Light harvesting components of photosynthetic organisms are complex, coupled, many-body quantum systems, in which electronic coherence has recently been shown to survive for relatively long time scales despite the decohering effects of their environments. Within this context, we analyze entanglement in multi-chromophoric light harvesting complexes, and establish methods for quantification of entanglement by presenting necessary and sufficient conditions for entanglement and by deriving a measure of global entanglement. These methods are then applied to the Fenna-MatthewsOlson (FMO) protein to extract the initial state and temperature dependencies of entanglement. We show that while FMO in natural conditions largely contains bipartite entanglement between dimerized chromophores, a small amount of long-range and multipartite entanglement exists even at physiological temperatures. This constitutes the first rigorous quantification of entanglement in a biological system. Finally, we discuss the practical utilization of entanglement in densely packed molecular aggregates such as light harvesting complexes.Unlike in classical physics, within quantum mechanics one can have maximal knowledge of a composite physical system and still not be able to assign a definite state to its constituent elements without reference to their relation to each other [1, 2]. Such systems are called entangled, and entanglement is a characteristic quantum mechanical effect that has been widely investigated in recent years [3,4]. Entanglement is often viewed as a fragile and exotic property, and in the quantum information context, where it is used as a resource for information processing tasks, precisely engineered entangled states of interest can indeed be both fragile and difficult to manufacture. However, it has also been recognized that entanglement is a natural feature of coherent evolution, and recently, there has been an effort to expand the realms in which entanglement can be shown to exist rigorously, particularly in "natural" systems -i.e., not ones manufactured in laboratory conditions. Signatures of entanglement, a characteristically quantum feature, have been demonstrated in thermal states of bulk systems at low temperatures and between parties at macroscopic length scales [5]. Additionally, several recent studies have focused on the dynamics of entanglement in damped, driven, or generally non-equilibrium quantum systems [6][7][8][9]. The dynamics of entanglement in open systems can be extremely nontrivial -especially in many-body systems -and the precise influence of non-Hamiltonian dynamics on entanglement is poorly understood. In a result particularly relevant to this work, it is shown in Ref.[8] that entanglement can be continuously generated and destroyed by non-equilibrium effects in an environment where no static entanglement exists. The possibility of entanglement in noisy non-equilibrium systems at high temperatures intimates the question: can we observe entanglement in the complex non-equilibrium chemical and biological ...

The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separated by more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory.

We introduce a new continuous-variable quantum key distribution (CV-QKD) protocol, self-referenced CV-QKD, that eliminates the need for transmission of a high-power local oscillator between the communicating parties. In this protocol, each signal pulse is accompanied by a reference pulse (or a pair of twin reference pulses), used to align Alice's and Bob's measurement bases. The method of phase estimation and compensation based on the reference pulse measurement can be viewed as a quantum analog of intradyne detection used in classical coherent communication, which extracts the phase information from the modulated signal. We present a proof-of-principle, fiber-based experimental demonstration of the protocol and quantify the expected secret key rates by expressing them in terms of experimental parameters. Our analysis of the secret key rate fully takes into account the inherent uncertainty associated with the quantum nature of the reference pulse(s) and quantifies the limit at which the theoretical key rate approaches that of the respective conventional protocol that requires local oscillator transmission. The self-referenced protocol greatly simplifies the hardware required for CV-QKD, especially for potential integrated photonics implementations of transmitters and receivers, with minimum sacrifice of performance. As such, it provides a pathway towards scalable integrated CV-QKD transceivers, a vital step towards large-scale QKD networks.

It has been suggested that excitation transport in photosynthetic light harvesting complexes features speedups analogous to those found in quantum algorithms. Here we compare the dynamics in these light harvesting systems to the dynamics of quantum walks, in order to elucidate the limits of such quantum speedups. For the Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria, we show that while there is indeed speedup at short times, this is short lived (70 fs) despite longer lived (ps) quantum coherence. Remarkably, this time scale is independent of the details of the decoherence model. More generally, we show that the distinguishing features of light-harvesting complexes not only limit the extent of quantum speedup but also reduce rates of diffusive transport. These results suggest that quantum coherent effects in biological systems are optimized for efficiency or robustness rather than the more elusive goal of quantum speedup.