11 research outputs found
Engineering Dynamical Sweet Spots to Protect Qubits from 1/ Noise
Protecting superconducting qubits from low-frequency noise is essential for
advancing superconducting quantum computation. Based on the application of a
periodic drive field, we develop a protocol for engineering dynamical sweet
spots which reduce the susceptibility of a qubit to low-frequency noise. Using
the framework of Floquet theory, we prove rigorously that there are manifolds
of dynamical sweet spots marked by extrema in the quasi-energy differences of
the driven qubit. In particular, for the example of fluxonium biased slightly
away from half a flux quantum, we predict an enhancement of pure-dephasing by
three orders of magnitude. Employing the Floquet eigenstates as the
computational basis, we show that high-fidelity single- and two-qubit gates can
be implemented while maintaining dynamical sweet-spot operation. We further
confirm that qubit readout can be performed by adiabatically mapping the
Floquet states back to the static qubit states, and subsequently applying
standard measurement techniques. Our work provides an intuitive tool to encode
quantum information in robust, time-dependent states, and may be extended to
alternative architectures for quantum information processing
Floquet-engineered enhancement of coherence times in a driven fluxonium qubit
We use the quasienergy structure that emerges when a fluxonium
superconducting circuit is driven periodically to encode quantum information
with dynamically induced flux-insensitive sweet spots. The framework of Floquet
theory provides an intuitive description of these high-coherence working points
located away from the half-flux symmetry point of the undriven qubit. This
approach offers flexibility in choosing the flux bias point and the energy of
the logical qubit states as shown in [\textit{Huang et al., 2020}]. We
characterize the response of the system to noise in the modulation amplitude
and DC flux bias, and experimentally demonstrate an optimal working point which
is simultaneously insensitive against fluctuations in both. We observe a
40-fold enhancement of the qubit coherence times measured with Ramsey-type
interferometry at the dynamical sweet spot compared with static operation at
the same bias point.Comment: 12 pages, 7 figure
Universal gates for protected superconducting qubits using optimal control
We employ quantum optimal control theory to realize quantum gates for two
protected superconducting circuits: the heavy-fluxonium qubit and the 0-
qubit. Utilizing automatic differentiation facilitates the simultaneous
inclusion of multiple optimization targets, allowing one to obtain
high-fidelity gates with realistic pulse shapes. For both qubits, disjoint
support of low-lying wave functions prevents direct population transfer between
the computational-basis states. Instead, optimal control favors dynamics
involving higher-lying levels, effectively lifting the protection for a
fraction of the gate duration. For the 0- qubit, offset-charge dependence
of matrix elements among higher levels poses an additional challenge for gate
protocols. To mitigate this issue, we randomize the offset charge during the
optimization process, steering the system towards pulse shapes insensitive to
charge variations. Closed-system fidelities obtained are 99% or higher, and
show slight reductions in open-system simulations.Comment: 12 pages, 6 figure
Accurate methods for the analysis of strong-drive effects in parametric gates
The ability to perform fast, high-fidelity entangling gates is an important
requirement for a viable quantum processor. In practice, achieving fast gates
often comes with the penalty of strong-drive effects that are not captured by
the rotating-wave approximation. These effects can be analyzed in simulations
of the gate protocol, but those are computationally costly and often hide the
physics at play. Here, we show how to efficiently extract gate parameters by
directly solving a Floquet eigenproblem using exact numerics and a perturbative
analytical approach. As an example application of this toolkit, we study the
space of parametric gates generated between two fixed-frequency transmon qubits
connected by a parametrically driven coupler. Our analytical treatment, based
on time-dependent Schrieffer-Wolff perturbation theory, yields closed-form
expressions for gate frequencies and spurious interactions, and is valid for
strong drives. From these calculations, we identify optimal regimes of
operation for different types of gates including SWAP, controlled-Z, and
CNOT. These analytical results are supplemented by numerical Floquet
computations from which we directly extract drive-dependent gate parameters.
This approach has a considerable computational advantage over full simulations
of time evolutions. More generally, our combined analytical and numerical
strategy allows us to characterize two-qubit gates involving parametrically
driven interactions, and can be applied to gate optimization and cross-talk
mitigation such as the cancellation of unwanted ZZ interactions in multi-qubit
architectures.Comment: 20 pages, 9 figures, 62 reference
Protected qubits, Floquet engineering and crosstalk suppression with superconducting circuits
Over the past decade, quantum circuits have been transitioning from being useful solely in fundamental physics research to having applications in a wide variety of fields. This has been made possible by the advancements in the coherence, coupling and optimal control of various elements of these quantum circuits. The experiments presented in this thesis solve critical challenges for the above mentioned areas. We provide the first experimental realization of a protected qubit having simultaneous robustness to relaxation and dephasing processes. We show a 40-fold improvement in the coherence time in fluxonium qubit by harnessing insights from Floquet engineering. Furthermore, we also demonstrate a coupling architecture for suppressing qubit-qubit crosstalk. The above works unlock new directions for improving the state of quantum systems
Recommended from our members
Experimental Realization of a Protected Superconducting Circuit Derived from the 0 - π Qubit
Encoding a qubit in logical quantum states with wave functions characterized by disjoint support and robust energies can offer simultaneous protection against relaxation and pure dephasing. One of the most promising candidates for such a fully protected superconducting qubit is the 0-Ď€ circuit [Brooks et al., Phys. Rev. A 87, 052306 (2013)]. Here we realize the proposed circuit topology in an experimentally obtainable parameter regime, where the ground-state degeneracy is lifted but the qubit is still largely noise protected. More precisely, the logical states of this qubit feature disjoint support and are exponentially protected against relaxation and exponentially (first order) protected against dephasing due to charge (flux) noise. We name the resultant device the "soft 0-Ď€ qubit."Multitone spectroscopy measurements reveal the energy-level structure of the system, which can be precisely described by a simple two-mode Hamiltonian. Using a Raman-type protocol, we exploit a higher-lying charge-insensitive energy level of the device to realize coherent population transfer and logical operations. The measured relaxation (T1 = 1.6 ms) and dephasing (TR = 9 ÎĽs, T2E = 25ÎĽs) times demonstrate that the soft 0-Ď€ circuit not only broadens the family of superconducting qubits but also constitutes an important step toward quantum computing with intrinsically protected superconducting qubits
Recommended from our members
Experimental Realization of a Protected Superconducting Circuit Derived from the 0-Ď€ Qubit
Encoding a qubit in logical quantum states with wave functions characterized by disjoint support and robust energies can offer simultaneous protection against relaxation and pure dephasing. One of the most promising candidates for such a fully protected superconducting qubit is the 0–π circuit [Brooks et al., Phys. Rev. A 87, 052306 (2013)]. Here we realize the proposed circuit topology in an experimentally obtainable parameter regime, where the ground-state degeneracy is lifted but the qubit is still largely noise protected. More precisely, the logical states of this qubit feature disjoint support and are exponentially protected against relaxation and exponentially (first order) protected against dephasing due to charge (flux) noise. We name the resultant device the “soft 0–π qubit.” Multitone spectroscopy measurements reveal the energy-level structure of the system, which can be precisely described by a simple two-mode Hamiltonian. Using a Raman-type protocol, we exploit a higher-lying charge-insensitive energy level of the device to realize coherent population transfer and logical operations. The measured relaxation (T1 = 1.6 ms) and dephasing (TR = 9 μs, T2E = 25 μs) times demonstrate that the soft 0–π circuit not only broadens the family of superconducting qubits but also constitutes an important step toward quantum computing with intrinsically protected superconducting qubits