Given that many fundamental questions in neuroscience are still open, it seems pertinent to explore whether the brain might use other physical modalities than the ones that have been discovered so far. In particular it is well established that neurons can emit photons, which prompts the question whether these biophotons could serve as signals between neurons, in addition to the well-known electro-chemical signals. For such communication to be targeted, the photons would need to travel in waveguides. Here we show, based on detailed theoretical modeling, that myelinated axons could serve as photonic waveguides, taking into account realistic optical imperfections. We propose experiments, both in vivo and in vitro, to test our hypothesis. We discuss the implications of our results, including the question whether photons could mediate long-range quantum entanglement in the brain.

Given that many fundamental questions in neuroscience are still open, it seems pertinent to explore whether the brain might use other physical modalities than the ones that have been discovered so far. In particular it is well established that neurons can emit photons, which prompts the question whether these biophotons could serve as signals between neurons, in addition to the well-known electro-chemical signals. For such communication to be targeted, the photons would need to travel in waveguides. Here we show, based on detailed theoretical modeling, that myelinated axons could serve as photonic waveguides, taking into account realistic optical imperfections. We propose experiments, both in vivo and in vitro, to test our hypothesis. We discuss the implications of our results, including the question whether photons could mediate long-range quantum entanglement in the brain.The human brain is a dynamic physical system of unparalleled complexity. While neuroscience has made great strides, many fundamental questions are still unanswered 1 , including the processes underlying memory formation 2 , the working principle of anesthesia 3 , and-most fundamentally-the generation of conscious experience 4-6 . It therefore seems pertinent to explore whether the brain might generate, transmit and store information using other physical modalities than the ones that have been discovered so far.In the present work we focus on the question whether biophotons could serve as a supplementary information carrier in the brain in addition to the well established electro-chemical signals. Biophotons are the quanta of light spanning the near-UV to near-IR frequency range. They are produced mostly by electronically excited molecular species in a variety of oxidative metabolic processes 7,8 in cells. They may play a role in cell to cell communication 7,9 , and have been observed in many organisms, including humans, and in different parts of the body, including the brain [10][11][12][13][14][15] . Photons in the brain could serve as ideal candidates for information transfer. They travel tens of millions of times faster than a typical electrical neural signal and are not prone to thermal noise at body temperature owing to their relatively high energies. It is conceivable that evolution might have found a way to utilize these precious high-energy resources for information transfer, even if they were just the by-products of metabolism to begin with. Most of the required molecular machinery seems to exist in living cells such as neurons 16 . Mitochondrial respiration 17,18 or lipid oxidation 19 could serve as sources, and centrosomes 20 or chromophores in the mitochondria 21 could serve as detectors. However, one crucial element for optical communication is not well established, namely the existence of physical links to connect all of these spatially separated agents in a selective way. The only viable way to achieve targeted optical communication in the dense and (seemingly) disordered brain environment is for the photons to travel in wavegu...

We design a quantum repeater architecture, necessary for long distance quantum networks, using the recently proposed microwave cat state qubits, formed and manipulated via interaction between a superconducting nonlinear element and a microwave cavity. These qubits are especially attractive for repeaters because in addition to serving as excellent computational units with deterministic gate operations, they also have coherence times long enough to deal with the unavoidable propagation delays. Since microwave photons are too low in energy to be able to carry quantum information over long distances, as an intermediate step, we expand on a recently proposed microwave to optical transduction protocol using excited states of a rare-earth ion (Er 3+ ) doped crystal. To enhance the entanglement distribution rate, we propose to use spectral multiplexing by employing an array of cavities at each node. We compare our achievable rates with direct transmission and with two other promising repeater approaches, and show that ours could be higher in appropriate regimes, even in the presence of realistic imperfections and noise, while maintaining reasonably high fidelities of the final state. Thus, in the short term, our work could be directly useful for secure quantum communication, whereas in the long term, we can envision a large scale distributed quantum computing network built on our architecture. velop a new repeater scheme using microwave cavities and transducers. This could also be useful in allocating some resources of relatively nearby quantum computing nodes to serve as repeater links to connect more distant nodes. We calculate the entanglement distribution rates, and compare those with direct transmission, with the well-known ensemble-based Duan-Lukin-Cirac-Zoller (DLCZ) repeater protocol [56], and with a recently proposed single-emitter-based approach in rare-earth (RE) ion doped crystals [65]. We conclude that our approach could yield higher rates in suitable regimes. We also estimate the fidelities of our final entangled states in the presence of realistic noise and imperfections, and we find them to be sufficiently high to perform useful quantum communication tasks, even without entanglement purification or quantum error correction. The latter protocols are likely to be needed for more complex tasks such as distributed quantum computing, and we anticipate that it should be possible to incorporate them in the present framework.The paper is organised as follows: In Sec. II, we briefly describe the qubit and the gates, following ref. [33,34]. In Sec. III, we discuss the entanglement generation scheme between distant qubits, which includes a description of the transduction protocol. Sec. IV deals with our proposal for a quantum repeater with the same architecture. In Sec. V, we provide some additional implementation details pertinent to our proposal. In Sec. VI, we estimate the rates and fidelities of our final entangled states, and make pertinent comparisons with other schemes. In Sec. VII, we draw our conclusions an...

Motivated by recent studies of circuit complexity in weakly interacting scalar field theory, we explore the computation of circuit complexity in Z2 Even Effective Field Theories (Z2 EEFTs). We consider a massive free field theory with higher-order Wilsonian operators such as ϕ4, ϕ6, and ϕ8. To facilitate our computation, we regularize the theory by putting it on a lattice. First, we consider a simple case of two oscillators and later generalize the results to N oscillators. This study was carried out for nearly Gaussian states. In our computation, the reference state is an approximately Gaussian unentangled state, and the corresponding target state, calculated from our theory, is an approximately Gaussian entangled state. We compute the complexity using the geometric approach developed by Nielsen, parameterizing the path-ordered unitary transformation and minimizing the geodesic in the space of unitaries. The contribution of higher-order operators to the circuit complexity in our theory is discussed. We also explore the dependency of complexity on other parameters in our theory for various cases.

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