Quantum key distribution (QKD) has established itself as a groundbreaking technology, showcasing inherent security features that are fundamentally proven. Qubit-based QKD protocols that rely on binary encoding encounter an inherent constraint related to the secret key capacity. This limitation restricts the maximum secret key capacity to one bit per photon. On the other hand, qudit-based QKD protocols have their advantages in scenarios where photons are scarce and noise is present, as they enable the transmission of more than one secret bit per photon. While proof-of-principle entangled-based qudit QKD systems have been successfully demonstrated over the years, the current limitation lies in the maximum distribution distance, which remains at 20 km fiber distance. Moreover, in these entangled high-dimensional QKD systems, the witness and distribution of quantum steering have not been shown before. Here we present a high-dimensional time-bin QKD protocol based on energy-time entanglement that generates a secure finite-length key capacity of 2.39 bit/coincidences and secure cryptographic finite-length keys at 0.24 Mbits s−1in a 50 km optical fiber link. Our system is built entirely using readily available commercial off-the-shelf components, and secured by nonlocal dispersion cancellation technique against collective Gaussian attacks. Furthermore, we set new records for witnessing both energy-time entanglement and quantum steering over different fiber distances. When operating with a quantum channel loss of 39 dB, our system retains its inherent characteristic of utilizing large-alphabet. This enables us to achieve a secure key rate of 0.30 kbits s−1and a secure key capacity of 1.10 bit/coincidences, considering finite-key effects. Our experimental results closely match the theoretical upper bound limit of secure cryptographic keys in high-dimensional time-bin QKD protocols (Mower
In a recent work (Halverson
- Award ID(s):
- 2019786
- NSF-PAR ID:
- 10363314
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Machine Learning: Science and Technology
- Volume:
- 3
- Issue:
- 1
- ISSN:
- 2632-2153
- Page Range / eLocation ID:
- Article No. 015027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Abstract et al 2013Phys. Rev. A87 062322; Zhanget al 2014Phys. Rev. Lett. 112 120506), and outperform recent state-of-the-art qubit-based QKD protocols in terms of secure key throughput using commercial single-photon detectors (Wengerowskyet al 2019Proc. Natl Acad. Sci. 116 6684; Wengerowskyet al 2020npj Quantum Inf. 6 5; Zhanget al 2014Phys. Rev. Lett. 112 120506; Zhanget al 2019Nat. Photon. 13 839; Liuet al 2019Phys. Rev. Lett. 122 160501; Zhanget al 2020Phys. Rev. Lett. 125 010502; Weiet al 2020Phys. Rev. X10 031030). The simple and robust entanglement-based high-dimensional time-bin protocol presented here provides potential for practical long-distance quantum steering and QKD with multiple secure bits-per-coincidence, and higher secure cryptographic keys compared to mature qubit-based QKD protocols. -
Abstract The generalized Wilson loop operator interpolating between the supersymmetric and the ordinary Wilson loop in
SYM theory provides an interesting example of renormalization group flow on a line defect: the scalar coupling parameterζ has a non-trivial beta function and may be viewed as a running coupling constant in a 1D defect QFT. In this paper we continue the study of this operator, generalizing previous results for the beta function and Wilson loop expectation value to the case of an arbitrary representation of the gauge group and beyond the planar limit. Focusing on the scalar ladder limit where the generalized Wilson loop reduces to a purely scalar line operator in a free adjoint theory, and specializing to the case of the rankk symmetric representation ofSU (N ), we also consider a certain ‘semiclassical’ limit wherek is taken to infinity with the productkζ 2fixed. This limit can be conveniently studied using a 1D defect QFT representation in terms ofN commuting bosons. Using this representation, we compute the beta function and the circular loop expectation value in the largek limit, and use it to derive constraints on the structure of the beta function for general representation. We discuss the corresponding 1D RG flow and comment on the consistency of the results with the 1D defect version of the F-theorem. -
Abstract The renormalization group (RG) is a class of theoretical techniques used to explain the collective physics of interacting, many-body systems. It has been suggested that the RG formalism may be useful in finding and interpreting emergent low-dimensional structure in complex systems outside of the traditional physics context, such as in biology or computer science. In such contexts, one common dimensionality-reduction framework already in use is information bottleneck (IB), in which the goal is to compress an ‘input’ signal
X while maximizing its mutual information with some stochastic ‘relevance’ variableY . IB has been applied in the vertebrate and invertebrate processing systems to characterize optimal encoding of the future motion of the external world. Other recent work has shown that the RG scheme for the dimer model could be ‘discovered’ by a neural network attempting to solve an IB-like problem. This manuscript explores whether IB and any existing formulation of RG are formally equivalent. A class of soft-cutoff non-perturbative RG techniques are defined by families of non-deterministic coarsening maps, and hence can be formally mapped onto IB, and vice versa. For concreteness, this discussion is limited entirely to Gaussian statistics (GIB), for which IB has exact, closed-form solutions. Under this constraint, GIB has a semigroup structure, in which successive transformations remain IB-optimal. Further, the RG cutoff scheme associated with GIB can be identified. Our results suggest that IB can be used toimpose a notion of ‘large scale’ structure, such as biological function, on an RG procedure. -
Abstract The Dicke model—a paradigmatic example of superradiance in quantum optics—describes an ensemble of atoms which are collectively coupled to a leaky cavity mode. As a result of the cooperative nature of these interactions, the system’s dynamics is captured by the behavior of a single mean-field, collective spin. In this mean-field limit, it has recently been shown that the interplay between photon losses and periodic driving of light–matter coupling can lead to time-crystalline-like behavior of the collective spin (Gong
et al 2018Phys. Rev. Lett. 120 040404). In this work, we investigate whether such a Dicke time crystal (TC) is stable to perturbations that explicitly break the mean-field solvability of the conventional Dicke model. In particular, we consider the addition of short-range interactions between the atoms which breaks the collective coupling and leads to complex many-body dynamics. In this context, the interplay between periodic driving, dissipation and interactions yields a rich set of dynamical responses, including long-lived and metastable Dicke-TCs, where losses can cool down the many-body heating resulting from the continuous pump of energy from the periodic drive. Specifically, when the additional short-range interactions are ferromagnetic, we observe time crystalline behavior at non-perturbative values of the coupling strength, suggesting the possible existence of stable dynamical order in a driven-dissipative quantum many-body system. These findings illustrate the rich nature of novel dynamical responses with many-body character in quantum optics platforms. -
Abstract Due to its construction, the nonperturbative renormalizationgroup (RG) evolution of the constant, field-independent term (which is constantwith respect to field variations but depends on the RG scale k ) requiresspecial care within the Functional Renormalization Group (FRG) approach.In several instances, the constant term of the potential has nophysical meaning. However, there are special cases where it receives importantapplications. In low dimensions ( d = 1), in a quantum mechanical model, thisterm is associated with the ground-state energy of the anharmonic oscillator.In higher dimensions ( d = 4), it is identical to the Λ termof the Einstein equationsand it plays a role in cosmic inflation. Thus, instatistical field theory, in flat space, the constant term could be associatedwith the free energy, while in curved space, it could be naturally associatedwith the cosmological constant. It is known that one has to use a subtractionmethod for the quantum anharmonic oscillator in d = 1 to remove the k 2 term that appears in the RGflow in its high-energy (UV) limit in order to recover thecorrect results for the ground-state energy. The subtraction is needed becausethe Gaussian fixed point is missing in the RG flow once the constant term isincluded. However, if the Gaussian fixed point is there, no furthersubtraction is required. Here, we propose a subtraction method for k 4 and k 2 terms of the UV scaling of the RG equations for d = 4 dimensions if theGaussian fixed point is missing in the RG flow with the constant term. Finally,comments on the application of our results to cosmological models are provided.more » « less