L. DiCarlo

Deterministic entanglement of superconducting qubits by parity measurement and feedback

  1. D. Ristè,
  2. M. Dukalski,
  3. C. A. Watson,
  4. G. de Lange,
  5. M. J. Tiggelman,
  6. Ya. M. Blanter,
  7. K. W. Lehnert,
  8. R. N. Schouten,
  9. and L. DiCarlo
The stochastic evolution of quantum systems during measurement is arguably the most enigmatic feature of quantum mechanics. Measuring a quantum system typically steers it towards a
classical state, destroying any initial quantum superposition and any entanglement with other quantum systems. Remarkably, the measurement of a shared property between non-interacting quantum systems can generate entanglement starting from an uncorrelated state. Of special interest in quantum computing is the parity measurement, which projects a register of quantum bits (qubits) to a state with an even or odd total number of excitations. Crucially, a parity meter must discern the two parities with high fidelity while preserving coherence between same-parity states. Despite numerous proposals for atomic, semiconducting, and superconducting qubits, realizing a parity meter creating entanglement for both even and odd measurement results has remained an outstanding challenge. We realize a time-resolved, continuous parity measurement of two superconducting qubits using the cavity in a 3D circuit quantum electrodynamics (cQED) architecture and phase-sensitive parametric amplification. Using postselection, we produce entanglement by parity measurement reaching 77% concurrence. Incorporating the parity meter in a feedback-control loop, we transform the entanglement generation from probabilistic to fully deterministic, achieving 66% fidelity to a target Bell state on demand. These realizations of a parity meter and a feedback-enabled deterministic measurement protocol provide key ingredients for active quantum error correction in the solid state.
arxiv pdf

Partial-measurement back-action and non-classical weak values in a superconducting circuit

  1. J. P. Groen,
  2. D. Ristè,
  3. L. Tornberg,
  4. J. Cramer,
  5. P. C. de Groot,
  6. T. Picot,
  7. G. Johansson,
  8. and L. DiCarlo
We realize indirect partial measurement of a transmon qubit in circuit quantum electrodynamics by interaction with an ancilla qubit and projective ancilla measurement with a dedicated
readout resonator. Accurate control of the interaction and ancilla measurement basis allows tailoring the measurement strength and operator. The tradeoff between measurement strength and qubit back-action is characterized through the distortion of a qubit Rabi oscillation imposed by ancilla measurement in different bases. Combining partial and projective qubit measurements, we provide the solid-state demonstration of the correspondence between a non-classical weak value and the violation of a Leggett-Garg inequality.
arxiv pdf

Feedback control of a solid-state qubit using high-fidelity projective measurement

  1. D. Ristè,
  2. C. C. Bultink,
  3. K. W. Lehnert,
  4. and L. DiCarlo
We demonstrate feedback control of a superconducting transmon qubit using discrete, projective measurement and conditional coherent driving. Feedback realizes a fast and deterministic
qubit reset to a target state with 2.4% error averaged over input superposition states, and cooling of the transmon from 16% spurious excitation to 3%. This closed-loop qubit control is necessary for measurement-based protocols such as quantum error correction and teleportation.
arxiv pdf

Initialization by measurement of a two-qubit superconducting circuit

  1. D. Ristè,
  2. J. G. van Leeuwen,
  3. H.-S. Ku,
  4. K. W. Lehnert,
  5. and L. DiCarlo
We demonstrate initialization by joint measurement of two transmon qubits in 3D circuit quantum electrodynamics. Homodyne detection of cavity transmission is enhanced by Josephson parametric
amplification to discriminate the two-qubit ground state from single-qubit excitations non-destructively and with 98.1% fidelity. Measurement and postselection of a steady-state mixture with 4.7% residual excitation per qubit achieve 98.8% fidelity to the ground state, thus outperforming passive initialization.
arxiv pdf