Quantum states displaying quantum correlations, play a very important role in the field of quantum information processing and have been posited to lie at the heart of quantum computational speedup. This talk will focus on the quantification of quantum correlations, the distinction from their classical counterparts, and their behavior under decoherence. The discovery of the intriguing phenomenon that certain kinds of quantum correlations remain impervious to noise up to a specfic point in time and then suddenly decay, has generated immense recent interest. We exploit dynamical decoupling (DD) sequences to prolong the persistence of time-invariant quantum correlations in a system of two NMR qubits decohering in independent dephasing environments[1]. DD sequences have found widespread application in quantum information processing, as strategies for protecting quantum states against decoherence. For a quantum system coupled to a bath, the DD sequence decouples the system and bath by adding a suitable decoupling interaction, periodic with a cycle time, to the over-all system-bath Hamiltonian. We experimentally prepare two-qubit Bell-diagonal quantum states that interact with individual noise channels and demonstrate that we are able to freeze quantum correlations over long time scales via dynamical decoupling. Our results have impor-tant implications for experimental quantum control and for quantum information processing
situations where persistent quantum correlations have to be maintained. Using frequent measurements to project a quantum system back to its initial state and hence slow down its time evolution is a phenomenon known as the quantum Zeno effect. An interesting quantum Zeno-type strategy for state preservation, achieved using a sequence of nonperiodic short duration pulses, has been termed the super-Zeno scheme. We have experi-mentally applied the super-Zeno scheme for preservation of a quantum state by freezing state evolution (one-dimensional subspace protection) and for subspace preservation by preventing leakage of population to an orthogonal subspace (two-dimensional subspace protection) [2]. The change in the experimental density matrices was tracked by carrying out full state tomog-raphy at several time points. We use the delity measure for thene-dimensional case and the leakage (fraction) into the orthogonal subspace for the two-dimensional case, as qualitative indicators to estimate the resemblance of the density matrix at a later time to the initially prepared NMR density matrix. For the case of entangled states, we additionally ompute an entanglement parameter to indicate the presence of entanglement in the state at dierent times. We experimentally demonstrate that the super-Zeno scheme is able to successfully conne state evolution to the one- or two-dimensional subspace being protected. Uhrig dynamical decoupling (UDD) sequences are designed such that the pulse timing in the DD sequence is tailored to produce higher-order cancellations in the Magnus expan-sion of the eective average Hamiltonian, thereby achieving system-bath decoupling to a higher order and hence stronger noise protection. While UDD schemes can well protect states against single- and two-axis noise (i.e., pure dephasing and/or pure bit- ip), they are not 1 able to protect against general three-axis decoherence. Nested UDD (NUDD) schemes were hence proposed to protect multiqubit systems in generic quantum baths to arbitrary decou-pling orders by nesting several UDD layers. We experimentally implemented a three-layer nested UDD sequence on an NMR quantum information processor and explored its efficiency in protecting arbitrary states in a two-dimensional subspace of two qubits. The scheme is suffciently general as it does not assume prior information about the explicit form of the system-bath coupling. The experiments were highly demanding, with the control operations being complicated and involving manipulations of both qubits simultaneously. However, our
results demonstrate that such systematic NUDD schemes can be experimentally implemented, and are able to protect multiqubit states in systems that are arbitrarily coupled to quantum baths. Our study points the way to the realistic protection of fragile quantum states up to
high orders and against arbitrary noise.