Nicole Yunger Halpern

Superconducting antiqubits achieve optimal phase estimation via unitary inversion

  1. Xingrui Song,
  2. Surihan Sean Borjigin,
  3. Flavio Salvati,
  4. Yu-Xin Wang,
  5. Nicole Yunger Halpern,
  6. David R. M. Arvidsson-Shukur,
  7. and Kater Murch
A positron is equivalent to an electron traveling backward through time. Casting transmon superconducting qubits as akin to electrons, we simulate a positron with a transmon subject
to particular resonant and off-resonant drives. We call positron-like transmons „antiqubits.“ An antiqubit’s effective gyromagnetic ratio equals the negative of a qubit’s. This fact enables us to time-invert a unitary implemented on a transmon by its environment. We apply this platform-specific unitary inversion, with qubit–antiqubit entanglement, to achieve a quantum advantage in phase estimation: consider measuring the strength of a field that points in an unknown direction. An entangled qubit–antiqubit sensor offers the greatest possible sensitivity (amount of Fisher information), per qubit, per application of the field. We prove this result theoretically and observe it experimentally. This work shows how antimatter, whether real or simulated, can enable platform-specific unitary inversion and benefit quantum information processing.
arxiv pdf

Weak Measurement of Superconducting Qubit Reconciles Incompatible Operators

  1. Jonathan T. Monroe,
  2. Nicole Yunger Halpern,
  3. Taeho Lee,
  4. and Kater W. Murch
Traditional uncertainty relations dictate a minimal amount of noise in incompatible projective quantum measurements. However, not all measurements are projective. Weak measurements
are minimally invasive methods for obtaining partial state information without projection. Recently, weak measurements were shown to obey an uncertainty relation cast in terms of entropies. We experimentally test this entropic uncertainty relation with strong and weak measurements of a superconducting transmon qubit. A weak measurement, we find, can reconcile two strong measurements‘ incompatibility, via backaction on the state. Mathematically, a weak value—a preselected and postselected expectation value—lowers the uncertainty bound. Hence we provide experimental support for the physical interpretation of the weak value as a determinant of a weak measurement’s ability to reconcile incompatible operations.
arxiv pdf