15 research outputs found
Anyonic interference and braiding phase in a Mach-Zehnder Interferometer
The fractional quantum Hall states have long been predicted to be a testing
ground of fractional (anyonic) exchange statistics. These topological states
harbor quasiparticles with fractional charges of both abelian and non-abelian
characters. The quasiparticles' charge is commonly determined by shot noise
measurements (1, 2), and states' statistics can be revealed by appropriately
interfering the quasiparticles. While the multipath Fabry-Perot electronic
interferometer (FPI) is easier to fabricate, it is often plagued by Coulomb
interactions (3), its area breathes with the magnetic field (4), and its bulk's
charges tend to fluctuate (5). Recent FPI experiments employing adequate
screening allowed an observation of Aharonov-Bohm (AB) interference at bulk
filling =1/3 (6). In the current work, we chose to employ an
interaction-free, two-path, Mach-Zehnder interferometer (MZI), tuned to bulk
filling =2/5. Interfering the outer =1/3 mode (with the inner
=1/15 mode screening out the bulk), we observed a 'dressed AB'
periodicity, with a combined 'bare AB' flux periodicity of three flux-quanta
(3) and the 'braiding phase' 2/3. This unique interference
resulted with an AB periodicity of a single flux-quantum. Moreover, the
visibility of the interference, , deviated markedly from that of the
electronic one , agreeing with the theoretically expected
visibility, . With the two non-equivalent
drains of the MZI, the fractional visibility peaked away from the ubiquitous
transmission-half of the MZI. We provide simple theoretical arguments that
support our results. The MZI proves to be a powerful tool that can be used to
probe further the statistics of more complex anyonic quasiparticles
Comparing and combining measurement-based and driven-dissipative entanglement stabilization
We demonstrate and contrast two approaches to the stabilization of qubit
entanglement by feedback. Our demonstration is built on a feedback platform
consisting of two superconducting qubits coupled to a cavity which are measured
by a nearly-quantum-limited measurement chain and controlled by high-speed
classical logic circuits. This platform is used to stabilize entanglement by
two nominally distinct schemes: a "passive" reservoir engineering method and an
"active" correction based on conditional parity measurements. In view of the
instrumental roles that these two feedback paradigms play in quantum
error-correction and quantum control, we directly compare them on the same
experimental setup. Further, we show that a second layer of feedback can be
added to each of these schemes, which heralds the presence of a high-fidelity
entangled state in realtime. This "nested" feedback brings about a marked
entanglement fidelity improvement without sacrificing success probability.Comment: 40 pages, 12 figure
Demonstrating Quantum Error Correction that Extends the Lifetime of Quantum Information
The remarkable discovery of Quantum Error Correction (QEC), which can
overcome the errors experienced by a bit of quantum information (qubit), was a
critical advance that gives hope for eventually realizing practical quantum
computers. In principle, a system that implements QEC can actually pass a
"break-even" point and preserve quantum information for longer than the
lifetime of its constituent parts. Reaching the break-even point, however, has
thus far remained an outstanding and challenging goal. Several previous works
have demonstrated elements of QEC in NMR, ions, nitrogen vacancy (NV) centers,
photons, and superconducting transmons. However, these works primarily
illustrate the signatures or scaling properties of QEC codes rather than test
the capacity of the system to extend the lifetime of quantum information over
time. Here we demonstrate a QEC system that reaches the break-even point by
suppressing the natural errors due to energy loss for a qubit logically encoded
in superpositions of coherent states, or cat states of a superconducting
resonator. Moreover, the experiment implements a full QEC protocol by using
real-time feedback to encode, monitor naturally occurring errors, decode, and
correct. As measured by full process tomography, the enhanced lifetime of the
encoded information is 320 microseconds without any post-selection. This is 20
times greater than that of the system's transmon, over twice as long as an
uncorrected logical encoding, and 10% longer than the highest quality element
of the system (the resonator's 0, 1 Fock states). Our results illustrate the
power of novel, hardware efficient qubit encodings over traditional QEC
schemes. Furthermore, they advance the field of experimental error correction
from confirming the basic concepts to exploring the metrics that drive system
performance and the challenges in implementing a fault-tolerant system
Quantum-classical processing and benchmarking at the pulse-level
Towards the practical use of quantum computers in the NISQ era, as well as
the realization of fault-tolerant quantum computers that utilize quantum error
correction codes, pressing needs have emerged for the control hardware and
software platforms. In particular, a clear demand has arisen for platforms that
allow classical processing to be integrated with quantum processing. While
recent works discuss the requirements for such quantum-classical processing
integration that is formulated at the gate-level, pulse-level discussions are
lacking and are critically important. Moreover, defining concrete performance
benchmarks for the control system at the pulse-level is key to the necessary
quantum-classical integration. In this work, we categorize the requirements for
quantum-classical processing at the pulse-level, demonstrate these requirements
with a variety of use cases, including recently published works, and propose
well-defined performance benchmarks for quantum control systems. We utilize a
comprehensive pulse-level language that allows embedding universal classical
processing in the quantum program and hence allows for a general formulation of
benchmarks. We expect the metrics defined in this work to form a solid basis to
continue to push the boundaries of quantum computing via control systems,
bridging the gap between low-level and application-level implementations with
relevant metrics.Comment: 22 page
Quantum bits with Josephson junctions
Already in the first edition of this book (Barone and Paterno, "Fundamentals
and Physics and Applications of the Josephson Effect", Wiley 1982), a great
number of interesting and important applications for Josephson junctions were
discussed. In the decades that have passed since then, several new applications
have emerged. This chapter treats one such new class of applications: quantum
optics and quantum information processing (QIP) based on superconducting
circuits with Josephson junctions. In this chapter, we aim to explain the
basics of superconducting quantum circuits with Josephson junctions and
demonstrate how these systems open up new prospects, both for QIP and for the
study of quantum optics and atomic physics.Comment: 30 pages, 10 figures. Book chapter for a new edition of Barone and
Paterno's "Fundamentals and Physics and Applications of the Josephson
Effect". Final versio
The topological in-equivalence of Hall bar and Corbino geometries in coordinate space: Screening theory and direct transport experiments
Here we discuss the effect of topology on the quantum Hall effect taking into account the direct Coulomb interactions, considering two distinct geometries, namely the Hall bar and the Corbino disc. The consequences of interactions are underestimated in the standard approaches to explain the quantized Hall effect. However, the local distributions of the electron number density, the electrochemical potential, and current distributions depend on electron–electron interactions. Accounting for the direct Coulomb interaction and realistic boundary conditions results in local variations of compressibility—namely metal-like compressible and (topological) insulator-like incompressible regions. Within the framework of the screening theory, we show in the coordinate space that for both geometries, the bulk is compressible within most of the magnetic field interval corresponding to a quantized Hall plateau. The non-incompressible bulk throughout the plateau directly contrasts the standard explanation of the quantized Hall effect but is confirmed by our transport experiments. Our experimental results with two inner contacts within the Hall bar imply that the QHE plateaus scattering free transport along the sample edges even if the bulk of the sample is clearly in a compressible state. The scattering free transport is thereby supported by incompressible stripes. Our results confirm that the often promoted analogy in coordinate space between the quantized Hall effect and topological insulators is invalid throughout the entire plateau. We conclude that the equivalence of Hall and Corbino geometries is questionable. In addition, family relations of quantized Hall effect and topological insulators are doubtful. Finally, we propose experiments which will enable us to distinguish the topological properties of the two geometries in the coordinate space