Generating Spatially Entangled Itinerant Photons with Waveguide Quantum Electrodynamics

  1. Bharath Kannan,
  2. Daniel Campbell,
  3. Francisca Vasconcelos,
  4. Roni Winik,
  5. David Kim,
  6. Morten Kjaergaard,
  7. Philip Krantz,
  8. Alexander Melville,
  9. Bethany M. Niedzielski,
  10. Jonilyn Yoder,
  11. Terry P. Orlando,
  12. Simon Gustavsson,
  13. and William D. Oliver
Realizing a fully connected network of quantum processors requires the ability to distribute quantum entanglement. For distant processing nodes, this can be achieved by generating,
routing, and capturing spatially entangled itinerant photons. In this work, we demonstrate deterministic generation of such photons using superconducting transmon qubits that are directly coupled to a waveguide. In particular, we generate two-photon N00N states and show that the state and spatial entanglement of the emitted photons can be tuned via the qubit frequencies. Using quadrature amplitude detection, we reconstruct the moments and correlations of the photonic modes and demonstrate state preparation fidelities of 84%. Our results provide a path towards realizing quantum communication and teleportation protocols using non-classical, spatially entangled itinerant photons.

Multi-level Quantum Noise Spectroscopy

  1. Youngkyu Sung,
  2. Antti Vepsäläinen,
  3. Jochen Braumüller,
  4. Fei Yan,
  5. Joel I-Jan Wang,
  6. Morten Kjaergaard,
  7. Roni Winik,
  8. Philip Krantz,
  9. Andreas Bengtsson,
  10. Alexander J. Melville,
  11. Bethany M. Niedzielski,
  12. Mollie E. Schwartz,
  13. David K. Kim,
  14. Jonilyn L. Yoder,
  15. Terry P. Orlando,
  16. Simon Gustavsson,
  17. and William D. Oliver
System noise identification is crucial to the engineering of robust quantum systems. Although existing quantum noise spectroscopy (QNS) protocols measure an aggregate amount of noise
affecting a quantum system, they generally cannot distinguish between the underlying processes that contribute to it. Here, we propose and experimentally validate a spin-locking-based QNS protocol that exploits the multi-level energy structure of a superconducting qubit to achieve two notable advances. First, our protocol extends the spectral range of weakly anharmonic qubit spectrometers beyond the present limitations set by their lack of strong anharmonicity. Second, the additional information gained from probing the higher-excited levels enables us to identify and distinguish contributions from different underlying noise mechanisms.

Characterizing and optimizing qubit coherence based on SQUID geometry

  1. Jochen Braumüller,
  2. Leon Ding,
  3. Antti Vepsäläinen,
  4. Youngkyu Sung,
  5. Morten Kjaergaard,
  6. Tim Menke,
  7. Roni Winik,
  8. David Kim,
  9. Bethany M. Niedzielski,
  10. Alexander Melville,
  11. Jonilyn L. Yoder,
  12. Cyrus F. Hirjibehedin,
  13. Terry P. Orlando,
  14. Simon Gustavsson,
  15. and William D. Oliver
The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is 1/f flux noise. To understand its origin and find ways to minimize its impact, we systematically
study flux noise amplitudes in more than 50 flux qubits with varied SQUID geometry parameters and compare our results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Our data are in agreement with an extension of the previously proposed model, based on numerical simulations of the current distribution in the investigated SQUIDs. Our results and detailed model provide a guide for minimizing the flux noise susceptibility in future circuits.

Two-qubit spectroscopy of spatiotemporally correlated quantum noise in superconducting qubits

  1. Uwe von Lüpke,
  2. Félix Beaudoin,
  3. Leigh M. Norris,
  4. Youngkyu Sung,
  5. Roni Winik,
  6. Jack Y. Qiu,
  7. Morten Kjaergaard,
  8. David Kim,
  9. Jonilyn Yoder,
  10. Simon Gustavsson,
  11. Lorenza Viola,
  12. and William D. Oliver
Noise that exhibits significant temporal and spatial correlations across multiple qubits can be especially harmful to both fault-tolerant quantum computation and quantum-enhanced metrology.
However, a complete spectral characterization of the noise environment of even a two-qubit system has not been reported thus far. We propose and experimentally validate a protocol for two-qubit dephasing noise spectroscopy based on continuous control modulation. By combining ideas from spin-locking relaxometry with a statistically motivated robust estimation approach, our protocol allows for the simultaneous reconstruction of all the single-qubit and two-qubit cross-correlation spectra, including access to their distinctive non-classical features. Only single-qubit control manipulations and state-tomography measurements are employed, with no need for entangled-state preparation or readout of two-qubit observables. While our experimental validation uses two superconducting qubits coupled to a shared engineered noise source, our methodology is portable to a variety of dephasing-dominated qubit architectures. By pushing quantum noise spectroscopy beyond the single-qubit setting, our work paves the way to characterizing spatiotemporal correlations in both engineered and naturally occurring noise environments.

Hybrid Quantum Error Correction in Qubit Architectures

  1. Lasse Bjørn Kristensen,
  2. Morten Kjaergaard,
  3. Christian Kraglund Andersen,
  4. and Nikolaj Thomas Zinner
Noise and errors are inevitable parts of any practical implementation of a quantum computer. As a result, large-scale quantum computation will require ways to detect and correct errors
on quantum information. Here, we present such a quantum error correcting scheme for correcting the dominant error sources, phase decoherence and energy relaxation, in qubit architectures, using a hybrid approach combining autonomous correction based on engineered dissipation with traditional measurement-based quantum error correction. Using numerical simulations with realistic device parameters for superconducting circuits, we show that this scheme can achieve a 5- to 10-fold increase in storage-time while using only six qubits for the encoding and two ancillary qubits for the operation of the autonomous part of the scheme, providing a potentially large reduction of qubit overhead compared to typical measurement-based error correction schemes. Furthermore, the scheme relies on standard interactions and qubit driving available in most major quantum computing platforms, making it implementable in a wide range of architectures.

Superconducting Qubits: Current State of Play

  1. Morten Kjaergaard,
  2. Mollie E. Schwartz,
  3. Jochen Braumüller,
  4. Philip Krantz,
  5. Joel I-Jan Wang,
  6. Simon Gustavsson,
  7. and William D. Oliver
Superconducting qubits are leading candidates in the race to build a quantum computer capable of realizing computations beyond the reach of modern supercomputers. The superconducting
qubit modality has been used to demonstrate prototype algorithms in the `noisy intermediate scale quantum‘ (NISQ) technology era, in which non-error-corrected qubits are used to implement quantum simulations and quantum algorithms. With the recent demonstrations of multiple high fidelity two-qubit gates as well as operations on logical qubits in extensible superconducting qubit systems, this modality also holds promise for the longer-term goal of building larger-scale error-corrected quantum computers. In this brief review, we discuss several of the recent experimental advances in qubit hardware, gate implementations, readout capabilities, early NISQ algorithm implementations, and quantum error correction using superconducting qubits. While continued work on many aspects of this technology is certainly necessary, the pace of both conceptual and technical progress in the last years has been impressive, and here we hope to convey the excitement stemming from this progress.

A Quantum Engineer’s Guide to Superconducting Qubits

  1. Philip Krantz,
  2. Morten Kjaergaard,
  3. Fei Yan,
  4. Terry P. Orlando,
  5. Simon Gustavsson,
  6. and William D. Oliver
The aim of this review is to provide quantum engineers with an introductory guide to the central concepts and challenges in the rapidly accelerating field of superconducting quantum
circuits. Over the past twenty years, the field has matured from a predominantly basic research endeavor to one that increasingly explores the engineering of larger-scale superconducting quantum systems. Here, we review several foundational elements — qubit design, noise properties, qubit control, and readout techniques — developed during this period, bridging fundamental concepts in circuit quantum electrodynamics (cQED) and contemporary, state-of-the-art applications in gate-model quantum computation.

A tunable coupling scheme for implementing high-fidelity two-qubit gates

  1. Fei Yan,
  2. Philip Krantz,
  3. Youngkyu Sung,
  4. Morten Kjaergaard,
  5. Dan Campbell,
  6. Joel I.J. Wang,
  7. Terry P. Orlando,
  8. Simon Gustavsson,
  9. and William D. Oliver
The prospect of computational hardware with quantum advantage relies critically on the quality of quantum gate operations. Imperfect two-qubit gates is a major bottleneck for achieving
scalable quantum information processors. Here, we propose a generalizable and extensible scheme for a two-qubit coupler switch that controls the qubit-qubit coupling by modulating the coupler frequency. Two-qubit gate operations can be implemented by operating the coupler in the dispersive regime, which is non-invasive to the qubit states. We investigate the performance of the scheme by simulating a universal two-qubit gate on a superconducting quantum circuit, and find that errors from known parasitic effects are strongly suppressed. The scheme is compatible with existing high-coherence hardware, thereby promising a higher gate fidelity with current technologies.

Distinguishing coherent and thermal photon noise in a circuit QED system

  1. Fei Yan,
  2. Dan Campbell,
  3. Philip Krantz,
  4. Morten Kjaergaard,
  5. David Kim,
  6. Jonilyn L. Yoder,
  7. David Hover,
  8. Adam Sears,
  9. Andrew J. Kerman,
  10. Terry P. Orlando,
  11. Simon Gustavsson,
  12. and William D. Oliver
In the cavity-QED architecture, photon number fluctuations from residual cavity photons cause qubit dephasing due to the AC Stark effect. These unwanted photons originate from a variety
of sources, such as thermal radiation, leftover measurement photons, and crosstalk. Using a capacitively-shunted flux qubit coupled to a transmission line cavity, we demonstrate a method that identifies and distinguishes coherent and thermal photons based on noise-spectral reconstruction from time-domain spin-locking relaxometry. Using these measurements, we attribute the limiting dephasing source in our system to thermal photons, rather than coherent photons. By improving the cryogenic attenuation on lines leading to the cavity, we successfully suppress residual thermal photons and achieve T1-limited spin-echo decay time. The spin-locking noise spectroscopy technique can readily be applied to other qubit modalities for identifying general asymmetric non-classical noise spectra.

Superconducting Gatemon Qubit based on a Proximitized Two-Dimensional Electron Gas

  1. Lucas Casparis,
  2. Malcolm R. Connolly,
  3. Morten Kjaergaard,
  4. Natalie J. Pearson,
  5. Anders Kringhøj,
  6. Thorvald W. Larsen,
  7. Ferdinand Kuemmeth,
  8. Tiantian Wang,
  9. Candice Thomas,
  10. Sergei Gronin,
  11. Geoffrey C. Gardner,
  12. Michael J. Manfra,
  13. Charles M. Marcus,
  14. and Karl D. Petersson
The coherent tunnelling of Cooper pairs across Josephson junctions (JJs) generates a nonlinear inductance that is used extensively in quantum information processors based on superconducting
circuits, from setting qubit transition frequencies and interqubit coupling strengths, to the gain of parametric amplifiers for quantum-limited readout. The inductance is either set by tailoring the metal-oxide dimensions of single JJs, or magnetically tuned by parallelizing multiple JJs in superconducting quantum interference devices (SQUIDs) with local current-biased flux lines. JJs based on superconductor-semiconductor hybrids represent a tantalizing all-electric alternative. The gatemon is a recently developed transmon variant which employs locally gated nanowire (NW) superconductor-semiconductor JJs for qubit control. Here, we go beyond proof-of-concept and demonstrate that semiconducting channels etched from a wafer-scale two-dimensional electron gas (2DEG) are a suitable platform for building a scalable gatemon-based quantum computer. We show 2DEG gatemons meet the requirements by performing voltage-controlled single qubit rotations and two-qubit swap operations. We measure qubit coherence times up to ~2 us, limited by dielectric loss in the 2DEG host substrate.