Christopher Eichler

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

Correlations and entanglement of microwave photons emitted in a cascade decay

  1. Simone Gasparinetti,
  2. Marek Pechal,
  3. Jean-Claude Besse,
  4. Mintu Mondal,
  5. Christopher Eichler,
  6. and Andreas Wallraff
An excited emitter decays by radiating a photon into a quantized mode of the electromagnetic field, a process known as spontaneous emission. If the emitter is driven to a higher excited
state, it radiates multiple photons in a cascade decay. Atomic and biexciton cascades have been exploited as sources of polarization-entangled photon pairs. Because the photons are emitted sequentially, their intensities are strongly correlated in time, as measured in a double-beam coincidence experiment. Perhaps less intuitively, their phases can also be correlated, provided a single emitter is deterministically prepared into a superposition state, and the emitted radiation is detected in a phase-sensitive manner and with high efficiency. Here we have met these requirements by using a superconducting artificial atom, coherently driven to its second-excited state and decaying into a well-defined microwave mode. Our results highlight the coherent nature of cascade decay and demonstrate a novel protocol to generate entanglement between itinerant field modes.
arxiv pdf