arXiv subjects

Oleg V. Astafiev

Publications and source records attributed to Oleg V. Astafiev.

11 recordsLinked to original sources

arXiv2026

Single Artificial Atom SASER

Lasing - an effect of orthodox quantum mechanics - was discovered in 1955 and recognized by the Nobel Prize in 1964 due to its fundamentality. Nowadays, lasers and masers routinely work with electromagnetic waves and consist of a resonator with an active medium - usually a system of atoms with population inversion mechanism. Amazingly, quantum mechanics remains valid even when electromagnetic waves are replaced by vibrations of a crystal lattice, and, therefore, photons by phonons, even though are not fundamental particles. By implementing acoustic resonators coupled to an atom with a mechanism of population inversion, the lasing effect in sound can be achieved. In this paper, we demonstrate the single artificial atom SASER (Sound Amplification by Stimulated Emission of Radiation) action by utilizing a surface acoustic wave (SAW) resonator on quartz coupled to a deliberately designed superconducting three-level quantum system (artificial atom), in which population inversion is realized. The SASER operates in the ultrasound range at a frequency about 3 GHz. Acoustic-to-electric signals are converted via piezo-electric effect and the circuit elements; an artificial atom and input/outputs are coupled via the acoustic waves. We observe amplification of the waves and their strong self-emission with a significant narrowing of the linewidth. The phonon number generated in the system exceeds 90.

quant-ph↗

arXiv2026

The microwave phase locking in Bloch transistor

Recent experimental demonstration of the quantum coherent phase slip and current quantization in the superconductors, the fundamental phenomena dual to the coherent Cooper pair tunnelling and voltage quantization (Shapiro steps), enables the development of a new quantum device, the Bloch transistor (BT). BT has a unique functionality: it can deliver quantized non-dissipative current to the quantum circuit. BT consists of two coupled Josephson Junctions (JJ) in the regime of coherent quantum phase slip. At the heart of the BT operation is a new mechanism for phase-locking the Bloch oscillations in JJs to microwaves via induced charge. The charge phase locking allows not only quantization of current but also gate voltage control of this quantisation through the Aharonov-Casher effect. We study the operation of the BT and analyse its parameters. BT technology is scalable and compatible with other superconducting quantum devices, making it part of an emerging cryogenic quantum technology platform

cond-mat.mes-hall↗

arXiv2025

Variational preparation of entangled states in a system of transmon qubits

The conventional method for generating entangled states in qubit systems relies on applying precise two-qubit entangling gates alongside single-qubit rotations. However, achieving high-fidelity entanglement demands high accuracy in two-qubit operations, requiring complex calibration protocols. In this work, we use a minimally calibrated two-qubit iSwap-like gate, tuned via straightforward parameter optimization (flux pulse amplitude and duration), to prepare Bell states and GHZ states experimentally in systems of two and three transmon qubits. By integrating this gate into a variational quantum algorithm (VQA), we bypass the need for intricate calibration while maintaining high fidelity. Our proposed methodology employs variational quantum algorithms (VQAs) to create the target quantum state through imperfect multiqubit operations. Furthermore, we experimentally demonstrate a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality for Bell states, confirming their high fidelity of preparation.

quant-ph↗

arXiv2024

High quality quasinormal modes of phononic crystals for quantum acoustodynamics

Phononic crystals are a promising platform for the study of quantum acoustodynamics. In a recent experiment, the interaction of a superconducting quantum bit with modes of a phononic crystal has been demonstrated. The field of these modes is localized in a compact area, providing high values of the coupling constant with the qubit. However, the Q-factor of phononic crystal modes is strongly limited (1050) due to a phonon emission from the crystal ends. For further use of phononic crystals in research in the field of quantum acoustodynamics, it is desirable to overcome this limitation in the quality factor. In this work, we have proposed a structure consisting of a phononic crystal placed between the Bragg mirrors. Our simulations predict that the Q-factor in such a structure can reach (100000). We demonstrate experimental results in which this structure has a Q-factor (60,000), which is 60 times higher than that of an acoustic crystal of the same size.

quant-ph↗

arXiv2024

Highly stable aluminum air-bridges with stiffeners

Air-bridges play a critical role in the performance of microwave circuits integrated with superconducting quantum bits, and their mechanical stability is predominant for reliable operation. This study is devoted to the technological issues that lead to air-bridge instability. We propose an optimized bridge geometry designed to enhance mechanical resilience. Through systematic testing, we established that bridges incorporating this novel geometry achieved complete stability for lengths up to 170 micrometers in our technological processes. The findings provide an insight into the problem and a practical solution for technologists that faced with the challenges of air-bridge stability. The implementation of our technology has the potential to significantly improve the mechanical robustness of air-bridges in multi-qubit circuits for quantum computation.

quant-ph↗

arXiv2024

Feasibility of the Josephson voltage and current standards on a single chip

The quantum Josephson voltage standard is well established across the metrology community for many years. It relies on the synchronisation of the flux tunneling in the S/I/S Josepson junctions (JJ) with the microwave radiation (MW). The phenomenon is called the Shapiro steps. Together with the Quantum Hall resistance standard, the voltage standard forms the foundation of electrostatic metrology. The current is then defined as the ratio of the voltage and resistance. Realisation of the quantum current standard, would close the electrostatic metrological triangle of voltage-resistance-current. The current quantisation, the inverse Shapiro steps, was recently shown using the superconducting nanowires and small JJ. The effect is a synchronization of the MW with the Cooper pair tunnelling. This paves the way to combine the JJ voltage and current standards on the same chip and demonstrate feasibility of the multi-standard operation. We show the voltage and current quantization on the same chip up to frequency of 10 GHz, corresponding to the amplitudes 0.021 mV and 3.23 nA respectively. The accuracy of the voltage and current quantisation, however, is relatively low, 35 ppk and 100 ppk respectively. We discuss measures to optimise the JJs, circuit and environment to boost the amplitude and accuracy of the standards.

cond-mat.supr-con↗

arXiv2024

Hybrid quantum learning with data re-uploading on a small-scale superconducting quantum simulator

Supervised quantum learning is an emergent multidisciplinary domain bridging between variational quantum algorithms and classical machine learning. Here, we study experimentally a hybrid classifier model accelerated by a quantum simulator - a linear array of four superconducting transmon artificial atoms - trained to solve multilabel classification and image recognition problems. We train a quantum circuit on simple binary and multi-label tasks, achieving classification accuracy around 95%, and a hybrid model with data re-uploading with accuracy around 90% when recognizing handwritten decimal digits. Finally, we analyze the inference time in experimental conditions and compare the performance of the studied quantum model with known classical solutions.

quant-ph↗

arXiv2023

A phononic crystal coupled to a transmission line via an artificial atom

We study a phononic crystal interacting with an artificial atom { a superconducting quantum system { in the quantum regime. The phononic crystal is made of a long lattice of narrow metallic stripes on a quatz surface. The artificial atom in turn interacts with a transmission line therefore two degrees of freedom of different nature, acoustic and electromagnetic, are coupled with a single quantum object. A scattering spectrum of propagating electromagnetic waves on the artificial atom visualizes acoustic modes of the phononic crystal. We simulate the system and found quasinormal modes of our phononic crystal and their properties. The calculations are consistent with the experimentally found modes, which are fitted to the dispersion branches of the phononic crystal near the first Brillouin zone edge. Our geometry allows to realize effects of quantum acoustics on a simple and compact phononic crystal.

quant-ph↗

arXiv2023

Resonance Fluorescence from a two-level artificial atom strongly coupled to a single-mode cavity

We experimentally demonstrate the resonance fluorescence of a two-level artificial atom strongly coupled to a single-mode cavity field. The effect was theoretically predicted thirty years ago by Savage [Phys. Rev. Lett. 63, 1376 (1989)]. The system consists of a superconducting qubit circuit and a one-dimensional transmission line resonator. In addition, a one-dimensional transmission line strongly coupled to the atom serves as an open space. The effect takes place, when a microwave field is applied to the cavity, which in turn is resonantly coupled to the atom. The fluorescence spectrum is measured via the emission into the transmission line. We find that the central peak is determined by the atom spontaneous emission to the open space and the widths of side peaks are largely determined by the coherent interaction between the atom and the cavity, that is, the fluorescence spectrum here is very different from that of the Mollow triplet. We also derive analytical form for the spectrum. Our experimental results agree well with theoretical calculations.

quant-ph↗

arXiv2021

Quantum versus Classical Regime in Circuit Quantum Acoustodynamics

We experimentally study a circuit quantum acoustodynamics system, which consists of a superconducting artificial atom, coupled to both a two-dimensional surface acoustic wave resonator and a one-dimensional microwave transmission line. The strong coupling between the artificial atom and the acoustic wave resonator is confirmed by the observation of the vacuum Rabi splitting at the base temperature of dilution refrigerator. We show that the propagation of microwave photons in the microwave transmission line can be controlled by a few phonons in the acoustic wave resonator. Furthermore, we demonstrate the temperature effect on the measurements of the Rabi splitting and temperature induced transitions from high excited dressed states. We find that the spectrum structure of two-peak for the Rabi splitting becomes into those of several peaks, and gradually disappears with the increase of the environmental temperature $T$. The quantum-to-classical transition is observed around the crossover temperature $T_{c}$, which is determined via the thermal fluctuation energy $k_{B}T$ and the characteristic energy level spacing of the coupled system. Experimental results agree well with the theoretical simulations via the master equation of the coupled system at different effective temperatures.

quant-ph↗

arXiv2018

Mixing of coherent waves on a single three-level artificial atom

We report coherent frequency conversion in the gigahertz range via three-wave mixing on a single artificial atom in open space. All frequencies involved are in vicinity of transition frequencies of the three-level atom. A cyclic configuration of levels is therefore essential, which we have realised with an artificial atom based on the flux qubit geometry. The atom is continuously driven at two transition frequencies and we directly measure the coherent emission at the sum or difference frequency. Our approach enables coherent conversion of the incoming fields into the coherent emission at a designed frequency in prospective devices of quantum electronics.

quant-ph↗