69 research outputs found
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
Chiral quantum optics with giant atoms
In quantum optics, it is common to assume that atoms are pointlike objects compared to the wavelength of the electromagnetic field they interact with. However, this dipole approximation is not always valid, e.g., if atoms couple to the field at multiple discrete points. Previous work has shown that superconducting qubits coupled to a one-dimensional waveguide can behave as such "giant atoms"and then can interact through the waveguide without decohering, a phenomenon that is not possible with small atoms. Here, we show that this decoherence-free interaction is also possible when the coupling to the waveguide is chiral, i.e., when the coupling depends on the propagation direction of the light. Furthermore, we derive conditions under which the giant atoms in such chiral architectures exhibit dark states. In particular, we show that unlike small atoms, giant atoms in a chiral waveguide can reach a dark state even without being excited by a coherent drive. We also show that in the driven-dissipative regime, dark states can be populated faster in giant atoms. The results presented here lay a foundation for applications based on giant atoms in quantum simulations and quantum networks with chiral settings
Detecting quantum speedup of random walks with machine learning
We explore the use of machine-learning techniques to detect quantum speedup
in random walks on graphs. Specifically, we investigate the performance of
three different neural-network architectures (variations on fully connected and
convolutional neural networks) for identifying linear, cyclic, and random
graphs that yield quantum speedups in terms of the hitting time for reaching a
target node after starting in another node of the graph. Our results indicate
that carefully building the data set for training can improve the performance
of the neural networks, but all architectures we test struggle to classify
large random graphs and generalize from training on one graph size to testing
on another. If classification accuracy can be improved further, valuable
insights about quantum advantage may be gleaned from these neural networks, not
only for random walks, but more generally for quantum computing and quantum
transport.Comment: 15 pages, 8 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
The XYZ hexagonal stabilizer code
We consider a topological stabilizer code on a honeycomb grid, the "XYZ"
code. The code is inspired by the Kitaev honeycomb model and is a simple
realization of a "matching code" discussed by Wootton [J. Phys. A: Math. Theor.
48, 215302 (2015)], with a specific implementation of the boundary. It utilizes
weight-six () plaquette stabilizers and weight-two () link
stabilizers on a planar hexagonal grid composed of qubits for code
distance , with weight-three stabilizers at the boundary, stabilizing one
logical qubit. We study the properties of the code using maximum-likelihood
decoding, assuming perfect stabilizer measurements. For pure , , or
noise, we can solve for the logical failure rate analytically, giving a
threshold of 50%. In contrast to the rotated surface code and the XZZX code,
which have code distance only for pure noise, here the code distance
is for both pure and pure noise. Thresholds for noise with
finite bias are similar to the XZZX code, but with markedly lower
sub-threshold logical failure rates. The code possesses distinctive syndrome
properties with unidirectional pairs of plaquette defects along the three
directions of the triangular lattice for isolated errors, which may be useful
for efficient matching-based or other approximate decoding.Comment: 15 pages, 7 figure
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