The science of quantum information has arisen over the last two decades centered on manipulation individual quanta information, known as bits or qubits. Quantum computers, cryptography, and teleportation are among most celebrated ideas that have emerged from this new field. It was realized later using continuous-variable carriers, instead qubits, constitutes an extremely powerful alternative approach to processing. This review focuses processes rely any combination Gaussian states,...

A fully general approach to the security analysis of continuous-variable quantum key distribution (CV-QKD) is presented. Provided that channel estimated via covariance matrix quadratures, Gaussian attacks are shown be optimal against all collective eavesdropping strategies. The proof made strikingly simple by combining a physical model measurement, an entanglement-based description CV-QKD, and recent powerful result on extremality states [M. M. Wolf et al., Phys. Rev. Lett. 96, 080502 (2006)].

We report on the implementation of a reverse-reconciliated coherent-state continuous-variable quantum key distribution system, with which we generated secret keys at rate more than $2\phantom{\rule{0.3em}{0ex}}\mathrm{kb}∕\mathrm{s}$ over $25\phantom{\rule{0.3em}{0ex}}\mathrm{km}$ optical fiber. Time multiplexing is used to transmit both signal and phase reference in same Our system includes all experimental aspects required for field setup. Real-time reverse reconciliation achieved by using...

We prove the security of Gaussian continuous-variable quantum key distribution with coherent states against arbitrary attacks in finite-size regime. In contrast to previously known proofs principle (based on de Finetti theorem), our result is applicable practically relevant This achieved using a novel proof approach, which exploits phase-space symmetries protocols as well postselection technique introduced by Christandl, Koenig, and Renner [Phys. Rev. Lett. 102, 020504 (2009)].

We propose a feasible optical setup allowing for loophole-free Bell test with efficient homodyne detection. A non-Gaussian entangled state is generated from two-mode squeezed vacuum by subtracting single photon each mode, using beam splitters and standard low-efficiency single-photon detectors. violation exceeding 1% achievable 6 dB light efficiency around 95%. detailed feasibility analysis, based upon the recent experimental generation of single-mode states, suggests that this method opens...

We define the direct and reverse secret-key capacities of a memoryless quantum channel as optimal rates that entanglement-based quantum-key-distribution protocols can reach by using single forward classical communication (direct reconciliation) or feedback (reverse reconciliation). In particular, capacity be positive for antidegradable channels, where no strategy is known to secure. This property explicitly shown in continuous variable framework considering arbitrary one-mode Gaussian channels.

The non-Gaussian operations effected by adding or subtracting a photon on the entangled optical beams emerging from parametric down-conversion process have been suggested to enhance entanglement. Heralded addition subtraction is, as matter of fact, at heart continuous-variable entanglement distillation. use such processes has recently experimentally demonstrated in context generation coherent-state superpositions verification canonical commutation relations. Here, we carry out systematic...

A continuous-variable quantum key distribution protocol based on squeezed states and heterodyne detection is introduced shown to attain higher secret rates over a noisy line than any other one-way Gaussian protocol. This increased resistance channel noise can be understood as resulting from purposely adding the signal that converted into key. notion of noise-enhanced tolerance also provides better physical insight poorly discrepancies between previously defined families protocols.

Closed timelike curves (CTCs) are trajectories in spacetime that effectively travel backwards time: a test particle following CTC can interact with its former self the past. A widely accepted quantum theory of CTCs was proposed by Deutsch. Here we analyze an alternative formulation based on teleportation and postselection, show it is inequivalent to Deutsch's. The predictions or retrodictions our be simulated experimentally: report results experiment illustrating how particular ``grandfather...

This paper discusses the quantum mechanics of closed timelike curves (CTC) and other potential methods for time travel. We analyze a specific proposal such travel, description CTCs based on post-selected teleportation (P-CTCs). compare theory P-CTCs to previously proposed theories travel: is physically inequivalent Deutsch's CTCs, but it consistent with path-integral approaches (which are best suited analyzing field in curved spacetime). derive dynamical equations that chronology-respecting...

As a promising candidate for exhibiting quantum computational supremacy, Gaussian boson sampling (GBS) is designed to exploit the ease of experimental preparation states. However, sufficiently large and inevitable noise might render GBS classically simulable. In this work, we formalize intuition by establishing sufficient condition approximate polynomial-time classical simulation noisy GBS-in form an inequality between input squeezing parameter, overall transmission rate, quality photon...

The semi-device-independent approach provides a framework for prepare-and-measure quantum protocols using devices whose behavior must not be characterized nor trusted, except single assumption on the dimension of Hilbert space characterizing carriers. Here, we propose instead to constrain carriers through bound mean value well-chosen observable. This modified is physically better motivated than and closer description actual experiments. In particular, consider optical schemes where source...

We introduce an algorithm for the classical simulation of Gaussian boson sampling that is quadratically faster than previously known methods. The complexity exponential in number photon pairs detected, not photons, and directly proportional to time required calculate a probability amplitude pure state. main innovation use auxiliary conditioning variables reduce problem computation pure-state amplitudes, which most computationally expensive step calculation loop hafnian. implement benchmark...

We propose a scheme for the conditional generation of arbitrary finite superpositions Fock states in single mode traveling optical field. The setup requires source squeezed vacuum states, beam splitters, strong coherent beams, photodetectors with single-photon sensitivity, and final squeezer. If we want to generate superposition which is sufficient several applications, then last squeezer not even needed. thrust this method that it achieves high fidelity without requiring efficiency or...

In this letter we define a family of entanglement distribution protocols assisted by feedback classical communication that gives an operational interpretation to reverse coherent information, i.e., the symmetric counterpart well known information. This lead definition new capacity exceeds unassisted for some interesting channels.

A longstanding open problem in quantum information theory is to find the classical capacity of an optical communication link, modeled as a Gaussian bosonic channel. It has been conjectured that this achieved by random coding coherent states using isotropic distribution phase space. We show proving minimum entropy conjecture for quantum-limited amplifier actually sufficient confirm conjecture, and we provide strong argument towards proof exploiting connection between entanglement majorization theory.

Photon losses are among the strongest imperfections affecting multi-photon interference. Despite their importance, little is known about effect on boson sampling experiments. In this work we show that using classical computers, one can efficiently simulate interference in all architectures suffer from an exponential decay of transmission with depth circuit, such as integrated photonic circuits or optical fibers. We prove either circuit large enough it be simulated by thermal noise algorithm...

The current shift in the quantum optics community towards large-size experiments -- with many modes and photons necessitates new classical simulation techniques that go beyond usual phase space formulation of mechanics. To address this pressing demand we formulate linear language tensor network states. As a toy model, extensively analyze correlations time-bin interference single fiber loop. We then generalize our results to more complex setups identify different classes architectures for...

An intercept-resend attack on a continuous-variable quantum-key-distribution protocol is investigated experimentally. By varying the interception fraction, one can implement family of attacks where eavesdropper totally controls channel parameters. In general, such add excess noise in channel, and may also result non-Gaussian output distributions. We characterize measurements needed to detect these attacks, evaluate experimentally information rates available legitimate users eavesdropper. The...

BosonSampling is a problem where quantum computer offers provable speedup over classical computers. Its main feature that it can be solved with current linear optics technology, without the need for full computer. In this work, we investigate whether an experimentally realistic BosonSampler really solve any fault-tolerance mechanism. More precisely, study how unavoidable errors linked to imperfect calibration of optical elements affect final result computation. We show fidelity each element...

Quantum mechanics promises computational powers beyond the reach of classical computers. Current technology is on brink an experimental demonstration superior power quantum computation compared to devices. For such a be meaningful, noise must not affect device; this occurs when algorithm can use simulate system. In work, we demonstrate which simulates boson sampling, advantage based many-body interference indistinguishable bosons, in presence optical loss. Finding level where approximation...