High dynamic-range quantum sensing of magnons and their dynamics using a superconducting qubit

  1. Sonia Rani,
  2. Xi Cao,
  3. Alejandro E. Baptista,
  4. Axel Hoffmann,
  5. and Wolfgang Pfaff
Magnons can endow quantum devices with new functionalities. Assessing their potential requires precise characterization of magnon properties. Here, we use a superconducting qubit to
probe magnons in a ferrimagnet over a range of about 2000 excitations. Using qubit control and parametrically induced qubit-magnon interactions we demonstrate few-excitation sensitive detection of magnons and are able to accurately resolve their decay. These results introduce quantum circuits as high-dynamic range probes for magnons and provide an avenue toward sensitive detection of nontrivial magnon dynamics.

A high-efficiency plug-and-play superconducting qubit network

  1. Michael Mollenhauer,
  2. Abdullah Irfan,
  3. Xi Cao,
  4. Supriya Mandal,
  5. and Wolfgang Pfaff
Modular architectures are a promising approach to scale quantum devices to the point of fault tolerance and utility. Modularity is particularly appealing for superconducting qubits,
as monolithically manufactured devices are limited in both system size and quality. Constructing complex quantum systems as networks of interchangeable modules can overcome this challenge through `Lego-like‘ assembly, reconfiguration, and expansion, in a spirit similar to modern classical computers. First prototypical superconducting quantum device networks have been demonstrated. Interfaces that simultaneously permit interchangeability and high-fidelity operations remain a crucial challenge, however. Here, we demonstrate a high-efficiency interconnect based on a detachable cable between superconducting qubit devices. We overcome the inevitable loss in a detachable connection through a fast pump scheme, enabling inter-module SWAP efficiencies at the 99%-level in less than 100 ns. We use this scheme to generate high-fidelity entanglement and operate a distributed logical dual-rail qubit. At the observed ~1% error rate, operations through the interconnect are at the threshold for fault-tolerance. These results introduce a modular architecture for scaling quantum processors with reconfigurable and expandable networks.

Parametrically controlled chiral interface for superconducting quantum devices

  1. Xi Cao,
  2. Abdullah Irfan,
  3. Michael Mollenhauer,
  4. Kaushik Singirikonda,
  5. and Wolfgang Pfaff
Nonreciprocal microwave routing plays a crucial role for measuring quantum circuits, and allows for realizing cascaded quantum systems for generating and stabilizing entanglement between
non-interacting qubits. The most commonly used tools for implementing directionality are ferrite-based circulators. These devices are versatile, but suffer from excess loss, a large footprint, and fixed directionality. For utilizing nonreciprocity in scalable quantum circuits it is desirable to develop efficient integration of low-loss and in-situ controllable directional elements. Here, we report the design and experimental realization of a controllable directional interface that may be integrated directly with superconducting qubits. In the presented device, nonreciprocity is realized through a combination of interference and phase-controlled parametric pumping. We have achieved a maximum directionality of around 30\,dB, and the performance of the device is predicted quantitatively from independent calibration measurements. Using the excellent agreement of model and experiment, we predict that the circuit will be useable as a chiral qubit interface with inefficiencies at the one-percent level or below. Our work provides a route toward isolator-free qubit readout schemes and high-fidelity entanglement generation in all-to-all connected networks of superconducting quantum devices.

Parametrically-controlled microwave-photonic interface for the fluxonium

  1. Ke Nie,
  2. Aayam Bista,
  3. Kaicheung Chow,
  4. Wolfgang Pfaff,
  5. and Angela Kou
Converting quantum information from stationary qubits to traveling photons enables both fast qubit initialization and efficient generation of flying qubits for redistribution of quantum
information. This conversion can be performed using cavity sideband transitions. In the fluxonium, however, direct cavity sideband transitions are forbidden due to parity symmetry. Here we circumvent this parity selection rule by using a three-wave mixing element to couple the fluxonium to a resonator. We experimentally demonstrate a scheme for interfacing the fluxonium with traveling photons through microwave-induced parametric conversion. We perform fast reset on the fluxonium qubit, initializing it with > 95% ground state population. We then implement controlled release and temporal shaping of a flying photon, useful for quantum state transfer and remote entanglement. The simplicity and flexibility of our demonstrated scheme enables fluxonium-based remote entanglement architectures.

Loss resilience of driven-dissipative remote entanglement in chiral waveguide quantum electrodynamics

  1. Abdullah Irfan,
  2. Mingxing Yao,
  3. Andrew Lingenfelter,
  4. Xi Cao,
  5. Aashish A. Clerk,
  6. and Wolfgang Pfaff
Establishing limits of entanglement in open quantum systems is a problem of fundamental interest, with strong implications for applications in quantum information science. Here, we
study limits of entanglement stabilization between remote qubits. We theoretically investigate the loss resilience of driven-dissipative entanglement between remote qubits coupled to a chiral waveguide. We find that by coupling a pair of storage qubits to the two driven qubits, the steady state can be tailored such that the storage qubits show a degree of entanglement that is higher than what can be achieved with only two driven qubits coupled to the waveguide. By reducing the degree of entanglement of the driven qubits, we show that the entanglement between the storage qubits becomes more resilient to waveguide loss. Our analytical and numerical results offer insights into how waveguide loss limits the degree of entanglement in this driven-dissipative system, and offers important guidance for remote entanglement stabilization in the laboratory, for example using superconducting circuits.

A modular quantum computer based on a quantum state router

  1. Chao Zhou,
  2. Pinlei Lu,
  3. Matthieu Praquin,
  4. Tzu-Chiao Chien,
  5. Ryan Kaufman,
  6. Xi Cao,
  7. Mingkang Xia,
  8. Roger Mong,
  9. Wolfgang Pfaff,
  10. David Pekker,
  11. and Michael Hatridge
In this work, we present the design of a superconducting, microwave quantum state router which can realize all-to-all couplings among four quantum modules. Each module consists of a
single transmon, readout mode, and communication mode coupled to the router. The router design centers on a parametrically driven, Josephson-junction based three-wave mixing element which generates photon exchange among the modules‘ communication modes. We first demonstrate SWAP operations among the four communication modes, with an average full-SWAP time of 760 ns and average inter-module gate fidelity of 0.97, limited by our modes‘ coherences. We also demonstrate photon transfer and pairwise entanglement between the modules‘ qubits, and parallel operation of simultaneous SWAP gates across the router. These results can readily be extended to faster and higher fidelity router operations, as well as scaled to support larger networks of quantum modules.

A gate-tunable, field-compatible fluxonium

  1. Marta Pita-Vidal,
  2. Arno Bargerbos,
  3. Chung-Kai Yang,
  4. David J. van Woerkom,
  5. Wolfgang Pfaff,
  6. Nadia Haider,
  7. Peter Krogstrup,
  8. Leo P. Kouwenhoven,
  9. Gijs de Lange,
  10. and Angela Kou
Circuit quantum electrodynamics, where photons are coherently coupled to artificial atoms built with superconducting circuits, has enabled the investigation and control of macroscopic
quantum-mechanical phenomena in superconductors. Recently, hybrid circuits incorporating semiconducting nanowires and other electrostatically-gateable elements have provided new insights into mesoscopic superconductivity. Extending the capabilities of hybrid flux-based circuits to work in magnetic fields would be especially useful both as a probe of spin-polarized Andreev bound states and as a possible platform for topological qubits. The fluxonium is particularly suitable as a readout circuit for topological qubits due to its unique persistent-current based eigenstates. In this Letter, we present a magnetic-field compatible hybrid fluxonium with an electrostatically-tuned semiconducting nanowire as its non-linear element. We operate the fluxonium in magnetic fields up to 1T and use it to observe the φ0-Josephson effect. This combination of gate-tunability and field-compatibility opens avenues for the exploration and control of spin-polarized phenomena using superconducting circuits and enables the use of the fluxonium as a readout device for topological qubits.

On-demand quantum state transfer and entanglement between remote microwave cavity memories

  1. Christopher Axline,
  2. Luke Burkhart,
  3. Wolfgang Pfaff,
  4. Mengzhen Zhang,
  5. Kevin Chou,
  6. Philippe Campagne-Ibarcq,
  7. Philip Reinhold,
  8. Luigi Frunzio,
  9. S.M. Girvin,
  10. Liang Jiang,
  11. M.H. Devoret,
  12. and R. J. Schoelkopf
Modular quantum computing architectures require fast and efficient distribution of quantum information through propagating signals. Here we report rapid, on-demand quantum state transfer
between two remote superconducting cavity quantum memories through traveling microwave photons. We demonstrate a quantum communication channel by deterministic transfer of quantum bits with 76% fidelity. Heralding on errors induced by experimental imperfection can improve this to 87% with a success probability of 0.87. By partial transfer of a microwave photon, we generate remote entanglement at a rate that exceeds photon loss in either memory by more than a factor of three. We further show the transfer of quantum error correction code words that will allow deterministic mitigation of photon loss. These results pave the way for scaling superconducting quantum devices through modular quantum networks.

Schrodinger’s catapult: Launching multiphoton quantum states from a microwave cavity memory

  1. Wolfgang Pfaff,
  2. Christopher J Axline,
  3. Luke D Burkhart,
  4. Uri Vool,
  5. Philip Reinhold,
  6. Luigi Frunzio,
  7. Liang Jiang,
  8. Michel H. Devoret,
  9. and Robert J. Schoelkopf
Encoding quantum states in complex multiphoton fields can overcome loss during signal transmission in a quantum network. Transmitting quantum information encoded in this way requires
that locally stored states can be converted to propagating fields. Here we experimentally show the controlled conversion of multiphoton quantum states, like „Schr\“odinger cat“ states, from a microwave cavity quantum memory into propagating modes. By parametric conversion using the nonlinearity of a single Josephson junction, we can release the cavity state in ~500 ns, about 3 orders of magnitude faster than its intrinsic lifetime. This `catapult‘ faithfully converts arbitrary cavity fields to traveling signals with an estimated efficiency of > 90%, enabling on-demand generation of complex itinerant quantum states. Importantly, the release process can be controlled precisely on fast time scales, allowing us to generate entanglement between the cavity and the traveling mode by partial conversion. Our system can serve as the backbone of a microwave quantum network, paving the way towards error-correctable distribution of quantum information and the transfer of highly non-classical states to hybrid quantum systems.

A coaxial line architecture for integrating and scaling 3D cQED systems

  1. Christopher Axline,
  2. Matthew Reagor,
  3. Reinier W. Heeres,
  4. Philip Reinhold,
  5. Chen Wang,
  6. Kevin Shain,
  7. Wolfgang Pfaff,
  8. Yiwen Chu,
  9. Luigi Frunzio,
  10. and Robert J. Schoelkopf
Numerous loss mechanisms can limit coherence and scalability of planar and 3D-based circuit quantum electrodynamics (cQED) devices, particularly due to their packaging. The low loss
and natural isolation of 3D enclosures make them good candidates for coherent scaling. We introduce a coaxial transmission line device architecture with coherence similar to traditional 3D cQED systems. Measurements demonstrate well-controlled external and on-chip couplings, a spectrum absent of cross-talk or spurious modes, and excellent resonator and qubit lifetimes. We integrate a resonator-qubit system in this architecture with a seamless 3D cavity, and separately pattern a qubit, readout resonator, Purcell filter and high-Q stripline resonator on a single chip. Device coherence and its ease of integration make this a promising tool for complex experiments.