### Browsing by Author "Paraoanu, G. S."

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Item Autler-Townes Effect in a Superconducting Three-Level System(American Physical Society (APS), 2009) Sillanpää, Mika A.; Li, Jian; Cicak, Katarina; Altomare, Fabio; Park, Jae I.; Simmonds, Raymond W.; Paraoanu, G. S.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWhen a three-level quantum system is irradiated by an intense coupling field resonant with one of the three possible transitions, the absorption peak of an additional probe field involving the remaining level is split. This process is known in quantum optics as the Autler-Townes effect. We observe these phenomena in a superconducting Josephson phase qubit, which can be considered an “artificial atom” with a multilevel quantum structure. The spectroscopy peaks can be explained reasonably well by a simple three-level Hamiltonian model. Simulation of a more complete model (including dissipation, higher levels, and cross coupling) provides excellent agreement with all of the experimental data.Item Broadband Continuous-Variable Entanglement Generation Using a Kerr-Free Josephson Metamaterial(American Physical Society, 2022-08) Perelshtein, M. R.; Petrovnin, K. V.; Vesterinen, V.; Hamedani Raja, S.; Lilja, I.; Will, M.; Savin, A.; Simbierowicz, S.; Jabdaraghi, R. N.; Lehtinen, J. S.; Grönberg, L.; Hassel, J.; Prunnila, M. P.; Govenius, J.; Paraoanu, G. S.; Hakonen, P. J.; Quantum Circuits and Noise; Superconducting Qubits and Circuit QED; VTT Technical Research Centre of Finland; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsEntangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising superconducting, nonlinear, asymmetric inductive elements to generate frequency-entangled photons from vacuum fluctuations at a rate of 2 giga entangled bits per second spanning over the 4-GHz bandwidth. The device is operated as a traveling-wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we demonstrate single-mode squeezing in such devices - 3.1±0.7dB below the zero-point level at half of modulation frequency.Item Broadband Continuous-Variable Entanglement Generation Using a Kerr-Free Josephson Metamaterial(American Physical Society, 2022-08-23) Perelshtein, M. R.; Petrovnin, K. V.; Vesterinen, V.; Hamedani Raja, S.; Lilja, I.; Will, M.; Savin, A.; Simbierowicz, S.; Jabdaraghi, R. N.; Lehtinen, J. S.; Grönberg, L.; Hassel, J.; Prunnila, M. P.; Govenius, J.; Paraoanu, G. S.; Hakonen, P. J.; Department of Applied Physics; Quantum Circuits and Correlations; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QED; VTT Technical Research Centre of FinlandEntangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising superconducting, nonlinear, asymmetric inductive elements to generate frequency-entangled photons from vacuum fluctuations at a rate of 2 giga entangled bits per second spanning over the 4-GHz bandwidth. The device is operated as a traveling-wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we demonstrate single-mode squeezing in such devices - 3.1±0.7dB below the zero-point level at half of modulation frequency.Item Contact doping, Klein tunneling, and asymmetry of shot noise in suspended graphene(2016-03-09) Laitinen, Antti; Paraoanu, G. S.; Oksanen, Mika; Craciun, Monica F.; Russo, Saverio; Sonin, Edouard; Hakonen, Pertti; Department of Applied Physics; Quantum Circuits and Correlations; Superconducting Qubits and Circuit QED; University of Exeter; Hebrew University of JerusalemThe inherent asymmetry of the electric transport in graphene is attributed to Klein tunneling across barriers defined by pn interfaces between positively and negatively charged regions. By combining conductance and shot noise experiments, we determine the main characteristics of the tunneling barrier (height and slope) in a high-quality suspended sample with Au/Cr/Au contacts. We observe an asymmetric resistance Rodd=100-70Ω across the Dirac point of the suspended graphene at carrier density |nG|=(0.3-4)×1011cm-2, while the Fano factor displays a nonmonotonic asymmetry in the range Fodd∼0.03-0.1. Our findings agree with analytical calculations based on the Dirac equation with a trapezoidal barrier. Comparison between the model and the data yields the barrier height for tunneling, an estimate of the thickness of the pn interface dItem Cross-coupling effects in circuit-QED stimulated Raman adiabatic passage(2018-04-19) Vepsäläinen, A.; Paraoanu, G. S.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QEDStimulated Raman adiabatic passage is a quantum protocol that can be used for robust state preparation in a three-level system. It has been commonly employed in quantum optics, but recently this technique has drawn attention also in circuit quantum electrodynamics. The protocol relies on two slowly varying drive pulses that couple the initial and the target state via an intermediate state, which remains unpopulated. Here we study the detrimental effect of the parasitic couplings of the drives into transitions other than those required by the protocol. The effect is most prominent in systems with almost harmonic energy level structure, such as the transmon. We show that under these conditions in the presence of decoherence there exists an optimal STIRAP amplitude for population transfer.Item Decoherence, Autler-Townes effect, and dark states in two-tone driving of a three-level superconducting system(American Physical Society (APS), 2011) Li, Jian; Paraoanu, G. S.; Cicak, Katarina; Altomare, Fabio; Park, Jae I.; Simmonds, Raymond W.; Sillanpää, Mika A.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe present a detailed theoretical analysis of a multilevel quantum system coupled to two radiation fields and subject to decoherence. We concentrate on an effect known from quantum optics as Autler-Townes splitting, which has been recently demonstrated experimentally [M. A. Sillanpää et al., Phys. Rev. Lett. 103, 193601 (2009)] in a superconducting phase qubit. In the three-level approximation, we derive analytical solutions and describe how they can be used to extract the decoherence rates and to account for the measurement data. Better agreement with the experiment can be obtained by extending this model to five levels. Finally, we investigate the stationary states created in the experiment and show that their structure is close to that of dark states.Item Detecting virtual photons in ultrastrongly coupled superconducting quantum circuits(American Physical Society, 2024-01) Giannelli, L.; Paladino, E.; Grajcar, M.; Paraoanu, G. S.; Falci, G.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QED; National Research Council of Italy; Comenius University BratislavaLight-matter interaction and understanding the fundamental physics behind is essential for emerging quantum technologies. Solid-state devices may explore new regimes where coupling strengths are "ultrastrong", i.e., comparable to the energies of the subsystems. New exotic phenomena occur the common root of many of them being the fact that the entangled vacuum contains virtual photons. They herald the lack of conservation of the number of excitations which is the witness of ultrastrong coupling breaking the U(1) symmetry. Despite more than a decade of research, the detection of ground-state virtual photons still awaits demonstration. In this work, we recognize the "conspiring"set of experimental challenges and show how to overcome them, thus providing a solution to this long-standing problem. We find that combining a superinductor-based unconventional "light fluxonium"qudit and coherent control yields a highly efficient, faithful, and selective conversion of virtual photons into real ones. This enables their detection with resources available to present-day quantum technologies.Item Dynamical Autler-Townes control of a phase qubit(Nature Publishing Group, 2012) Li, Jian; Paraoanu, G. S.; Cicak, Katarina; Altomare, Fabio; Park, Jae I.; Simmonds, Raymond W.; Sillanpää, Mika A.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceRouters, switches, and repeaters are essential components of modern information-processing systems. Similar devices will be needed in future superconducting quantum computers. In this work we investigate experimentally the time evolution of Autler-Townes splitting in a superconducting phase qubit under the application of a control tone resonantly coupled to the second transition. A three-level model that includes independently determined parameters for relaxation and dephasing gives excellent agreement with the experiment. The results demonstrate that the qubit can be used as a ON/OFF switch with 100 ns operating time-scale for the reflection/transmission of photons coming from an applied probe microwave tone. The ON state is realized when the control tone is sufficiently strong to generate an Autler-Townes doublet, suppressing the absorption of the probe tone photons and resulting in a maximum of transmissioItem Finite-time quantum Stirling heat engine(IOP Publishing Ltd., 2021-03) Hamedani Raja, S.; Maniscalco, S.; Paraoanu, G. S.; Pekola, J. P.; Lo Gullo, N.; Department of Applied Physics; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTF; Quantum Phenomena and Devices; University of TurkuWe study the thermodynamic performance of a finite-time non-regenerative quantum Stirling-like cycle used as a heat engine. We consider specifically the case in which the working substance (WS) is a two-level system (TLS). The Stirling cycle is made of two isochoric transformations separated by a compression and an expansion stroke during which the WS is in contact with a thermal reservoir. To describe these two strokes we derive a non-Markovian master equation which allows to study the real-time dynamics of a driven open quantum system with arbitrary fast driving. Following the real-time dynamics of the WS using this master equation, the endpoints of the isotherms can deviate from the equilibrium thermal states. The role of this deviation in the performance of the heat engine is addressed. We found that the finite-time dynamics and thermodynamics of the cycle depend non-trivially on the different time scales at play. In particular, driving the WS at a time scale comparable to the resonance time of the bath enhances the performance of the cycle and allows for an efficiency higher than the efficiency of the quasistatic cycle, but still below the Carnot bound. However, by adding thermalization of the WS with the baths at the end of compression/expansion processes one recovers the conventional scenario in which efficiency decreases by speeding up the processes. In addition, the performance of the cycle is dependent on the compression/expansion speeds asymmetrically, which suggests new freedom in optimizing quantum heat engines. The maximum output power and the maximum efficiency are obtained almost simultaneously when the real-time endpoints of the compression/expansion processes are considered instead of the equilibrium thermal endpoint states. However, the net extractable work always declines by speeding up the drive.Item General solution of the time evolution of two interacting harmonic oscillators(American Physical Society, 2021-02-11) Bruschi, David Edward; Paraoanu, G. S.; Fuentes, Ivette; Wilhelm, Frank K.; Schell, Andreas W.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QED; Universität des Saarlandes; University of Southampton; Saarland University; Physikalisch-Technische BundesanstaltWe study the time evolution of an ideal system composed of two harmonic oscillators coupled through a quadratic Hamiltonian with arbitrary interaction strength. We solve its dynamics analytically by employing tools from symplectic geometry. In particular, we use this result to completely characterize the dynamics of the two oscillators interacting in the ultrastrong-coupling regime with additional single-mode squeezing on both oscillators, as well as higher-order terms. Furthermore, we compute quantities of interest, such as the average number of excitations and the correlations that are established between the two subsystems due to the evolution. We find that this model predicts a second-order phase transition and we compute the critical exponents and the critical value. We also provide an exact decoupling of the time evolution in terms of simple quantum optical operations, which can be used for practical implementations and studies. Finally, we show how our techniques can be extended to include more oscillators and higher-order interactions.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 Non-local parity measurements and the quantum pigeonhole effect(2018-08-01) Paraoanu, G. S.; Department of Applied PhysicsThe pigeonhole principle upholds the idea that by ascribing to three different particles either one of two properties, we necessarily end up in a situation when at least two of the particles have the same property. In quantum physics, this principle is violated in experiments involving postselection of the particles in appropriately-chosen states. Here, we give two explicit constructions using standard gates and measurements that illustrate this fact. Intriguingly, the procedures described are manifestly non-local, which demonstrates that the correlations needed to observe the violation of this principle can be created without direct interactions between particles.Item Observation of the Bloch-Siegert shift in a driven quantum-to-classical transition(2017-07-07) Pietikäinen, I.; Danilin, S.; Sampath Kumar, Karthikeyan; Vepsäläinen, A.; Golubev, D. S.; Tuorila, J.; Paraoanu, G. S.; Department of Applied Physics; Superconducting Qubits and Circuit QED; Quantum Phenomena and Devices; Multiscale Statistical and Quantum Physics; University of OuluWe show that the counter-rotating terms of the dispersive qubit-cavity Rabi model can produce relatively large and nonmonotonic Bloch-Siegert shifts in the cavity frequency as the system is driven through a quantum-to-classical transition. Using a weak microwave probe tone, we demonstrate experimentally this effect by monitoring the resonance frequency of a microwave cavity coupled to a transmon and driven by a microwave field with varying power. In the weakly driven regime (quantum phase), the Bloch-Siegert shift appears as a small constant frequency shift, while for a strong drive (classical phase) it presents an oscillatory behavior as a function of the number of photons in the cavity. The experimental results are in agreement with numerical simulations based on the quasienergy spectrum.Item Optimal superadiabatic population transfer and gates by dynamical phase corrections(2018-04-01) Vepsäläinen, A.; Danilin, S.; Paraoanu, G. S.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QEDIn many quantum technologies adiabatic processes are used for coherent quantum state operations, offering inherent robustness to errors in the control parameters. The main limitation is the long operation time resulting from the requirement of adiabaticity. The superadiabatic method allows for faster operation, by applying counterdiabatic driving that corrects for excitations resulting from the violation of the adiabatic condition. In this article we show how to construct the counterdiabatic Hamiltonian in a system with forbidden transitions by using two-photon processes and how to correct for the resulting time-dependent ac-Stark shifts in order to enable population transfer with unit fidelity. We further demonstrate that superadiabatic stimulated Raman passage can realize a robust unitary NOT-gate between the ground state and the second excited state of a three-level system. The results can be readily applied to a three-level transmon with the ladder energy level structure.Item Optimized emulation of quantum magnetometry via superconducting qubits(American Physical Society, 2023-05) Gusarov, N. N.; Perelshtein, M. R.; Hakonen, P. J.; Paraoanu, G. S.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Circuits and Correlations; Superconducting Qubits and Circuit QEDQuantum magnetometry based on adaptive phase estimation allows for Heisenberg precision while avoiding creation and maintenance of complex entangled states. However, the absolute sensitivity is limited by the nonoptimal use of quantum resources provided by multiple-qubit devices and algorithmic realizations of the protocol. Here, addressing both issues, we advance the time-ascending phase estimation protocol by numerical improvements of Bayesian learning, i.e., sequential updating of the field distribution, and optimal exploitation of resources provided by unentangled qubits with limited coherence. Such algorithmic improvements are used to evaluate the absolute sensitivity both on a simulator and by pulsed-transmon experiments conducted on the IBMQ platform, where we take advantage of high coherence time. In addition, we compare the proficiency of separable and entangled states for magnetometry and show that, in practice, separable states provide superior performance. Flux-sensing emulation experiments demonstrate that a sensitivity of (0.17-1.74)μφ0(Hz)-1 (where φ0 is the flux quantum) for a single-qubit magnetometer and (0.06-0.65)μφ0(Hz)-1 for a five-qubit magnetometer can be achieved for slowly oscillating 1-10kHz magnetic fields, which is comparable to more established experimental platforms for magnetometry.Item Photon blockade and the quantum-to-classical transition in the driven-dissipative Josephson pendulum coupled to a resonator(American Physical Society, 2019-06-20) Pietikäinen, I.; Tuorila, J.; Golubev, D. S.; Paraoanu, G. S.; Department of Applied Physics; Multiscale Statistical and Quantum Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Computing and Devices; Quantum Phenomena and Devices; Superconducting Qubits and Circuit QED; University of OuluWe investigate the driven quantum phase transition between the oscillating motion and the classical nearly free rotations of the Josephson pendulum coupled to a harmonic oscillator in the presence of dissipation. We refer to this as the Josephson-Rabi model. This model describes the standard setup of circuit quantum electrodynamics, where typically a transmon device is embedded in a superconducting cavity. We find that by treating the system quantum mechanically this transition occurs at higher drive powers than expected from an all-classical treatment, which is a consequence of the quasiperiodicity originating in the discrete energy spectrum of the bound states. We calculate the photon number in the resonator and show that its dependence on the drive power is nonlinear. In addition, the resulting multiphoton blockade phenomenon is sensitive to the truncation of the number of states in the transmon, which limits the applicability of the standard Jaynes-Cummings model as an approximation for the pendulum-oscillator system. We calculate the nth-order correlation functions of the blockaded microwave photons and observe the differences between the rotating-wave approximation and the full multilevel Josephson-Rabi Hamiltonian with the counter-rotating terms included. Finally, we compare two different approaches to dissipation, namely the Floquet-Born-Markov and the Lindblad formalisms.Item Quantum metrology with a transmon qutrit(2018-02-27) Shlyakhov, A. R.; Zemlyanov, V. V.; Suslov, M. V.; Lebedev, A. V.; Paraoanu, G. S.; Lesovik, G. B.; Blatter, G.; Department of Applied Physics; Superconducting Qubits and Circuit QED; Centre of Excellence in Quantum Technology, QTF; Moscow Institute of Physics and Technology; University of ZurichMaking use of coherence and entanglement as metrological quantum resources allows us to improve the measurement precision from the shot-noise or quantum limit to the Heisenberg limit. Quantum metrology then relies on the availability of quantum engineered systems that involve controllable quantum degrees of freedom which are sensitive to the measured quantity. Sensors operating in the qubit mode and exploiting their coherence in a phase-sensitive measurement have been shown to approach the Heisenberg scaling in precision. Here, we show that this result can be further improved by operating the quantum sensor in the qudit mode, i.e., by exploiting d rather than two levels. Specifically, we describe the metrological algorithm for using a superconducting transmon device operating in a qutrit mode as a magnetometer. The algorithm is based on the base-3 semiquantum Fourier transformation and enhances the quantum theoretical performance of the sensor by a factor of 2. Even more, the practical gain of our qutrit implementation is found in a reduction of the number of iteration steps of the quantum Fourier transformation by the factor ln(2)/ln(3)≈0.63 compared to the qubit mode. We show that a two-tone capacitively coupled radio-frequency signal is sufficient for implementation of the algorithm.Item Quantum-enhanced magnetometry by phase estimation algorithms with a single artificial atom(2018-06-29) Danilin, S.; Lebedev, A. V.; Vepsalainen, A.; Lesovik, G. B.; Blatter, G.; Paraoanu, G. S.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QED; Quantum Circuits and Correlations; Swiss Federal Institute of Technology ZurichPhase estimation algorithms are key protocols in quantum information processing. Besides applications in quantum computing, they can also be employed in metrology as they allow for fast extraction of information stored in the quantum state of a system. Here, we implement two suitably modified phase estimation procedures, the Kitaev and the semiclassical Fourier-transform algorithms, using an artificial atom realized with a superconducting transmon circuit. We demonstrate that both algorithms yield a flux sensitivity exceeding the classical shot-noise limit of the device, allowing one to approach the Heisenberg limit. Our experiment paves the way for the use of superconducting qubits as metrological devices which are potentially able to outperform the best existing flux sensors with a sensitivity enhanced by few orders of magnitude.Item Solving Large-Scale Linear Systems of Equations by a Quantum Hybrid Algorithm(WILEY-VCH VERLAG, 2022-07) Perelshtein, M. R.; Pakhomchik, A. I.; Melnikov, A. A.; Novikov, A. A.; Glatz, A.; Paraoanu, G. S.; Vinokur, V. M.; Lesovik, G. B.; Department of Applied Physics; Quantum Circuits and Correlations; Centre of Excellence in Quantum Technology, QTF; Superconducting Qubits and Circuit QED; Terra Quantum AG; Moscow Institute of Physics and Technology; Argonne National LaboratoryToday's intermediate-scale quantum computers, although imperfect, already perform computational tasks that are manifestly beyond the capabilities of modern classical supercomputers. However, so far, quantum-enabled large-scale solutions have been realized only for limited set of problems. Here a hybrid algorithm based on phase estimation and classical optimization of the circuit width and depth is employed for solving a specific class of large linear systems of equations ubiquitous to many areas of science and engineering. A classification of linear systems based on the entanglement properties of the associated phase-estimation unitary operation is introduced, enabling a highly efficient search for solutions that is facilitated by a straightforward matrix-to-circuit map. A 217-dimensional problem is implemented on several IBM quantum computer superconducting quantum processors, a record-breaking result for a linear system solved by a quantum computer. Demonstrated realisation sets a clear benchmark in the quest for the future quantum speedup in the linear systems of equations solution.Item Stimulated Raman adiabatic passage in a three-level superconducting circuit(2016-02-23) Kumar, K. S.; Vepsäläinen, Antti; Danilin, S.; Paraoanu, G. S.; Department of Applied Physics; Superconducting Qubits and Circuit QEDThe adiabatic manipulation of quantum states is a powerful technique that opened up new directions in quantum engineering-enabling tests of fundamental concepts such as geometrical phases and topological transitions, and holding the promise of alternative models of quantum computation. Here we benchmark the stimulated Raman adiabatic passage for circuit quantum electrodynamics by employing the first three levels of a transmon qubit. In this ladder configuration, we demonstrate a population transfer efficiency >80% between the ground state and the second excited state using two adiabatic Gaussian-shaped control microwave pulses. By doing quantum tomography at successive moments during the Raman pulses, we investigate the transfer of the population in time domain. Furthermore, we show that this protocol can be reversed by applying a third adiabatic pulse, we study a hybrid nondiabatic-adiabatic sequence, and we present experimental results for a quasi-degenerate intermediate level.