Long-range connectivity in a superconducting quantum processor using a ring resonator

  1. Sumeru Hazra,
  2. Anirban Bhattacharjee,
  3. Madhavi Chand,
  4. Kishor V. Salunkhe,
  5. Sriram Gopalakrishnan,
  6. Meghan P. Patankar,
  7. and R. Vijay
Qubit coherence and gate fidelity are typically considered the two most important metrics for characterizing a quantum processor. An equally important metric is inter-qubit connectivity
as it minimizes gate count and allows implementing algorithms efficiently with reduced error. However, inter-qubit connectivity in superconducting processors tends to be limited to nearest neighbour due to practical constraints in the physical realization. Here, we introduce a novel superconducting architecture that uses a ring resonator as a multi-path coupling element with the qubits uniformly distributed throughout its circumference. Our planar design provides significant enhancement in connectivity over state of the art superconducting processors without any additional fabrication complexity. We theoretically analyse the qubit connectivity and experimentally verify it in a device capable of supporting up to twelve qubits where each qubit can be connected to nine other qubits. Our concept is scalable, adaptable to other platforms and has the potential to significantly accelerate progress in quantum computing, annealing, simulations and error correction.

Multi-mode superconducting circuits for realizing strongly coupled multi-qubit processor units

  1. Tanay Roy,
  2. Madhavi Chand,
  3. Sumeru Hazra,
  4. Suman Kundu,
  5. Kedar Damle,
  6. and R. Vijay
Inter-qubit coupling and qubit connectivity in a processor are crucial for achieving high fidelity multi-qubit gates and efficient implementation of quantum algorithms. Typical superconducting
processors employ relatively weak transverse inter-qubit coupling which are activated via frequency tuning or microwave drives. Here, we propose a class of multi-mode superconducting circuits which realize multiple transmon qubits with all-to-all longitudinal coupling. These „artificial molecules“ directly implement a multi-dimensional Hilbert space that can be easily manipulated due to the always-on longitudinal coupling. We describe the basic technique to analyze such circuits, compute the relevant properties and discuss how to optimize them to create efficient small-scale quantum processors with universal programmability.

Implementation of pairwise longitudinal coupling in a three-qubit superconducting circuit

  1. Tanay Roy,
  2. Suman Kundu,
  3. Madhavi Chand,
  4. Sumeru Hazra,
  5. N. Nehra,
  6. R. Cosmic,
  7. A. Ranadive,
  8. Meghan P. Patankar,
  9. Kedar Damle,
  10. and R. Vijay
We present the „trimon“, a multi-mode superconducting circuit implementing three qubits with all-to-all longitudinal coupling. This always-on interaction enables simple
implementation of generalized controlled-NOT gates which form a universal set. Further, two of the three qubits are protected against Purcell decay while retaining measurability. We demonstrate high-fidelity state swapping operations between two qubits and characterize the coupling of all three qubits to a neighbouring transmon qubit. Our results offer a new paradigm for multi-qubit architecture with applications in quantum error correction, quantum simulations and quantum annealing.

A two-fold quantum delayed-choice experiment in a superconducting circuit

  1. K. Liu,
  2. Y. Xu,
  3. W. Wang,
  4. Shi-Biao Zheng,
  5. Tanay Roy,
  6. Suman Kundu,
  7. Madhavi Chand,
  8. A. Ranadive,
  9. R. Vijay,
  10. Y. P. Song,
  11. L. M. Duan,
  12. and L. Sun
We propose and experimentally demonstrate a two-fold quantum delayed-choice experiment where wave or particle nature of a superconducting interfering device can be post-selected twice
after the interferometer. The wave-particle complementarity is controlled by a quantum which-path detector (WPD) in a superposition of its on and off states implemented through a superconducting cavity. The WPD projected to its on state records which-path information, which manifests the particle nature and destroys the interference associated with wave nature of the system. In our experiment, we can recover the interference signal through a quantum eraser even if the WPD has selected out the particle nature in the first round of delayed-choice detection, showing that a quantum WPD adds further unprecedented controllability to test of wave-particle complementarity through the peculiar quantum delayed-choice measurements.

Broadband parametric amplification with impedance engineering: Beyond the gain-bandwidth product

  1. Tanay Roy,
  2. Suman Kundu,
  3. Madhavi Chand,
  4. Vadiraj A. M.,
  5. A. Ranadive,
  6. N. Nehra,
  7. Meghan P. Patankar,
  8. J. Aumentado,
  9. A. A. Clerk,
  10. and R. Vijay
We present an impedance engineered Josephson parametric amplifier capable of providing bandwidth beyond the traditional gain-bandwidth product. We achieve this by introducing a positive
linear slope in the imaginary component of the input impedance seen by the Josephson oscillator using a λ/2 transformer. Our theoretical model predicts an extremely flat gain profile with a bandwidth enhancement proportional to the square root of amplitude gain. We experimentally demonstrate a nearly flat 20 dB gain over a 640 MHz band, along with a mean 1-dB compression point of -110 dBm and near quantum-limited noise. The results are in good agreement with our theoretical model.