arXiv subjects

Arkady Fedorov

Publications and source records attributed to Arkady Fedorov.

At least 19 recordsLinked to original sources

arXiv2026

Can a quantum circuit detect the Unruh effect?

The Unruh effect predicts that an accelerating observer perceives the Minkowski vacuum as a thermal bath, yet direct detection remains experimentally inaccessible. Its timelike counterpart, arising from the entanglement of massless fields between the future and past light cones, offers a more feasible route but requires a detector whose transition frequency follows a specific conformal-time scaling. We propose and analyze a practical implementation of such a detector using superconducting fluxonium circuits, which naturally provide two quasi-degenerate ground states and a tunable excited state, forming an effective $\Lambda$-system. By modulating the excited-state transition frequency in Minkowski time, the detector accumulates a geometric phase associated with the timelike Unruh effect. Open-system simulations predict $\sim 10\%$ shift in the ground-state population within $530$ ns, representing a three-order-of-magnitude sensitivity enhancement over two-level Unruh-DeWitt detectors. These results establish a realistic quantum-circuit platform for experimentally probing the timelike Unruh effect and, more broadly, for testing fundamental nature of quantum fields using engineered quantum systems.

quant-ph↗

arXiv2026

Reducing TLS loss in tantalum CPW resonators using titanium sacrificial layers

We demonstrate a substantial reduction in two-level system loss in tantalum coplanar waveguide resonators fabricated on high-resistivity silicon substrates through the use of an ultrathin titanium sacrificial layer. A 0.2nm titanium film, deposited atop pre-sputtered {\alpha}-tantalum, acts as a solid-state oxygen getter that chemically modifies the native Ta oxide at the metal-air interface. After device fabrication, the titanium layer is removed using buffered oxide etchant, leaving behind a chemically reduced Ta oxide surface. Subsequent high-vacuum annealing further suppresses two-level system loss. Resonators treated with this process exhibit internal quality factors Qi exceeding an average of 1.5 million in the single-photon regime across ten devices, over three times higher than otherwise identical devices lacking the titanium layer. These results highlight the critical role of interfacial oxide chemistry in superconducting loss and reinforce atomic-scale surface engineering as an effective approach to improving coherence in tantalum-based quantum circuits. The method is compatible with existing fabrication workflows applicable to tantalum films, offering a practical route to further extending T1 lifetimes in superconducting qubits.

quant-ph↗

arXiv2025

Microwave Circulation in an Extended Josephson Junction Ring

Circulators are nonreciprocal devices that enable directional signal routing. Nonreciprocity, which requires time-reversal symmetry breaking, can be produced in waveguides in which the propagation medium moves relative to the waveguide at a moderate fraction of the wave speed. Motivated by this effect, here we propose a design for nonreciprocal microwave transmission based on an extended, annular Josephson junction, in which the propagation medium consists of a train of moving fluxons. We show how to harness this to build a high-quality resonant microwave circulator, and we theoretically evaluate the anticipated performance of such a device.

quant-ph↗

arXiv2025

Open-Source Highly Parallel Electromagnetic Simulations for Superconducting Circuits

Electromagnetic simulations form an indispensable part of the design and optimization process for superconducting quantum devices. Although several commercial platforms exist, open-source alternatives optimized for high-performance computing remain limited. To address this gap, we introduce SQDMetal, a Python-based API that integrates Qiskit Metal (IBM), Gmsh, Palace (AWS), and Paraview (Kitware) into an open-source, highly parallel simulation workflow for superconducting quantum circuits. SQDMetal enables accurate, efficient, and scalable simulations while remaining community-driven and free from commercial constraints. In this work, we validate SQDMetal through mesh convergence studies which benchmark SQDMetal against COMSOL Multiphysics and Ansys, demonstrating excellent agreement for both eigenmode and electrostatic (capacitance) simulations. Furthermore, we simulate superconducting resonators and transmon qubits, showing reasonable agreement with experimental measurements. SQDMetal also supports advanced capabilities, including Hamiltonian extraction via the energy participation ratio (EPR) method, incorporation of kinetic inductance effects, and full 3D modelling of device geometry for improved predictive accuracy. By unifying open-source tools into a single framework, SQDMetal lowers the barriers to entry for community members seeking to access high-performance simulations to assist in the design and optimization of their devices.

quant-ph↗

arXiv2025

Powering Quantum Computation with Quantum Batteries

Executing quantum logic in cryogenic quantum computers requires a continuous energy supply from room-temperature control electronics. This dependence on external energy sources creates scalability limitations due to control channel density and heat dissipation. Here, we propose quantum batteries (QBs) as intrinsic quantum energy sources for quantum computation, enabling the thermodynamic limit of zero dissipation for unitary gates. Unlike classical power sources, QBs maintain quantum coherence with their load - a property that, while theoretically studied, remains unexploited in practical quantum technologies. We demonstrate that initializing a bosonic QB in a Fock state can supply the energy required for arbitrary unitary gates regardless of the circuit's depth, via the recycling of pre-charged energy. Crucially, allowing QB-qubit entanglement during computation lowers the QB initial energy requirements below established energy-fidelity bounds. This scheme facilitates a universal gate set controlled by a single parameter per qubit, its resonant frequency. The relative detuning of each qubit from the QB resonant frequency gives rise to qualitatively two gate types, off-resonance and around-resonance. The former facilitates dispersive gates which allow multi-qubit parity probing while the latter enables energy exchange between the QB and the qubits, driving both population transfer and entanglement generation. This mechanism utilizes the all-to-all connectivity of the shared resonator architecture to go beyond the standard single- and two-qubit native gates of current platforms with multi-qubit gate timescales of few pi/g, where g is the qubit-resonator coupling. The resultant speed-up includes also superextensive gates between symmetric Dicke states, characteristic of QB systems.

quant-ph↗

arXiv2024

Experimental realization of qubit-state-controlled directional edge states in waveguide QED

We experimentally realise the theoretical proposal for in-situ tunable photonic edge states emerging from qubits coupled to a waveguide with a photonic bandgap. These edge states are directional, exhibiting theoretically zero population in the opposite direction. Our experiment implements a tunable Rice-Mele waveguide configuration, where the directionality of edge states is controlled in-situ by varying the qubit energy. The Rice-Mele waveguide is constructed using lumped resonators coupled to a standard Xmon qubit. We demonstrate the existence of these edge states both actively, via waveguide transmission, and passively, through qubit emission via an edge state. We estimate a 99.4% fidelity in the directionality, constrained by our measurement noise floor. These results hold significant promise for the development of long-range qubit couplers with effectively zero crosstalk.

quant-ph↗

arXiv2024

Using coherent feedback for a periodic clock

A driven linear oscillator and a feedback mechanism are two necessary elements of any classical periodic clock. Here, we introduce a novel, fully quantum clock using a driven oscillator in the quantum regime and coherent quantum feedback. We show that if we treat the model semiclassically, this system supports limit cycles, or self-sustained oscillations, as needed for a periodic clock. We then analyse the noise of the system quantum mechanically and prove that the accuracy of this clock is higher compared to the clock implemented with the classical measurement feedback. We experimentally implement the model using two superconducting cavities with incorporated Josephson junctions and microwave circulators for the realisation of the quantum feedback. We confirm the appearance of the limit cycle and study the clock accuracy both in frequency and time domains. Under specific conditions of noisy driving, we observe that the clock oscillations are more coherent than the drive, pointing towards the implementation of a quantum autonomous clock.

quant-ph↗

arXiv2024

Fano-enhanced low-loss on-chip superconducting microwave circulator

Ferrite-free circulators that are passive and readily integratable on a chip are highly sought-after in quantum technologies based on superconducting circuits. In our previous work, we implemented such a circulator using a three-Josephson-junction loop that exhibited unambiguous nonreciprocity and signal circulation, but required junction energies to be within $1\%$ of design values. This tolerance is tighter than standard junction fabrication methods provide, so we propose and demonstrate a design improvement that relaxes the required junction fabrication precision, allowing for higher device performance and fabrication yield. Specifically, we introduce large direct capacitive couplings between the waveguides to create strong Fano scattering interference. We measure enhanced `circulation fidelity' above $97\%$, with optimised on-resonance insertion loss of $0.2$~dB, isolation of $18$~dB, and power reflectance of $-15$~dB, in good agreement with model calculations.

quant-ph↗

arXiv2023

How Real is Incomputability in Physics?

A physical system is determined by a finite set of initial conditions and "laws" represented by equations. The system is computable if we can solve the equations in all instances using a "finite body of mathematical knowledge". In this case, if the laws of the system can be coded into a computer program, then given the initial conditions of the system, one can compute the system's evolution. Are there incomputable physical systems? This question has been theoretically studied in the last 30-40 years. In this paper, we experimentally show for the first time the strong incomputability of a quantum experiment, namely the outputs of a quantum random number generator. Moreover, the experimental results are robust and statistically significant.

quant-ph↗

arXiv2023

Passive microwave circulation on a superconducting chip

Building large-scale superconducting quantum circuits will require miniaturisation and integration of supporting devices including microwave circulators, which are currently bulky, stand-alone components. Here we report the realisation of a passive on-chip circulator which is made from a loop consisting of three tunnel-coupled superconducting islands, with DC-only control fields. We observe the effect of quasiparticle tunnelling, and we dynamically classify the system into different quasiparticle sectors. When tuned for circulation, the device exhibits strongly non-reciprocal 3-port scattering, with average on-resonance insertion loss of 2 dB, isolation of 14 dB, power reflectance of -11 dB, and a bandwidth of 200 MHz.

quant-ph↗

arXiv2023

Multi-time quantum process tomography on a superconducting qubit

Current quantum technologies are at the cusp of becoming useful, but still face formidable obstacles such as noise. Noise severely limits the ability to scale quantum devices to the point that they would offer an advantage over classical devices. To understand the sources of noise it is necessary to fully characterise the quantum processes occurring across many time steps; only this would reveal any time-correlated noise called non-Markovian. Previous efforts have attempted such a characterisation but obtained only a limited reconstruction of such multi-time processes. In this work, we fully characterise a multi-time quantum process on superconducting hardware using in-house and cloud-based quantum processors. We achieve this by employing sequential measure-and-prepare operations combined with post-processing. Employing a recently developed formalism for multi-time processes, we detect general multi-time correlated noise. We also detect quantum correlated noise which demonstrates that part of the noise originates from quantum sources, such as physically nearby qubits on the chip.

quant-ph↗

arXiv2023

Advancements in Superconducting Microwave Cavities and Qubits for Quantum Information Systems

Superconducting microwave cavities with ultra-high Q-factors are revolutionizing the field of quantum computing, offering long coherence times exceeding 1 ms, which is critical for realizing scalable multi-qubit quantum systems with low error rates. In this work, we provide an in-depth analysis of recent advances in ultra-high Q-factor cavities, integration of Josephson junction-based qubits, and bosonic-encoded qubits in 3D cavities. We examine the sources of quantum state dephasing caused by damping and noise mechanisms in cavities and qubits, highlighting the critical challenges that need to be addressed to achieve even higher coherence times. We critically survey the latest progress made in implementing single 3D qubits using superconducting materials, normal metals, and multi-qubit and multi-state quantum systems. Our work sheds light on the promising future of this research area, including novel materials for cavities and qubits, modes with nontrivial topological properties, error correction techniques for bosonic qubits, and new light-matter interaction effects.

quant-ph↗

arXiv2023

Near-field localization of the boson peak on tantalum films for superconducting quantum devices

Superconducting circuits are among the most advanced quantum computing technologies, however their performance is currently limited by losses found in surface oxides and disordered materials. Here, we identify and spatially localize a near-field signature of loss centers on tantalum films using terahertz scattering-type scanning near-field optical microscopy (s-SNOM). Making use of terahertz nanospectroscopy, we observe a localized excess vibrational mode around 0.5 THz and identify this resonance as the boson peak, a signature of amorphous materials. Grazing-incidence wide-angle x-ray scattering (GIWAXS) shows that oxides on freshly solvent-cleaned samples are amorphous, whereas crystalline phases emerge after aging in air. By localizing defect centers at the nanoscale, our characterization techniques and results will inform the optimization of fabrication procedures for new low-loss superconducting circuits.

cond-mat.mes-hall↗

arXiv2022

Qubit-controlled directional edge states in waveguide QED

We propose an in-situ tunable chiral quantum system, composed of a quantum emitter coupled to a waveguide based on the Rice-Mele model (where we alternate both the on-site potentials and tunnel couplings between sites in the waveguide array). Specifically, we show that the chirality of photonic bound state, that emerges in the bandgap of the waveguide, depends only on the energy of the qubit; a parameter that is easy to tune in many artificial atoms. In contrast to previous proposals that have either shown imperfect chirality or fixed directionality, our waveguide QED scheme achieves both perfect chirality and the capability to switch the directionality on demand with just one tunable element in the device. We also show that our model is easy to implement in both state-of-the-art superconducting circuit and quantum dot architectures. The results show technological promise in creating long-range couplers between qubits while maintaining, in principle, zero crosstalk.

quant-ph↗

arXiv2022

Double Upconversion for Superconducting Qubit Control realised using Microstrip Filters

Superconducting qubits provide a promising platform for physically realising quantum computers at scale. Such devices require precision control at microwave frequencies. Common practice is to synthesise such control signals using IQ modulation, requiring calibration of a in-phase (I) and quadrature (Q) signals alongside two DC offsets to generate pure tones. This paper presents an economic physical implementation of an alternative method referred to as double upconversion which requires considerably less hardware calibration and physical resources to operate a qubit. A physical circuit was created using standard PCB design techniques for microstrip filters and two common RF mixers. This circuit was then utilised to successfully control a superconducting transmon qubit. When using proper RF shielding, qubit tones were demonstrated with over 70dB of spurious-free dynamic range across the entire operational spectrum of a transmon qubit.

quant-ph↗

arXiv2022

Passive superconducting circulator on a chip

An on-chip microwave circulator that is compatible with superconducting devices is a key element for scale-up of superconducting circuits. Previous approaches to integrating circulators on chip involve either external driving that requires extra microwave lines or a strong magnetic field that would compromise superconductivity. Here we report the first proof-of-principle realisation of a passive on-chip circulator which is made from a superconducting loop interrupted by three notionally-identical Josephson junctions and is tuned with only DC control fields. Our experimental results shows evidence for nonreciprocal scattering, and excellent agreement with theoretical simulations. We also present a detailed analysis of quasiparticle tunneling in our device using a hidden Markov model. By reducing the junction asymmetry and utilising the known methods of protection from quasiparticles, we anticipate that Josephson-loop circulator will become ubiquitous in superconducting circuits.

quant-ph↗

arXiv2022

Bound states in microwave QED: Crossover from waveguide to cavity regime

Light-matter interaction at the single-quantum level is the heart of many regimes of high fundamental importance to modern quantum technologies. Strong interaction of a qubit with a single photon of an electromagnetic field mode is described by the cavity/circuit electrodynamics (QED) regime which is one of the most advanced platforms for quantum computing. The opposite regime of the waveguide QED, where qubits interact with a continuum of modes in an infinite one-dimensional space, is also at the focus of recent research revealing novel quantum phenomena. Despite the demonstration of several key features of waveguide QED, the transition from an experimentally realizable finite-size system to the theoretically assumed infinite device size is neither rigorously justified nor fully understood. In this paper, we formulate a unifying theory which under a minimal set of standard approximations accounts for physical boundaries of a system in all parameter domains. Considering two qubits in a rectangular waveguide which naturally exhibits a low frequency cutoff we are able to account for infinite number of modes and obtain an accurate description of the waveguide transmission, a life-time of a qubit-photon bound state and the exchange interaction between two qubit-photon bounds states. For verification, we compare our theory to experimental data obtained for two superconducting qubits in a rectangular waveguide demonstrating how the infinite size limit of waveguide QED emerges in a finite-size system. Our theory can be straightforwardly extended to other waveguides such as the photonic crystal and coupled cavity arrays.

quant-ph↗

arXiv2022

Effect of Measurement Backaction on Quantum Clock Precision Studied with a Superconducting Circuit

We theoretically and experimentally study the precision of a quantum clock near zero temperature, explicitly accounting for the effect of continuous measurement. The clock is created by a superconducting transmon qubit dispersively coupled to an open coplanar resonator. The cavity and qubit are driven by coherent fields, and the cavity output is monitored with a quantum-noise-limited amplifier. When the continuous measurement is weak, it induces persistent coherent oscillations (with fluctuating periods) in the conditional moments of the qubit's energy probability distribution, which are manifest in the output of the resonator. On the other hand, strong continuous measurement leads to an incoherent cycle of quantum jumps. We theoretically find an equality for the precision of the clock in each regime. Independently from the equalities, we derive a kinetic uncertainty relation for the precision, and find that both equalities satisfy this uncertainty relation. Finally, we experimentally verify that our quantum clock obeys the kinetic uncertainty relation for the precision, thus making an explicit link between the (kinetic) thermodynamic behavior of the clock and its precision, and achieving an experimental test of a kinetic uncertainty relation in the quantum domain.

quant-ph↗