arXiv subjects

V. N. Smelyanskiy

Publications and source records attributed to V. N. Smelyanskiy.

At least 19 recordsLinked to original sources

arXiv2022

Learning Noise via Dynamical Decoupling of Entangled Qubits

Noise in entangled quantum systems is difficult to characterize due to many-body effects involving multiple degrees of freedom. This noise poses a challenge to quantum computing, where two-qubit gate performance is critical. Here, we develop and apply multi-qubit dynamical decoupling sequences that characterize noise that occurs during two-qubit gates. In our superconducting system comprised of Transmon qubits with tunable couplers, we observe noise that is consistent with flux fluctuations in the coupler that simultaneously affects both qubits and induces noise in their entangling parameter. The effect of this noise on the qubits is very different from the well-studied single-qubit dephasing. Additionally, steps are observed in the decoupled signals, implying the presence of non-Gaussian noise.

quant-ph↗

arXiv2021

Removing leakage-induced correlated errors in superconducting quantum error correction

Quantum computing can become scalable through error correction, but logical error rates only decrease with system size when physical errors are sufficiently uncorrelated. During computation, unused high energy levels of the qubits can become excited, creating leakage states that are long-lived and mobile. Particularly for superconducting transmon qubits, this leakage opens a path to errors that are correlated in space and time. Here, we report a reset protocol that returns a qubit to the ground state from all relevant higher level states. We test its performance with the bit-flip stabilizer code, a simplified version of the surface code for quantum error correction. We investigate the accumulation and dynamics of leakage during error correction. Using this protocol, we find lower rates of logical errors and an improved scaling and stability of error suppression with increasing qubit number. This demonstration provides a key step on the path towards scalable quantum computing.

quant-ph↗

arXiv2020

Probing Environmental Spin Polarization with Superconducting Flux Qubits

We present measurements of the dynamics of a polarized magnetic environment coupled to the flux degree of freedom of rf-SQUID flux qubits. The qubits are used as both sources of polarizing field and detectors of the environmental polarization. We probe dynamics at timescales from 5 $μ$s to 5 ms and at temperatures between 12.5 and 22 mK. The measured polarization versus temperature provides strong evidence for a phase transition at a temperature of $5.7\pm 0.3$ mK. Furthermore, the environmental polarization grows initially as $\sqrt{t}$, consistent with spin diffusion dynamics. However, spin diffusion model deviates from data at long timescales, suggesting that a different phenomenon is responsible for the low-frequency behavior. A simple $1/f$ model can fit the data at all time scales but it requires empirical low- and high-frequency cutoffs. We argue that these results are consistent with an environment comprised of random clusters of spins, with fast spin diffusion dynamics within the clusters and slow fluctuations of the total moments of the clusters.

quant-ph↗

arXiv2019

Diabatic gates for frequency-tunable superconducting qubits

We demonstrate diabatic two-qubit gates with Pauli error rates down to $4.3(2)\cdot 10^{-3}$ in as fast as 18 ns using frequency-tunable superconducting qubits. This is achieved by synchronizing the entangling parameters with minima in the leakage channel. The synchronization shows a landscape in gate parameter space that agrees with model predictions and facilitates robust tune-up. We test both iSWAP-like and CPHASE gates with cross-entropy benchmarking. The presented approach can be extended to multibody operations as well.

quant-ph↗

arXiv2018

Fluctuations of Energy-Relaxation Times in Superconducting Qubits

Superconducting qubits are an attractive platform for quantum computing since they have demonstrated high-fidelity quantum gates and extensibility to modest system sizes. Nonetheless, an outstanding challenge is stabilizing their energy-relaxation times, which can fluctuate unpredictably in frequency and time. Here, we use qubits as spectral and temporal probes of individual two-level-system defects to provide direct evidence that they are responsible for the largest fluctuations. This research lays the foundation for stabilizing qubit performance through calibration, design, and fabrication.

quant-ph↗

arXiv2016

Observation of classical-quantum crossover of 1/f flux noise and its paramagnetic temperature dependence

By analyzing the dissipative dynamics of a tunable gap flux qubit, we extract both sides of its two-sided environmental flux noise spectral density over a range of frequencies around $2k_BT/h \approx 1\,\rm{GHz}$, allowing for the observation of a classical-quantum crossover. Below the crossover point, the symmetric noise component follows a $1/f$ power law that matches the magnitude of the $1/f$ noise near $1\,{\rm{Hz}}$. The antisymmetric component displays a 1/T dependence below $100\,\rm{mK}$, providing dynamical evidence for a paramagnetic environment. Extrapolating the two-sided spectrum predicts the linewidth and reorganization energy of incoherent resonant tunneling between flux qubit wells.

quant-ph↗

arXiv2014

Donor Spin Qubits in Ge-based Phononic Crystals

We propose qubits based on shallow donor electron spins in germanium. Spin-orbit interaction for donor spins in germanium is in many orders of magnitude stronger than in silicon. In a uniform bulk material it leads to very short spin lifetimes. However the lifetime increases dramatically when the donor is placed into a quasi-2D phononic crystal and the energy of the Zeeman splitting is tuned to lie within a phonon bandgap. In this situation single phonon processes are suppressed by energy conservation. The remaining two-phonon decay channel is very slow. The Zeeman splitting within the gap can be fine tuned to induce a strong, long-range coupling between the spins of remote donors via exchange by virtual phonons. This, in turn, opens a very efficient way to manipulate the quits. We explore various geometries of phononic crystals in order to maximize the coherent qubit-qubit coupling while keeping the decay rate minimal. We find that phononic crystals with unit cell sizes of 100-150 nm are viable candidates for quantum computing applications and suggest several spin-resonance experiments to verify our theoretical predictions.

cond-mat.mes-hall↗

arXiv2012

Large Stark Effect for Li Donor Spins in Si

We study the effect of a static electric field on lithium donor spins in silicon. The anisotropy of the effective mass leads to the anisotropy of the quadratic Stark susceptibility, which we determined using the Dalgarno-Lewis exact summation method. The theory is asymptotically exact in the field domain below Li-donor ionization threshold, relevant to the Stark-tuning electron spin resonance experiments. To obtain the generalized Stark susceptibilities at arbitrary fields, we propose a new variational wave function which reproduces the exact results in the low-field limit. With the calculated susceptibilities at hand, we are able to predict and analyze several important physical effects. First, we observe that the energy level shifts due to the quadratic Stark effect for Li donors in Si are equivalent to, and can be mapped onto, those produced by an external stress. Second, we demonstrate that the Stark effect anisotropy, combined with the unique valley-orbit splitting of a Li donor in Si, spin-orbit interaction and specially tuned external stress, may lead to a very strong modulation of the donor spin $g$-factor by the electric field. Third, we investigate the influence of random strains on the $g$-factor shifts and quantify the random strain limits and requirements to Si material purity necessary to observe the $g$-factor Stark shifts experimentally. Finally, we discuss possible implications of our results for quantum information processing with Li spin qubits in Si.

cond-mat.mes-hall↗

arXiv2011

Cavitation-induced ignition of cryogenic hydrogen-oxygen fluids

The Challenger disaster and purposeful experiments with liquid hydrogen (H2) and oxygen (Ox) tanks demonstrated that cryogenic H2/Ox fluids always self-ignite in the process of their mixing. Here we propose a cavitation-induced self-ignition mechanism that may be realized under these conditions. In one possible scenario, self-ignition is caused by the strong shock waves generated by the collapse of pure Ox vapor bubble near the surface of the Ox liquid that may initiate detonation of the gaseous H2/Ox mixture adjacent to the gas-liquid interface. This effect is further enhanced by H2/Ox combustion inside the collapsing bubble in the presence of admixed H2 gas.

physics.flu-dyn↗

arXiv2010

First order phase transition in the Quantum Adiabatic Algorithm

We simulate the quantum adiabatic algorithm (QAA) for the exact cover problem for sizes up to N=256 using quantum Monte Carlo simulations incorporating parallel tempering. At large N we find that some instances have a discontinuous (first order) quantum phase transition during the evolution of the QAA. This fraction increases with increasing N and may tend to 1 for N -> infinity.

cond-mat.stat-mech↗

arXiv2008

Size dependence of the minimum excitation gap in the Quantum Adiabatic Algorithm

We study the typical (median) value of the minimum gap in the quantum version of the Exact Cover problem using Quantum Monte Carlo simulations, in order to understand the complexity of the quantum adiabatic algorithm (QAA) for much larger sizes than before. For a range of sizes, N <= 128, where the classical Davis-Putnam algorithm shows exponential median complexity, the QAA shows polynomial median complexity. The bottleneck of the algorithm is an isolated avoided crossing point of a Landau-Zener type (collision between the two lowest energy levels only).

cond-mat.dis-nn↗

arXiv2008

Chiral Symmetry and Electron Spin Relaxation of Lithium Donors in Silicon

We report theoretical and experimental studies of the longitudinal electron spin and orbital relaxation time of interstitial Li donors in $^{28}$Si. We predict that despite the near-degeneracy of the ground-state manifold the spin relaxation times are extremely long for the temperatures below 0.3 K. This prediction is based on a new finding of the chiral symmetry of the donor states, which presists in the presence of random strains and magnetic fields parallel to one of the cubic axes. Experimentally observed kinetics of magnetization reversal at 2.1 K and 4.5 K are in a very close agreement with the theory. To explain these kinetics we introduced a new mechanism of spin decoherence based on a combination of a small off-site displacement of the Li atom and an umklapp phonon process. Both these factors weakly break chiral symmetry and enable the long-term spin relaxation.

cond-mat.other↗

arXiv2008

Model based IVHM system for the solid rocket booster

We report progress in the development of a model-based hybrid probabilistic approach to an on-board IVHM for solid rocket boosters (SRBs) that can accommodate the abrupt changes of the model parameters in various nonlinear dynamical off-nominal regimes. The work is related to the ORION mission program. Specifically, a case breach fault for SRBs is considered that takes into account burning a hole through the rocket case, as well as ablation of the nozzle throat under the action of hot gas flow. A high-fidelity model (HFM) of the fault is developed in FLUENT in cylindrical symmetry. The results of the FLUENT simulations are shown to be in good agreement with quasi-stationary approximation and analytical solution of a system of one-dimensional partial differential equations (PDEs) for the gas flow in the combustion chamber and in the hole through the rocket case.

physics.data-an↗

arXiv2008

Statistical Mechanics of the Quantum K-Satisfiability problem

We study the quantum version of the random $K$-Satisfiability problem in the presence of the external magnetic field $Γ$ applied in the transverse direction. We derive the replica-symmetric free energy functional within static approximation and the saddle-point equation for the order parameter: the distribution $P[h(m)]$ of functions of magnetizations. The order parameter is interpreted as the histogram of probability distributions of individual magnetizations. In the limit of zero temperature and small transverse fields, to leading order in $Γ$ magnetizations $m \approx 0$ become relevant in addition to purely classical values of $m \approx \pm 1$. Self-consistency equations for the order parameter are solved numerically using Quasi Monte Carlo method for K=3. It is shown that for an arbitrarily small $Γ$ quantum fluctuations destroy the phase transition present in the classical limit $Γ=0$, replacing it with a smooth crossover transition. The implications of this result with respect to the expected performance of quantum optimization algorithms via adiabatic evolution are discussed. The replica-symmetric solution of the classical random $K$-Satisfiability problem is briefly revisited. It is shown that the phase transition at T=0 predicted by the replica-symmetric theory is of continuous type with atypical critical exponents.

cond-mat.dis-nn↗

arXiv2008

Distribution of Fluctuational Paths in Noise-Driven Systems

Dynamics of a system that performs a large fluctuation to a given state is essentially deterministic: the distribution of fluctuational paths peaks sharply at a certain optimal path along which the system is most likely to move. For the general case of a system driven by colored Gaussian noise, we provide a formulation of the variational problem for optimal paths. We also consider the prehistory problem, which makes it possible to analyze the shape of the distribution of fluctuational paths that arrive at a given state. We obtain, and solve in the limiting case, a set of linear equations for the characteristic width of this distribution.

cond-mat.stat-mech↗

arXiv2006

Complete spin polarization of degenerate electrons in semiconductors near ferromagnetic contacts

We show that spin polarization of electron density in nonmagnetic degenerate semiconductors can achieve 100%. This effect is realized in ferromagnet-semiconductor $FM-n^{+}$-$n$ junctions even at moderate spin selectivity of the $FM-n^{+}$ contact when the electrons are extracted from the heavily doped $n^{+}-$semiconductor into the ferromagnet. We derived a general equation relating spin polarization of the current to that of the electron density in nonmagnetic semiconductors. We found that the effect of the complete spin polarization is achieved near $n^{+}$-$n$ interface when an effective diffusion coefficient goes to zero in this region while the diffusion current remains finite.

cond-mat.other↗

arXiv2006

Quantum Adiabatic Evolution Algorithm and Quantum Phase Transition in 3-Satisfiability Problem

In this paper we show that the performance of the quantum adiabatic algorithm is determined by phase transitions in underlying problem in the presence of transverse magnetic field $Γ$. We show that the quantum version of random Satisfiability problem with 3 bits in a clause (3-SAT) has a first-order quantum phase transition. We analyze the phase diagram $γ=γ(Γ)$ where $γ$ is an average number of clauses per binary variable in 3-SAT. The results are obtained in a closed form assuming replica symmetry and neglecting time correlations at small values of the transverse field $Γ$. In the limit of $Γ=0$ the value of $γ(0)\approx$ 5.18 corresponds to that given by the replica symmetric treatment of a classical random 3-SAT problem. We demonstrate the qualitative similarity between classical and quantum versions of this problem.

cond-mat.dis-nn↗

arXiv2006

Electronic Control and Readout of Qubit States in Solid State Quantum Computing Systems

We demonstrate that an $n^+/i/n^+$ junction is the most suitable candidate for electronic control and readout of qubit states in quantum computing systems based on shallow impurities. The signature of this system is that the $n^+-$regions serve as metallic electrodes separated form the $i-$region by a self-induced barrier (internal workfunction). The $n^+/i/n^+$ system mimics the properties of a metal-vacuum-metal junction with the qubit (impurity atom) placed in a ``vacuum'' $i$-region between two ``metallic'' $n^+$ electrodes. We will show that the self-induced barrier exists in a sufficiently wide range of the concentration of dopants in the $n^+$-semiconductor (e.g. up to $10^{21}$ cm$^{-3}$ for Si) and its height can be controlled by tuning the doping level. A shallow donor placed in a vacuum $i$-region will be populated with one electron in equilibrium. In the case of Li donor in Si the $n^+$-electrodes will be used for a precision placement of the Li atom during the growth process; for voltage control and manipulation of the qubit states; and for a qubit readout by means of the optically stimulated resonant tunnelling. Another important feature of our system is that the qubit states (first two lowest energy levels of Li in Si) are separated by an energy gap from a continuum of the many-body states of the controlling electrodes.

cond-mat.other↗