149 research outputs found
Tunable coupler to fully decouple superconducting qubits
Enhancing the capabilities of superconducting quantum hardware, requires
higher gate fidelities and lower crosstalk, particularly in larger scale
devices, in which qubits are coupled to multiple neighbors. Progress towards
both of these objectives would highly benefit from the ability to fully control
all interactions between pairs of qubits. Here we propose a new coupler model
that allows to fully decouple dispersively detuned Transmon qubits from each
other, i.e. ZZ-crosstalk is completely suppressed while maintaining a maximal
localization of the qubits' computational basis states. We further reason that,
for a dispersively detuned Transmon system, this can only be the case if the
anharmonicity of the coupler is positive at the idling point. A simulation of a
40ns CZ-gate for a lumped element model suggests that achievable process
infidelity can be pushed below the limit imposed by state-of-the-art coherence
times of Transmon qubits. On the other hand, idle gates between qubits are no
longer limited by parasitic interactions. We show that our scheme can be
applied to large integrated qubit grids, where it allows to fully isolate a
pair of qubits, that undergoes a gate operation, from the rest of the chip
while simultaneously pushing the fidelity of gates to the limit set by the
coherence time of the individual qubits.Comment: 6 pages, 4 figure
Diagnosing magnetars with transient cooling
Transient X-ray emission, with an approximate t^{-0.7} decay, was observed
from SGR 1900+14 over 40 days following the the giant flare of 27 Aug 1998. We
calculate in detail the diffusion of heat to the surface of a neutron star
through an intense 10^{14}-10^{15} G magnetic field, following the release of
magnetic energy in its outer layers. We show that the power law index, the
fraction of burst energy in the afterglow, and the return to persistent
emission can all be understood if the star is composed of normal baryonic
material.Comment: 9 pages, 1 eps figur
Entanglement Stabilization using Parity Detection and Real-Time Feedback in Superconducting Circuits
Fault tolerant quantum computing relies on the ability to detect and correct
errors, which in quantum error correction codes is typically achieved by
projectively measuring multi-qubit parity operators and by conditioning
operations on the observed error syndromes. Here, we experimentally demonstrate
the use of an ancillary qubit to repeatedly measure the and parity
operators of two data qubits and to thereby project their joint state into the
respective parity subspaces. By applying feedback operations conditioned on the
outcomes of individual parity measurements, we demonstrate the real-time
stabilization of a Bell state with a fidelity of in up to 12
cycles of the feedback loop. We also perform the protocol using Pauli frame
updating and, in contrast to the case of real-time stabilization, observe a
steady decrease in fidelity from cycle to cycle. The ability to stabilize
parity over multiple feedback rounds with no reduction in fidelity provides
strong evidence for the feasibility of executing stabilizer codes on timescales
much longer than the intrinsic coherence times of the constituent qubits.Comment: 12 pages, 10 figures. Update: Fig. 5 correcte
The Jacobi orientation and the two-variable elliptic genus
We explain the relationship between the sigma orientation and Witten genus on
the one hand and the two-variable elliptic genus on the other. We show that if
E is an elliptic spectrum, then the Theorem of the Cube implies the existence
of canonical SU-orientation of the associated spectrum of Jacobi forms. In the
case of the elliptic spectrum associated to the Tate curve, this gives the
two-variable elliptic genus. We also show that the two-variable genus arises as
an instance of the circle-equivariant sigma orientation.Comment: Revised to better exhibit complex orientation of
MSU^(CP^\infty_{-infty}
Studying Light-Harvesting Models with Superconducting Circuits
The process of photosynthesis, the main source of energy in the animate
world, converts sunlight into chemical energy. The surprisingly high efficiency
of this process is believed to be enabled by an intricate interplay between the
quantum nature of molecular structures in photosynthetic complexes and their
interaction with the environment. Investigating these effects in biological
samples is challenging due to their complex and disordered structure. Here we
experimentally demonstrate a new approach for studying photosynthetic models
based on superconducting quantum circuits. In particular, we demonstrate the
unprecedented versatility and control of our method in an engineered three-site
model of a pigment protein complex with realistic parameters scaled down in
energy by a factor of . With this system we show that the excitation
transport between quantum coherent sites disordered in energy can be enabled
through the interaction with environmental noise. We also show that the
efficiency of the process is maximized for structured noise resembling
intramolecular phononic environments found in photosynthetic complexes.Comment: 8+12 pages, 4+12 figure
Realizing a Deterministic Source of Multipartite-Entangled Photonic Qubits
Sources of entangled electromagnetic radiation are a cornerstone in quantum
information processing and offer unique opportunities for the study of quantum
many-body physics in a controlled experimental setting. While multi-mode
entangled states of radiation have been generated in various platforms, all
previous experiments are either probabilistic or restricted to generate
specific types of states with a moderate entanglement length. Here, we
demonstrate the fully deterministic generation of purely photonic entangled
states such as the cluster, GHZ, and W state by sequentially emitting microwave
photons from a controlled auxiliary system into a waveguide. We tomographically
reconstruct the entire quantum many-body state for up to photonic modes
and infer the quantum state for even larger from process tomography. We
estimate that localizable entanglement persists over a distance of
approximately ten photonic qubits, outperforming any previous deterministic
scheme
Intermodulation Distortion in a Josephson Traveling Wave Parametric Amplifier
Josephson traveling wave parametric amplifiers enable the amplification of
weak microwave signals close to the quantum limit with large bandwidth, which
has a broad range of applications in superconducting quantum computing and in
the operation of single-photon detectors. While the large bandwidth allows for
their use in frequency-multiplexed detection architectures, an increased number
of readout tones per amplifier puts more stringent requirements on the dynamic
range to avoid saturation. Here, we characterize the undesired mixing processes
between the different frequency-multiplexed tones applied to a Josephson
traveling wave parametric amplifier, a phenomenon also known as intermodulation
distortion. The effect becomes particularly significant when the amplifier is
operated close to its saturation power. Furthermore, we demonstrate that
intermodulation distortion can lead to significant crosstalk and reduction of
fidelity for multiplexed readout of superconducting qubits. We suggest using
large detunings between the pump and signal frequencies to mitigate crosstalk.
Our work provides insights into the limitations of current Josephson traveling
wave parametric amplifiers and highlights the importance of performing further
research on these devices.Comment: 11 pages, 12 figure
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