842 research outputs found
Undoing measurement-induced dephasing in circuit QED
We analyze the backaction of homodyne detection and photodetection on
superconducting qubits in circuit quantum electrodynamics. Although both
measurement schemes give rise to backaction in the form of stochastic phase
rotations, which leads to dephasing, we show that this can be perfectly undone
provided that the measurement signal is fully accounted for. This result
improves upon that of Phys. Rev. A, 82, 012329 (2010), showing that the method
suggested can be made to realize a perfect two-qubit parity measurement. We
propose a benchmarking experiment on a single qubit to demonstrate the method
using homodyne detection. By analyzing the limited measurement efficiency of
the detector and bandwidth of the amplifier, we show that the parameter values
necessary to see the effect are within the limits of existing technology
Designing frequency-dependent relaxation rates and Lamb shift for a giant artificial atom
In traditional quantum optics, where the interaction between atoms and light
at optical frequencies is studied, the atoms can be approximated as point-like
when compared to the wavelength of light. So far, this relation has also been
true for artificial atoms made out of superconducting circuits or quantum dots,
interacting with microwave radiation. However, recent and ongoing experiments
using surface acoustic waves show that a single artificial atom can be coupled
to a bosonic field at several points wavelengths apart. Here, we theoretically
study this type of system. We find that the multiple coupling points give rise
to a frequency dependence in the coupling strength between the atom and its
environment, and also in the Lamb shift of the atom. The frequency dependence
is given by the discrete Fourier transform of the coupling point coordinates
and can therefore be designed. We discuss a number of possible applications for
this phenomenon, including tunable coupling, single-atom lasing, and other
effects that can be achieved by designing the relative coupling strengths of
different transitions in a multi-level atom.Comment: 14 pages, 8 figure
Simple preparation of Bell and GHZ states using ultrastrong-coupling circuit QED
The ability to entangle quantum systems is crucial for many applications in
quantum technology, including quantum communication and quantum computing.
Here, we propose a new, simple, and versatile setup for deterministically
creating Bell and Greenberger-Horne-Zeilinger (GHZ) states between photons of
different frequencies in a two-step protocol. The setup consists of a quantum
bit (qubit) coupled ultrastrongly to three photonic resonator modes. The only
operations needed in our protocol are to put the qubit in a superposition
state, and then tune its frequency in and out of resonance with sums of the
resonator-mode frequencies. By choosing which frequency we tune the qubit to,
we select which entangled state we create. We show that our protocol can be
implemented with high fidelity using feasible experimental parameters in
state-of-the-art circuit quantum electrodynamics. One possible application of
our setup is as a node distributing entanglement in a quantum network.Comment: 15 pages, 7 figure
The giant acoustic atom --- a single quantum system with a deterministic time delay
We investigate the quantum dynamics of a single transmon qubit coupled to
surface acoustic waves (SAWs) via two distant connection points. Since the
acoustic speed is five orders of magnitude slower than the speed of light, the
travelling time between the two connection points needs to be taken into
account. Therefore, we treat the transmon qubit as a giant atom with a
deterministic time delay. We find that the spontaneous emission of the system,
formed by the giant atom and the SAWs between its connection points, initially
decays polynomially in the form of pulses instead of a continuous exponential
decay behaviour, as would be the case for a small atom. We obtain exact
analytical results for the scattering properties of the giant atom up to
two-phonon processes by using a diagrammatic approach. We find that two peaks
appear in the inelastic (incoherent) power spectrum of the giant atom, a
phenomenon which does not exist for a small atom. The time delay also gives
rise to novel features in the reflectance, transmittance, and second-order
correlation functions of the system. Furthermore, we find the short-time
dynamics of the giant atom for arbitrary drive strength by a numerically exact
method for open quantum systems with a finite-time-delay feedback loop.Comment: To be published on Physical Review
Photodetection probability in quantum systems with arbitrarily strong light-matter interaction
Cavity-QED systems have recently reached a regime where the light-matter
interaction strength amounts to a non-negligible fraction of the resonance
frequencies of the bare subsystems. In this regime, it is known that the usual
normal-order correlation functions for the cavity-photon operators fail to
describe both the rate and the statistics of emitted photons. Following
Glauber's original approach, we derive a simple and general quantum theory of
photodetection, valid for arbitrary light-matter interaction strengths. Our
derivation uses Fermi's golden rule, together with an expansion of system
operators in the eigenbasis of the interacting light-matter system, to arrive
at the correct photodetection probabilities. We consider both narrow- and
wide-band photodetectors. Our description is also valid for point-like
detectors placed inside the optical cavity. As an application, we propose a
gedanken experiment confirming the virtual nature of the bare excitations that
enrich the ground state of the quantum Rabi model.Comment: 9 pages, 1 figur
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