Fiber-Optics Implementation of an Asymmetric Phase-Covariant Quantum Cloner

Abstract

We present the experimental realization of optimal symmetric and asymmetric phase-covariant 1 → 2 cloning of qubit states using fiber optics. State of each qubit is encoded into a single photon which can propagate through two optical fibers. The operation of our device is based on one-and two-photon interference. We have demonstrated creation of two copies of any state of a qubit from the equator of the Bloch sphere. The measured fidelities of both copies are close to the theoretical values and they surpass the theoretical maximum obtainable with the universal cloner.PACS numbers: 03.67.-a, 42.50.-p, 32.80.-t The quantum no-cloning theorem [1] lies at the heart of quantum information theory. The apparently simple observation that perfect copying of unknown quantum states is impossible has profound consequences. On the fundamental side, it prevents superluminal communication with entangled states, thereby guaranteeing the peaceful coexistence of quantum mechanics and theory of relativity. On the practical side, this theorem is behind the security of the quantum key distribution schemes which rely on the fact that any attempt to measure or copy an unknown quantum state results in the disturbance of this state. Going beyond the no-cloning theorem, Bužek and Hillery in a seminal paper introduced the concept of the universal approximate quantum cloning machine that optimally approximates the forbidden transformation |ψ → |ψ |ψ [2]. Today, optimal quantum cloners are known for many different cases and scenarios [3,4]. During recent years, growing attention has been paid to the experimental implementation of quantum cloning machines and, in particular, optimal cloning of polarization states of single photons via stimulated parametric downconversion or via photon bunching on a beam splitter has been successfully demonstrated [5,6,7,8,9,10].Besides giving an insight into the fundamental limits on distribution of quantum information, the quantum cloning machines turned out to be very efficient eavesdropping attacks on the quantum key distribution protocols [11,12,13,14]. In this context one is particularly interested in the asymmetric quantum cloners that produce two copies with different fidelities. In this way, the eavesdropper can control the trade-off between the information gained on a secret cryptographic key and the amount of noise added to the copy which is sent down the channel to the authorized receiver. While the theory of optimal asymmetric quantum copying machines is well established (see, e.g. the recent reviews [3,4]), the experimental optical realization of such machines has received considerably less attention. This might be attributed to the fact that the asymmetric cloning operations exhibit much less symmetry than the corresponding symmetric ones. To the best of our knowledge, asymmetric quantum cloning of single-photon states has been so far achieved only in a single experiment, where universal asymmetric copying of polarization states was performed by means of partial quantum teleportation [15].In...

Temporal steering and security of quantum key distribution with mutually unbiased bases against individual attacks

“…The relation between optimal quantum cloning and the security of these QKD protocols was studied in various works (see, e.g., Refs. [38][39][40][41][42] and references therein). This connection to optimal quantum copying is anticipated since the security of the MUB-based QKD protocols is guaranteed by the impossibility of ideal copying of an unknown quantum state [43,44].…”

Experimental temporal quantum steering

“…The security of these QKD protocols is naturally related to optimal quantum cloning and it was studied in various works (see, e.g., Refs. [36,[40][41][42][43] and references therein). It was recently shown that the TS parameter S N depends on the average QBER r N or the average fidelity F N = 1 − r N of the states received by Bob [33].…”

Near-infrared Hong-Ou-Mandel interference on a silicon quantum photonic chip

“…For balanced chip-scale splitting, we note that multi-mode interference [35] and Y-splitters are also good elements for physical realization. Directional couplers on the other hand provides differential and accurate thermal tuning on the SR, enabling controlled asymmetries such as for various C-NOT gate [53], quantum cloning [54,55], and Fock state filtration [56,57] applications. Supported by these designs, the devices were next fabricated at the Institute of Microelectronics.…”