42 research outputs found
Deterministic protocol for mapping a qubit to coherent state superpositions in a cavity
We introduce a new gate that transfers an arbitrary state of a qubit into a
superposition of two quasi-orthogonal coherent states of a cavity mode, with
opposite phases. This qcMAP gate is based on conditional qubit and cavity
operations exploiting the energy level dispersive shifts, in the regime where
they are much stronger than the cavity and qubit linewidths. The generation of
multi-component superpositions of quasi-orthogonal coherent states, non-local
entangled states of two resonators and multi-qubit GHZ states can be
efficiently achieved by this gate
Hardware-efficient autonomous quantum error correction
We propose a new method to autonomously correct for errors of a logical qubit
induced by energy relaxation. This scheme encodes the logical qubit as a
multi-component superposition of coherent states in a harmonic oscillator, more
specifically a cavity mode. The sequences of encoding, decoding and correction
operations employ the non-linearity provided by a single physical qubit coupled
to the cavity. We layout in detail how to implement these operations in a
practical system. This proposal directly addresses the task of building a
hardware-efficient and technically realizable quantum memory.Comment: 12 pages,6 figure
Control of Localized Multiple Excitation Dark States in Waveguide QED
Subradiant excited states in finite chains of two-level quantum emitters
coupled to a one-dimensional reservoir are a resource for superior photon
storage and controlled photon manipulation. Typically, states storing multiple
excitations exhibit fermionic correlations and are thus characterized by an
anti-symmetric wavefunction, which makes them hard to prepare experimentally.
Here we identify a class of quasi-localized dark states with up to half of the
qubits excited, which appear for lattice constants that are an integer multiple
of the guided-mode wavelength. They allow for a high-fidelity preparation and
minimally invasive read out in state-of-the-art setups. In particular, we
suggest an experimental implementation using a coplanar wave-guide coupled to
superconducting transmon qubits on a chip. As free space and intrinsic losses
are minimal, virtually perfect dark states can be achieved even for a low
number of qubits, enabling fast preparation and manipulation with high
fidelity.Comment: 11 pages, 6 figure
Visualizing the emission of a single photon with frequency and time resolved spectroscopy
At the dawn of Quantum Physics, Wigner and Weisskopf obtained a full
analytical description (a \textit{photon portrait}) of the emission of a single
photon by a two-level system, using the basis of frequency modes (Weisskopf and
Wigner, "Zeitschrift f\"ur Physik", 63, 1930). A direct experimental
reconstruction of this portrait demands an accurate measurement of a time
resolved fluorescence spectrum, with high sensitivity to the off-resonant
frequencies and ultrafast dynamics describing the photon creation. In this work
we demonstrate such an experimental technique in a superconducting waveguide
Quantum Electrodynamics (wQED) platform, using single transmon qubit and two
coupled transmon qubits as quantum emitters. In both scenarios, the photon
portraits agree quantitatively with the predictions of the input-output theory
and qualitatively with Wigner-Weisskopf theory. We believe that our technique
allows not only for interesting visualization of fundamental principles, but
may serve as a tool, e.g. to realize multi-dimensional spectroscopy in
waveguide Quantum Electrodynamics.Comment: 18 pages, 10 figures including appendice
Compatibility and noncontextuality for sequential measurements
A basic assumption behind the inequalities used for testing noncontextual
hidden variable models is that the observables measured on the same individual
system are perfectly compatible. However, compatibility is not perfect in
actual experiments using sequential measurements. We discuss the resulting
"compatibility loophole" and present several methods to rule out certain hidden
variable models which obey a kind of extended noncontextuality. Finally, we
present a detailed analysis of experimental imperfections in a recent trapped
ion experiment and apply our analysis to that case.Comment: 15 pages, 2 figures, v2: problem with latex solve