101 research outputs found
Measurement of a Vacuum-Induced Geometric Phase
Berry's geometric phase naturally appears when a quantum system is driven by
an external field whose parameters are slowly and cyclically changed. A
variation in the coupling between the system and the external field can also
give rise to a geometric phase, even when the field is in the vacuum state or
any other Fock state. Here we demonstrate the appearance of a vacuum-induced
Berry phase in an artificial atom, a superconducting transmon, interacting with
a single mode of a microwave cavity. As we vary the phase of the interaction,
the artificial atom acquires a geometric phase determined by the path traced
out in the combined Hilbert space of the atom and the quantum field. Our
ability to control this phase opens new possibilities for the geometric
manipulation of atom-cavity systems also in the context of quantum information
processing.Comment: 5 + 6 page
Microwave photon-mediated interactions between semiconductor qubits
The realization of a coherent interface between distant charge or spin qubits
in semiconductor quantum dots is an open challenge for quantum information
processing. Here we demonstrate both resonant and non-resonant photon-mediated
coherent interactions between double quantum dot charge qubits separated by
several tens of micrometers. We present clear spectroscopic evidence of the
collective enhancement of the resonant coupling of two qubits. With both qubits
detuned from the resonator we observe exchange coupling between the qubits
mediated by virtual photons. In both instances pronounced bright and dark
states governed by the symmetry of the qubit-field interaction are found. Our
observations are in excellent quantitative agreement with master-equation
simulations. The extracted two-qubit coupling strengths significantly exceed
the linewidths of the combined resonator-qubit system. This indicates that this
approach is viable for creating photon-mediated two-qubit gates in quantum dot
based systems.Comment: 14 pages, 10 figures and 6 table
Superconducting quantum simulator for topological order and the toric code
Topological order is now being established as a central criterion for
characterizing and classifying ground states of condensed matter systems and
complements categorizations based on symmetries. Fractional quantum Hall
systems and quantum spin liquids are receiving substantial interest because of
their intriguing quantum correlations, their exotic excitations and prospects
for protecting stored quantum information against errors. Here we show that the
Hamiltonian of the central model of this class of systems, the Toric Code, can
be directly implemented as an analog quantum simulator in lattices of
superconducting circuits. The four-body interactions, which lie at its heart,
are in our concept realized via Superconducting Quantum Interference Devices
(SQUIDs) that are driven by a suitably oscillating flux bias. All physical
qubits and coupling SQUIDs can be individually controlled with high precision.
Topologically ordered states can be prepared via an adiabatic ramp of the
stabilizer interactions. Strings of qubit operators, including the stabilizers
and correlations along non-contractible loops, can be read out via a capacitive
coupling to read-out resonators. Moreover, the available single qubit
operations allow to create and propagate elementary excitations of the Toric
Code and to verify their fractional statistics. The architecture we propose
allows to implement a large variety of many-body interactions and thus provides
a versatile analog quantum simulator for topological order and lattice gauge
theories
Improved Parameter Targeting in 3D-Integrated Superconducting Circuits through a Polymer Spacer Process
Three-dimensional device integration facilitates the construction of
superconducting quantum information processors with more than several tens of
qubits by distributing elements such as control wires, qubits, and resonators
between multiple layers. The frequencies of resonators and qubits in
flip-chip-bonded multi-chip modules depend on the details of their
electromagnetic environment defined by the conductors and dielectrics in their
vicinity. Accurate frequency targeting therefore requires precise control of
the separation between chips and minimization of their relative tilt. Here, we
describe a method to control the inter-chip separation by using polymer
spacers. Compared to an identical process without spacers, we reduce the
measured planarity error by a factor of 3.5, to a mean tilt of 76(35) rad,
and the deviation from the target inter-chip separation by a factor of ten, to
a mean of 0.4(8) m. We apply this process to coplanar waveguide resonator
samples and observe chip-to-chip resonator frequency variations below 50 MHz
( 1 %). We measure internal quality factors of at the
single-photon level, suggesting that the added spacers are compatible with
low-loss device fabrication.Comment: 7 pages + 7 pages appendice
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
Observation of the Crossover from Photon Ordering to Delocalization in Tunably Coupled Resonators
Networks of nonlinear resonators offer intriguing perspectives as quantum
simulators for non-equilibrium many-body phases of driven-dissipative systems.
Here, we employ photon correlation measurements to study the radiation fields
emitted from a system of two superconducting resonators, coupled nonlinearly by
a superconducting quantum interference device (SQUID). We apply a
parametrically modulated magnetic flux to control the linear photon hopping
rate between the two resonators and its ratio with the cross-Kerr rate. When
increasing the hopping rate, we observe a crossover from an ordered to a
delocalized state of photons. The presented coupling scheme is intrinsically
robust to frequency disorder and may therefore prove useful for realizing
larger-scale resonator arrays
Calibration of Drive Non-Linearity for Arbitrary-Angle Single-Qubit Gates Using Error Amplification
The ability to execute high-fidelity operations is crucial to scaling up
quantum devices to large numbers of qubits. However, signal distortions
originating from non-linear components in the control lines can limit the
performance of single-qubit gates. In this work, we use a measurement based on
error amplification to characterize and correct the small single-qubit rotation
errors originating from the non-linear scaling of the qubit drive rate with the
amplitude of the programmed pulse. With our hardware, and for a 15-ns pulse,
the rotation angles deviate by up to several degrees from a linear model. Using
purity benchmarking, we find that control errors reach , which
accounts for half of the total gate error. Using cross-entropy benchmarking, we
demonstrate arbitrary-angle single-qubit gates with coherence-limited errors of
and leakage below . While the exact
magnitude of these errors is specific to our setup, the presented method is
applicable to any source of non-linearity. Our work shows that the
non-linearity of qubit drive line components imposes a limit on the fidelity of
single-qubit gates, independent of improvements in coherence times, circuit
design, or leakage mitigation when not corrected for
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