Microwave Quantum Link between Superconducting Circuits Housed in Spatially Separated Cryogenic Systems

  1. Paul Magnard,
  2. Simon Storz,
  3. Philipp Kurpiers,
  4. Josua Schär,
  5. Fabian Marxer,
  6. Janis Luetolf,
  7. Jean-Claude Besse,
  8. Mihai Gabureac,
  9. Kevin Reuer,
  10. Abdulkadir Akin,
  11. Baptiste Royer,
  12. Alexandre Blais,
  13. and Andreas Wallraff
Superconducting circuits are a strong contender for realizing quantum computing systems, and are also successfully used to study quantum optics and hybrid quantum systems. However,
their cryogenic operation temperatures and the current lack of coherence-preserving microwave-to-optical conversion solutions have hindered the realization of superconducting quantum networks either spanning different cryogenics systems or larger distances. Here, we report the successful operation of a cryogenic waveguide coherently linking transmon qubits located in two dilution refrigerators separated by a physical distance of five meters. We transfer qubit states and generate entanglement on-demand with average transfer and target state fidelities of 85.8 % and 79.5 %, respectively, between the two nodes of this elementary network. Cryogenic microwave links do provide an opportunity to scale up systems for quantum computing and create local area quantum communication networks over length scales of at least tens of meters.

Quantum communication with time-bin encoded microwave photons

  1. Philipp Kurpiers,
  2. Marek Pechal,
  3. Baptiste Royer,
  4. Paul Magnard,
  5. Theo Walter,
  6. Johannes Heinsoo,
  7. Yves Salathé,
  8. Abdulkadir Akin,
  9. Simon Storz,
  10. Jean-Claude Besse,
  11. Simone Gasparinetti,
  12. Alexandre Blais,
  13. and Andreas Wallraff
Heralding techniques are useful in quantum communication to circumvent losses without resorting to error correction schemes or quantum repeaters. Such techniques are realized, for example,
by monitoring for photon loss at the receiving end of the quantum link while not disturbing the transmitted quantum state. We describe and experimentally benchmark a scheme that incorporates error detection in a quantum channel connecting two transmon qubits using traveling microwave photons. This is achieved by encoding the quantum information as a time-bin superposition of a single photon, which simultaneously realizes high communication rates and high fidelities. The presented scheme is straightforward to implement in circuit QED and is fully microwave-controlled, making it an interesting candidate for future modular quantum computing architectures.

Engineering cryogenic setups for 100-qubit scale superconducting circuit systems

  1. Sebastian Krinner,
  2. Simon Storz,
  3. Philipp Kurpiers,
  4. Paul Magnard,
  5. Johannes Heinsoo,
  6. Raphael Keller,
  7. Janis Luetolf,
  8. Christopher Eichler,
  9. and Andreas Wallraff
A robust cryogenic infrastructure in form of a wired, thermally optimized dilution refrigerator is essential for present and future solid-state based quantum processors. Here, we engineer
an extensible cryogenic setup, which minimizes passive and active heat loads, while guaranteeing rapid qubit control and readout. We review design criteria for qubit drive lines, flux lines, and output lines used in typical experiments with superconducting circuits and describe each type of line in detail. The passive heat load of stainless steel and NbTi coaxial cables and the active load due to signal dissipation are measured, validating our robust and extensible concept for thermal anchoring of attenuators, cables, and other microwave components. Our results are important for managing the heat budget of future large-scale quantum computers based on superconducting circuits.

Fast and Unconditional All-Microwave Reset of a Superconducting Qubit

  1. Paul Magnard,
  2. Philipp Kurpiers,
  3. Baptiste Royer,
  4. Theo Walter,
  5. Jean-Claude Besse,
  6. Simone Gasparinetti,
  7. Marek Pechal,
  8. Johannes Heinsoo,
  9. Simon Storz,
  10. Alexandre Blais,
  11. and Andreas Wallraff
Active qubit reset is a key operation in many quantum algorithms, and particularly in error correction codes. Here, we experimentally demonstrate a reset scheme of a three level transmon
artificial atom coupled to a large bandwidth resonator. The reset protocol uses a microwave-induced interaction between the |f,0⟩ and |g,1⟩ states of the coupled transmon-resonator system, with |g⟩ and |f⟩ denoting the ground and second excited states of the transmon, and |0⟩ and |1⟩ the photon Fock states of the resonator. We characterize the reset process and demonstrate reinitialization of the transmon-resonator system to its ground state with 0.2% residual excitation in less than 500ns. Our protocol is of practical interest as it has no requirements on the architecture, beyond those for fast and efficient single-shot readout of the transmon, and does not require feedback.

Deterministic Quantum State Transfer and Generation of Remote Entanglement using Microwave Photons

  1. Philipp Kurpiers,
  2. Paul Magnard,
  3. Theo Walter,
  4. Baptiste Royer,
  5. Marek Pechal,
  6. Johannes Heinsoo,
  7. Yves Salathé,
  8. Abdulkadir Akin,
  9. Simon Storz,
  10. Jean-Claude Besse,
  11. Simone Gasparinetti,
  12. Alexandre Blais,
  13. and Andreas Wallraff
Sharing information coherently between nodes of a quantum network is at the foundation of distributed quantum information processing. In this scheme, the computation is divided into
subroutines and performed on several smaller quantum registers connected by classical and quantum channels. A direct quantum channel, which connects nodes deterministically, rather than probabilistically, is advantageous for fault-tolerant quantum computation because it reduces the threshold requirements and can achieve larger entanglement rates. Here, we implement deterministic state transfer and entanglement protocols between two superconducting qubits fabricated on separate chips. Superconducting circuits constitute a universal quantum node capable of sending, receiving, storing, and processing quantum information. Our implementation is based on an all-microwave cavity-assisted Raman process which entangles or transfers the qubit state of a transmon-type artificial atom to a time-symmetric itinerant single photon. We transfer qubit states at a rate of 50kHz using the emitted photons which are absorbed at the receiving node with a probability of 98.1±0.1% achieving a transfer process fidelity of 80.02±0.07%. We also prepare on demand remote entanglement with a fidelity as high as 78.9±0.1%. Our results are in excellent agreement with numerical simulations based on a master equation description of the system. This deterministic quantum protocol has the potential to be used as a backbone of surface code quantum error correction across different nodes of a cryogenic network to realize large-scale fault-tolerant quantum computation in the circuit quantum electrodynamic architecture.