Error filtration is a method for encoding the quantum state of a single particle into a higher dimensional Hilbert space in such a way that it becomes less sensitive to noise. We have realized a fiber optics demonstration of this method and illustrated its potentialities by carrying out the optical part of a quantum key distribution scheme over a line whose phase noise is too high for a standard implementation of BB84 to be secure. By filtering out the noise, a bit error rate of 15.3% +/- 0.1%, which is beyond the security limit, can be reduced to 10.6% +/- 0.1%, thereby guaranteeing the cryptographic security.
Error filtration is a method for encoding the quantum state of a single particle into a higher dimensional Hilbert space in such a way that it becomes less sensitive to noise. We have realized a fiber optics demonstration of this method and illustrated its potentialities by carrying out the optical part of a quantum key distribution scheme over a line whose phase noise is too high for a standard implementation of BB84 to be secure. By filtering out the noise, a bit error rate of 15.3% +/- 0.1%, which is beyond the security limit, can be reduced to 10.6% +/- 0.1%, thereby guaranteeing the cryptographic security.
We show how the techniques developed for long distance quantum key distribution in op-tical fibers can be used to demonstrate other quantum information processing and com-munication protocols. We present a fiber optics realization of the Deutsch-Jozsa and Bernstein-Vazirani algorithms. We describe a method, called ”error filtration”, for re-ducing errors in quantum communication channels, and present an experimental imple-mentation thereof. We discuss the cryptographic primitive of string flipping, and present an experimental implementation which has higher security than achievable using any classical protocol.
Improving the tolerance of channel noise is an important task for devising and implementing quantum key distribution (QKD) protocols. Quantum phase-flip error rejection (QPFER) code [Phys. Rev. Lett.92, 077902 (2004)PRLTAO0031-900710.1103/PhysRevLett.92.077902] has been introduced by Wang to increase the tolerable phase-flip noise of QKD implementations. However, an experiment that demonstrates its advantages is still missing. Here, we experimentally verify the QPFER code with the assistance of two photon quantum states generated by spontaneous parametric downconversion. The methods of parity check and postselection are introduced to the protocol for achieving the phase-flipping rejection. Comparing with the standard realization of the single photon polarization encoding BB84 scheme, the quantum error rate after decoding is obviously reduced when the probability of channel noise is less than 25%. The experiment results also show that QPFER protocol can reduce error rate, obtain a higher key rate, and be robust in the noisy channel when the noise level is in a proper region.
In this thesis, Quantum Key Distribution and quantum communication methods based on high-dimensional spatial coding of light are developed. Their security is based on the fundamental quantum nature of light and profits from the highdimensional Hilbert spaces offered intrinsically by imaging optics. An overview of classical information theory and quantum information is given to introduce the field of quantum cryptography. The advantages of larger dimensional alphabets and the security of the standard two-bases BB84 protocol is shown. The spatial states used for encoding are introduced. An analytic expression for the upper bound on the mutual information for these states is derived, including multiphoton states, detector noise and beam broadening. High-dimensional encoding of single photons is experimentally realized, reaching 10.5 bit per received photon. The dependence of the mutual information on the number of detector pixels is discussed and the experimental values are compared with the theoretical upper bound. It was shown that a standard error-correcting LDPC code is sufficient to achieve practically error-free communication. By adding a second mutually unbiased basis, a large-alphabet QKD system is experimentally realized and characterized. The security of this BB84-like protocol is analyzed in terms of intercept-resend and collective attacks. The key rate after postprocessing is analyzed under realistic circumstances, including finite key length. Finally, a new quantum communication method is demonstrated, which is based on encoding information into wavefronts decomposed over guided modes of a multimode fiber. At the end a step back is made and the similarities and differences of several quantum authentication and quantum cryptography schemes as well as Quantum Data Locking are discussed. This involves comparison of the characteristics of the classical channel, the quantum channel and the necessary dimension of the Hilbert space.
At the fundamental level, quantum communication is ultimately limited by noise. For instance, quantum signals cannot be amplified without the introduction of noise in the amplified states. Furthermore, photon loss reduces the signal-to-noise ratio, accentuating the effect of noise. Thus, most of the efforts in quantum communications have been directed towards overcoming noise to achieve longer communication distances, larger secret key rates, or to operate in noisier environmental conditions. Here, we propose and experimentally demonstrate a platform for quantum communication based on ultrafast optical techniques. In particular, our scheme enables the experimental realization of high-rates and quantum signal filtering approaching a single spectro-temporal mode, resulting in a dramatic reduction in channel noise. By experimentally realizing a 1-ps optically induced temporal gate, we show that ultrafast time filtering can result in an improvement in noise tolerance by a factor of up to 1200 compared to a 2-ns electronic filter enabling daytime quantum key distribution or quantum communication in bright fibers.
Quantum key distribution (QKD) uniquely allows the distribution of cryptographic keys with security verified by quantum mechanical limits.Both protocol execution and subsequent applications require the assistance of classical data communication channels.While using separate fibers is one option, it is economically more viable if data and quantum signals are simultaneously transmitted through a single fiber.However, noise-photon contamination arising from the intense data signal has severely restricted both the QKD distances and secure key rates.Here, we exploit a novel temporal-filtering effect for noisephoton rejection.This allows high-bit-rate QKD over fibers up to 90 km in length and populated with error-free bidirectional Gb=s data communications.With a high-bit rate and range sufficient for important information infrastructures, such as smart cities and 10-Gbit Ethernet, QKD is a significant step closer toward wide-scale deployment in fiber networks.
Abstract As quantum computing continues to demonstrate its potential to break through existing cryptographic systems, quantum key distribution (QKD) provides a novel approach to protect communications in the post-quantum era, but presents complex challenges in terms of research and widespread adoption. Although the resource-intensive nature of QKD setups limits experimental studies to specialized laboratories, we present an end-to-end simulation that models photon generation, transmission, and detection, thereby lowering the barrier to testing hypotheses regarding experimental setups under realistic conditions. In this study, a fiber optic-based simulation was utilized along with a quantum circuit and a detailed noise model within the BB84 protocol to recreate an experimental setup. Our BB84 simulation retained ≈50% of transmitted bits after sifting, matching the experimental expectations, and produced quantum bit error rate (QBER) values that rose with increasing channel length. Our model incorporates customizable noise, such as multi-photon emissions and channel disturbances, directly into the key exchange process, yielding QBER values of ≈4.5% at 20 km and ≈11% at 120 km, consistent with physical systems under the same conditions. Our findings support the development of QKD as a scalable solution for securing the quantum internet and future communication networks.