17 research outputs found
Sequential modular position and momentum measurements of a trapped ion mechanical oscillator
The non-commutativity of position and momentum observables is a hallmark
feature of quantum physics. However this incompatibility does not extend to
observables which are periodic in these base variables. Such modular-variable
observables have been suggested as tools for fault-tolerant quantum computing
and enhanced quantum sensing. Here we implement sequential measurements of
modular variables in the oscillatory motion of a single trapped ion, using
state-dependent displacements and a heralded non-destructive readout. We
investigate the commutative nature of modular variable observables by
demonstrating no-signaling-in-time between successive measurements, using a
variety of input states. In the presence of quantum interference, which we
enhance using squeezed input states, measurements of different periodicity show
signaling-in-time. The sequential measurements allow us to extract two-time
correlators for modular variables, which we use to violate a Leggett-Garg
inequality. The experiments involve control and coherence of multi-component
superpositions of up to 8 coherent, squeezed or Fock state wave-packets.
Signaling-in-time as well as Leggett-Garg inequalities serve as efficient
quantum witnesses which we probe here with a mechanical oscillator, a system
which has a natural crossover from the quantum to the classical regime.Comment: 6 pages, 3 figures and supplemental informatio
Time-dependent Hamiltonian estimation for Doppler velocimetry of trapped ions
The time evolution of a closed quantum system is connected to its Hamiltonian
through Schroedinger's equation. The ability to estimate the Hamiltonian is
critical to our understanding of quantum systems, and allows optimization of
control. Though spectroscopic methods allow time-independent Hamiltonians to be
recovered, for time-dependent Hamiltonians this task is more challenging. Here,
using a single trapped ion, we experimentally demonstrate a method for
estimating a time-dependent Hamiltonian of a single qubit. The method involves
measuring the time evolution of the qubit in a fixed basis as a function of a
time-independent offset term added to the Hamiltonian. In our system the
initially unknown Hamiltonian arises from transporting an ion through a static,
near-resonant laser beam. Hamiltonian estimation allows us to estimate the
spatial dependence of the laser beam intensity and the ion's velocity as a
function of time. This work is of direct value in optimizing transport
operations and transport-based gates in scalable trapped ion quantum
information processing, while the estimation technique is general enough that
it can be applied to other quantum systems, aiding the pursuit of high
operational fidelities in quantum control.Comment: 10 pages, 8 figure