Perfect Single Photon Source Research Articles

Free-Space and Fiber-Integrated Measurement-Device-Independent Quantum Key Distribution under High Background Noise.

Measurement-device-independent quantum key distribution (MDI QKD) provides immunity against all attacks targeting measurement devices. It is essential to implement MDI QKD in the future global-scale quantum communication network. Toward this goal, we demonstrate a robust MDI QKD fully covering daytime, overcoming the high background noise that prevents BB84 protocol even when using a perfect single-photon source. Based on this, we establish a hybrid quantum communication network that integrates free-space and fiber channels through Hong-Ou-Mandle (HOM) interference. Additionally, we investigate the feasibility of implementing HOM interference with moving satellites. Our results serve as a significant cornerstone for future integrated space-ground quantum communication networks that incorporate measurement-device-independent security.

SUPER Scheme in Action: Experimental Demonstration of Red-Detuned Excitation of a Quantum Emitter.

The quest for the perfect single-photon source includes finding the optimal protocol for exciting the quantum emitter. Coherent optical excitation was, up until now, achieved by tuning the laser pulses to the transition frequency of the emitter, either directly or in average. Recently, it was theoretically discovered that an excitation with two red-detuned pulses is also possible where neither of which would yield a significant upper-level population individually. We show that the so-called swing-up of quantum emitter population (SUPER) scheme can be implemented experimentally with similar properties to existing schemes by precise amplitude shaping of a broadband pulse. Because of its truly off-resonant nature, this scheme has the prospect of powering high-purity photon sources with superior photon count rate.

The quest for a perfect single-photon source

A quantum-dot-based single-photon source with a polarized single-photon efficiency of 60% and an indistinguishability of 0.975 is demonstrated by introducing a small asymmetry in a photonic cavity structure.

Round-robin differential-phase-shift quantum key distribution with heralded pair-coherent sources

Round-robin differential-phase-shift (RRDPS) quantum key distribution (QKD) scheme provides an effective way to overcome the signal disturbance from the transmission process. However, most RRDPS-QKD schemes use weak coherent pulses (WCPs) as the replacement of the perfect single-photon source. Considering the heralded pair-coherent source (HPCS) can efficiently remove the shortcomings of WCPs, we propose a RRDPS-QKD scheme with HPCS in this paper. Both infinite-intensity decoy-state method and practical three-intensity decoy-state method are adopted to discuss the tight bound of the key rate of the proposed scheme. The results show that HPCS is a better candidate for the replacement of the perfect single-photon source, and both the key rate and the transmission distance are greatly increased in comparison with those results with WCPs when the length of the pulse trains is small. Simultaneously, the performance of the proposed scheme using three-intensity decoy states is close to that result using infinite-intensity decoy states when the length of pulse trains is small.

Measurement-Device-Independent Quantum Key Distribution Over a 404km Optical Fiber.

Measurement-device-independent quantum key distribution (MDIQKD) with the decoy-state method negates security threats of both the imperfect single-photon source and detection losses. Lengthening the distance and improving the key rate of quantum key distribution (QKD) are vital issues in practical applications of QKD. Herein, we report the results of MDIQKD over 404km of ultralow-loss optical fiber and 311km of a standard optical fiber while employing an optimized four-intensity decoy-state method. This record-breaking implementation of the MDIQKD method not only provides a new distance record for both MDIQKD and all types of QKD systems but also, more significantly, achieves a distance that the traditional Bennett-Brassard 1984 QKD would not be able to achieve with the same detection devices even with ideal single-photon sources. This work represents a significant step toward proving and developing feasible long-distance QKD.

A long-distance quantum key distribution scheme based on pre-detection of optical pulse with auxiliary state**Project supported by the National Natural Science Foundation of China (Grant No. 61372076), the Programme of Introducing Talents of Discipline to Universities, China (Grant No. B08038), and the Fundamental Research Funds for the Central Universities, China (Grant No. K5051201021).

We construct a circuit based on PBS and CNOT gates, which can be used to determine whether the input pulse is empty or not according to the detection result of the auxiliary state, while the input state will not be changed. The circuit can be treated as a pre-detection device. Equipping the pre-detection device in the front of the receiver of the quantum key distribution (QKD) can reduce the influence of the dark count of the detector, hence increasing the secure communication distance significantly. Simulation results show that the secure communication distance can reach 516 km and 479 km for QKD with perfect single photon source and decoy-state QKD with weak coherent photon source, respectively.

Security of counterfactual quantum cryptography

Recently, a "counterfactual" quantum key distribution scheme was proposed by Tae-Gon Noh [1]. In this scheme, two legitimate distant peers may share secret keys even when the information carriers are not traveled in the quantum channel. We find that this protocol is equivalent to an entanglement distillation protocol (EDP). According to this equivalence, a strict security proof and the asymptotic key bit rate are both obtained when perfect single photon source is applied and Trojan-horse attack can be detected. We also find that the security of this scheme is deeply related with not only the bit error rate but also the yields of photons. And our security proof may shed light on security of other two-way protocols.

New method of decoy state quantum key distribution with a heralded single-photon source

We propose a new method of decoy state quantum key distribution with a heralded single-photon source. Alice uses the parametric down-conversion to generate entangled photon-pairs, one of the pair is used as heralding photon. According to the results of the trigger detector the heralded photons are divided into trigger and non-trigger sets.The states of the photons in the trigger set are used as the signal states and the ones in the non-trigger sets are used as the decoy states. Because of the efficiency of the trigger detector, the two sets both have photons. The yield and error rate of the single-photon are estimated through the yield and error rate of the two sets. This method does not need changing the intensity of the photon source, is easy to implement. Analysis results show that this method can reach the same security distance as with a perfect single photon source; the key generation rate is increased a lot compared with the method with a heralded photon source and is approximately two thirds the rate of the three intensities decoy state quantum key distribution with a heralded photon source.

Quantum key distribution with passive decoy state selection

We propose a quantum key distribution scheme which closely matches the performance of a perfect single photon source. It nearly attains the physical upper bound in terms of key generation rate and maximally achievable distance. Our scheme relies on a practical setup based on a parametric downconversion source and present day, nonideal photon-number detection. Arbitrary experimental imperfections which lead to bit errors are included. We select decoy states by classical postprocessing. This allows one to improve the effective signal statistics and achievable distance.

Comment on “Quantum Key Distribution with Blind Polarization Bases”

We show an eavesdropping scheme, by which the eavesdropper can achieve the full information of the key against the protocol [Kye et al., PRL 95 040501 (2005)] with a probability of unity and will not be discovered by the the legitimate users, even in the case that they have the perfect single-photon source and the loseless channel.

SECURITY OF QUANTUM KEY DISTRIBUTION WITH STRONG PHASE-REFERENCE PULSE

In the BB84 protocol with a perfect single photon source, the key rate decreases linearly with the transmission η of the channel. If we simply replace this source with a weak coherent-state pulse, the key rate drops more rapidly (as O(η2)) since the presence of multiple photons favors the eavesdropper. Here we discuss the unconditional security of a quantum key distribution protocol in which bit values are encoded in the phase of a weak coherent-state pulse relative to a strong reference pulse, which is essentially the one proposed by Bennett in 1992 (the B92 scheme). We show that in the limit of high loss in the transmission channel, we can construct a secret key with a rate proportional to the transmission η of the channel.

Higher-security thresholds for quantum key distribution by improved analysis of dark counts

We discuss the potential of quantum key distribution (QKD) for long distance communication by proposing a new analysis of the errors caused by dark counts. We give sufficient conditions for a considerable improvement of the key generation rates and the security thresholds of well-known QKD protocols such as Bennett-Brassard 1984, Phoenix-Barnett-Chefles 2000, and the six-state protocol. This analysis is applicable to other QKD protocols like Bennett 1992. We examine two scenarios: a sender using a perfect single-photon source and a sender using a Poissonian source.