### Browsing by Author "Dogra, Shruti"

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Item Coherent interaction-free detection of microwave pulses with a superconducting circuit(Nature Publishing Group, 2022-12) Dogra, Shruti; McCord, John J.; Paraoanu, Gheorghe Sorin; Department of Applied Physics; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTFThe interaction-free measurement is a fundamental quantum effect whereby the presence of a photosensitive object is determined without irreversible photon absorption. Here we propose the concept of coherent interaction-free detection and demonstrate it experimentally using a three-level superconducting transmon circuit. In contrast to standard interaction-free measurement setups, where the dynamics involves a series of projection operations, our protocol employs a fully coherent evolution that results, surprisingly, in a higher probability of success. We show that it is possible to ascertain the presence of a microwave pulse resonant with the second transition of the transmon, while at the same time avoid exciting the device onto the third level. Experimentally, this is done by using a series of Ramsey microwave pulses coupled into the first transition and monitoring the ground-state population.Item Coherent interaction-free detection of noise(American Physical Society, 2024-09) McCord, John J.; Dogra, Shruti; Paraoanu, Gheorghe Sorin; Department of Applied Physics; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTFThe measurement and characterization of noise is a flourishing area of research in mesoscopic physics. In this work, we propose interaction-free measurements as a noise-detection technique, exploring two conceptually different schemes: the coherent and the projective realizations. These detectors consist of a qutrit whose second transition is resonantly coupled to an oscillatory field that may have noise in amplitude or phase. For comparison, we consider a more standard detector previously discussed in this context: a qubit coupled in a similar way to the noise source. We find that the qutrit scheme offers clear advantages, allowing precise detection and characterization of the noise, while the qubit does not. Finally, we study the signature of noise correlations in the detector's signal.Item Experimental demonstration of robustness under scaling errors for superadiabatic population transfer in a superconducting circuit(ROYAL SOC, 2022-12-26) Dogra, Shruti; Vepsäläinen, Antti; Paraoanu, Gheorghe Sorin; Department of Applied Physics; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTFWe study experimentally and theoretically the transfer of population between the ground state and the second excited state in a transmon circuit by the use of superadiabatic stimulated Raman adiabatic passage (saSTIRAP). We show that the transfer is remarkably resilient against variations in the amplitudes of the pulses (scaling errors), thus demostrating that the superadiabatic process inherits certain robustness features from the adiabatic one. In particular, we provide new evidence of a plateau that appears at high values of the counterdiabatic pulse strength, which goes beyond the usual framework of saSTIRAP. This article is part of the theme issue 'Shortcuts to adiabaticity: theoretical, experimental and interdisciplinary perspectives'.Item High-fidelity robust qubit control by phase-modulated pulses(American Physical Society, 2024-01) Kuzmanović, Marko; Björkman, Isak; McCord, John J.; Dogra, Shruti; Paraoanu, Gheorghe Sorin; Department of Applied Physics; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsWe present a set of robust and high-fidelity pulses that realize paradigmatic operations such as the transfer of the ground state population into the excited state and arbitrary X/Y rotations on the Bloch sphere. These pulses are based on the phase modulation of the control field. We provide an experimental proof-of-concept of these operations by using a transmon qubit, demonstrating resilience against deviations in the drive amplitude of more than ≈20%, and/or detuning from the qubit transition frequency in the order of 10MHz. This modulation scheme is straightforward to implement in practice and can be deployed to any other qubit-based experimental platform.Item Majorana representation of adiabatic and superadiabatic processes in three-level systems(American Physical Society, 2020-10-15) Dogra, Shruti; Vepsalainen, Antti; Paraoanu, G. S.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QEDWe show that stimulated Raman adiabatic passage (STIRAP) and its superadiabatic version (saSTIRAP) have a natural geometric two-star representation on the Majorana sphere. In the case of STIRAP, we find that the evolution is confined to a vertical plane. A faster evolution can be achieved in the saSTIRAP protocol, which employs a counterdiabatic Hamiltonian to nullify the nonadiabatic excitations. We derive this Hamiltonian in the Majorana picture, and we observe how, under realistic experimental parameters, the counterdiabatic term corrects the trajectory of the Majorana stars toward the dark state. We also introduce a spin-1 average vector and present its evolution during the two processes, demonstrating that it provides a measure of nonadiabaticity. We show that the Majorana representation can be used as a sensitive tool for the detection of process errors due to ac Stark shifts and nonadiabatic transitions. Finally, we provide an extension of these results to mixed states and processes with decoherence.Item Perfect stimulated Raman adiabatic passage with imperfect finite-time pulses(Institute of Physics Publishing, 2022-09-07) Dogra, Shruti; Paraoanu, Gheorghe Sorin; Department of Applied Physics; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTFWe present a well-tailored sequence of two Gaussian-pulsed drives that achieves perfect population transfer in stimulated Raman adiabatic passage. We give a theoretical analysis of the optimal truncation and relative placement of the Stokes and pump pulses. Further, we obtain the power and the duration of the protocol for a given pulse width. Importantly, the duration of the protocol required to attain a desired value of fidelity depends only logarithmically on the infidelity. Subject to optimal truncation of the drives and with reference to the point of fastest transfer, we obtain a new adiabaticity criteria, which is remarkably simple and effective.Item Quantum process tomography of adiabatic and superadiabatic Raman passage(American Institute of Physics, 2021-06-16) Dogra, Shruti; Paraoanu, Gheorghe Sorin; Department of Applied Physics; Lesovik, Gordey; Vinokur, Valeril; Perelshtein, Mikhail; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QEDQuantum control methods for three-level systems have become recently an important direction of research in quantum information science and technology. Here we present numerical simulations using realistic experimental parameters for quantum process tomography in STIRAP (stimulated Raman adiabatic passage) and saSTIRAP (superadiabatic STIRAP). Specifically, we identify a suitable basis in the operator space as the identity operator together with the 8 Gell-Mann operators, and we calculate the corresponding process matrices, which have 9 × 9=81 elements. We discuss these results for the ideal decoherence-free case, as well as for the experimentally-relevant case with decoherence included.Item Quantum simulation of parity-time symmetry breaking with a superconducting quantum processor(Nature Publishing Group, 2021-12) Dogra, Shruti; Melnikov, Artem A.; Paraoanu, Gheorghe Sorin; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QED; Aalto UniversityThe observation of genuine quantum effects in systems governed by non-Hermitian Hamiltonians has been an outstanding challenge in the field. Here we simulate the evolution under such Hamiltonians in the quantum regime on a superconducting quantum processor by using a dilation procedure involving an ancillary qubit. We observe the parity-time (PT)-symmetry breaking phase transition at the exceptional points, obtain the critical exponent, and show that this transition is associated with a loss of state distinguishability. In a two-qubit setting, we show that the entanglement can be modified by local operations.Item Quantum simulation of the pseudo-Hermitian Landau-Zener-Stückelberg-Majorana effect(American Physical Society, 2024-04) Kivelä, Feliks; Dogra, Shruti; Paraoanu, Gheorghe Sorin; Department of Applied Physics; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTFWhile the Hamiltonians used in standard quantum mechanics are Hermitian, it is also possible to extend the theory to non-Hermitian Hamiltonians. Particularly interesting are non-Hermitian Hamiltonians satisfying parity-time (PT) symmetry, or more generally pseudo-Hermiticity, since such non-Hermitian Hamiltonians can still exhibit real eigenvalues. In this article, we present a quantum simulation of the time-dependent non-Hermitian non-PT-symmetric Hamiltonian used in a pseudo-Hermitian extension of the Landau-Zener-Stückelberg-Majorana (LZSM) model. The simulation is implemented on a superconducting processor by using Naimark dilation to transform a non-Hermitian Hamiltonian for one qubit into a Hermitian Hamiltonian for a qubit and an ancilla; postselection on the ancilla state ensures that the qubit undergoes nonunitary time evolution corresponding to the original non-Hermitian Hamiltonian. We observe properties such as the dependence of transition probabilities on time and the replacement of conservation of total probability by other dynamical invariants in agreement with predictions based on a theoretical treatment of the pseudo-Hermitian LZSM system.Item Theory of coherent interaction-free detection of pulses(American Physical Society, 2023-07) McCord, John J.; Dogra, Shruti; Paraoanu, Gheorghe Sorin; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsQuantum physics allows an object to be detected even in the absence of photon absorption, by the use of so-called interaction-free measurements. We provide a formulation of this protocol using a three-level system, where the object to be detected is a pulse coupled resonantly into the second transition. In the original formulation of interaction-free measurements, the absorption is associated with a projection operator onto the third state. We perform an in-depth analytical and numerical analysis of the coherent protocol, where coherent interaction between the object and the detector replaces the projective operators, resulting in higher detection efficiencies. We provide approximate asymptotic analytical results to support this finding. We find that our protocol reaches the Heisenberg limit when evaluating the Fisher information at small strengths of the pulses we aim to detect - in contrast to the projective protocol that can only reach the standard quantum limit. We also demonstrate that the coherent protocol remains remarkably robust under errors such as pulse rotation phases and strengths, the effect of relaxation rates and detunings, as well as different thermalized initial states.