arXiv subjects

Markus Brink

Publications and source records attributed to Markus Brink.

17 recordsLinked to original sources

arXiv2022

Weakly Flux-Tunable Superconducting Qubit

Flux-tunable qubits are a useful resource for superconducting quantum processors. They can be used to perform cPhase gates, facilitate fast reset protocols, avoid qubit-frequency collisions in large processors, and enable certain fast readout schemes. However, flux-tunable qubits suffer from a trade-off between their tunability range and sensitivity to flux noise. Optimizing this trade-off is particularly important for enabling fast, high-fidelity, all-microwave cross-resonance gates in large, high-coherence processors. This is mainly because cross-resonance gates set stringent conditions on the frequency landscape of neighboring qubits, which are difficult to satisfy with non-tunable transmons due to their relatively large fabrication imprecision. To solve this problem, we realize a coherent, flux-tunable, transmon-like qubit, which exhibits a frequency tunability range as small as 43 MHz, and whose frequency, anharmonicity and tunability range are set by a few experimentally achievable design parameters. Such a weakly tunable qubit is useful for avoiding frequency collisions in a large lattice while limiting its susceptibility to flux noise.

quant-ph↗

arXiv2021

High-fidelity qubit readout using interferometric directional Josephson devices

Nonreciprocal microwave devices, such as circulators and isolators, are needed in high-fidelity qubit readout schemes to unidirectionally route the readout signals and protect the qubits against noise coming from the output chain. However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magnetic materials. Here, we perform a fast (750 ns) high-fidelity (95%) quantum nondemolition readout of a coherent superconducting qubit ($T_{1}=52$ $μs$, $T_{\rm{2E}}=35$ $μs$) without any nonreciprocal magnetic devices. We employ in our readout chain a microwave-controlled qubit-Readout Multi-Chip Module (qRMCM) that integrates interferometric directional Josephson devices consisting of an isolator and a reconfigurable isolator/amplifier device and an off-chip low-pass filter. Using the qRMCM, we demonstrate isolation up to 45 dB within 13 MHz, when both directional devices are operated as isolators, and low-noise amplification in excess of 10 dB within a dynamical bandwidth of $10$ MHz, when the reconfigurable device is operated as an amplifier. We also demonstrate using the variable isolation of the qRMCM an in-situ enhancement of the qubit coherence times $T_{\rmφ}$ and $T_{\rm{2E}}$ by two orders of magnitude (i.e., from $T_{\rmφ}=T_{\rm{2E}}=0.5$ $μs$ to $T_{\rmφ}=90$ $μs$ and $T_{\rm{2E}}=50$ $μs$). Furthermore, by directly comparing the qRMCM performance to a state-of-art configuration (with $T_{\rm{2E}}\approx 2T_{1}$) that employs a pair of wideband magnetic isolators, we find that the excess pure dephasing measured with the qRMCM (for which $T_{\rm{2E}}\approx T_{1}$) is likely limited by residual thermal photon population in the readout resonator. Improved versions of the qRMCM could replace magnetic circulators and isolators in large superconducting quantum processors.

quant-ph↗

arXiv2021

Investigating microwave loss of SiGe using superconducting transmon qubits

Silicon-Germanium (SiGe) is a material that possesses a multitude of applications ranging from transistors to eletro-optical modulators and quantum dots. The diverse properties of SiGe also make it attractive to implementations involving superconducting quantum computing. Here we demonstrate the fabrication of transmon quantum bits on SiGe layers and investigate the microwave loss properties of SiGe at cryogenic temperatures and single photon microwave powers. We find relaxation times of up to 100 $μ$s, corresponding to a quality factor Q above 4 M for large pad transmons. The high Q values obtained indicate that the SiGe/Si heterostructure is compatible with state of the art performance of superconducting quantum circuits.

cond-mat.mes-hall↗

arXiv2020

High-fidelity superconducting quantum processors via laser-annealing of transmon qubits

Scaling the number of qubits while maintaining high-fidelity quantum gates remains a key challenge for quantum computing. Presently, superconducting quantum processors with >50-qubits are actively available. For such systems, fixed-frequency transmons are attractive due to their long coherence and noise immunity. However, scaling fixed-frequency architectures proves challenging due to precise relative frequency requirements. Here we employ laser annealing to selectively tune transmon qubits into desired frequency patterns. Statistics over hundreds of annealed qubits demonstrate an empirical tuning precision of 18.5 MHz, with no measurable impact on qubit coherence. We quantify gate error statistics on a tuned 65-qubit processor, with median two-qubit gate fidelity of 98.7%. Baseline tuning statistics yield a frequency-equivalent resistance precision of 4.7 MHz, sufficient for high-yield scaling beyond 1000-qubit levels. Moving forward, we anticipate selective laser annealing to play a central role in scaling fixed-frequency architectures.

quant-ph↗

arXiv2020

Laser-annealing Josephson junctions for yielding scaled-up superconducting quantum processors

As superconducting quantum circuits scale to larger sizes, the problem of frequency crowding proves a formidable task. Here we present a solution for this problem in fixed-frequency qubit architectures. By systematically adjusting qubit frequencies post-fabrication, we show a nearly ten-fold improvement in the precision of setting qubit frequencies. To assess scalability, we identify the types of 'frequency collisions' that will impair a transmon qubit and cross-resonance gate architecture. Using statistical modeling, we compute the probability of evading all such conditions, as a function of qubit frequency precision. We find that without post-fabrication tuning, the probability of finding a workable lattice quickly approaches 0. However with the demonstrated precisions it is possible to find collision-free lattices with favorable yield. These techniques and models are currently employed in available quantum systems and will be indispensable as systems continue to scale to larger sizes.

quant-ph↗

arXiv2020

Demonstration of quantum volume 64 on a superconducting quantum computing system

We improve the quality of quantum circuits on superconducting quantum computing systems, as measured by the quantum volume, with a combination of dynamical decoupling, compiler optimizations, shorter two-qubit gates, and excited state promoted readout. This result shows that the path to larger quantum volume systems requires the simultaneous increase of coherence, control gate fidelities, measurement fidelities, and smarter software which takes into account hardware details, thereby demonstrating the need to continue to co-design the software and hardware stack for the foreseeable future.

quant-ph↗

arXiv2020

Suppression of Unwanted $ZZ$ Interactions in a Hybrid Two-Qubit System

Mitigating crosstalk errors, whether classical or quantum mechanical, is critically important for achieving high-fidelity entangling gates in multi-qubit circuits. For weakly anharmonic superconducting qubits, unwanted $ZZ$ interactions can be suppressed by combining qubits with opposite anharmonicity. We present experimental measurements and theoretical modeling of two-qubit gate error for gates based on the cross resonance interaction between a capacitively shunted flux qubit and a transmon and demonstrate the elimination of the $ZZ$ interaction.

quant-ph↗

arXiv2019

Real-time decoding of stabilizer measurements in a bit-flip code

Although qubit coherence times and gate fidelities are continuously improving, logical encoding is essential to achieve fault tolerance in quantum computing. In most encoding schemes, correcting or tracking errors throughout the computation is necessary to implement a universal gate set without adding significant delays in the processor. Here we realize a classical control architecture for the fast extraction of errors based on multiple cycles of stabilizer measurements and subsequent correction. We demonstrate its application on a minimal bit-flip code with five transmon qubits, showing that real-time decoding and correction based on multiple stabilizers is superior in both speed and fidelity to repeated correction based on individual cycles. Furthermore, the encoded qubit can be rapidly measured, thus enabling conditional operations that rely on feed-forward, such as logical gates. This co-processing of classical and quantum information will be crucial in running a logical circuit at its full speed to outpace error accumulation.

quant-ph↗

arXiv2018

Active protection of a superconducting qubit with an interferometric Josephson isolator

Nonreciprocal microwave devices play several critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They separate input from output, impose unidirectional routing of readout signals, and protect the quantum systems from unwanted noise originated by the output chain. However, state-of-the-art, cryogenic circulators and isolators are disadvantageous in scalable superconducting quantum processors because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the Josephson mixers, which play the role of the magnetic field in a Faraday medium. To demonstrate the applicability of this Josephson-based isolator for quantum measurements, we incorporate it into the output line of a superconducting qubit, coupled to a fast resonator and a Purcell filter. We also utilize a wideband, superconducting directional coupler for coupling the readout signals into and out of the qubit-resonator system and a quantum-limited Josephson amplifier for boosting the readout fidelity. By using this novel quantum setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing more than 20 dB of protection against amplified noise reflected off the Josephson amplifier.

quant-ph↗

arXiv2017

Hardware-efficient Variational Quantum Eigensolver for Small Molecules and Quantum Magnets

Quantum computers can be used to address molecular structure, materials science and condensed matter physics problems, which currently stretch the limits of existing high-performance computing resources. Finding exact numerical solutions to these interacting fermion problems has exponential cost, while Monte Carlo methods are plagued by the fermionic sign problem. These limitations of classical computational methods have made even few-atom molecular structures problems of practical interest for medium-sized quantum computers. Yet, thus far experimental implementations have been restricted to molecules involving only Period I elements. Here, we demonstrate the experimental optimization of up to six-qubit Hamiltonian problems with over a hundred Pauli terms, determining the ground state energy for molecules of increasing size, up to BeH2. This is enabled by a hardware-efficient variational quantum eigensolver with trial states specifically tailored to the available interactions in our quantum processor, combined with a compact encoding of fermionic Hamiltonians and a robust stochastic optimization routine. We further demonstrate the flexibility of our approach by applying the technique to a problem of quantum magnetism. Across all studied problems, we find agreement between experiment and numerical simulations with a noisy model of the device. These results help elucidate the requirements for scaling the method to larger systems, and aim at bridging the gap between problems at the forefront of high-performance computing and their implementation on quantum hardware.

quant-ph↗

arXiv2017

Multi-Path Interferometric Josephson Directional Amplifier for Qubit Readout

We realize and characterize a quantum-limited, directional Josephson amplifier suitable for qubit readout. The device consists of two nondegenerate, three-wave-mixing amplifiers that are coupled together in an interferometric scheme, embedded in a printed circuit board. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the device, which plays the role of the magnetic field in a Faraday medium. Directional amplification and reflection-gain elimination are induced via wave interference between multiple paths in the system. We measure and discuss the main figures of merit of the device and show that the experimental results are in good agreement with theory. An improved version of this directional amplifier is expected to eliminate the need for bulky, off-chip isolation stages that generally separate quantum systems and preamplifiers in high-fidelity, quantum-nondemolition measurement setups.

physics.ins-det↗

arXiv2017

Gyrator operation using Josephson mixers

Nonreciprocal microwave devices, such as circulators, are useful in routing quantum signals in quantum networks and protecting quantum systems against noise coming from the detection chain. However, commercial, cryogenic circulators, now in use, are unsuitable for scalable superconducting quantum architectures due to their appreciable size, loss, and inherent magnetic field. We report on the measurement of a key nonreciprocal element, i.e., the gyrator, which can be used to realize a circulator. Unlike state-of-the-art gyrators, which use a magneto-optic effect to induce a phase shift of $π$ between transmitted signals in opposite directions, our device uses the phase nonreciprocity of a Josephson-based three-wave-mixing device. By coupling two of these mixers and operating them in noiseless frequency-conversion mode, we show that the device acts as a nonreciprocal phase shifter whose phase shift is controlled by the phase difference of the microwave tones driving the mixers. Such a device could be used to realize a lossless, on-chip, superconducting circulator suitable for quantum-information-processing applications.

cond-mat.supr-con↗

arXiv2017

Demonstration of weight-four parity measurements in the surface code architecture

We present parity measurements on a five-qubit lattice with connectivity amenable to the surface code quantum error correction architecture. Using all-microwave controls of superconducting qubits coupled via resonators, we encode the parities of four data qubit states in either the $X$- or the $Z$-basis. Given the connectivity of the lattice, we perform full characterization of the static $Z$-interactions within the set of five qubits, as well as dynamical $Z$-interactions brought along by single- and two-qubit microwave drives. The parity measurements are significantly improved by modifying the microwave two-qubit gates to dynamically remove non-ideal $Z$ errors.

quant-ph↗

arXiv2016

Time-multiplexed amplification in a hybrid-less and coil-less Josephson parametric converter

Josephson parametric converters (JPCs) are superconducting devices capable of performing nondegenerate, three-wave mixing in the microwave domain without losses. One drawback limiting their use in scalable quantum architectures is the large footprint of the auxiliary circuit needed for their operation, in particular, the use of off-chip, bulky, broadband hybrids and magnetic coils. Here, we realize a JPC which eliminates the need for these bulky components. The pump drive and flux bias are applied in the new device through an on-chip, lossless, three-port power divider and on-chip flux line, respectively. We show that the new design considerably simplifies the circuit and reduces the footprint of the device while maintaining a comparable performance to state-of-the-art JPCs. Furthermore, we exploit the tunable bandwidth property of the JPC and the added capability of applying alternating currents to the flux line in order to switch the resonance frequencies of the device, hence demonstrating time-multiplexed amplification of microwave tones that are separated by more than the dynamical bandwidth of the amplifier. Such a measurement technique can potentially serve to perform time-multiplexed, high-fidelity readout of superconducting qubits.

cond-mat.supr-con↗

arXiv2016

Observation of Collective Coulomb Blockade in a Gate-controlled Linear Quantum-dot Array

The quantum transport of electrons in an artificial atom, such as a quantum dot (QD), is governed by the Coulomb blockade (CB) effects, revealing the ground-state charge configuration of the electronic system under interplays of the on-site strong Coulomb interactions. In a coherently-coupled QD array, i.e. artificial molecules, the phenomenon of collective CB (CCB) was predicted by theoretical studies circa two decades ago but its evidence remains controversial. Here, we present direct evidence for the observation of CCB in a six-quantum-dot array (QDA) under high magnetic fields at 20 mK. The coherent inter-dot coupling is enhanced and mediated via the Quantum Hall edge states of the GaAs sample substrate. Two continuously fine-tuned gate voltages enable the quantum dot conductance spectrum to undergo a localization to delocalization transition process which manifests as an emergence and a collapse of CCB. The transition between these two distinct quantum phases is analogous to the Mott-Hubbard metal-insulator transition. The gate-voltage-dependent conductance map thus obtained makes it possible to utilize QDA as an on-chip laboratory for studying CCB and Mott physics. Our QDA device provides a platform for developing engineered QD materials, qubit systems and artificial molecular devices in the future.

cond-mat.str-el↗

arXiv2009

Coherent oscillations between classically separable quantum states of a superconducting loop

Ten years ago, coherent oscillations between two quantum states of a superconducting circuit differing by the presence or absence of a single Cooper pair on a metallic island were observed for the first time. This result immediately stimulated the development of several other types of superconducting quantum circuits behaving as artificial atoms, thus bridging mesoscopic and atomic physics. Interestingly, none of these circuits fully implements the now almost 30 year old proposal of A. J. Leggett to observe coherent oscillations between two states differing by the presence or absence of a single fluxon trapped in the superconducting loop interrupted by a Josephson tunnel junction. This phenomenon of reversible quantum tunneling between two classically separable states, known as Macroscopic Quantum Coherence (MQC), is regarded crucial for precision tests of whether macroscopic systems such as circuits fully obey quantum mechanics. In this article, we report the observation of such oscillations with sub-GHz frequency and quality factor larger than 500. We achieved this result with two innovations. First, our ring has an inductance four orders of magnitude larger than that considered by Leggett, combined with a junction in the charging regime, a parameter choice never addressed in previous experiments. Second, readout is performed with a novel dispersive scheme which eliminates the electromagnetic relaxation process induced by the measurement circuit (Purcell effect). Moreover, the reset of the system to its ground state is naturally built into this scheme, working even if the transition energy is smaller than that of temperature fluctuations. As we argue, the MQC transition could therefore be, contrary to expectations, the basis of a superconducting qubit of improved coherence and readout fidelity.

cond-mat.mes-hall↗

arXiv2003

Tuning carbon nanotube bandgaps with strain

We show that the band structure of a carbon nanotube (NT) can be dramatically altered by mechanical strain. We employ an atomic force microscope tip to simultaneously vary the NT strain and to electrostatically gate the tube. We show that strain can open a bandgap in a metallic NT and modify the bandgap in a semiconducting NT. Theoretical work predicts that bandgap changes can range between +100meV and -100meV per 1% stretch, depending on NT chirality, and our measurements are consistent with this predicted range.

cond-mat.mes-hall↗