14 research outputs found
SQUID magnetometry from nanometer to centimeter length scales
The development of Superconducting QUantum Interference Device (SQUID)-based magnetometer for two applications, in vivo prepolarized, ultra-low field MRI of humans and dispersive readout of SQUIDs for micro- and nano-scale magnetometery, are the focus of this thesis
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
Confining the state of light to a quantum manifold by engineered two-photon loss
Physical systems usually exhibit quantum behavior, such as superpositions and
entanglement, only when they are sufficiently decoupled from a lossy
environment. Paradoxically, a specially engineered interaction with the
environment can become a resource for the generation and protection of quantum
states. This notion can be generalized to the confinement of a system into a
manifold of quantum states, consisting of all coherent superpositions of
multiple stable steady states. We have experimentally confined the state of a
harmonic oscillator to the quantum manifold spanned by two coherent states of
opposite phases. In particular, we have observed a Schrodinger cat state
spontaneously squeeze out of vacuum, before decaying into a classical mixture.
This was accomplished by designing a superconducting microwave resonator whose
coupling to a cold bath is dominated by photon pair exchange. This experiment
opens new avenues in the fields of nonlinear quantum optics and quantum
information, where systems with multi-dimensional steady state manifolds can be
used as error corrected logical qubits
Non-Poissonian Quantum Jumps of a Fluxonium Qubit due to Quasiparticle Excitations
As the energy relaxation time of superconducting qubits steadily improves,
non-equilibrium quasiparticle excitations above the superconducting gap emerge
as an increasingly relevant limit for qubit coherence. We measure fluctuations
in the number of quasiparticle excitations by continuously monitoring the
spontaneous quantum jumps between the states of a fluxonium qubit, in
conditions where relaxation is dominated by quasiparticle loss. Resolution on
the scale of a single quasiparticle is obtained by performing quantum
non-demolition projective measurements within a time interval much shorter than
, using a quantum limited amplifier (Josephson Parametric Converter). The
quantum jumps statistics switches between the expected Poisson distribution and
a non-Poissonian one, indicating large relative fluctuations in the
quasiparticle population, on time scales varying from seconds to hours. This
dynamics can be modified controllably by injecting quasiparticles or by seeding
quasiparticle-trapping vortices by cooling down in magnetic field
Architectures for Multinode Superconducting Quantum Computers
Many proposals to scale quantum technology rely on modular or distributed
designs where individual quantum processors, called nodes, are linked together
to form one large multinode quantum computer (MNQC). One scalable method to
construct an MNQC is using superconducting quantum systems with optical
interconnects. However, a limiting factor of these machines will be internode
gates, which may be two to three orders of magnitude noisier and slower than
local operations. Surmounting the limitations of internode gates will require a
range of techniques, including improvements in entanglement generation, the use
of entanglement distillation, and optimized software and compilers, and it
remains unclear how improvements to these components interact to affect overall
system performance, what performance from each is required, or even how to
quantify the performance of each. In this paper, we employ a `co-design'
inspired approach to quantify overall MNQC performance in terms of hardware
models of internode links, entanglement distillation, and local architecture.
In the case of superconducting MNQCs with microwave-to-optical links, we
uncover a tradeoff between entanglement generation and distillation that
threatens to degrade performance. We show how to navigate this tradeoff, lay
out how compilers should optimize between local and internode gates, and
discuss when noisy quantum links have an advantage over purely classical links.
Using these results, we introduce a roadmap for the realization of early MNQCs
which illustrates potential improvements to the hardware and software of MNQCs
and outlines criteria for evaluating the landscape, from progress in
entanglement generation and quantum memory to dedicated algorithms such as
distributed quantum phase estimation. While we focus on superconducting devices
with optical interconnects, our approach is general across MNQC
implementations.Comment: 23 pages, white pape
Conductive shield for ultra-low-field magnetic resonance imaging: theory and measurements of eddy currents
Peer reviewe