Tunable broadband polarization retarders.

We theoretically propose a type of tunable polarization retarder, which is composed of sequences of half-wave and quarter-wave polarization retarders, allowing operation at broad spectral bandwidth. The constituent retarders are composed of stacked standard half-wave retarders and quarter-wave retarders rotated at designated angles relative to their fast polarization axes. The proposed composite retarder (CR) can be tuned to an arbitrary value of the retardance by varying the middle retarder alone while maintaining its broadband spectral bandwidth intact.

Defying Conventional Wisdom in Spectroscopy: Power Narrowing on IBM Quantum.

Power broadening-the broadening of the spectral line profile of a two-state quantum transition as the amplitude of the driving field increases-is a well-known and thoroughly examined phenomenon in spectroscopy. It typically occurs in continuous-wave driving when the intensity of the radiation field increases beyond the saturation intensity of the transition. In pulsed-field excitation, linear power broadening occurs for a pulse of rectangular temporal shape. Pulses with smooth shapes are known to exhibit much less power broadening, e.g., logarithmic for a Gaussian pulse shape. It has been predicted, but never experimentally verified, that pulse shapes which vanish in time as ∼|t|^{-λ} should exhibit the opposite effect-power narrowing-in which the postpulse transition line width decreases as the amplitude of the driving pulse increases. In this Letter, power narrowing is demonstrated for a class of powers-of-Lorentzian pulse shapes on the IBM Quantum processor ibmq_manila. Reduction of the line width by a factor of over 10 is observed when increasing the pulse area from π to 7π, in a complete reversal of the power broadening paradigm. Moreover, thorough study is conducted on the truncation of the pulse wings which introduces a (small) power-broadened term which prevents power narrowing from reaching extreme values. In the absence of other power broadening mechanisms, Lorentzian pulses truncated at sufficiently small values can achieve as narrow line profiles as desired.

Multipass quantum process tomography.

We introduce a method to enhance the precision and accuracy of Quantum Process Tomography (QPT) by mitigating the errors caused by state preparation and measurement (SPAM), readout and shot noise. Instead of performing QPT solely on a single gate, we propose performing QPT on a sequence of multiple applications of the same gate. The method involves the measurement of the Pauli transfer matrix (PTM) by standard QPT of the multipass process, and then deduce the single-process PTM by two alternative approaches: an iterative approach which in theory delivers the exact result for small errors, and a linearized approach based on solving the Sylvester equation. We examine the efficiency of these two approaches through simulations on IBM Quantum using IBMQ_QASM_SIMULATOR. Compared to the Randomized Benchmarking type of methods, the proposed method delivers the entire PTM rather than a single number (fidelity). Compared to standard QPT, our method delivers PTM with much higher accuracy and precision because it greatly reduces the SPAM, readout and shot noise errors. We use the proposed method to experimentally determine the PTM and the fidelity of the CNOT gate on the quantum processor IBMQ_MANILA (Falcon r5.11L).