78 research outputs found
Measuring the quadrature coherence scale on a cloud quantum computer
Coherence underlies quantum phenomena, yet it is manifest in classical
theories; delineating coherence's role is a fickle business. The quadrature
coherence scale (QCS) was invented to remove such ambiguity, quantifying
quantum features of any single-mode bosonic system without choosing a preferred
orientation of phase space. The QCS is defined for any state, reducing to
well-known quantities in appropriate limits, including Gaussian and pure
states, and perhaps most importantly for a coherence measure, it is highly
sensitive to decoherence. Until recently, it was unknown how to measure the
QCS; we here report on an initial measurement of the QCS for squeezed light and
thermal states of light. This is performed using Xanadu's machine Borealis,
accessed through the cloud, which offers the configurable beam splitters and
photon-number-resolving detectors essential for measuring the QCS. The data and
theory match well, certifying the usefulness of interferometers and
photon-counting devices in certifying quantumness.Comment: 11 pages including 4 figures and 1 appendix; close to published
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Beyond transcoherent states: Field states for effecting optimal coherent rotations on single or multiple qubits
Semiclassically, laser pulses can be used to implement arbitrary
transformations on atomic systems; quantum mechanically, residual atom-field
entanglement spoils this promise. Transcoherent states are field states that
fix this problem in the fully quantized regime by generating perfect coherence
in an atom initially in its ground or excited state. We extend this fully
quantized paradigm in four directions: First, we introduce field states that
transform an atom from its ground or excited state to any point on the Bloch
sphere without residual atom-field entanglement. The best strong pulses for
carrying out rotations by angle are are squeezed in photon-number
variance by a factor of . Next, we investigate implementing
rotation gates, showing that the optimal Gaussian field state for enacting a
pulse on an atom in an arbitrary, unknown initial state is number
squeezed by less: . Third, we extend these
investigations to fields interacting with multiple atoms simultaneously,
discovering once again that number squeezing by is optimal for
enacting pulses on all of the atoms simultaneously, with small
corrections on the order of the ratio of the number of atoms to the average
number of photons. Finally, we find field states that best perform arbitrary
rotations by through nonlinear interactions involving -photon
absorption, where the same optimal squeezing factor is found to be
. Backaction in a wide variety of atom-field interactions can
thus be mitigated by squeezing the control fields by optimal amounts.Comment: Updated formatting following acceptance in Quantu
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