Publications
2025
- A learning agent-based approach to the characterization of open quantum systemsLorenzo Fioroni , Ivan Rojkov , and Florentin ReiterarXiv:quant-ph/2501.05350 (2025)
Characterizing quantum processes is crucial for the execution of quantum algorithms on available quantum devices. A powerful framework for this purpose is the Quantum Model Learning Agent (QMLA) which characterizes a given system by learning its Hamiltonian via adaptive generations of informative experiments and their validation against simulated models. Identifying the incoherent noise of a quantum device in addition to its coherent interactions is, however, as essential. Precise knowledge of such imperfections of a quantum device allows to devise strategies to mitigate detrimental effects, for example via quantum error correction. We introduce the open Quantum Model Learning Agent (oQMLA) framework to account for Markovian noise through the Liouvillian formalism. By simultaneously learning the Hamiltonian and jump operators, oQMLA independently captures both the coherent and incoherent dynamics of a system. The added complexity of open systems necessitates advanced algorithmic strategies. Among these, we implement regularization to steer the algorithm towards plausible models and an unbiased metric to evaluate the quality of the results. We validate our implementation in simulated scenarios of increasing complexity, demonstrating its robustness to hardware-induced measurement errors and its ability to characterize systems using only local operations. Additionally, we develop a scheme to interface oQMLA with a publicly available superconducting quantum computer, showcasing its practical utility. These advancements represent a significant step toward improving the performance of quantum hardware and contribute to the broader goal of advancing quantum technologies and their applications.
@article{fioroni_learning_2025, title = {A learning agent-based approach to the characterization of open quantum systems}, author = {Fioroni, Lorenzo and Rojkov, Ivan and Reiter, Florentin}, journal = {arXiv:quant-ph/2501.05350}, year = {2025}, month = jan, }
2024
- Two-Qubit Operations for Finite-Energy Gottesman-Kitaev-Preskill EncodingsIvan Rojkov , Paul Moser Röggla , Martin Wagener , Moritz Fontboté-Schmidt , Stephan Welte , Jonathan Home , and Florentin ReiterPhys. Rev. Lett. 133, 100601 (2024)
We present techniques for performing two-qubit gates on Gottesman-Kitaev-Preskill (GKP) codes with finite energy, and find that operations designed for ideal infinite-energy codes create undesired entanglement when applied to physically realistic states. We demonstrate that this can be mitigated using recently developed local error-correction protocols, and evaluate the resulting performance. We also propose energy-conserving finite-energy gate implementations which largely avoid the need for further correction.
@article{two_qubit_rojkov_2023, title = {Two-Qubit Operations for Finite-Energy Gottesman-Kitaev-Preskill Encodings}, author = {Rojkov, Ivan and R{\"o}ggla, Paul Moser and Wagener, Martin and Fontbot{\'e}-Schmidt, Moritz and Welte, Stephan and Home, Jonathan and Reiter, Florentin}, journal = {Phys. Rev. Lett.}, volume = {133}, number = {10}, pages = {100601}, year = {2024}, month = aug, publisher = {American Physical Society}, doi = {10.1103/PhysRevLett.133.100601}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.133.100601}, } - Stabilization of cat-state manifolds using nonlinear reservoir engineeringIvan Rojkov , Matteo Simoni , Elias Zapusek , Florentin Reiter , and Jonathan HomearXiv:quant-ph/2407.18087 (2024)
We introduce a novel reservoir engineering approach for stabilizing multi-component Schrödinger’s cat manifolds. The fundamental principle of the method lies in the destructive interference at crossings of gain and loss Hamiltonian terms in the coupling of an oscillator to a zero-temperature auxiliary system, which are nonlinear with respect to the oscillator’s energy. The nature of these gain and loss terms is found to determine the rotational symmetry, energy distributions, and degeneracy of the resulting stabilized manifolds. Considering these systems as bosonic error-correction codes, we analyze their properties with respect to a variety of errors, including both autonomous and passive error correction, where we find that our formalism gives straightforward insights into the nature of the correction. We give example implementations using the anharmonic laser-ion coupling of a trapped ion outside the Lamb-Dicke regime as well as nonlinear superconducting circuits. Beyond the dissipative stabilization of standard cat manifolds and novel rotation symmetric codes, we demonstrate that our formalism allows for the stabilization of bosonic codes linked to cat states through unitary transformations, such as quadrature-squeezed cats. Our work establishes a design approach for creating and utilizing codes using nonlinearity, providing access to novel quantum states and processes across a range of physical systems.
@article{stabilization_rojkov_2024, title = {Stabilization of cat-state manifolds using nonlinear reservoir engineering}, author = {Rojkov, Ivan and Simoni, Matteo and Zapusek, Elias and Reiter, Florentin and Home, Jonathan}, journal = {arXiv:quant-ph/2407.18087}, year = {2024}, month = jul, }
2023
- Acceleration of inferred neural responses to oddball targets in an individual with bilateral amygdala lesion compared to healthy controlsAslan Abivardi , Christoph W Korn , Ivan Rojkov , Samuel Gerster , Rene Hurlemann , and Dominik R BachScientific Reports 13, 14550 (2023)
Detecting unusual auditory stimuli is crucial for discovering potential threat. Locus coeruleus (LC), which coordinates attention, and amygdala, which is implicated in resource prioritization, both respond to deviant sounds. Evidence concerning their interaction, however, is sparse. Seeking to elucidate if human amygdala affects estimated LC activity during this process, we recorded pupillary responses during an auditory oddball and an illuminance change task, in a female with bilateral amygdala lesions (BG) and in n = 23 matched controls. Neural input in response to oddballs was estimated via pupil dilation, a reported proxy of LC activity, harnessing a linear-time invariant system and individual pupillary dilation response function (IRF) inferred from illuminance responses. While oddball recognition remained intact, estimated LC input for BG was compacted to an impulse rather than the prolonged waveform seen in healthy controls. This impulse had the earliest response mean and highest kurtosis in the sample. As a secondary finding, BG showed enhanced early pupillary constriction to darkness. These findings suggest that LC-amygdala communication is required to sustain LC activity in response to anomalous sounds. Our results provide further evidence for amygdala involvement in processing deviant sound targets, although it is not required for their behavioral recognition.
@article{abivardi2023acceleration, title = {Acceleration of inferred neural responses to oddball targets in an individual with bilateral amygdala lesion compared to healthy controls}, author = {Abivardi, Aslan and Korn, Christoph W and Rojkov, Ivan and Gerster, Samuel and Hurlemann, Rene and Bach, Dominik R}, journal = {Scientific Reports}, volume = {13}, number = {1}, pages = {14550}, year = {2023}, month = sep, publisher = {Nature Publishing Group UK London}, url = {https://www.nature.com/articles/s41598-023-41357-1}, } - Characterization of Coherent Errors in Noisy Quantum DevicesNoah Kaufmann , Ivan Rojkov , and Florentin ReiterarXiv:quant-ph/2307.08741 (2023)
Characterization of quantum devices generates insights into their sources of disturbances. State-of-the-art characterization protocols often focus on incoherent noise and eliminate coherent errors when using Pauli or Clifford twirling techniques. This approach biases the structure of the effective noise and adds a circuit and sampling overhead. We motivate the extension of an incoherent local Pauli noise model to coherent errors and present a practical characterization protocol for an arbitrary gate layer. We demonstrate our protocol on a superconducting hardware platform and identify the leading coherent errors. To verify the characterized noise structure, we mitigate its coherent and incoherent components using a gate-level coherent noise mitigation scheme in conjunction with probabilistic error cancellation. The proposed characterization procedure opens up possibilities for device calibration, hardware development, and improvement of error mitigation and correction techniques.
@article{characterization_kaufmann_2023, title = {Characterization of Coherent Errors in Noisy Quantum Devices}, author = {Kaufmann, Noah and Rojkov, Ivan and Reiter, Florentin}, journal = {arXiv:quant-ph/2307.08741}, year = {2023}, month = jul, }
2022
- Bias in Error-Corrected Quantum SensingIvan Rojkov , David Layden , Paola Cappellaro , Jonathan Home , and Florentin ReiterPhys. Rev. Lett. 128, 140503 (2022)
The sensitivity afforded by quantum sensors is limited by decoherence. Quantum error correction (QEC) can enhance sensitivity by suppressing decoherence, but it has a side effect: it biases a sensor’s output in realistic settings. If unaccounted for, this bias can systematically reduce a sensor’s performance in experiment, and also give misleading values for the minimum detectable signal in theory. We analyze this effect in the experimentally motivated setting of continuous-time QEC, showing both how one can remedy it, and how incorrect results can arise when one does not.
@article{bias_rojkov_2022, title = {Bias in Error-Corrected Quantum Sensing}, author = {Rojkov, Ivan and Layden, David and Cappellaro, Paola and Home, Jonathan and Reiter, Florentin}, journal = {Phys. Rev. Lett.}, volume = {128}, issue = {14}, pages = {140503}, numpages = {6}, year = {2022}, month = apr, publisher = {American Physical Society}, doi = {10.1103/PhysRevLett.128.140503}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.128.140503}, } - Generation of a Maximally Entangled State Using Collective Optical PumpingMaciej Malinowski , Chi Zhang , Vlad Negnevitsky , Ivan Rojkov , Florentin Reiter , T-L Nguyen , Martin Stadler , Daniel Kienzler , Karan K Mehta , and Jonathan P HomePhys. Rev. Lett. 128, 080503 (2022)
We propose and implement a novel scheme for dissipatively pumping two qubits into a singlet Bell state. The method relies on a process of collective optical pumping to an excited level, to which all states apart from the singlet are coupled. We apply the method to deterministically entangle two trapped Ca+ 40 ions. Within 16 pumping cycles, an initially separable state is transformed into one with 83 (1)% singlet fidelity, and states with initial fidelity of⪆ 70% converge onto a fidelity of 93 (1)%. We theoretically analyze the performance and error susceptibility of the scheme and find it to be insensitive to a large class of experimentally relevant noise sources.
@article{generation_malinowski_2022, title = {Generation of a Maximally Entangled State Using Collective Optical Pumping}, author = {Malinowski, Maciej and Zhang, Chi and Negnevitsky, Vlad and Rojkov, Ivan and Reiter, Florentin and Nguyen, T-L and Stadler, Martin and Kienzler, Daniel and Mehta, Karan K and Home, Jonathan P}, journal = {Phys. Rev. Lett.}, volume = {128}, issue = {8}, pages = {080503}, numpages = {7}, year = {2022}, month = feb, publisher = {American Physical Society}, doi = {10.1103/PhysRevLett.128.080503}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.128.080503}, }