Search results for "dephasing"
showing 10 items of 73 documents
Coupling-assisted Landau-Majorana-Stückelberg-Zener transition in a system of two interacting spin qubits
2019
We analyse a system of two interacting spin-qubits subjected to a Landau-Majorana-Stuckelberg-Zener (LMSZ) ramp. We prove that LMSZ transitions of the two spin-qubits are possible without an external transverse static field since its role is played by the coupling between the spin-qubits. We show how such a physical effect could be exploited to estimate the strength of the interaction between the two spin-qubits and to generate entangled states of the system by appropriately setting the slope of the ramp. Moreover, the study of effects of the coupling parameters on the time-behaviour of the entanglement is reported. Finally, our symmetry-based approach allows us to discuss also effects stem…
Implications of surface noise for the motional coherence of trapped ions
2016
Electric noise from metallic surfaces is a major obstacle towards quantum applications with trapped ions due to motional heating of the ions. Here, we discuss how the same noise source can also lead to pure dephasing of motional quantum states. The mechanism is particularly relevant at small ion-surface distances, thus imposing a new constraint on trap miniaturization. By means of a free induction decay experiment, we measure the dephasing time of the motion of a single ion trapped 50~$\mu$m above a Cu-Al surface. From the dephasing times we extract the integrated noise below the secular frequency of the ion. We find that none of the most commonly discussed surface noise models for ion trap…
Temporal phase control of bound-bound and bound-free two-photon transitions in NO with two time-delayed cross-polarized pulses
2001
0953-4075; Temporal phase control over the multi-photoionization of NO is investigated experimentally The coherent excitation of the molecular system is achieved by a sequence of two phase-related non-Fourier-transform-limited laser pulses delivered by a nanosecond dye laser, The phase-locked pulses are produced by making use of an actively stabilized interferometer with a tunable pathlength difference. As the coherence time of the laser is comparable with the inhomogenous dephasing time of the medium, the measurements are performed with temporally overlapped pulses. In order to avoid the modulation of the ionic signal coming from optical interference, the linear polarizations of the two pu…
Decoherence in circuits of small Josephson junctions
2001
We discuss dephasing by the dissipative electromagnetic environment and by measurement in circuits consisting of small Josephson junctions. We present quantitative estimates and determine in which case the circuit might qualify as a quantum bit. Specifically, we analyse a three junction Cooper pair pump and propose a measurement to determine the decoherence time $\tau_\phi$.
Hidden entanglement in the presence of random telegraph dephasing noise
2012
Entanglement dynamics of two noninteracting qubits, locally affected by random telegraph noise at pure dephasing, exhibits revivals. These revivals are not due to the action of any nonlocal operation, thus their occurrence may appear paradoxical since entanglement is by definition a nonlocal resource. We show that a simple explanation of this phenomenon may be provided by using the (recently introduced) concept of "hidden" entanglement, which signals the presence of entanglement that may be recovered with the only help of local operations.
Coherent Control of the Rotational Degree of Freedom of a Two-Ion Coulomb Crystal.
2019
We demonstrate the preparation and coherent control of the angular momentum state of a two-ion crystal. The ions are prepared with an average angular momentum of 7850ℏ freely rotating at 100 kHz in a circularly symmetric potential, allowing us to address rotational sidebands. By coherently exciting these motional sidebands, we create superpositions of states separated by up to four angular momentum quanta. Ramsey experiments show the expected dephasing of the superposition which is dependent on the number of quanta separating the states. These results demonstrate coherent control of a collective motional state described as a quantum rotor in trapped ions. Moreover, our Letter offers an expa…
Memory-assisted long-distance phase-matching quantum key distribution
2019
We propose a scheme that generalizes the loss scaling properties of twin-field or phase-matching quantum key distribution (QKD) related to a channel of transmission $\eta_{total}$ from $\sqrt{\eta_{total}}$ to $\sqrt[2n]{\eta_{total}}$ by employing n-1 memory stations with spin qubits and n beam-splitter stations including optical detectors. Our scheme's resource states are similar to the coherent-state-based light-matter entangled states of a previous hybrid quantum repeater, but unlike the latter our scheme avoids the necessity of employing 2n-1 memory stations and writing the transmitted optical states into the matter memory qubits. The full scaling advantage of this memory-assisted phas…
Entanglement production by quantum error correction in the presence of correlated environment
2003
We analyze the effect of a quantum error correcting code on the entanglement of encoded logical qubits in the presence of a dephasing interaction with a correlated environment. Such correlated reservoir introduces entanglement between physical qubits. We show that for short times the quantum error correction interprets such entanglement as errors and suppresses it. However for longer time, although quantum error correction is no longer able to correct errors, it enhances the rate of entanglement production due to the interaction with the environment.
Robustness of quantum memories based on Majorana zero modes
2012
We analyze the rate at which quantum information encoded in zero-energy Majorana modes is lost in the presence of perturbations. We show that information can survive for times that scale exponentially with the size of the chain both in the presence of quenching and time-dependent quadratic dephasing perturbations, even when the latter have spectral components above the system's energy gap. The origin of the robust storage, namely the fact that a sudden quench affects in the same way both parity sectors of the original spectrum, is discussed, together with the memory performance at finite temperatures and in the presence of particle exchange with a bath.
Quantum synchronization as a local signature of super- and subradiance
2017
We study the relationship between the collective phenomena of super- and subradiance and spontaneous synchronization of quantum systems. To this aim we revisit the case of two detuned qubits interacting through a pure dissipative bosonic environment, which contains the minimal ingredients for our analysis. By using the Liouville formalism, we are able to find analytically the ultimate connection between these phenomena. We find that dynamical synchronization is due to the presence of longstanding coherence between the ground state of the system and the subradiant state. We finally show that, under pure dissipation, the emergence of spontaneous synchronization and of subradiant emission occu…