Phonon engineering of atomic-scale defects in superconducting quantum circuits

  1. Mo Chen,
  2. John Clai Owens,
  3. Harald Putterman,
  4. Max Schäfer,
  5. and Oskar Painter
Noise within solid-state systems at low temperatures, where many of the degrees of freedom of the host material are frozen out, can typically be traced back to material defects that
support low-energy excitations. These defects can take a wide variety of microscopic forms, and for amorphous materials are broadly described using generic models such as the tunneling two-level systems (TLS) model. Although the details of TLS, and their impact on the low-temperature behavior of materials have been studied since the 1970s, these states have recently taken on further relevance in the field of quantum computing, where the limits to the coherence of superconducting microwave quantum circuits are dominated by TLS. Efforts to mitigate the impact of TLS have thus far focused on circuit design, material selection, and material surface treatment. In this work, we take a new approach that seeks to directly modify the properties of TLS through nanoscale-engineering. This is achieved by periodically structuring the host material, forming an acoustic bandgap that suppresses all microwave-frequency phonons in a GHz-wide frequency band around the operating frequency of a transmon qubit superconducting quantum circuit. For embedded TLS that are strongly coupled to the electric qubit, we measure a pronounced increase in relaxation time by two orders of magnitude when the TLS transition frequency lies within the acoustic bandgap, with the longest T1 time exceeding 5 milliseconds. Our work paves the way for in-depth investigation and coherent control of TLS, which is essential for deepening our understanding of noise in amorphous materials and advancing solid-state quantum devices.

Demonstrating a long-coherence dual-rail erasure qubit using tunable transmons

  1. Harry Levine,
  2. Arbel Haim,
  3. Jimmy S.C. Hung,
  4. Nasser Alidoust,
  5. Mahmoud Kalaee,
  6. Laura DeLorenzo,
  7. E. Alex Wollack,
  8. Patricio Arrangoiz-Arriola,
  9. Amirhossein Khalajhedayati,
  10. Yotam Vaknin,
  11. Aleksander Kubica,
  12. Aashish A. Clerk,
  13. David Hover,
  14. Fernando Brandão,
  15. Alex Retzker,
  16. and Oskar Painter
Quantum error correction with erasure qubits promises significant advantages over standard error correction due to favorable thresholds for erasure errors. To realize this advantagein practice requires a qubit for which nearly all errors are such erasure errors, and the ability to check for erasure errors without dephasing the qubit. We experimentally demonstrate that a „dual-rail qubit“ consisting of a pair of resonantly-coupled transmons can form a highly coherent erasure qubit, where the erasure error rate is given by the transmon T1 but for which residual dephasing is strongly suppressed, leading to millisecond-scale coherence within the qubit subspace. We show that single-qubit gates are limited primarily by erasure errors, with erasure probability perasure=2.19(2)×10−3 per gate while the residual errors are ∼40 times lower. We further demonstrate mid-circuit detection of erasure errors while introducing <0.1% dephasing error per check. Finally, we show that the suppression of transmon noise allows this dual-rail qubit to preserve high coherence over a broad tunable operating range, offering an improved capacity to avoid frequency collisions. This work establishes transmon-based dual-rail qubits as an attractive building block for hardware-efficient quantum error correction.[/expand]

Non-classical microwave-optical photon pair generation with a chip-scale transducer

  1. Srujan Meesala,
  2. Steven Wood,
  3. David Lake,
  4. Piero Chiappina,
  5. Changchun Zhong,
  6. Andrew D. Beyer,
  7. Matthew D. Shaw,
  8. Liang Jiang,
  9. and Oskar Painter
Modern computing and communication technologies such as supercomputers and the internet are based on optically connected networks of microwave frequency information processors. In recent
years, an analogous architecture has emerged for quantum networks with optically distributed entanglement between remote superconducting quantum processors, a leading platform for quantum computing. Here we report an important milestone towards such networks by observing non-classical correlations between photons in an optical link and a superconducting electrical circuit. We generate such states of light through a spontaneous parametric down-conversion (SPDC) process in a chip-scale piezo-optomechanical transducer. The non-classical nature of the emitted light is verified by observing anti-bunching in the microwave state conditioned on detection of an optical photon. Such a transducer can be readily connected to a superconducting quantum processor, and serve as a key building block for optical quantum networks of microwave frequency qubits.

A scalable superconducting quantum simulator with long-range connectivity based on a photonic bandgap metamaterial

  1. Xueyue Zhang,
  2. Eunjong Kim,
  3. Daniel K. Mark,
  4. Soonwon Choi,
  5. and Oskar Painter
Synthesis of many-body quantum systems in the laboratory can provide further insight into the emergent behavior of quantum materials. While the majority of engineerable many-body systems,
or quantum simulators, consist of particles on a lattice with local interactions, quantum systems featuring long-range interactions are particularly difficult to model and interesting to study due to the rapid spatio-temporal growth of entanglement in such systems. Here we present a scalable quantum simulator architecture based on superconducting transmon qubits on a lattice, with interactions mediated by the exchange of photons via a metamaterial waveguide quantum bus. The metamaterial waveguide enables extensible scaling of the system and multiplexed qubit read-out, while simultaneously protecting the qubits from radiative decay. As an initial demonstration of this platform, we realize a 10-qubit simulator of the one-dimensional Bose-Hubbard model, with in situ tunability of both the hopping range and the on-site interaction. We characterize the Hamiltonian of the system using a measurement-efficient protocol based on quantum many-body chaos, uncovering the remnant phase of Bloch waves of the metamaterial bus in the long-range hopping terms. We further study the many-body quench dynamics of the system, revealing through global bit-string statistics the predicted crossover from integrability to ergodicity as the hopping range is extended beyond nearest-neighbor. Looking forward, the metamaterial quantum bus may be extended to a two-dimensional lattice of qubits, and used to generate other spin-like lattice interactions or tailored lattice connectivity, expanding the accessible Hamiltonians for analog quantum simulation using superconducting quantum circuits.

Building a fault-tolerant quantum computer using concatenated cat codes

  1. Christopher Chamberland,
  2. Kyungjoo Noh,
  3. Patricio Arrangoiz-Arriola,
  4. Earl T. Campbell,
  5. Connor T. Hann,
  6. Joseph Iverson,
  7. Harald Putterman,
  8. Thomas C. Bohdanowicz,
  9. Steven T. Flammia,
  10. Andrew Keller,
  11. Gil Refael,
  12. John Preskill,
  13. Liang Jiang,
  14. Amir H. Safavi-Naeini,
  15. Oskar Painter,
  16. and Fernando G.S.L. Brandão
We present a comprehensive architectural analysis for a fault-tolerant quantum computer based on cat codes concatenated with outer quantum error-correcting codes. For the physical hardware,
we propose a system of acoustic resonators coupled to superconducting circuits with a two-dimensional layout. Using estimated near-term physical parameters for electro-acoustic systems, we perform a detailed error analysis of measurements and gates, including CNOT and Toffoli gates. Having built a realistic noise model, we numerically simulate quantum error correction when the outer code is either a repetition code or a thin rectangular surface code. Our next step toward universal fault-tolerant quantum computation is a protocol for fault-tolerant Toffoli magic state preparation that significantly improves upon the fidelity of physical Toffoli gates at very low qubit cost. To achieve even lower overheads, we devise a new magic-state distillation protocol for Toffoli states. Combining these results together, we obtain realistic full-resource estimates of the physical error rates and overheads needed to run useful fault-tolerant quantum algorithms. We find that with around 1,000 superconducting circuit components, one could construct a fault-tolerant quantum computer that can run circuits which are intractable for classical supercomputers. Hardware with 32,000 superconducting circuit components, in turn, could simulate the Hubbard model in a regime beyond the reach of classical computing.

Quantum electrodynamics in a topological waveguide

  1. Eunjong Kim,
  2. Xueyue Zhang,
  3. Vinicius S. Ferreira,
  4. Jash Banker,
  5. Joseph K. Iverson,
  6. Alp Sipahigil,
  7. Miguel Bello,
  8. Alejandro Gonzalez-Tudela,
  9. Mohammad Mirhosseini,
  10. and Oskar Painter
While designing the energy-momentum relation of photons is key to many linear, non-linear, and quantum optical phenomena, a new set of light-matter properties may be realized by employing
the topology of the photonic bath itself. In this work we investigate the properties of superconducting qubits coupled to a metamaterial waveguide based on a photonic analog of the Su-Schrieffer-Heeger model. We explore topologically-induced properties of qubits coupled to such a waveguide, ranging from the formation of directional qubit-photon bound states to topology-dependent cooperative radiation effects. Addition of qubits to this waveguide system also enables direct quantum control over topological edge states that form in finite waveguide systems, useful for instance in constructing a topologically protected quantum communication channel. More broadly, our work demonstrates the opportunity that topological waveguide-QED systems offer in the synthesis and study of many-body states with exotic long-range quantum correlations.

Quantum transduction of optical photons from a superconducting qubit

  1. Mohammad Mirhosseini,
  2. Alp Sipahigil,
  3. Mahmoud Kalaee,
  4. and Oskar Painter
Bidirectional conversion of electrical and optical signals lies at the foundation of the global internet. Such converters are employed at repeater stations to extend the reach of long-haul
fiber optic communication systems and within data centers to exchange high-speed optical signals between computers. Likewise, coherent microwave-to-optical conversion of single photons would enable the exchange of quantum states between remotely connected superconducting quantum processors, a promising quantum computing hardware platform. Despite the prospects of quantum networking, maintaining the fragile quantum state in such a conversion process with superconducting qubits has remained elusive. Here we demonstrate the conversion of a microwave-frequency excitation of a superconducting transmon qubit into an optical photon. We achieve this using an intermediary nanomechanical resonator which converts the electrical excitation of the qubit into a single phonon by means of a piezoelectric interaction, and subsequently converts the phonon to an optical photon via radiation pressure. We demonstrate optical photon generation from the qubit with a signal-to-noise greater than unity by recording quantum Rabi oscillations of the qubit through single-photon detection of the emitted light over an optical fiber. With proposed improvements in the device and external measurement set-up, such quantum transducers may lead to practical devices capable of realizing new hybrid quantum networks, and ultimately, distributed quantum computers.

Collapse and Revival of an Artificial Atom Coupled to a Structured Photonic Reservoir

  1. Vinicius S. Ferreira,
  2. Jash Banker,
  3. Alp Sipahigil,
  4. Matthew H. Matheny,
  5. Andrew J. Keller,
  6. Eunjong Kim,
  7. Mohammad Mirhosseini,
  8. and Oskar Painter
A structured electromagnetic reservoir can result in novel dynamics of quantum emitters. In particular, the reservoir can be tailored to have a memory of past interactions with emitters,
in contrast to memory-less Markovian dynamics of typical open systems. In this Article, we investigate the non-Markovian dynamics of a superconducting qubit strongly coupled to a superconducting slow-light waveguide reservoir. Tuning the qubit into the spectral vicinity of the passband of this waveguide, we find non-exponential energy relaxation as well as substantial changes to the qubit emission rate. Further, upon addition of a reflective boundary to one end of the waveguide, we observe revivals in the qubit population on a timescale 30 times longer than the inverse of the qubit’s emission rate, corresponding to the round-trip travel time of an emitted photon. By tuning of the qubit-waveguide interaction strength, we probe a crossover between Markovian and non-Markovian qubit emission dynamics. These attributes allow for future studies of multi-qubit circuits coupled to structured reservoirs, in addition to constituting the necessary resources for generation of multiphoton highly entangled states.

Superconducting metamaterials for waveguide quantum electrodynamics

  1. Mohammad Mirhosseini,
  2. Eunjong Kim,
  3. Vinicius S. Ferreira,
  4. Mahmoud Kalaee,
  5. Alp Sipahigil,
  6. Andrew J. Keller,
  7. and Oskar Painter
The embedding of tunable quantum emitters in a photonic bandgap structure enables the control of dissipative and dispersive interactions between emitters and their photonic bath. Operation
in the transmission band, outside the gap, allows for studying waveguide quantum electrodynamics in the slow-light regime. Alternatively, tuning the emitter into the bandgap results in finite range emitter-emitter interactions via bound photonic states. Here we couple a transmon qubit to a superconducting metamaterial with a deep sub-wavelength lattice constant (λ/60). The metamaterial is formed by periodically loading a transmission line with compact, low loss, low disorder lumped element microwave resonators. We probe the coherent and dissipative dynamics of the system by measuring the Lamb shift and the change in the lifetime of the transmon qubit. Tuning the qubit frequency in the vicinity of a band-edge with a group index of ng=450, we observe an anomalous Lamb shift of 10 MHz accompanied by a 24-fold enhancement in the qubit lifetime. In addition, we demonstrate selective enhancement and inhibition of spontaneous emission of different transmon transitions, which provide simultaneous access to long-lived metastable qubit states and states strongly coupled to propagating waveguide modes.

Superconducting qubits on silicon substrates for quantum device integration

  1. Andrew J. Keller,
  2. Paul B. Dieterle,
  3. Michael Fang,
  4. Brett Berger,
  5. Johannes M. Fink,
  6. and Oskar Painter
We present the fabrication and characterization of transmon qubits formed from aluminum Josephson junctions on two different silicon-based substrates: (i) high-resistivity silicon (Si)
and (ii) silicon-on-insulator (SOI). Key to the qubit fabrication process is the use of an anhydrous hydrofluoric vapor process which removes silicon surface oxides without attacking aluminum, and in the case of SOI substrates, selectively removes the lossy buried oxide underneath the qubit region. For qubits with a transition frequency of approximately 5GHz we find qubit lifetimes and coherence times comparable to those attainable on sapphire substrates (T1,Si=27μs, T2,Si=6.6μs; T1,SOI=3.5μs, T2,SOI=2.2μs). This qubit fabrication process in principle permits co-fabrication of silicon photonic and mechanical elements, providing a route towards chip-scale integration of electro-opto-mechanical transducers for quantum networking of superconducting microwave quantum circuits.