Quantum Counterfactual Communication is the recently-proposed idea of using quantum physics to send messages between two parties, without any matter/energy transfer associated with the bits sent. While this has excited massive interest, both for potential ‘unhackable’ communication, and insight into the foundations of quantum mechanics, it has been asked whether this process is essentially quantum, or could be performed classically. We examine counterfactual communication, both classical and quantum, and show that the protocols proposed so far for sending signals that don’t involve matter/energy transfer associated with the bits sent must be quantum, insofar as they require wave-particle duality.
Exchange-Free Computation on an Unknown Qubit at a Distance (2021, New Journal of Physics – https://iopscience.iop.org/article/10.1088/1367-2630/abd3c4)
We present a way of directly manipulating an arbitrary qubit, without exchange of particles. This includes as an application the preparation of an arbitrary state at Alice by Bob, exchange-free. Hence, we are able to propose an exchange-free protocol that allows one party to directly enact, by means of a suitable program, any computation on a remote second party’s unknown qubit. We go on to show how to realise this in the exchange-free control of a universal two-qubit gate, thus opening the possibility of directly enacting any desired algorithm on a remote programmable quantum circuit.
Counterfactual Ghost Imaging (2020, arXiv – https://arxiv.org/abs/2010.14292)
We give a protocol for ghost imaging in a way that is always counterfactual – while imaging an object, no light interacts with that object. This extends the idea of counterfactuality beyond communication, showing how this interesting phenomenon can be leveraged for metrology. Given, in the infinite limit, no photons ever go to the imaged object, it presents a method of imaging even the most light-sensitive of objects without damaging them. Even when not in the infinite limit, it still provides a many-fold improvement in visibility and signal-to-noise ratio over previous protocols, with up to multiple orders of magnitude reduction in absorbed intensity.
Deterministic Teleportation and Universal Computation Without Particle Exchange (2020, arXiv – https://arxiv.org/abs/2009.05564)
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.
Backscatter and Spontaneous Four-Wave Mixing in Micro-Ring Resonators (2020, arXiv – https://arxiv.org/pdf/2001.05761.pdf)
We model backscatter for electric fields propagating through optical micro-ring resonators, as occurring both in-ring and in-coupler. These provide useful tools for modelling transmission and in-ring fields in these optical devices. We then discuss spontaneous four-wave mixing and use the models to obtain heralding efficiencies and rates. We observe a trade-off between these, which becomes more extreme as the rings become more strongly backscattered.
We show counterfactual definiteness separates classical from quantum physics, by analysing Bell’s Theorem. By comparing what it prohibited by various interpretations, we show most interpretations just require counterfactual semi-definiteness (the definiteness of possible options available after a measurement event), rather than full counterfactual indefiniteness. While less definite than classical counterfactual definiteness, it allows us a far more sophisticated tool to consider the physical interpretation of multi-valued variables in a way not yet done. Working from this, we further consider its relation to how counterfactual possibilities interact.
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 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.