arXiv subjects

Vladimir E. Manucharyan

Publications and source records attributed to Vladimir E. Manucharyan.

At least 37 records · Page 2Linked to original sources

arXiv2022

Theory of strong down-conversion in multi-mode cavity and circuit QED

We revisit the superstrong coupling regime of multi-mode cavity quantum electrodynamics (QED), defined to occur when the frequency of vacuum Rabi oscillations between the qubit and the nearest cavity mode exceeds the cavity's free spectral range. A novel prediction is made that the cavity's linear spectrum, measured in the vanishing power limit, can acquire an intricate fine structure associated with the qubit-induced cascades of coherent single-photon down-conversion processes. This many-body effect is hard to capture by a brute-force numerics and it is sensitive to the light-matter coupling parameters both in the infra-red and the ultra-violet limits. We focused at the example case of a superconducting fluxonium qubit coupled to a long transmission line section. The conversion rate in such a circuit QED setup can readily exceed a few MHz, which is plenty to overcome the usual decoherence processes. Analytical calculations were made possible by an unconventional gauge choice, in which the qubit circuit interacts with radiation via the flux/charge variable in the low-/high-frequency limits, respectively. Our prediction of the fine spectral structure lays the foundation for the "strong down-conversion" regime in quantum optics, in which a single photon excited in a non-linear medium spontaneously down-converts faster than it is absorbed.

quant-ph↗

arXiv2022

Tuning the inductance of Josephson junction arrays without SQUIDs

It is customary to use arrays of superconducting quantum interference devices (SQUIDs) for implementing magnetic field-tunable inductors. Here, we demonstrate an equivalent tunability in a (SQUID-free) array of single Al/AlOx/Al Josephson tunnel junctions. With the proper choice of junction geometry, a perpendicularly applied magnetic field bends along the plane of the superconductor and focuses into the tunnel barrier region due to a demagnetization effect. Consequently, the Josephson inductance can be efficiently modulated by the Fraunhoffer-type supercurrent interference. The elimination of SQUIDs not only simplifies the device design and fabrication, but also facilitates a denser packing of junctions and, hence, a higher inductance per unit length. As an example, we demonstrate a transmission line, the wave impedance of which is field-tuned in the range of $4-8~\textrm{k}Ω$, centered around the important value of the resistance quantum $h/(2e)^2 \approx 6.5~\textrm{k}Ω$.

cond-mat.supr-con↗

arXiv2022

Fast Flux Entangling Gate for Fluxonium Circuits

We analyze a high-fidelity two-qubit gate using fast flux pulses on superconducting fluxonium qubits. The gate is realized by temporarily detuning magnetic flux through fluxonium loop away from the half flux quantum sweet spot. We simulate dynamics of two capacitively coupled fluxoniums during the flux pulses and optimize the pulse parameters to obtain a highly accurate $\sqrt{i\mathrm{swap}}$-like entangling gate. We also evaluate the effect of the flux noise and qubit relaxation on the gate fidelity. Our results demonstrate that the gate error remains below $10^{-4}$ for currently achievable magnitude of the flux noise and qubit relaxation time.

quant-ph↗

arXiv2022

CNOT gates for fluxonium qubits via selective darkening of transitions

We analyze the cross-resonance effect for fluxonium circuits and investigate a two-qubit gate scheme based on selective darkening of a transition. In this approach, two microwave pulses at the frequency of the target qubit are applied simultaneously with a proper ratio between their amplitudes to achieve a controlled-NOT operation. We study in detail coherent gate dynamics and calculate gate error. With nonunitary effects accounted for, we demonstrate that gate error below $10^{-4}$ is possible for realistic hardware parameters. This number is facilitated by long coherence times of computational transitions and strong anharmonicity of fluxoniums, which easily prevents excitation to higher excited states during the gate microwave drive.

quant-ph↗

arXiv2022

Demonstration of the Two-Fluxonium Cross-Resonance Gate

The superconducting fluxonium qubit has a great potential for high-fidelity quantum gates with its long coherence times and strong anharmonicity at the half flux quantum sweet spot. However, current implementations of two-qubit gates compromise fluxonium's coherence properties by requiring either a temporary population of the non-computational states or tuning the magnetic flux off the sweet spot. Here we realize a fast all-microwave cross-resonance gate between two capacitively-coupled fluxoniums with the qubit dynamics well confined to the computational space. We demonstrate a direct CNOT gate in 70 ns with fidelity up to $\mathcal{F}=0.9949(6)$ despite the limitations of a sub-optimal measurement setup and device coherence. Our results project a possible pathway towards reducing the two-qubit error rate below $10^{-4}$ with present-day technologies.

quant-ph↗

arXiv2022

Down-conversion of a single photon as a probe of many-body localization

Decay of a particle into more particles is a ubiquitous phenomenon to interacting quantum systems, taking place in colliders, nuclear reactors, or solids. In a non-linear medium, even a single photon would decay by down-converting (splitting) into lower frequency photons with the same total energy, at a rate given by Fermi's Golden Rule. However, the energy conservation condition cannot be matched precisely if the medium is finite and only supports quantized modes. In this case, the photon's fate becomes the long-standing question of many-body localization (MBL), originally formulated as a gedanken experiment for the lifetime of a single Fermi-liquid quasiparticle confined to a quantum dot. Here we implement such an experiment using a superconducting multi-mode cavity, the non-linearity of which was tailored to strongly violate the photon number conservation. The resulting interaction attempts to convert a single photon excitation into a shower of low-energy photons, but fails due to the MBL mechanism, which manifests as a striking spectral fine structure of multi-particle resonances at the cavity's standing wave mode frequencies. Each resonance was identified as a many-body state of radiation composed of photons from a broad frequency range, and not obeying the Fermi's Golden Rule theory. Our result introduces a new platform to explore fundamentals of MBL without having to control many atoms or qubits.

quant-ph↗

arXiv2021

Local Control of Supercurrent Density in Epitaxial Planar Josephson Junctions

The critical current response to an applied out-of-plane magnetic field in a Josephson junction provides insight into the uniformity of its current distribution. In Josephson junctions with semiconducting weak links, the carrier density and, therefore the overall current distribution could be modified electrostatically via metallic gates. Here, we show local control of the current distribution in an epitaxial Al-InAs Josephson junction equipped with five mini-gates. We demonstrate that not only can the junction width be electrostatically defined but also we can locally adjust the current profile to form superconducting quantum interference devices. Our studies show enhanced edge conduction in such long junctions, which can be eliminated by mini-gates to create a uniform current distribution.

cond-mat.mes-hall↗

arXiv2021

Proposal for entangling gates on fluxonium qubits via a two-photon transition

We propose a family of microwave-activated entangling gates on two capacitively coupled fluxonium qubits. A microwave pulse applied to either qubit at a frequency near the half-frequency of the $|00\rangle - |11\rangle$ transition induces two-photon Rabi oscillations with a negligible leakage outside the computational subspace, owing to the strong anharmonicity of fluxoniums. By adjusting the drive frequency, amplitude, and duration, we obtain the gate family that is locally equivalent to the fermionic-simulation gates such as $\sqrt{\rm SWAP}$-like and controlled-phase gates. The gate error can be tuned below $10^{-4}$ for a pulse duration under 100 ns without excessive circuit parameter matching. Given that the fluxonium coherence time can exceed 1 ms, our gate scheme is promising for large-scale quantum processors.

quant-ph↗

arXiv2021

Excitation dynamics in inductively coupled fluxonium circuits

We propose a near-term quantum simulator based on the fluxonium qubits inductively coupled to form a chain. This system provides long coherence time, large anharmonicity, and strong coupling, making it suitable to study Ising spin models. At the half-flux quantum sweet spot, the system is described by the transverse field Ising model (TFIM). We evaluate the propagation of qubit excitations through the system. As disorder increases, the excitations become localized. A single qubit measurement using the circuit QED methods is sufficient to identify localization transition without introducing tunable couplers. We argue that inductively coupled fluxoniums provide opportunities to study localization and many-body effects in highly coherent quantum systems.

quant-ph↗

arXiv2021

Photon-instanton collider implemented by a superconducting circuit

Instantons, spacetime-localized quantum field tunneling events, are ubiquitous in correlated condensed matter and high energy systems. However, their direct observation through collisions with conventional particles has not been considered possible. We show how recent advances in circuit quantum electrodynamics, specifically, the realization of galvanic coupling of a transmon qubit to a high-impedance transmission line, allows the observation of inelastic collisions of single microwave photons with instantons (phase slips). We develop a formalism for calculating the photon-instanton cross section, which should be useful in other quantum field theoretical contexts. In particular, we show that the inelastic scattering probability can significantly exceed the effect of conventional Josephson quartic anharmonicity, and reach order-unity values.

cond-mat.supr-con↗

arXiv2021

Millisecond coherence in a superconducting qubit

Increasing the degree of control over physical qubits is a crucial component of quantum computing research. We report a superconducting qubit of fluxonium type with the Ramsey coherence time reaching $T_2^*= 1.48 \pm 0.13 \mathrm{~ms}$, which exceeds the state of the art value by an order of magnitude. As a result, the average single-qubit gate fidelity grew above $0.9999$, surpassing, to our knowledge, any other solid-state quantum system. Furthermore, by measuring energy relaxation of the parity-forbidden transition to second excited state, we exclude the effect of out-of-equilibrium quasiparticles on coherence in our circuit. Combined with recent demonstrations of two-qubit gates on fluxoniums, our result paves the way for the next generation of quantum processors.

quant-ph↗

arXiv2021

Arbitrary controlled-phase gate on fluxonium qubits using differential ac-Stark shifts

Large scale quantum computing motivates the invention of two-qubit gate schemes that not only maximize the gate fidelity but also draw minimal resources. In the case of superconducting qubits, the weak anharmonicity of transmons imposes profound constraints on the gate design, leading to increased complexity of devices and control protocols. Here we demonstrate a resource-efficient control over the interaction of strongly-anharmonic fluxonium qubits. Namely, applying an off-resonant drive to non-computational transitions in a pair of capacitively-coupled fluxoniums induces a $\textrm{ZZ}$-interaction due to unequal ac-Stark shifts of the computational levels. With a continuous choice of frequency and amplitude, the drive can either cancel the static $\textrm{ZZ}$-term or increase it by an order of magnitude to enable a controlled-phase (CP) gate with an arbitrary programmed phase shift. The cross-entropy benchmarking of these non-Clifford operations yields a sub $1\%$ error, limited solely by incoherent processes. Our result demonstrates the advantages of strongly-anharmonic circuits over transmons in designing the next generation of quantum processors.

quant-ph↗

arXiv2021

Superstrong coupling in circuit quantum electrodynamics

Vacuum fluctuations fundamentally affect an atom by inducing a finite excited state lifetime along with a Lamb shift of its transition frequency. Here we report the reverse effect: modification of vacuum modes by a single atom in circuit quantum electrodynamics. Our one-dimensional vacuum is a long section of a high wave impedance (comparable to resistance quantum) superconducting transmission line. It is directly wired to a transmon qubit circuit. Owing to the combination of high impedance and galvanic connection, the transmon's spontaneous emission linewidth can greatly exceed the discrete transmission line modes spacing. This condition defines a previously unexplored "superstrong" coupling regime of quantum electrodynamics where many frequency-resolved vacuum modes hybridize with a single atom. We establish this regime by observing the spontaneous emission line of the transmon, revealed through the mode-by-mode measurement of the vacuum's density of states. The linewidth as well as the atom-induced dispersive photon-photon interaction are accurately described by a physically transparent Caldeira-Leggett model, with the transmon's quartic non-linearity treated as a perturbation. Non-perturbative modification of vacuum, including inelastic scattering of single photons, can be enabled by the superstrong coupling regime upon replacing the transmon by more anharmonic qubits, with broad implications for simulating quantum impurity models of many-body physics.

cond-mat.supr-con↗

arXiv2021

The superconducting quasicharge qubit

The non-dissipative non-linearity of a Josephson junction converts macroscopic superconducting circuits into artificial atoms, enabling some of the best controlled quantum bits (qubits) today. Three fundamental types of superconducting qubits are known, each reflecting a distinct behavior of quantum fluctuations in a Cooper pair condensate: single charge tunneling (charge qubit), single flux tunneling (flux qubit), and phase oscillations (phase qubit). Yet, the dual nature of charge and flux suggests that circuit atoms must come in pairs. Here we introduce the missing one, named "blochnium". It exploits a coherent insulating response of a single Josephson junction that emerges from the extension of phase fluctuations beyond the $2π$-interval. Evidence for such effect was found in an out-of-equilibrium dc-transport through junctions connected to high-impedance leads, although a full consensus is absent to date. We shunt a weak junction with an exceptionally high-value inductance -- the key technological innovation behind our experiment -- and measure the rf-excitation spectrum as a function of external magnetic flux through the resulting loop. The junction's insulating character manifests by the vanishing flux-sensitivity of the qubit transition between the ground and the first excited states, which nevertheless rapidly recovers for transitions to higher energy states. The spectrum agrees with a duality mapping of blochnium onto transmon, which replaces the external flux by the offset charge and introduces a new collective quasicharge variable in place of the superconducting phase. Our result unlocks the door to an unexplored regime of macroscopic quantum dynamics in ultrahigh-impedance circuits, which may have applications to quantum computing and quantum metrology of direct current.

cond-mat.supr-con↗

arXiv2021

Inelastic scattering of a photon by a quantum phase-slip

Spontaneous decay of a single photon is a notoriously inefficient process in nature irrespective of the frequency range. We report that a quantum phase-slip fluctuation in high-impedance superconducting waveguides can split a single incident microwave photon into a large number of lower-energy photons with a near unit probability. The underlying inelastic photon-photon interaction has no analogs in non-linear optics. Instead, the measured decay rates are explained without adjustable parameters in the framework of a new model of a quantum impurity in a Luttinger liquid. Our result connects circuit quantum electrodynamics to critical phenomena in two-dimensional boundary quantum field theories, important in the physics of strongly-correlated systems. The photon lifetime data represents a rare example of verified and useful quantum many-body simulation.

cond-mat.supr-con↗

arXiv2020

Fast logic with slow qubits: microwave-activated controlled-Z gate on low-frequency fluxoniums

We demonstrate a controlled-Z gate between capacitively coupled fluxonium qubits with transition frequencies $72.3~\textrm{MHz}$ and $136.3~\textrm{MHz}$. The gate is activated by a $61.6~\textrm{ns}$ long pulse at the frequency between non-computational transitions $|10\rangle - |20\rangle$ and $|11\rangle - |21\rangle$, during which the qubits complete only $4$ and $8$ Larmor periods, respectively. The measured gate error of $(8\pm1)\times 10^{-3}$ is limited by decoherence in the non-computational subspace, which will likely improve in the next generation devices. Although our qubits are about fifty times slower than transmons, the two-qubit gate is faster than microwave-activated gates on transmons, and the gate error is on par with the lowest reported. Architectural advantages of low-frequency fluxoniums include long qubit coherence time, weak hybridization in the computational subspace, suppressed residual $ZZ$-coupling rate (here $46~\mathrm{kHz}$), and absence of either excessive parameter matching or complex pulse shaping requirements.

quant-ph↗

arXiv2020

Electron shelving of a superconducting artificial atom

Interfacing stationary qubits with propagating photons is a fundamental problem in quantum technology. Cavity quantum electrodynamics (CQED) invokes a mediator degree of freedom in the form of a far-detuned cavity mode, the adaptation of which to superconducting circuits (cQED) proved remarkably fruitful. The cavity both blocks the qubit emission and it enables a dispersive readout of the qubit state. Yet, a more direct (cavityless) interface is possible with atomic clocks, in which an orbital cycling transition can scatter photons depending on the state of a hyperfine or quadrupole qubit transition. Originally termed "electron shelving", such a conditional fluorescence phenomenon is the cornerstone of many quantum information platforms, including trapped ions, solid state defects, and semiconductor quantum dots. Here we apply the shelving idea to circuit atoms and demonstrate a conditional fluorescence readout of fluxonium qubit placed inside a matched one-dimensional waveguide. Cycling the non-computational transition between ground and third excited states produces a microwave photon every 91 ns conditioned on the qubit ground state, while the qubit coherence time exceeds 50 us. The readout has a built-in quantum non-demolition property, allowing over 100 fluorescence cycles in agreement with a four-level optical pumping model. Our result introduces a resource-efficient alternative to cQED. It also adds a state-of-the-art quantum memory to the growing toolbox of waveguide QED.

cond-mat.mes-hall↗

arXiv2019

Quantum electrodynamics of a superconductor-insulator phase transition

A chain of Josephson junctions implements one of the simplest many-body models undergoing a superconductor-insulator (SI) quantum phase transition between states with zero and infinite resistance. Apart from zero resistance, the superconducting state is necessarily accompanied by a sound-like mode due to collective oscillations of the phase of the complex-valued order parameter. Exciting this phase mode results in transverse photons propagating along the chain. Surprisingly little is known about the fate of this mode upon entering the insulating state, where the order parameter's amplitude remains non-zero, but the phase ordering is "melted" by quantum fluctuations. Here we report momentum-resolved radio-frequency spectroscopy of collective modes in nanofabricated chains of Al/AlOx/Al tunnel junctions. We find that the phase mode survives remarkably far into the insulating regime, such that $\textrm{M}Ω$-resistance chains carry $\textrm{GHz}$-frequency alternating currents as nearly ideal superconductors. The insulator reveals itself through broadening and random frequency shifts of collective mode resonances, originated from intrinsic interactions. By pushing the chain parameters deeper into the insulating state, we achieved propagation with the speed of light down to $8\times 10^5~\textrm{m/s}$ and the wave impedance up to $23~\textrm{k}Ω$. The latter quantity exceeds the predicted critical impedance by an order of magnitude, which opens the problem of quantum electrodynamics of a Bose glass insulator for both theory and experiment. Notably, the effective fine structure constant of such a 1D vacuum exceeds a unity, promising transformative applications to quantum science and technology.

cond-mat.supr-con↗