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In this paper, we explicitly evaluate the one-shot quantum nonsignaling-assisted zero-error classical capacities ${\mathcal{M}}_0^{\mathrm{QNS}}$ for qubit channels. In particular, we show that for nonunital qubit channels ${\mathcal{M}}_0^{\mathrm{QNS}}=0$, which implies that in the one-shot setting nonunital qubit channels cannot transmit any information with zero probability of error even when assisted by quantum nonsignaling correlations. Furthermore, we show that for qubit channels ${\mathcal{M}}_0^{\mathrm{QNS}}$ equals the one-shot entanglement-assisted zero-error classical capacity. This means that for a single use of a qubit channel quantum nonsignaling correlations are not more powerful than shared entanglement.

By how much must the communication complexity of a function increase if we demand that the parties not only correctly compute the function but also return all registers (other than the one containing the answer) to their initial states at the end of the communication protocol? Protocols that achieve this are referred to as clean and the associated cost as the clean communication complexity. Here we present clean protocols for calculating the inner product of two $n$-bit strings, showing that (in the absence of preshared entanglement) at most $n+3$ qubits or $n+O(\sqrt{n})$ bits of communication are required. The quantum protocol provides inspiration for obtaining the optimal method to implement distributed cnot gates in parallel while minimizing the amount of quantum communication. For more general functions, we show that nearly all Boolean functions require close to $2n$ bits of classical communication to compute and close to $n$ qubits if the parties have access to preshared entanglement. Both of these values are maximal for their respective paradigms.

Symmetry plays a fundamental role in the security analysis of quantum key distribution (QKD). Here we review how symmetry is exploited in continuous-variable (CV) QKD to prove the optimality of Gaussian attacks in the finite-size regime. We then apply these results to improve the feasibility and the key rate of these protocols. First we show how to improve the feasibility of the energy test, which is one important routine aimed at establishing an upper bound on the effective dimensions of the otherwise infinite-dimensional Hilbert space of CV systems. Second, we show how the routine of parameter estimation can be made resource efficient in measurement-device independent QKD. These results show that all the raw data can be used both for key extraction and for the routines of the energy test and parameter estimation. Furthermore, the improved energy test does not require active symmetrization of the measured data, which is computationally demanding.

We propose a carrier synchronization scheme for continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) with a real local oscillator (LO). The middle node Charlie evaluates carrier frequency offsets and phase drift values based on the pilot signal sent by the legal parties Alice and Bob and a part of their public data. Alice and Bob then calibrate their modulation data according to the evaluation parameters to achieve carrier synchronization. We simulate the entire CV-MDI-QKD protocol and verify the feasibility of this scheme. The influences of pilot segment size and public data size on the excess noise are also analyzed. Besides, we experiment to measure the actual frequency offsets and phase drifts in the channel, which gives guidance on the actual implementation. This solution can ensure the implementation of CV-MDI-QKD with a real LO, paving the way for the practical application of this high-rate QKD protocol.

We present theoretical models for quantum-optical mode-selective filters and mirrors using continuous-variable teleportation. We call these devices telefilters and telemirrors, respectively. Both devices act as the identity channel on a mode of interest from an input multimode field while filtering or reflecting all the orthogonal modes. We utilize these models to analyze a causality problem in relativistic quantum optics, specifically the apparently acausal transmission and propagation of temporally delocalized wave packets through mode-selective mirrors. First, we show how telemirrors, and thus mode-selective operations generally, enact a fundamental time delay on such wave packets, which is necessary in order to prevent violations of causality. We next consider teleporting the independent temporal components of the input field separately and continuously, that is, performing operations on the fly. In this scenario, the telemirror transmits the mode of interest as well as orthogonal modes which carry with them uncorrelated noise. In this scenario, the device may be considered mode discriminating but not mode selective.

Gleason's theorem is a statement that, given some reasonable assumptions, the Born rule used to calculate probabilities in quantum mechanics is essentially unique [A. M. Gleason, Indiana Univ. Math. J. 6, 885 (1957)]. We show that Gleason's theorem contains within it also the structure of sequential measurements, and along with this the state update rule. We give a small set of axioms, which are physically motivated and analogous to those in Busch's proof of Gleason's theorem [P. Busch, Phys. Rev. Lett. 91, 120403 (2003)], from which the familiar Kraus operator form follows. An axiomatic approach has practical relevance as well as fundamental interest, in making clear those assumptions which underlie the security of quantum communication protocols. Interestingly, the two-time formalism is seen to arise naturally in this approach.

Alice wants to convey the value of a parameter to Bob with whom she does not share a reference frame. What physical object can she use for this task? Shall she encode this value into the angle between two physical vectors such as the angle between two spins? Can she benefit from using entanglement? We investigate these questions here and show that an entangled state of two qubits has three parameters that are invariant under changes of the reference frame. We also calculate for specific examples the average information gain for different circumstances, where one of these parameters is used for communication. We compare our result with the special case of separable states and find that entanglement enhances the information gain.

The monogamy relation for quantum correlations is not satisfied by all measures for all multiparty quantum states. We prove that an arbitrary quantum state which is nonmonogamous for negativity will become monogamous if a finite number of copies of the same state is provided. We refer to this as activation of nonmonogamous states. We also show that multiple copies of a state satisfy monogamy for negativity if it does so for a single copy. The results are true for all quantum states of an arbitrary number of parties. Moreover, we find that for a three-qubit pure state, we can find another state such that they individually violate the monogamy relation for negativity and when taken together they can satisfy the three-party monogamy relation. This holds for all three-qubit pure states except $1\%$ states from the W class, while it is true for all three-qubit pure states when a four-party monogamy relation is used to check for their activation. We connect monogamy of negativity with genuine multipartite entanglement.

Time-bin encoding of photons offers a robust kind of long-distance quantum communication over lossy channels. The diversity of material nodes in quantum networks operating at natural frequencies requires coherent frequency and wave-form conversion of information-carrying photons to provide an efficient quantum interface with optical fibers and various quantum memories, which has been extensively studied. However, the quantum frequency conversion with transfer of time-bin encoding between single photons of different wavelengths has not yet been explored. In this paper, we present a method for efficiently exchanging time-bin encoding in the conversion process between two photons propagating in cold tripod atoms driven by a strong laser field. The latter is designed to slow down photons and suppress their absorption due to electromagnetically induced transparency. Photons interact parametrically through modified atomic coherence, which is utilized also to achieve equal group velocities of photons. We demonstrate the ability of our model to generate entanglement between distant atoms that equally share the original quantum information stored in the ground-state polarization qubits of both atoms. The proposed method for frequency conversion based on modified atomic coherence is promising because it can simplify the implementation of reliable and high-speed quantum communication protocols based on single-photon entanglement.

The Bell inequality, and its substantial experimental violation, offers a seminal paradigm for showing that the world is not in fact locally realistic. Here, going beyond the scope of Bell's inequality on physical states, we show that quantum teleportation can be used to quantitatively characterize quantum correlations of physical processes. The validity of the proposed formalism is demonstrated by considering the problem of teleportation through a linear three-node quantum network. A hierarchy is derived between the Bell nonlocality, nonbilocality, steering, and nonlocality-steering hybrid correlations based on a process fidelity constraint. The proposed framework can be directly extended to reveal the nonlocality structure of teleportation through any linear many-node quantum network. The formalism provides a faithful identification of quantum teleportation and demonstrates the use of quantum-information processing as a means of quantitatively discriminating quantum correlations.

Due to turbulence and tracking errors, free-space optical channels involving mobile transceivers are characterized by a signal's partial loss or complete erasure. This work presents an error-correction protocol capable of protecting a signal passing through such channels by encoding it with an ancillary entangled bipartite state. Beyond its ability to offer protection under realistic channel conditions, our protocol has the ability to transmit both a quantum state and a classical symbol using the same encoded state. We show how the protocol can improve the fidelity of transmitted coherent states over a wide range of losses and erasure probabilities relative to nonencoded direct transmission. In addition, the use of ancillary non-Gaussian entangled bipartite states in the signal encoding is considered, showing how this can increase performance. Finally, we briefly discuss the application of our protocol to the transmission of more complex input states, such as multimode entangled states.

We show that the criticism raised by Dumont and Pollak [Dumont and Pollak, preceding Comment, Phys. Rev. A 108, 036201 (2023)] is based on a flawed understanding of our analysis.

A squeezed-displaced state can be used as an optical signal for efficient binary quantum optical communication. Here, we consider quantum optical communication where an atomic indirect measurement is used to decide the message encoded in a binary phase-shift-keying signal composed of squeezed-displaced states, and show that the incorrect decision probability of an atomic indirect measurement can be less than that of a homodyne measurement. We further show that an atomic indirect measurement can lower the incorrect decision probability even when the quantum channel is exposed to weak phase-diffusion noise.

We theoretically introduce and experimentally demonstrate the realization of a nonclassicality test that allows for arbitrarily low detection efficiency without invoking an extra assumption of independence of the devices. Our test and its implementation is set in a prepare-and-measure scenario with an upper limit on the classical communication capacity of the channel through which the systems are communicated. The essence for our test is the use of two preparation and two measurement devices, which are randomly paired in each round. Our work opens the possibility for experimental realizations of nonclassicality tests with off-the-shelf technology.

I present an explicit classical simulation of arbitrary quantum noise for quantum models in which one qubit interacts with a quantum bath. The classical model simulates the interaction of the bath and the qubit by random unitary evolutions. I show that any arbitrary quantum dynamics, including quantum dissipation, recurrence, and dephasing, can be simulated classically when one allows the unitary operators in the classical model to depend on the initial state of the system and bath. For initial mixed states of the system and nonproduct states of the system and bath, I demonstrate that random unitary expansion is still possible, in terms of a set of pure states.

We determine the ultimate classical information capacity of a linear time-invariant bosonic channel with additive phase-insensitive Gaussian noise. This channel can model fiber-optic communication at power levels below the threshold for significant nonlinear effects. We provide a general continuous-time result that gives the ultimate capacity for such a channel operating in the quasimonochromatic regime under an average power constraint. This ultimate capacity is compared with corresponding results for heterodyne and homodyne detection over the same channel.