76 research outputs found
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
Primary thermometry of propagating microwaves in the quantum regime
The ability to control and measure the temperature of propagating microwave
modes down to very low temperatures is indispensable for quantum information
processing, and may open opportunities for studies of heat transport at the
nanoscale, also in the quantum regime. Here we propose and experimentally
demonstrate primary thermometry of propagating microwaves using a transmon-type
superconducting circuit. Our device operates continuously, with a sensitivity
down to photons/\sqrt{\mbox{Hz}} and a bandwidth of 40 MHz.
We measure the thermal occupation of the modes of a highly attenuated coaxial
cable in a range of 0.001 to 0.4 thermal photons, corresponding to a
temperature range from 35 mK to 210 mK at a frequency around 5 GHz. To increase
the radiation temperature in a controlled fashion, we either inject calibrated,
wideband digital noise, or heat the device and its environment. This
thermometry scheme can find applications in benchmarking and characterization
of cryogenic microwave setups, temperature measurements in hybrid quantum
systems, and quantum thermodynamics
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
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