Experimental linear-optical implementation of a multifunctional optimal qubit cloner

Abstract

We present the first experimental implementation of a multifunctional device for the optimal cloning of one to two qubits. Previous implementations have always been designed to optimize the cloning procedure with respect to one single type of a priori information about the cloned state. In contrast, our all-in-one implementation is optimal for several prominent regimes such as universal cloning, phase-covariant cloning, and also the first ever realized mirror phase-covariant cloning, when the square of the expected value of Paulis Z operator is known in advance.In all these regimes the experimental device yields clones with almost maximum achievable aver-age fidelity (97.5% of theoretical limit). Our device has a wide range of possible applications in quantum information processing, especially in quantum communication. For instance, one can use it for incoherent and coherent attacks against a variety of cryptographic protocols , including the Bennett-Brassard 1984 protocol of quantum key distribution through the Pauli damping channels. It can be also applied as a state-dependent photon multiplier in practical quantum networks. Introduction. One of the most fundamental laws of nature, the so-called no-cloning theorem, states that an unknown quantum state cannot be perfectly copied. This fact has an imminent impact on quantum information processing. For instance, it allows designing inherently secure cryptographic protocols [1] or assures the impossibility of superluminal communication [2]. Although perfect quantum copying is impossible, one can still investigate how well such an operation can be approximated within the limits of physical laws. Despite some very intense research in this domain, many aspects of statedependent quantum cloning have not yet been fully investigated.Quantum cloning is one of the most intriguing topics in quantum physics. It is important not only because of its fundamental nature but also because of its immediate applications to quantum communications, including quantum cryptography. Similar to other important quantum information processing protocols, quantum cloning has undergone considerable development over the past two decades. The first design of an optimal cloning machine was suggested by Bužek and Hillery [3]. The cloner is called optimal when it gives the best results allowed by quantum mechanics. Moreover universal cloning (UC) should operate equally well for all possible qubit states [4][5][6][7][8]. In contrast, limiting cloning to a specific subset of qubit states, one can achieve a more precise cloning operation. A prominent example of this situation is phase-covariant cloning (PCC), where only qubit states with equal superposition of |0 and |1 are considered [9][10][11][12][13][14][15].In this Rapid Communication we address a question

Luminescence-induced noise in single photon sources based on BBO crystals

“…These values are based on recent experiments in the field of linear-optical quantum information processing [3,19,28,30]. Note that for the purposes of subsequent quantitative model, we have considered a pump wavelength of 267 nm (fourth harmonics of Nd-YAG laser) and ideal binary detectors with unity quantum efficiency.…”

Efficient amplification of photonic qubits by optimal quantum cloning

“…Other implementations use two-photon interference on a polarization sensitive beam splitter [23]. Here we will use the latter approach to experimentally demonstrate a principle of operation of a CBA.…”

“…The device functions as a phase-covariant and mirror phasecovariant cloner [23]. The optimal cloning transformation can be now written in the form of a unitary transformation…”

“…Thus, we conclude that the two types of amplifiers perform complementary tasks. The CBA employs the 1 → 2 multi-functional cloner [22,23] optimized for distributions with axial symmetry.…”

“…State-dependent cloners can exceed this fidelity for some subspaces of the sphere [13] using partial information about the input qubit distribution. Examples include the phase-covariant cloner [13,17] optimal for equatorial states; and its generalizations to phase-independent cloners [18][19][20][21][22][23], optimal for qubits with axially-symmetric distributions, e.g., Fisher [24], Brosseau [25], and Henyey-Greenstein [26] distributions. These distributions are used to describe many problems of directional statistics [27] in physics, astronomy, biology, geology and psychology.…”