Qiss

We revisit Hawking’s black hole radiation derivation, including the quantum state of the initial matter forming the black hole. We investigate how non-vacuum initial quantum states, at the past of a black hole geometry, influence the black hole radiation observed at future null infinity $( mathcal{I}^+)$. We further classify which of the initial state excitations are distinguishable from one another through measurements on the black hole radiation state. We use Algebraic Quantum Field Theory (AQFT) to provide a clear physical interpretation of the results, in terms of localised operations. We then take a concrete example of a black hole made of one large collapsing excitation of mass $M$ and compare it to a same-mass black hole formed due to the collapse of two smaller excitations, of mass $M/2$ each. We find using our formalism that the two cases yield different radiation states and can in principle be distinguished. Our results provide a mechanism for partial information recovery in evaporating black holes, classify what information is recoverable through stimulated emission, and a concrete understanding of the classification based on the AQFT localisation.

In the ever-evolving landscape of quantum cryptography, Device-independent Quantum Key Distribution (DI-QKD) stands out for its unique approach to ensuring security based not on the trustworthiness of the devices but on nonlocal correlations. Beginning with a contextual understanding of modern cryptographic security and the limitations of standard quantum key distribution methods, this review explores the pivotal role of nonclassicality and the challenges posed by various experimental loopholes for DI-QKD. Various protocols, security against individual, collective and coherent attacks, and the concept of self-testing are also examined, as well as the entropy accumulation theorem, and additional mathematical methods in formulating advanced security proofs. In addition, the burgeoning field of semi-device-independent models (measurement DI–QKD, Receiver DI–QKD, and One–sided DI–QKD) is also analyzed. The practical aspects are discussed through a detailed overview of experimental progress and the open challenges toward the commercial deployment in the future of secure communications.

Large-scale communication networks, such as the internet, rely on routing packets of data through multiple intermediate nodes to transmit information from a sender to a receiver. In this paper, we develop a model of a quantum communication network that routes information simultaneously along multiple paths passing through intermediate stations. We demonstrate that a quantum routing approach can in principle extend the distance over which information can be transmitted reliably. Surprisingly, the benefit of quantum routing also applies to the transmission of classical information: even if the transmitted data is purely classical, delocalising it on multiple routes can enhance the achievable transmission distance. Our findings highlight the potential of a future quantum internet not only for achieving secure quantum communication and distributed quantum computing but also for extending the range of classical data transmission.

Proposed experiments for obtaining empirical evidence for a quantum description of gravity in a table-top setting focus on detecting quantum information signatures, such as entanglement or non-Gaussianity production, in gravitationally interacting quantum systems. Here, we explore an alternative approach where the quantization of gravity could be inferred through measurements of macroscopic, thermodynamical quantities, without the need for addressability of individual quantum systems. To demonstrate the idea, we take as a case study a gravitationally self-interacting Bose gas, and consider its heat capacity. We find a clear-cut distinction between the predictions of a classical gravitational interaction and a quantum gravitational interaction in the heat capacity of the Bose gas.

We investigate the phenomenon of entanglement harvesting for a spacetime in quantum superposition, using two Unruh-DeWitt detectors interacting with a quantum scalar field where the spacetime background is modeled as a superposition of two quotient Minkowski spaces which are not related by diffeomorphisms. Our results demonstrate that the superposed nature of spacetime induces interference effects that can significantly enhance entanglement for both twisted and untwisted field. We compute the concurrence, which quantifies the harvested entanglement, as function of the energy gap of detectors and their separation. We find that it reaches its maximum when we condition the final spacetime superposition state to match the initial spacetime state. Notably, for the twisted field, the parameter region without entanglement exhibits a significant deviation from that observed in classical Minkowski space or a single quotient Minkowski space.

ZX-calculus has proved to be a useful tool for quantum technology with a wide range of successful applications. Most of these applications are of an algebraic nature. However, other tasks that involve differentiation and integration remain unreachable with current ZX techniques. Here we elevate ZX to an analytical perspective by realising differentiation and integration entirely within the framework of ZX-calculus. We explicitly illustrate the new analytic framework of ZX-calculus by applying it in context of quantum machine learning for the analysis of barren plateaus.

The quantization of gravity is widely believed to result in gravitons — particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single graviton exchange can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental clue of the quantization of gravity.

It is often stated that the second law of thermodynamics follows from the condition that at some given time in the past the entropy was lower than it is now. Formally, this condition is the statement that $E[S(t)|S(t_0)]$, the expected entropy of the universe at the current time $t$ conditioned on its value $S(t_0)$ at a time $t_0$ in the past, is an increasing function of $t $. We point out that in general this is incorrect. The epistemic axioms underlying probability theory say that we should condition expectations on all that we know, and on nothing that we do not know. Arguably, we know the value of the universe’s entropy at the present time $t$ at least as well as its value at a time in the past, $t_0$. However, as we show here, conditioning expected entropy on its value at two times rather than one radically changes its dynamics, resulting in a unexpected, very rich structure. For example, the expectation value conditioned on two times can have a maximum at an intermediate time between $t_0$ and $t$, i.e., in our past. Moreover, it can have a negative rather than positive time derivative at the present. In such “Boltzmann bridge” situations, the second law would not hold at the present time. We illustrate and investigate these phenomena for a random walk model and an idealized gas model, and briefly discuss the role of Boltzmann bridges in our universe.

Discrete-time Quantum Walks (QWs) are transportation models of single quantum particles over a lattice. Their evolution is driven through causal and local unitary operators. QWs are a powerful tool for quantum simulation of fundamental physics as some of them have a continuum limit converging to well-known physics partial differential equations, such as the Dirac or the Schr”odinger equation. In this work, we show how to recover the Dirac equation in (3+1)-dimensions with a QW evolving in a tetrahedral space. This paves the way to simulate the Dirac equation on a curved spacetime. This also suggests an ordered scheme for propagating matter over a spin network, of interest in Loop Quantum Gravity where matter propagation has remained an open problem.

We solve the infinite potential well problem using the methods of Heisenberg’s matrix mechanics. In addition to being of educational value, the matrix mechanics allows us to deal with various unphysical issues caused by this potential in a seemingly unproblematic fashion. We also show how to treat many particles within this representation.