about hardware characteristics on which the proof relies fails in practice., Comment: 12 pages, 12 figures

Quantum cryptography promises security based on the laws of physics with proofs of security against attackers of unlimited computational power. However, deviations from the original assumptions allow quantum hackers to compromise the system. We present a side channel attack that takes advantage of ventilation holes in optical devices to inject additional photons that can leak information about the secret key. We experimentally demonstrate light injection on an ID Quantique Clavis2 quantum key distribution platform and show that this may help an attacker to learn information about the secret key. We then apply the same technique to a prototype quantum random number generator and show that its output is biased by injected light. This shows that light injection is a potential security risk that should be addressed during the design of quantum information processing devices.

Quantum key distribution (QKD) promises information theoretic secure key as long as the device performs as assumed in the theoretical model. One of the assumptions is an absence of information leakage about individual photon detection outcomes of the receiver unit. Here we investigate the information leakage from a QKD receiver due to photon emission caused by detection events in single-photon detectors (backflash). We test commercial silicon avalanche photodiodes and a photomultiplier tube, and find that the former emit backflashes. We study the spectral, timing and polarization characteristics of these backflash photons. We experimentally demonstrate on a free-space QKD receiver that an eavesdropper can distinguish which detector has clicked inside it, and thus acquire secret information. A set of countermeasures both in theory and on the physical devices are discussed., Comment: 9 pages, 7 figures

We demonstrate the experimental feasibility of a Trojan-horse attack that remains nearly invisible to the single-photon detectors employed in practical quantum key distribution (QKD) systems, such as Clavis2 from ID Quantique. We perform a detailed numerical comparison of the attack performance against Scarani-Acin-Ribordy-Gisin (SARG04) QKD protocol at 1924nm versus that at 1536nm. The attack strategy was proposed earlier but found to be unsuccessful at the latter wavelength, as reported in N.~Jain et al., New J. Phys. 16, 123030 (2014). However at 1924nm, we show experimentally that the noise response of the detectors to bright pulses is greatly reduced, and show by modeling that the same attack will succeed. The invisible nature of the attack poses a threat to the security of practical QKD if proper countermeasures are not adopted., Comment: 8 pages, 3 figures, due to problem in the compilation of bibliography, we are uploading a corrected version

Detector-device-independent quantum key distribution (ddiQKD) held the promise of being robust to detector side-channels, a major security loophole in QKD implementations. In contrast to what has been claimed, however, we demonstrate that the security of ddiQKD is not based on post-selected entanglement, and we introduce various eavesdropping strategies that show that ddiQKD is in fact insecure against detector side-channel attacks as well as against other attacks that exploit device's imperfections of the receiver. Our attacks are valid even when the QKD apparatuses are built by the legitimate users of the system themselves, and thus free of malicious modifications, which is a key assumption in ddiQKD., 7 pages, 5 figures, 1 table

A security evaluation against the finite-key-size effect was performed for a commercial plug-and-play quantum key distribution (QKD) system. We demonstrate the ability of an eavesdropper to force the system to distill key from a smaller length of sifted-key. We also derive a key-rate equation that is specific for this system. This equation provides bounds above the upper bound of secure key under finite-key-size analysis. From this equation and our experimental data, we show that the keys that have been distilled from the smaller sifted-key size fall above our bound. Thus, their security is not covered by finite-key-size analysis. Experimentally, we could consistently force the system to generate the key outside of the bound. We also test manufacturer's software update. Although all the keys after the patch fall under our bound, their security cannot be guaranteed under this analysis. Our methodology can be used for security certification and standardization of QKD systems., 6 pages, 3 figures, accepted for publication in Quantum Science and Technology

Decoy-state quantum key distribution (QKD) is a standard technique in current quantum cryptographic implementations. Unfortunately, existing experiments have two important drawbacks: the state preparation is assumed to be perfect without errors and the employed security proofs do not fully consider the finite-key effects for general attacks. These two drawbacks mean that existing experiments are not guaranteed to be secure in practice. Here, we perform an experiment that for the first time shows secure QKD with imperfect state preparations over long distances and achieves rigorous finite-key security bounds for decoy-state QKD against coherent attacks in the universally composable framework. We quantify the source flaws experimentally and demonstrate a QKD implementation that is tolerant to channel loss despite the source flaws. Our implementation considers more real-world problems than most previous experiments and our theory can be applied to general QKD systems. These features constitute a step towards secure QKD with imperfect devices., Comment: 12 pages, 4 figures, updated experiment and theory

In free-space quantum key distribution (QKD), the sensitivity of the receiver's detector channels may depend differently on the spatial mode of incoming photons. Consequently, an attacker can control the spatial mode to break security. We experimentally investigate a standard polarization QKD receiver, and identify sources of efficiency mismatch in its optical scheme. We model a practical intercept-and-resend attack and show that it would break security in most situations. We show experimentally that adding an appropriately chosen spatial filter at the receiver's entrance is an effective countermeasure., Comment: 7 pages, 4 figures

We demonstrate the experimental feasibility of a Trojan-horse attack that remains nearly invisible to the single-photon detectors employed in practical quantum key distribution (QKD) systems, such as Clavis2 from ID Quantique. We perform a detailed numerical comparison of the attack performance against Scarani-Ac´ın-Ribordy-Gisin (SARG04) QKD protocol at 1924 nm versus that at 1536 nm. The attack strategy was proposed earlier but found to be unsuccessful at the latter wavelength, as reported in N. Jain et al., New J. Phys. 16, 123030 (2014). However at 1924 nm, we show experimentally that the noise response of the detectors to bright pulses is greatly reduced, and show by modeling that the same attack will succeed. The invisible nature of the attack poses a threat to the security of practical QKD if proper countermeasures are not adopted.

The security of quantum communication using a weak coherent source requires an accurate knowledge of the source's mean photon number. Finite calibration precision or an active manipulation by an attacker may cause the actual emitted photon number to deviate from the known value. We model effects of this deviation on the security of three quantum communication protocols: the Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol without decoy states, Scarani-Acin-Ribordy-Gisin 2004 (SARG04) QKD protocol, and a coin-tossing protocol. For QKD, we model both a strong attack using technology possible in principle, and a realistic attack bounded by today's technology. To maintain the mean photon number in two-way systems, such as plug-and-play and relativistic quantum cryptography schemes, bright pulse energy incoming from the communication channel must be monitored. Implementation of a monitoring detector has largely been ignored so far, except for ID Quantique's commercial QKD system Clavis2. We scrutinize this implementation for security problems, and show that designing a hack-proof pulse-energy-measuring detector is far from trivial. Indeed the first implementation has three serious flaws confirmed experimentally, each of which may be exploited in a cleverly constructed Trojan-horse attack. We discuss requirements for a loophole-free implementation of the monitoring detector., Comment: 15 pages, 13 figures, Improved Introduction and Conclusion, Published in Physical Review A, Accepted at QCrypt 2014, 4th international conference on quantum cryptography, September 1-5, 2014 in Paris, France http://2014.qcrypt.net/program/

Path of a photon was considered to be used as qubit for implementing quantum NOT gate. An experimental setup was designed using a Mach-Zehnder interferometer constructed from simple optical devices such as He-Ne lasers, beam splitters, mirrors, attenuators etc. and quantum interference was utilized for multi photon case. A number of experiments were performed and the outcome of the experiment was observed by taking a number of photographs of the output beam for a number of input intensities. The observations proved that the setup indeed performed the required NOT transformation with good reliability at multi photon case and thus can be usefully used as a quantum not gate.

Transverse modes of electromagnetic fields are proposed to be encoded as qubits for the implementation of a quantum NOT gate. The possibility of this encoding scheme was designed, simulated and verified through rigorous numerical analyses using the Beam propagation method. The proposed gate has the potential of being more compact and easily realizable than some other optical implementation proposals since it is based on planar light wave circuit technology and can be constructed very easily by using a planar light wave technology. The Results proved that the design is able to perform the required NOT transformation and the conclusion was reached that the model was a successful implementation of a quantum NOT gate.

Transverse modes of optical fields are proposed to be used as qubits for the implementation of quantum Hadamard gate. Mach-Zehnder Interferometer (MZI), phase shifter, y-junction beam splitters and other optical devices are utilized to realize a quantum Hadamard gate. The possibility of this encoding scheme was designed, simulated and verified through rigorous numerical analyses using the Beam propagation method. The proposed gate has the potential of being more compact and easily realizable than some other optical implementation proposals since it is based on planar light wave circuit technology and can be constructed very easily by using a Mach-Zehnder Interferometer. The Results proved that the design is able to perform the required Hadamard transformation and the conclusion was reached that the model was a successful implementation of a quantum Hadamard gate.