It is well known that the violation of Bell's inequality in form given by Clauser, Horne, Shimony, and Holt (CHSH) two-qubit systems requires entanglement, but not vice versa, i.e., there are entangled states which do violate CHSH inequality. Here we compare some standard entanglement measures with violations (as Horodecki measure) for generated Monte Carlo simulations. We describe have extremal according to negativity, concurrence, relative entropy a value violation. explicitly find these...

Amplitude damping changes entangled pure states into usually less-entangled mixed states. We show, however, that even local amplitude of one or two qubits can result in more than if compares the relative entropy entanglement (REE) for a given degree Bell--Clauser-Horne-Shimony-Holt inequality violation (referred to as nonlocality). By applying Monte Carlo simulations, we find maximally and show they are likely be optimal by checking Karush-Kuhn-Tucker conditions, which generalize method...

All-linear-optical scheme for fully featured quantum router is presented. This device directs the signal photonic qubit according to state of one control qubit. In introduction we formulate list requirements imposed on a router. Then describe our proposal showing exact principle operation linear-optical scheme. Subsequently provide generalization in order optimize success probability by means tunable controlled-phase gate. At end, show how can modify route multiple qubits using same

A bosonic state is commonly considered nonclassical (or quantum) if its Glauber-Sudarshan $P$ function not a classical probability density, which implies that only coherent states and their statistical mixtures are classical. We quantify the nonclassicality of single qubit, defined by vacuum single-photon states, applying following four well-known measures nonclassicality: (1) depth, $\ensuremath{\tau}$, related to minimal amount Gaussian noise changes nonpositive into positive one; (2)...

We implement an all-optical setup demonstrating kernel-based quantum machine learning for two-dimensional classification problems. In this hybrid approach, kernel evaluations are outsourced to projective measurements on suitably designed states encoding the training data, while model is processed a classical computer. Our two-photon proposal encodes data points in discrete, eight-dimensional feature Hilbert space. order maximize application range of deployable kernels, we optimize maps...

We present an error analysis of various tomographic protocols based on the linear inversion for reconstruction unknown two-qubit state. solve problem finding a protocol which is most robust against errors in terms lowest value (i.e., equal to 1) condition number, as required by Gastinel-Kahan theorem. In contrast, standard protocols, including those mutually unbiased bases, are nonoptimal determining all 16 elements density matrix. Our method measurements generalized Pauli operators, where...

Temporal steering, which is a temporal analog of Einstein-Podolsky-Rosen refers to quantum correlations between the initial and final state system. Our analysis steering inequalities in relation average bit error rates reveals interplay cloning, guarantees security key distribution based on mutually unbiased bases against individual attacks. The distributions analyzed here include Bennett-Brassard 1984 protocol six-state 1998 by Bruss. Moreover, we define steerable weight, enables us...

The security of quantum cryptography is guaranteed by the no-cloning theorem, which implies that an eavesdropper copying transmitted qubits in unknown states causes their disturbance. Nevertheless, real cryptographic systems some level disturbance has to be allowed cover, e.g., transmission losses. An can attack such replacing a noisy channel better one and performing approximate cloning disturb them but below noise assumed legitimate users. We experimentally demonstrate symmetric individual...

We propose a linear-optical scheme for an efficient amplification of photonic qubit based on interaction the signal mode with pair entangled ancillae. In contrast to previous proposal amplifier by Gisin et al. [Phys Rev. Lett. 105, 070501 (2010)], success probability our device does not decrease asymptotically zero increasing gain. Moreover, we show how can be used restore entanglement deteriorated transmission over lossy channel and calculate secure key rate device-independent quantum distribution.

Abstract Temporal steering is a form of temporal correlation between the initial and final state quantum system. It analogue famous Einstein-Podolsky-Rosen (spatial) steering. We demonstrate, by measuring photon polarization, that allows two parties to verify if they have been interacting with same particle, even no information about what happened particle in measurements. This first experimental study also performed tests, based on violation inequalities, security key distribution protocols...

Unknown quantum information cannot be perfectly copied (cloned). This statement is the bedrock of technologies and cryptography, including seminal scheme Wiesner's money, which was first quantum-cryptographic proposal. Surprisingly, to our knowledge, money has not been tested experimentally yet. Here, we revisit Wiesner idea, assuming a banknote an image encoded in polarization states single photons. We demonstrate that it possible use prepare ideally without making owner aware only...

We investigate quantum teleportation through dissipative channels and calculate fidelity as a function of damping rates. It is found that the average range states to be teleported depend on type rate in channel. Using fully entangled fraction, we derive two bounds rates channels: one beat classical limit second guarantee nonexistence any other copy with better fidelity. The effect initially distributed maximally state process presented; concurrence fraction shared are discussed. intend show...

If a single-mode nonclassical light is combined with the vacuum on beam splitter, then output state entangled. As proposed in [Phys. Rev. Lett. 94, 173602 (2005)], by measuring this output-state entanglement for balanced lossless one can quantify input-state nonclassicality. These measures of nonclassicality (referred to as potentials) be based, principle, various measures, leading negativity (NP) and concurrence (CP) potentials, potential relative entropy (REEP). We search maximal...

In this paper, we introduce a supervised learning technique that harnesses artificial neural networks, along with the outcomes of collective entanglement measurements, to estimate negativity quantum states in two-qubit and qubit-qutrit systems. The resulting deep-learned witnesses offer unique capability continuous sensitivity selectivity tuning. This instrument enables us explore tradeoff between detection, dimension not accessible previously employed analytical witnesses. particular,...

Detecting and quantifying quantum entanglement of a given unknown state poses problems that are fundamentally important for information processing. Surprisingly, no direct (i.e., without tomography) universal experimental implementation necessary sufficient test has been designed even general two-qubit state. Here we propose an method detecting collective witness, which is two-photon polarization entanglement. It allows us to detect any mixed establish tight upper lower bounds on its amount....

Linear-optical interferometers play a key role in designing circuits for quantum information processing and communications. Even though nested Mach-Zehnder appear easy to describe, there are occasions when they provide unintuitive results. This paper explains the results of highly discussed experiment performed by Danan et al. [Phys. Rev. Lett. 111, 240402 (2013).] using standard approach. We simple intuitive one-state vector formalism capable interpreting their experiment. Additionally, we...

We propose and experimentally implement an efficient procedure based on entanglement swapping to determine the Bell nonlocality measure of Horodecki et al. [Phys. Lett. A 200, 340 (1995)] fully entangled fraction Bennett Rev. 54, 3824 (1996)] arbitrary two-qubit polarization-encoded state. The corresponds amount violation Clauser-Horne-Shimony-Holt (CHSH) optimized over all measurement settings. By using simultaneously two copies a given state, we directly only six parameters. This is...

We present the first experimental implementation of a multifunctional device for optimal cloning one to two qubits. Previous implementations have always been designed optimize procedure with respect single type priori information about cloned state. In contrast, our ``all-in-one'' is several prominent regimes such as universal cloning, phase-covariant and also ever realized mirror when square expected value Pauli's $Z$ operator known in advance. all these yields clones almost maximum...

We describe a direct method to determine the negativity of an arbitrary two-qubit state in experiments. The is derived by analyzing relation between purity, negativity, and universal entanglement witness for entanglement. show how can be calculated from just three experimentally accessible moments partially transposed density matrix two-photon state. Moreover, we that given as function only six invariants, which are linear combinations nine invariants complete set 21 fundamental independent...

The Uhlmann-Jozsa fidelity (or, equivalently, the Bures distance) is a basic concept of quantum communication and information, which however very difficult to measure efficiently without recourse tomography. Here we propose direct experimental method estimate between two unknown two-qubit mixed states via measurement upper lower bounds fidelity, are referred as superfidelity subfidelity, respectively. Our enables first- second-order overlaps arbitrary states. In particular, can be applied...

Detection of entangled states is essential in both fundamental and applied quantum physics. However, this task proves to be challenging especially for general states. One can execute full state tomography but method time demanding complex systems. Other approaches use entanglement witnesses, these methods tend less lack reliability. Here, we demonstrate that ANN -- artificial neural networks provide a balance between approaches. In paper, make comparison performance against witness-based...

Abstract A Werner state, which is the singlet Bell state affected by white noise, a prototype example of states, can reveal hierarchy quantum entanglement, steering, and nonlocality controlling amount noise. However, experimental demonstrations this in sufficient necessary way (i.e., applying measures or universal witnesses these correlations) have been mainly based on full tomography, corresponding to measuring at least 15 real parameters two-qubit states. Here we report an demonstration...

We propose a quantum cloning machine, which clones qubit into two assuming known modulus of expectation value Pauli ${\ensuremath{\sigma}}_{z}$ matrix. The process is referred to as the mirror phase-covariant for input state priori less than that standard cloning. Analytical expressions describing transformation and fidelity are found. Extremal equations optimal derived analytically solved by generalizing method Fiur\'a\ifmmode \check{s}\else \v{s}\fi{}ek [Phys. Rev. A 64, 062310 (2001)]....

We experimentally demonstrate the migration of coherence between composite quantum systems and their subsystems. The are implemented using polarization states photons in two experimental setups. first setup is based on a linear optical controlled-phase gate second scheme utilizes effects nonlinear optics. Our experiment allows one to verify relation correlations subsystems system, which was given terms conservation law for maximal accessible by Svozil\'{\i}k et al. [J. al., Phys. Rev. Lett....