32 research outputs found
A simulation methodology for superconducting qubit readout fidelity
Qubit readout is a critical part of any quantum computer including the superconducting-qubit-based one. The readout fidelity is affected by the readout pulse width, readout pulse energy, resonator design, qubit design, qubit-resonator coupling, and the noise generated along the readout path. It is thus important to model and predict the fidelity based on various design parameters along the readout path. In this work, a simulation methodology for superconducting qubit readout fidelity is proposed and implemented using Matlab and Ansys HFSS to allow co-optimization in the readout path. As an example, parameters are taken from an actual superconducting-qubit-based quantum computer. Without any calibrations, the model is able to predict the readout error of the system as a function of the readout pulse power. It is found that the system can still maintain high fidelity even if the input power is reduced by 7 dB. This can be used to guide the design and optimization of a superconducting qubit readout system
Simulating noise on a quantum processor: interactions between a qubit and resonant two-level system bath
Material defects fundamentally limit the coherence times of superconducting
qubits, and manufacturing completely defect-free devices is not yet possible.
Therefore, understanding the interactions between defects and a qubit in a real
quantum processor design is essential. We build a model that incorporates the
standard tunneling model, the electric field distributions in the qubit, and
open quantum system dynamics, and draws from the current understanding of
two-level system (TLS) theory. Specifically, we start with one million TLSs
distributed on the surface of a qubit and pick the 200 systems that are most
strongly coupled to the qubit. We then perform a full Lindbladian simulation
that explicitly includes the coherent coupling between the qubit and the TLS
bath to model the time dependent density matrix of resonant TLS defects and the
qubit. We find that the 200 most strongly coupled TLSs can accurately describe
the qubit energy relaxation time. This work confirms that resonant TLSs located
in areas where the electric field is strong can significantly affect the qubit
relaxation time, even if they are located far from the Josephson junction.
Similarly, a strongly-coupled resonant TLS located in the Josephson junction
does not guarantee a reduced qubit relaxation time if a more strongly coupled
TLS is far from the Josephson junction. In addition to the coupling strengths
between TLSs and the qubit, the model predicts that the geometry of the device
and the TLS relaxation time play a significant role in qubit dynamics. Our work
can provide guidance for future quantum processor designs with improved qubit
coherence times.Comment: 8 pages, 5 figure
Enhancing Qubit Readout with Autoencoders
In addition to the need for stable and precisely controllable qubits, quantum
computers take advantage of good readout schemes. Superconducting qubit states
can be inferred from the readout signal transmitted through a dispersively
coupled resonator. This work proposes a novel readout classification method for
superconducting qubits based on a neural network pre-trained with an
autoencoder approach. A neural network is pre-trained with qubit readout
signals as autoencoders in order to extract relevant features from the data
set. Afterwards, the pre-trained network inner layer values are used to perform
a classification of the inputs in a supervised manner. We demonstrate that this
method can enhance classification performance, particularly for short and long
time measurements where more traditional methods present lower performance.Comment: 16 pages, 23 figure
Experimentally revealing anomalously large dipoles in the dielectric of a quantum circuit
Quantum two-level systems (TLSs) intrinsic to glasses induce decoherence in many modern quantum devices, such as superconducting qubits. Although the low-temperature physics of these TLSs is usually well-explained by a phenomenological standard tunneling model of independent TLSs, the nature of these TLSs, as well as their behavior out of equilibrium and at high energies above 1 K, remain inconclusive. Here we measure the non-equilibrium dielectric loss of TLSs in amorphous silicon using a superconducting resonator, where energies of TLSs are varied in time using a swept electric field. Our results show the existence of two distinct ensembles of TLSs, interacting weakly and strongly with phonons, where the latter also possesses anomalously large electric dipole moment. These results may shed new light on the low temperature characteristics of amorphous solids, and hold implications to experiments and applications in quantum devices using time-varying electric fields
Stochastic modeling of superconducting qudits in the dispersive regime
The field of superconducting quantum computing, based on Josephson junctions,
has recently seen remarkable strides in scaling the number of logical qubits.
In particular, the fidelities of one- and two-qubit gates have reached the
breakeven point with the novel error mitigation and correction methods.
Parallel to these advances is the effort to expand the Hilbert space within a
single junction or device by employing high-dimensional qubits, otherwise known
as qudits. Research has demonstrated the possibility of driving higher-order
transitions in a transmon or designing innovative multimode superconducting
circuits, termed multimons. These advances can significantly expand the
computational basis while simplifying the interconnects in a large-scale
quantum processor. In this work we extend the measurement theory of a
conventional superconducting qubit to that of a qudit, focusing on modeling the
dispersive quadrature measurement in an open quantum system. Under the Markov
assumption, the qudit Lindblad and stochastic master equations are formulated
and analyzed; in addition, both the ensemble-averaged and the quantum-jump
approach of decoherence analysis are detailed with analytical and numerical
comparisons. We verify our stochastic model with a series of experimental
results on a transmon-type qutrit, verifying the validity of our
high-dimensional formalism.Comment: 16-page main text, 6 figures, 15-page appendice