Measurement-device-independent Quantum Key Distribution Protocol Research Articles

Measurement-device-independenization of quantum key distribution protocols

Quantum key distribution (QKD) allows the legitimate partner to establish a secret key whose security only depends on physical laws. In recent years, research on QKD by employing insecure measurement devices, namely measurement-device-independent QKD (MDI-QKD) is increased. MDI-QKD removes all attacks on measurement devices and thus an untrusted third party can be employed for measuring. However, a weakness of previous MDI-QKD protocols is the need for joint measurements such as Bell measurements whose efficiency is low in practice. On the other hand, can all QKD protocols become measurement-device-independent remains a problem. In this paper, we present a scheme making prepare-measure QKD protocols become MDI-QKD protocols, called ′measurement−device−independenization′, which does not need to employ joint measurements and could be efficiently implemented by weak coherence sources. The protocol might look like the detector-device-independent (DDI) protocols but it is also secure under the Trojan horse attack. To illustrate this, we investigate the photon-number-adding (PNA) attack and present a scheme, called ′photon−number−purification′, which can also be employed to close loopholes for previous protocols such as DDI and plug-and-play ones.

Detecting a Photon-Number Splitting Attack in Decoy-State Measurement-Device-Independent Quantum Key Distribution via Statistical Hypothesis Testing.

Measurement-device-independent quantum key distribution (MDI-QKD) is innately immune to all detection-side attacks. Due to the limitations of technology, most MDI-QKD protocols use weak coherent photon sources (WCPs), which may suffer from a photon-number splitting (PNS) attack from eavesdroppers. Therefore, the existing MDI-QKD protocols also need the decoy-state method, which can resist PNS attacks very well. However, the existing decoy-state methods do not attend to the existence of PNS attacks, and the secure keys are only generated by single-photon components. In fact, multiphoton pulses can also form secure keys if we can confirm that there is no PNS attack. For simplicity, we only analyze the weaker version of a PNS attack in which a legitimate user’s pulse count rate changes significantly after the attack. In this paper, under the null hypothesis of no PNS attack, we first determine whether there is an attack or not by retrieving the missing information of the existing decoy-state MDI-QKD protocols via statistical hypothesis testing, extract a normal distribution statistic, and provide a detection method and the corresponding Type I error probability. If the result is judged to be an attack, we use the existing decoy-state method to estimate the secure key rate. Otherwise, all pulses with the same basis leading to successful Bell state measurement (BSM) events including both single-photon pulses and multiphoton pulses can be used to generate secure keys, and we give the formula of the secure key rate in this case. Finally, based on actual experimental data from other literature, the associated experimental results (e.g., the significance level is 5%) show the correctness of our method.

Unbalanced-basis-misalignment-tolerant measurement-device-independent quantum key distribution

Measurement-device-independent quantum key distribution (MDIQKD) is a revolutionary protocol since it is physically immune to all attacks on the detection side. However, the protocol still keeps the strict assumptions on the source side that specify that the four BB84 states must be perfectly prepared to ensure security. Some protocols release part of the assumptions in the encoding system to keep the practical security, but the performance would be dramatically reduced. In this work, we present a MDIQKD protocol that requires less knowledge of the encoding system to combat the troublesome modulation errors and fluctuations. We have also experimentally demonstrated the protocol. The result indicates a high performance and good security for practical applications. Its robustness and flexibility also exhibit a good value for complex scenarios such as the QKD networks.

Measurement-device-independent quantum key distribution protocol with phase post-selection

Measurement-device-independent quantum key distribution (MDI-QKD) protocol can remove all the loopholes of the detection devices and, thus, has attracted much attention. Based on the technique of single-photon interference, we propose a modified MDI-QKD protocol with phase post-selection. We prove the security of the announcement of the private phases in the X basis and show how to apply the phase post-selection method to the double-scanning four-intensity MDI-QKD protocol. The numerical results show that the phase post-selection method can significantly improve the key rates at all distances. In the double-scanning method, two parameters need to be scanned in the calculation of the final key rate, and the global parameter optimization is pretty time-consuming. We propose an accelerated method that can greatly reduce the running time of the global parameter optimization program. This makes the method practically useful in an unstable channel.

Measurement-device-independent quantum key distribution with classical Bob and no joint measurement

Measurement-device-independent quantum key distribution (MDI-QKD) provides a method for secret communication whose security does not rely on trusted measurement devices. In all existing MDI-QKD protocols, the participant Charlie has to perform the Bell state measurement or other joint measurements. Here we propose an MDI-QKD protocol which requires individual measurements only. Meanwhile, all operations of the receiver Bob are classical, without the need for preparing and measuring quantum systems. Thus the implementation of the protocol has a lower technical requirement on Bob and Charlie.

Polarization discriminated time‐bin phase‐encoding measurement‐device‐independent quantum key distribution

The correctness of measurement-device-independent quantum key distribution (MDI-QKD) has been demonstrated in tremendous amount of researches. There is an interesting proposal which can collect all coincidences by transforming two pairs of orthogonal polarization states into the encoding states of MDI-QKD. The attempt is meaningful. Here, based on this proposal, we realize a proof-of-principle demonstration. Only one phase modulator is applied to encode the qubits. The phase-reference-frame is aligned by modulating the phase modulator of the phase modulation unit. We modulate the signals into three intensities, and a secure key rate of bits per pulse is successfully demonstrated. Our experimental system simplifies the encoding process and has the potential to implement the reference-frame-independent MDI-QKD protocol.

Improved analysis of measurement-device-independent quantum key distribution with non-phase-randomized coherent states.

Measurement-device-independent quantum key distribution (MDI-QKD) can remove all detector side-channel attacks, which can be implemented with phase-randomized coherent states (PRCS) or non-phase-randomized coherent states (NPRCS). In this paper, we focus on the MDI-QKD protocol with NPRCS and provide an improved analysis. In contrast with the original MDI-QKD with NPRCS which modulates the same intensity of coherent states in the key and test bases, we propose to modulate different intensities of coherent states in the key and test bases. Simulation results show that the secret key rate and transmission distance of MDI-QKD with NPRCS can be significantly improved. Furthermore, it is noteworthy that the modulation of different intensities does not bring extra complexity for experimental researchers, which can be easily done by adding an intensity modulator.

Experimental measurement-device-independent quantum key distribution with the double-scanning method.

The measurement-device-independent quantum key distribution (MDI-QKD) can be immune to all detector side-channel attacks. Moreover, it can be easily implemented combining with the matured decoy-state methods under current technology. It, thus, seems a very promising candidate in practical implementation of quantum communications. However, it suffers from a severe finite-data-size effect in most existing MDI-QKD protocols, resulting in relatively low key rates. Recently, Jiang et al. [Phys. Rev. A103, 012402 (2021).PLRAAN1050-294710.1103/PhysRevA.103.012402] proposed a double-scanning method to drastically increase the key rate of MDI-QKD. Based on Jiang et al.'s theoretical work, here we for the first time, to the best of our knowledge, implement the double-scanning method into MDI-QKD and carry out corresponding experimental demonstration. With a moderate number of pulses of 1010, we can achieve 150 km secure transmission distance, which is impossible with all former methods. Therefore, our present work paves the way toward practical implementation of MDI-QKD.

Feasible high-dimensional measurement-device-independent quantum key distribution

Measurement-device-independent quantum key distribution (MDI-QKD) can resist all possible attacks from practical measurement devices. High-dimensional (HD) MDI-QKD in multiple degrees of freedom (DOFs) can efficiently increase the key generation rate and has strong noise and error resistance. However, previous HD MDI-QKD protocols in multiple DOFs required complete hyperentangled Bell state analysis (HBSA), which cannot be realized under current experimental conditions and largely limits the practicality of the HD MDI-QKD. In this paper, we propose an HD MDI-QKD protocol in multiple DOFs which adopts a linear-optical HBSA. This adoption of the linear-optical HBSA makes our HD MDI-QKD feasible under current experimental technology. The key generation rate of our MDI-QKD protocol is about eight times of the original MDI-QKD protocol. Our HD MDI-QKD protocol may have potential applications in current and future quantum communication fields.

Measurement-device-independent quantum key distribution protocols against collective noise

In order to eliminate the influence of the channel noise, two new measurement-device-independent quantum key distribution (MDI-QKD) protocols are proposed with logical quantum states. They can resist collective-dephasing noise and collective-rotation noise, respectively. This paper produces logical quantum states by adding the auxiliary light sources, the CNOT operations and the Hadamard transforms in the system model. The main light sources and auxiliary light sources are flexible and easily implemented, since they can be weak coherent state (WCS) sources, heralded single-photon sources (HSPSs) or heralded pair coherent state (HPCS) sources. To generate one key bit, the new MDI-QKD protocols only need one logical qubit with two particles so that they have high qubit efficiency. Moreover, the new protocols also use partial Bell-state measurement (BSM) which is very easily implemented with existing technologies.

Measurement-device-independent quantum key distribution with leaky sources

Measurement-device-independent quantum key distribution (MDI-QKD) can remove all detection side-channels from quantum communication systems. The security proofs require, however, that certain assumptions on the sources are satisfied. This includes, for instance, the requirement that there is no information leakage from the transmitters of the senders, which unfortunately is very difficult to guarantee in practice. In this paper we relax this unrealistic assumption by presenting a general formalism to prove the security of MDI-QKD with leaky sources. With this formalism, we analyze the finite-key security of two prominent MDI-QKD schemes—a symmetric three-intensity decoy-state MDI-QKD protocol and a four-intensity decoy-state MDI-QKD protocol—and determine their robustness against information leakage from both the intensity modulator and the phase modulator of the transmitters. Our work shows that MDI-QKD is feasible within a reasonable time frame of signal transmission given that the sources are sufficiently isolated. Thus, it provides an essential reference for experimentalists to ensure the security of implementations of MDI-QKD in the presence of information leakage.

Higher key rate of measurement-device-independent quantum key distribution through joint data processing

We propose a method named as double-scanning method, to improve the key rate of measurement-device-independent quantum key distribution (MDI-QKD) drastically. In the method, two parameters are scanned simultaneously to tightly estimate the counts of single-photon pairs and the phase-flip error rate jointly. Numerical results show that the method in this work can improve the key rate by $35\%-280\%$ in a typical experimental set-up. Besides, we study the optimization of MDI-QKD protocol with all parameters including the source parameters and failure probability parameters, over symmetric channel or asymmetric channel. Compared with the optimized results with only the source parameters, the all-parameter-optimization method could improve the key rate by about $10\%$.

Long-Distance Free-Space Measurement-Device-Independent Quantum Key Distribution.

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates, and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over a free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step toward satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.

Measurement-device-independent quantum key distribution of multiple degrees of freedom of a single photon

Measurement-device-independent quantum key distribution (MDI-QKD) provides us a powerful approach to resist all attacks at detection side. Besides the unconditional security, people also seek for high key generation rate, but MDI-QKD has relatively low key generation rate. In this paper, we provide an efficient approach to increase the key generation rate of MDI-QKD by adopting multiple degrees of freedom (DOFs) of single photons to generate keys. Compared with other high-dimension MDI-QKD protocols encoding in one DOF, our protocol is more flexible, for our protocol generating keys in independent subsystems and the detection failure or error in a DOF not affecting the information encoding in other DOFs. Based on above features, our MDI-QKD protocol may have potential application in future quantum communication field.

Experimental Hong-Ou-Mandel interference using two independent heralded single-photon sources.

Hong-Ou-Mandel (HOM) interference is one of the most important experimental phenomena in quantum optics. It has drawn considerable attention with respect to quantum cryptography and quantum communication because of the advent of the measurement device independent (MDI) quantum key distribution (QKD) protocol. Here, we realize HOM interference, having a visibility of approximately 38.1%, using two independent heralded single-photon sources (HSPSs). The HOM interference between two independent HSPSs is a core technology for realizing the long-distance MDI QKD protocol, the quantum coin-tossing protocol, and other quantum cryptography protocols.

The Decoy-State Measurement-Device-Independent Quantum Key Distribution with Heralded Single-Photon Source

Based on heralded single-photon source (HSPS), a decoy-state measurement-device-independent quantum key distribution (MDI-QKD) protocol is proposed in this paper. The MDI-QKD protocol mainly uses orbital angular momentum (OAM) states and pulse position modulation (PPM) technology to realize the coding of the signal states in heralded single-photon source. The three-intensity decoy states are used to avoid the attacks against the light source. Moreover, the formula of key generation rate is given by computing the lower bound of the yield of single-photon pairs and the upper bound of the error rate of single-photon pairs. Numerical simulation shows that the new MDI-QKD protocol has high key generation rate and low error rate. Moreover, the secure communication distance can be up to 450 km.

Measurement Device Independent Quantum Key Distribution Based on Orbital Angular Momentum under Parametric Light Source

On the one hand, existing measurement device independent quantum key distribution (MDI-QKD) protocols have usually adopted single photon source (SPS) and weak coherent photon (WCP), however, these protocols have suffered from multi-photon problem brought from photon splitter number attacks. On the other hand, the orbital angular momentum (OAM)-MDI-QKD protocol does not need to compare and adjust the reference frame, solving the dependency of the base in the MDI-QKD protocol. Given that, we propose the OAM-MDI-QKD protocol based on the parametric light sources which mainly include single-photon-added-coherent (SPACS) and heralded single-photon sources (HSPS). Due to the stability of OAM and the participation of parametric light sources, the performance of MDI-QKD protocol gradually approaches the ideal situation. Numerical simulation shows that compared with WCP scheme, HSPS and SPACS schemes have increased the maximum secure transmission distance by 30 km and 40 km respectively.

The enhanced measurement-device-independent quantum key distribution with heralded pair coherent state

With heralded pair coherent states (HPCS), orbital angular momentum (OAM) states and pulse position modulation (PPM) technology, a decoy-state measurement-device-independent quantum key distribution (MDI-QKD) protocol is proposed. OAM states and PPM technology are used to realize the coding of the signal states in the HPCS light source. The use of HPCS light source, OAM coding and PPM coding cannot only reduce the error rate but also improve the key generation rate and communication distance. The new MDI-QKD protocol also employs three-intensity decoy states to avoid the attacks against the light source. By calculating the error rate and key generation rate, the performance of the MDI-QKD protocol is analyzed. Numerical simulation shows that the protocol has very low error rate and very high key generation rate. Moreover, the maximum communication distance can reach 455 km.

Laser-seeding Attack in Quantum Key Distribution

Quantum key distribution (QKD) based on the laws of quantum physics allows the secure distribution of secret keys over an insecure channel. Unfortunately, imperfect implementations of QKD compromise its information-theoretical security. Measurement-device-independent quantum key distribution (MDI-QKD) is a promising approach to remove all side channels from the measurement unit, which is regarded as the "Achilles' heel" of QKD. An essential assumption in MDI-QKD is however that the sources are trusted. Here we experimentally demonstrate that a practical source based on a semiconductor laser diode is vulnerable to a laser seeding attack, in which light injected from the communication line into the laser results in an increase of the intensities of the prepared states. The unnoticed increase of intensity may compromise the security of QKD, as we show theoretically for the prepare-and-measure decoy-state BB84 and MDI-QKD protocols. Our theoretical security analysis is general and can be applied to any vulnerability that increases the intensity of the emitted pulses. Moreover, a laser seeding attack might be launched as well against decoy-state based quantum cryptographic protocols beyond QKD.

Measurement-device-independent quantum secure direct communication

Quantum secure direct communication (QSDC) is a unique technique, which supports the secure transmission of confidential information directly through a quantum channel without the need for a secret key and for ciphertext. Hence this secure communication protocol fundamentally differs from its conventional counterparts. In this article, we report the first measurement-device-independent (MDI) QSDC protocol relying on sequences of entangled photon pairs and single photons. Explicitly, it eliminates the security loopholes associated with the measurement device. Additionally, this MDI technique is capable of doubling the communication distance of its conventional counterpart operating without using our MDI technique. We also conceive a protocol associated with linear optical Bell-basis measurements, where only two of the four Bell-basis states could be measured. When the number of qubits in a sequence reduces to 1, the MDI-QSDC protocol degenerates to a deterministic MDI quantum key distribution protocol.