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Hatim Salih

Publications and source records attributed to Hatim Salih.

14 recordsLinked to original sources
arXiv2026

Gravitationally Induced Entanglement Across an Event Horizon

It has long been assumed that a particle---having crossed a black hole event horizon unentangled with another particle in the exterior universe---can no longer dynamically entangle with that particle. Here, we show within a gravitational time-delayed-potential model that entanglement can be created from scratch between freely falling spatial superpositions across an event horizon. Yet, escaping with this state radially requires non-inertial deceleration, triggering soft-graviton emission, and establishing a strict dephasing bound that grows with entangling phase, $\Gamma\ge\frac{729}{160\pi}\Phi$. Obtained within a constrained leading-quadrupole model, this decoheres the entangled state. By contrast, equivalent macroscopic optical masses allow local harvesting of entanglement tangentially, via quantum erasure, thus revealing a remarkable geometric duality: Spacetime irreversibly degrades entanglement under radial escape, while in principle allowing the transverse teleportation of its entangled state to infinity.

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arXiv2025

Reply to "Counterfactual communication not achieved yet -- A Comment on Salih et al. (2022)"

In his Comment on our recent paper ``The laws of physics do not prohibit counterfactual communication'', \textit{npj Quantum Information} (2022) 8:60, Popescu argues that the claims of the paper are invalid. Here, we refute his argument, showing that it is based on ignoring the specifics of what we set out to prove (that counterfactual communication is possible \emph{for post-selected particles}, and more specifically in these cases is not prohibited by the weak trace or consistent histories criteria for particle path), followed by an unwarranted simplification of the protocol. Moreover, the Comment's excursion into interpretation is misplaced. Our communication protocol is a precisely defined one that allows two remote parties, albeit rarely, to communicate an arbitrarily long binary message, with arbitrarily high accuracy. This is not a matter of interpretation -- as the concrete example given in our paper in question illustrates. As for our overarching claim that no particles are exchanged in the course of this communication, we have already demonstrated this both theoretically and experimentally, in the postselected case we consider, as per the weak trace and consistent histories criteria for path of a quantum particle.

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arXiv2022

From Counterportation to Local Wormholes

We propose an experimental realisation of the protocol for the counterfactual disembodied transport of an unknown qubit -- or what we call counterportation -- where sender and receiver, remarkably, exchange no particles. We employ cavity quantum electrodynamics, estimating resources for beating the classical fidelity limit -- except, unlike teleportation, no pre-shared entanglement nor classical communication are required. Our approach is multiple orders of magnitude more efficient in terms of physical resources than previously proposed implementations, paving the way for a demonstration using existing imperfect devices. Surprisingly, while such communication is intuitively explained in terms of "interaction-free" measurement and the Zeno effect, we show that neither is necessary, with far-reaching implications in support of an underlying physical reality. We go on to characterise an explanatory framework for counterportation starting from constructor theory: local wormholes. Conversely, a counterportation experiment demonstrating the traversability of space, by means of what is essentially a 2-qubit exchange-free quantum computer, can point to the existence in the lab of such traversable wormholes.

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arXiv2022

The laws of physics do not prohibit counterfactual communication

It has been conjectured that counterfactual communication is impossible, even for post-selected quantum particles. We strongly challenge this by proposing precisely such a counterfactual scheme where -- unambiguously -- none of Alice's photons that correctly contribute to her information about Bob's message have been to Bob. We demonstrate counterfactuality experimentally by means of weak measurements, and conceptually using consistent histories -- thus simultaneously satisfying both criteria without loopholes. Importantly, the fidelity of Alice learning Bob's bit can be made arbitrarily close to unity.

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arXiv2021

Exchange-Free Computation on an Unknown Qubit at a Distance

We present a way of directly manipulating an arbitrary qubit, without the exchange of any particles. This includes as an application the exchange-free preparation of an arbitrary quantum state at Alice by a remote classical Bob. As a result, we are able to propose a protocol that allows one party to directly enact, by means of a suitable program, any computation exchange-free on a remote second party's unknown qubit. Further, we show how to use this for the exchange-free control of a universal two-qubit gate, thus opening the possibility of directly enacting any desired algorithm remotely on a programmable quantum circuit.

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arXiv2020

Deterministic Teleportation and Universal Computation Without Particle Exchange

Teleportation is a cornerstone of quantum technologies, and has played a key role in the development of quantum information theory. Pushing the limits of teleportation is therefore of particular importance. Here, we apply a different aspect of quantumness to teleportation -- namely exchange-free, or counterfactual, computation at a distance. We propose a universal controlled-phase gate, where no particles are exchanged between control and target. This allows the full repertoire of quantum computation to me made exchange-free, including both complete Bell detection among two remote parties and so teleportation and telecloning. Further, we show that this gate, and the protocols based on it, is experimentally feasible, simulating the fidelity of our exchange-free teleportation and telecloning protocols for realistic bit-errors.

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arXiv2018

Counterfactual quantum erasure: spooky action without entanglement

We combine the eyebrow-raising quantum phenomena of erasure and counterfactuality for the first time, proposing a simple yet unusual quantum eraser: A distant Bob can decide to erase which-path information from Alice's photon, dramatically restoring interference, without previously-shared entanglement, and without Alice's photon ever leaving her lab.

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arXiv2018

Comment on "Particle Path Through a Nested Mach-Zehnder Interferometer"

In a recent paper, arXiv:1604.04596, Griffiths questioned - based on an informative consistent-histories (CH) argument - the counterfactuality, for one of the bit choices, of Salih et al.'s protocol for communicating without sending physical particles, Phys. Rev. Lett. 110, 170502 (2013). Here, we first show that for the Mach-Zehnder version used to explain our protocol, no family of consistent histories exists where any history has the photon travelling through the communication channel, thus rendering the question of whether the photon was in the communication channel meaningless from a CH viewpoint. We then show that for the actual Michelson-type protocol, there are consistent-histories families that include histories where the photon travels through the communication channel. We show that the probability of finding the photon in the communication channel is zero - thus proving complete counterfactuality.

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arXiv2016

Protocol for Counterfactually Transporting an Unknown Qubit

Quantum teleportation circumvents the uncertainty principle using dual channels: a quantum one consisting of previously-shared entanglement, and a classical one, together allowing the disembodied transport of an unknown quantum state over distance. It has recently been shown that a classical bit can be counterfactually communicated between two parties in empty space, "Alice" and "Bob". Here, by using our "dual" version of the chained quantum Zeno effect to achieve a counterfactual CNOT gate, we propose the first protocol for transporting an unknown qubit counterfactually, that is without any physical particles travelling between Alice and Bob - no classical channel and no previously-shared entanglement.

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arXiv2016

Quantum Erasure Cryptography

The phenomenon of quantum erasure has long intrigued physicists, but has surprisingly found limited practical application. Here, we propose an erasure-based protocol for quantum key distribution (QKD) that promises inherent security against detector attacks.

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arXiv2014

Tripartite Counterfactual Quantum Cryptography

We show how two distrustful parties, "Bob" and "Charlie", can share a secret key with the help of a mutually trusted "Alice", counterfactually - that is with no information-carrying particles travelling between any of the three parties.

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arXiv2014

Comment on "Asking Photons Where Have They Been"

We argue that, in the recent letter by A. Danan et al. [PRL 111, 240402 (2013), arXiv:1304.7469], if weak measurements are performed such that complete destructive interference is not disturbed, claims such as "some photons have been inside the inner interferometer but they never entered and never left" should not arise. Standard quantum mechanics not only explains the physics better than the two-state vector formulation the authors advocate, but it can tell a very different story.

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arXiv2013

Protocol for direct counterfactual quantum communication

It has long been assumed in physics that for information to travel between two parties in empty space, "Alice" and "Bob", physical particles have to travel between them. Here, using the "chained" quantum Zeno effect, we show how, in the ideal asymptotic limit, information can be transferred between Alice and Bob without any physical particles traveling between them.

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