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.Comment: One figure added supporting our claim that rotating mirrors A and B at exactly the same frequency, but out of phase, restores complete destructive interference. Minor changes made in order to make our main argument cleare

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.Comment: Minor improvement

From a Quantum Paradox to Counterportation

We uncover a new quantum paradox, where a simple question about two identical quantum systems reveals unsettlingly paradoxical answers when weak measurements are considered. Our resolution of the paradox, from within the weak measurement framework, amounts to a proof of counterfactuality for our generalised protocol (2014)---the first to do so---for sending an unknown qubit without any particles travelling between the communicating parties, i.e. counterfactually. The paradox and its resolution are reproduced from a consistent-histories viewpoint. We go on to propose a novel, experimentally feasible implementation of this counterfactual disembodied transport that we call counterportation, based on cavity quantum electrodynamics, estimating resources for beating the no-cloning fidelity limit---except that unlike teleportation no previously-shared entanglement nor classical communication are required. Our approach is up to several orders of magnitude more efficient in terms of physical resources than previously proposed techniques and is remarkably tolerant to device imperfections. Surprisingly, while counterfactual communication is intuitively explained in terms of interaction-free measurement and the Zeno effect, we show based on our proposed scheme that neither is necessary, with implications in support of an underlying physical reality.Comment: 31 pages, 4 figure

Do the laws of physics prohibit counterfactual communication?

It has been conjectured that counterfactual communication is impossible, even for post-selected quantum particles. We strongly challenge this by proposing exactly such a counterfactual scheme where---unambiguously---none of Alice's photons that make it has been to Bob. We demonstrate counterfactuality theoretically and experimentally by means of weak measurements, as well as conceptually using consistent histories. Importantly, the accuracy of Alice learning Bob's bit can be made arbitrarily close to unity with no trace left by Bob on Alice's photon.Comment: Experiment conducted in the lab, showing no weak trace from Bob at either D0 or D1. 5 pages, 5 figure

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.Comment: 5 pages, 4 figure

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 quantum weirdness to teleportation---namely exchange-free computation at a distance. The controlled-phase universal gate we propose, where no particles are exchanged between control and target, allows complete Bell detection among two remote parties, and is experimentally feasible. Our teleportation-with-a-twist, which we extend to telecloning, then requires no pre-shared entanglement between sender and receiver, nor classical communication, with the teleported state gradually appearing at its destination.Comment: Results extended to telecloning. 6+1 pages, 5 figure