Experimental protection against evolution of states in a subspace via a super-Zeno scheme on an NMR quantum information processor

Abstract

We experimentally demonstrate the freezing of evolution of quantum states in one-and two-dimensional subspaces of two qubits, on an NMR quantum information processor. State evolution was frozen and leakage of the state from its subspace to an orthogonal subspace was successfully prevented using super-Zeno sequences [Phys. Rev. Lett. 96, 100405 (2006)], comprising a set of radio frequency (rf) pulses punctuated by pre-selected time intervals. We demonstrate the efficacy of the scheme by preserving different types of states, including separable and maximally entangled states in one-and two-dimensional subspaces of two qubits. The change in the experimental density matrices was tracked by carrying out full state tomography at several time points. We use the fidelity measure for the one-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 density matrix. For the case of entangled states, we additionally compute an entanglement parameter to indicate the presence of entanglement in the state at different times. We experimentally demonstrate that the super-Zeno scheme is able to successfully confine state evolution to the one-or two-dimensional subspace being protected.

Experimental realization of quantum zeno dynamics

“…In multi-dimensional systems, however, Zeno subspaces are formed that can contain complex quantum states and dynamics: repeated observations create a barrier that blocks coherent evolution between subspaces, but leaves coherences and dynamics within those subspaces intact [10]. Analogous effects can also be realized through coherent control pulses or strong driving fields that decouple transitions between the subspaces [11][12][13][14][15][16][17][18] . Pioneering experiments have highlighted that the resulting non-trivial dynamics can be used to prepare exotic quantum states [19][20][21][22][23].…”

Experimentally freezing quantum discord in a dephasing environment using dynamical decoupling

“…This inherent immunity of such quantum correlations to environmental noise throws up new possibilities for the characterization of quantum behavior and its exploitation for quantum information processing. The peculiar "frozen" behavior of quantum discord in the presence of noise was experimentally investigated using photonic qubits [10] and NMR qubits [11,12]. The class of initial quantum states that exhibit this sudden transition in their decay rates was theoretically studied under the action of standard noise channels, and it was inferred that the type of states that display such behavior depends on the nature of the decohering channel being considered [13].…”

“…Protecting time-invariant discord:-Dynamical decoupling (DD) schemes, consisting of repeated sets of π pulses with tailored inter-pulse delays and phases, have played an important role in dealing with the debilitating effects of decoherence [20]. Several NMR QIP experiments have successfully used DD-type schemes to preserve quantum states [21,22].…”

Experimental creation of quantum Zeno subspaces by repeated multi-spin projections in diamond

“…In multi-dimensional systems; however, Zeno subspaces are formed that can contain complex quantum states and dynamics: repeated observations create a barrier that blocks coherent evolution between subspaces, but leaves coherences and dynamics within those subspaces intact 11 . Analogous effects can also be realized through coherent control pulses or strong driving fields that decouple transitions between the subspaces 12 13 14 15 16 17 18 19 . Pioneering experiments have highlighted that the non-trivial dynamics in Zeno subspaces can be used to prepare exotic quantum states 20 21 22 23 24 .…”