Frontiers of quantum physics

Abstract:

We propose a feasible approach to test quantum nonlocality with two qubits dispersively coupled to a driven cavity. Our proposal is based on spectral joint measurements of two qubits, i.e., their quantum states in the computational basis states {|kl,k,l=0,1} can be measured nondestructively by detecting the steady-state transmission spectra of the driven cavity. With this kind of measurements, the existence of Bell state can be robustly confirmed instead of conventional quantum state tomography. Then this kind of measurements is further utilized to test CHSH-Bell inequality. The advantage and feasibility of our proposal are also discussed. © Copyright EPLA, 2012.

Abstract:

With the advent of quantum information, the violation of a Bell inequality is used to witness the absence of an eavesdropper in cryptographic scenarios such as key distribution and randomness expansion. One of the key assumptions of Bell's theorem is the existence of experimental "free will," meaning that measurement settings can be chosen at random and independently by each party. The relaxation of this assumption potentially shifts the balance of power towards an eavesdropper. We consider a no-signaling model with reduced "free will" and bound the adversary’s capabilities in the task of randomness expansion.

Abstract:

Many intramolecular electron transfer (ET) reactions in biology are mediated by metal centers in proteins. This process is commonly described by a model of diffusive hopping according to the semiclassical theories of Marcus and Hopfield. However, recent studies have raised the possibility that nontrivial quantum mechanical effects play a functioning role in certain biomolecular processes. Here, we investigate the potential effects of quantum coherence in biological ET by extending the semiclassical model to allow for the possibility of quantum coherent phenomena using a quantum master equation based on the Holstein Hamiltonian. We test the model on the structurally defined chain of seven iron-sulfur clusters in nicotinamide adenine dinucleotide plus hydrogen:ubiquinone oxidoreductase (complex I), a crucial respiratory enzyme and one of the longest chains of metal centers in biology. Using experimental parameters where possible, we find that, in limited circumstances, a small quantum mechanical contribution can provide a marked increase in the ET rate above the semiclassical diffusive-hopping rate. Under typical biological conditions, our model reduces to well-known diffusive behavior. © 2012 American Physical Society.