75 research outputs found
Majorana fermions in superconducting nanowires without spin-orbit coupling
We show that confined Majorana fermions can exist in nanowires with proximity
induced s-wave superconducting pairing if the direction of an external magnetic
field rotates along the wire. The system is equivalent to nanowires with
Rashba-type spin-orbit coupling, with strength proportional to the derivative
of the field angle. For realistic parameters, we demonstrate that a set of
permanent magnets can bring a nearby nanowire into the topologically
non-trivial phase with localized Majorana modes at its ends. Without the
requirement of spin-orbit coupling this opens up for a new route for
demonstration and design of Majorana fermion systems.Comment: Fig. 2 correcte
Entangling transmons with low-frequency protected superconducting qubits
Novel qubits with intrinsic noise protection constitute a promising route for
improving the coherence of quantum information in superconducting circuits.
However, many protected superconducting qubits exhibit relatively low
transition frequencies, which could make their integration with conventional
transmon circuits challenging. In this work, we propose and study a scheme for
entangling a tunable transmon with a Cooper-pair parity-protected qubit, a
paradigmatic example of a low-frequency protected qubit that stores quantum
information in opposite Cooper-pair parity states on a superconducting island.
By tuning the external flux on the transmon, we show that non-computational
states can mediate a two-qubit entangling gate that preserves the Cooper-pair
parity independent of the detailed pulse sequence. Interestingly, the
entangling gate bears similarities to a controlled-phase gate in conventional
transmon devices. Hence, our results suggest that standard high-precision gate
calibration protocols could be repurposed for operating hybrid qubit devices
Scheme for parity-controlled multi-qubit gates with superconducting qubits
Multi-qubit parity measurements are at the core of many quantum error
correction schemes. Extracting multi-qubit parity information typically
involves using a sequence of multiple two-qubit gates. In this paper, we
propose a superconducting circuit device with native support for multi-qubit
parity-controlled gates (PCG). These are gates that perform rotations on a
parity ancilla based on the multi-qubit parity operator of adjacent qubits, and
can be directly used to perform multi-qubit parity measurements. The circuit
consists of a set of concatenated Josephson ring modulators and effectively
realizes a set of transmon-like qubits with strong longitudinal
nearest-neighbor couplings. PCGs are implemented by applying microwave drives
to the parity ancilla at specific frequencies. We investigate the scheme's
performance with numerical simulation using realistic parameter choices and
decoherence rates, and find that the device can perform four-qubit PCGs in 30
ns with process fidelity surpassing 99%. Furthermore, we study the effects of
parameter disorder and spurious coupling between next-nearest neighboring
qubits. Our results indicate that this approach to realizing PCGs constitute an
interesting candidate for near-term quantum error correction experiments.Comment: Some units contained typos and have been fixe
Zero-Energy Modes from Coalescing Andreev States in a Two-Dimensional Semiconductor-Superconductor Hybrid Platform
We investigate zero-bias conductance peaks that arise from coalescing subgap
Andreev states, consistent with emerging Majorana zero modes, in hybrid
semiconductor-superconductor wires defined in a two-dimensional InAs/Al
heterostructure using top-down lithography and gating. The measurements
indicate a hard superconducting gap, ballistic tunneling contact, and in-plane
critical fields up to ~T. Top-down lithography allows complex geometries,
branched structures, and straightforward scaling to multicomponent devices
compared to structures made from assembled nanowires.Comment: Includes Supplementary Materia
A tunable coupling scheme for implementing high-fidelity two-qubit gates
The prospect of computational hardware with quantum advantage relies
critically on the quality of quantum gate operations. Imperfect two-qubit gates
is a major bottleneck for achieving scalable quantum information processors.
Here, we propose a generalizable and extensible scheme for a two-qubit coupler
switch that controls the qubit-qubit coupling by modulating the coupler
frequency. Two-qubit gate operations can be implemented by operating the
coupler in the dispersive regime, which is non-invasive to the qubit states. We
investigate the performance of the scheme by simulating a universal two-qubit
gate on a superconducting quantum circuit, and find that errors from known
parasitic effects are strongly suppressed. The scheme is compatible with
existing high-coherence hardware, thereby promising a higher gate fidelity with
current technologies
Fast universal control of a flux qubit via exponentially tunable wave-function overlap
Fast, high fidelity control and readout of protected superconducting qubits
are fundamentally challenging due to their inherent insensitivity. We propose a
flux qubit variation which enjoys a tunable level of protection against
relaxation to resolve this outstanding issue. Our qubit design, the
double-shunted flux qubit (DSFQ), realizes a generic double-well potential
through its three junction ring geometry. One of the junctions is tunable,
making it possible to control the barrier height and thus the level of
protection. We analyze single- and two-qubit gate operations that rely on
lowering the barrier. We show that this is a viable method that results in high
fidelity gates as the non-computational states are not occupied during
operations. Further, we show how the effective coupling to a readout resonator
can be controlled by adjusting the externally applied flux while the DSFQ is
protected from decaying into the readout resonator. Finally, we also study a
double-loop gradiometric version of the DSFQ which is exponentially insensitive
to variations in the global magnetic field, even when the loop areas are
non-identical.Comment: 13 pages, 7 figure
Patient safety culture improvements depend on basic healthcare education:a longitudinal simulation-based intervention study at two Danish hospitals
BACKGROUND: A growing body of evidence supports the existence of an association between patient safety culture (PSC) and patient outcomes. PSC refers to shared perceptions and attitudes towards norms, policies and procedures related to patient safety. Existing literature shows that PSC varies among health professionals depending on their specific profession and specialty. However, these studies did not investigate whether PSC can be improved. This study investigates whether length of education is associated with improvements in PCS following a simulation intervention. METHODS: From April 2017 to November 2018, a cross-sectional intervention study was conducted at two regional hospitals in Denmark. Two groups with altogether 1230 health professionals were invited to participate. One group included nurses, midwives and radiographers; the other group included doctors. A train-the-trainer intervention approach was applied consisting of a 4-day simulation instructor course that emphasised team training, communication and leadership. Fifty-three healthcare professionals were trained as instructors. After the course, instructors performed in situ simulation in their own hospital environment. OUTCOMES: The Safety Attitude Questionnaire (SAQ), which has 6 dimensions and 32 items, was used to collect main outcome variables. All employees from both groups were surveyed before the intervention and again four and nine months after the intervention. RESULTS: Mean baseline scores were higher among doctors than among nurses, midwives and radiographers for all SAQ dimensions. At the second follow-up, four of six dimensions improved significantly (p ≤ 0.05) among nurses, midwives and radiographers, whereas no dimensions improved significantly among doctors. CONCLUSION: Over time, nurses, midwives and radiographers improved more in PSC attitudes than doctors did
Distinguishing coherent and thermal photon noise in a circuit QED system
In the cavity-QED architecture, photon number fluctuations from residual
cavity photons cause qubit dephasing due to the AC Stark effect. These unwanted
photons originate from a variety of sources, such as thermal radiation,
leftover measurement photons, and crosstalk. Using a capacitively-shunted flux
qubit coupled to a transmission line cavity, we demonstrate a method that
identifies and distinguishes coherent and thermal photons based on
noise-spectral reconstruction from time-domain spin-locking relaxometry. Using
these measurements, we attribute the limiting dephasing source in our system to
thermal photons, rather than coherent photons. By improving the cryogenic
attenuation on lines leading to the cavity, we successfully suppress residual
thermal photons and achieve -limited spin-echo decay time. The
spin-locking noise spectroscopy technique can readily be applied to other qubit
modalities for identifying general asymmetric non-classical noise spectra
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