It is generally impossible to probe a quantum system without disturbing it. However, it is possible to exploit the back action of quantum measurements and strong couplings to tailor and protect the coherent evolution of a quantum system. This is a profound and counterintuitive phenomenon known as quantum Zeno dynamics. Here we demonstrate quantum Zeno dynamics with a rubidium Bose–Einstein condensate in a five-level Hilbert space. We harness measurements and strong couplings to dynamically disconnect different groups of quantum states and constrain the atoms to coherently evolve inside a two-level subregion. In parallel to the foundational importance due to the realization of a dynamical superselection rule and the theory of quantum measurements, this is an important step forward in protecting and controlling quantum dynamics and, broadly speaking, quantum information processing.

Atom chips provide compact and robust platforms towards practical quantum technologies. A quick and faithful preparation of arbitrary input states for these systems is crucial but represents a very challenging experimental task. This is especially difficult when the dynamical evolution is noisy and unavoidable setup imperfections have to be considered. Here, we experimentally prepare with very high small errors different internal states of a Rubidium Bose-Einstein condensate realized on an atom chip. As a possible application of our scheme, we apply it to improve the sensitivity of an atomic interferometer.Microscopic magnetic traps, as atom chips [1,2], represent an important advance in the field of degenerate quantum gases towards their realization of practical quantum technological devices. Indeed, applications outside the laboratory depend on the compactness and robustness of these setups. Atom chips have already enabled, for instance, the demonstration of miniaturized interferometers [3,4], or the creation of squeezed atomic states [5]. Very recently, they have been also applied to the realization of atom interferometers based on non-classical motional states [6]. Furthermore, atom chips can be integrated with nanostructures [7], or photonic components [8], to implement new quantum information processing tools, or used as novel platforms for quantum simulations [9] and controllable coherent dynamics [10].As in most quantum technological applications, the ability to quickly and faithfully prepare arbitrary input states is of utmost importance. In particular, it is essential to speedup the initialization protocol of these quantum devices in order to reduce the effects of the inevitably present noise and decoherence sources. In this context, quantum optimal control provides powerful tools to tackle this problem by finding the optimal way to transform the system from an experimentally readily prepared initial condition to a desired state with high fidelity from which further quantum manipulations can be performed [11][12][13]. It has been successfully implemented in several physical systems, ranging from cold atoms [14] to molecules [15]. Recently optimal control algorithms have been developed and successfully applied to many-body quantum systems [16][17][18][19][20][21].In addition, optimal control allows one to speed up the state preparation process up to the ultimate bound imposed by quantum mechanics, the so-called quantum speed limit (QSL) [22][23][24][25]. Indeed, the change of a state into a different one cannot occur faster than a time scale that is inversely proportional to the associated energy scale. This is especially relevant for quantum systems like Bose-Einstein condensates (BECs) where it is crucial to perform some desired (coherent) task before dephasing processes unavoidably occur [26]. In this work we present experimental results obtained on a Rubidium ( 87 Rb) BEC, trapped on an atom chip, evolving in a five-level Hilbert space given by the five spin orientations of the F = 2 hyperfine groun...

The theoretical cornerstone of statistical mechanics is the ergodic assumption, i.e. the assumption that the time average of an observable equals its ensemble average. Here, we show how such a property is present when an open quantum system is continuously perturbed by an external environment effectively observing the system at random times while the system dynamics approaches the quantum Zeno regime. In this context, by large deviation theory we analytically show how the most probable value of the probability for the system to be in a given state eventually deviates from the non-stochastic case when the Zeno condition is not satisfied. We experimentally test our results with ultra-cold atoms prepared on an atom chip.

In the ideal quantum Zeno (QZ) effect, repeated quantum projective measurements can freeze the coherent dynamics of a quantum system. However, in the weak QZ regime, measurement backactions can allow the sensing of semi-classical field fluctuations. In this regard, we theoretically show how to combine the controlled manipulation of a quantum two-level system, used as a probe, with a sequence of projective measurements to have direct access to the noise correlation function. We experimentally test the effectiveness of the proposed noise sensing method on a properly engineered Bose-Einstein condensate of Rb 87 atoms realized on an atom chip. We believe that our QZ-based approach can open a new path towards novel quantum sensing devices.

We realize on an atom-chip, a practical, experimentally undemanding, tomographic reconstruction algorithm relying on the time-resolved measurements of the atomic population distribution among atomic internal states. More specifically, we estimate both the state density matrix, as well as the dephasing noise present in our system, by assuming complete knowledge of the Hamiltonian evolution. The proposed scheme is based on routinely performed measurements and established experimental procedures, hence providing a simplified methodology for quantum technological applications.

Atomic memories for flying photonic qubits are an essential ingredient for many applications like e.g. quantum repeaters. Verification of the coherent transfer of information from a light field to an atomic superposition is usually obtained using an optical read-out. In this paper we report the direct detection of the atomic coherence by means of atom interferometry. We experimentally verified both that a bichromatic laser field closing a Raman transition imprints a distinct, controllable phase on the atomic coherence and that it can be recovered after a variable time delay.

The authors report the use of the dressed chopped random basis optimal control algorithm to realize time‐reversal procedures. The latter are aimed for the implementation of quantum undo operations in quantum technology contexts as quantum computing and quantum communications. The last performed operation can be time‐reversed via the undo command so as to perfectly restore a condition in which any new operation, chosen by the external user, can be applied. By generalizing this concept, the undo command can also allow for the reversing of a quantum operation in a generic time instant of the past. Here, thanks to optimal time‐reversal routines, all these functionalities are experimentally implemented on the fivefold F=2$F=2$ Hilbert space of a Bose–Einstein condensate of non‐interacting 87Rb atoms in the ground state, realized with an atom chip. Each time‐reversal transformation is attained by designing an optimal modulated radio frequency field, achieving on average an accuracy of around 92% in any performed test. The experimental results are accompanied by a thermodynamic interpretation based on the Loschmidt echo. These findings are expected to promote the implementation of time‐reversal operations in a real scenario of gate‐based quantum computing with a more complex structure than the five‐level system considered here.

No abstract