Christian Lang

Low-Latency Digital Signal Processing for Feedback and Feedforward in Quantum Computing and Communication

  1. Yves Salathé,
  2. Philipp Kurpiers,
  3. Thomas Karg,
  4. Christian Lang,
  5. Christian Kraglund Andersen,
  6. Abdulkadir Akin,
  7. Christopher Eichler,
  8. and Andreas Wallraff
Quantum computing architectures rely on classical electronics for control and readout. Employing classical electronics in a feedback loop with the quantum system allows to stabilize
states, correct errors and to realize specific feedforward-based quantum computing and communication schemes such as deterministic quantum teleportation. These feedback and feedforward operations are required to be fast compared to the coherence time of the quantum system to minimize the probability of errors. We present a field programmable gate array (FPGA) based digital signal processing system capable of real-time quadrature demodulation, determination of the qubit state and generation of state-dependent feedback trigger signals. The feedback trigger is generated with a latency of 110ns with respect to the timing of the analog input signal. We characterize the performance of the system for an active qubit initialization protocol based on dispersive readout of a superconducting qubit and discuss potential applications in feedback and feedforward algorithms.
arxiv pdf

Demonstrating W-type Entanglement of Dicke-States in Resonant Cavity Quantum Electrodynamics

  1. Jonas A. Mlynek,
  2. Abdufarrukh A. Abdumalikov Jr,
  3. Johannes M. Fink,
  4. Lars Steffen,
  5. Matthias Baur,
  6. Christian Lang,
  7. Arjan F. van Loo,
  8. and Andreas Wallraff
Nonlinearity and entanglement are two important properties by which physical systems can be identified as non-classical. We study the dynamics of the resonant interaction of up to N=3
two-level systems and a single mode of the electromagnetic field sharing a single excitation dynamically. We observe coherent vacuum Rabi oscillations and their nonlinear speed up by tracking the populations of all qubits and the resonator in time. We use quantum state tomography to show explicitly that the dynamics generates maximally entangled states of the W class in a time limited only by the collective interaction rate. We use an entanglement witness and the threetangle to characterize the state whose fidelity F=78% is limited in our experiments by crosstalk arising during the simultaneous qubit manipulations which is absent in a sequential approach with F=91%.
arxiv pdf