Empowering high-dimensional quantum computing by traversing the dual bosonic ladder

  1. Long B. Nguyen,
  2. Noah Goss,
  3. Karthik Siva,
  4. Yosep Kim,
  5. Ed Younis,
  6. Bingcheng Qing,
  7. Akel Hashim,
  8. David I. Santiago,
  9. and Irfan Siddiqi
High-dimensional quantum information processing has emerged as a promising avenue to transcend hardware limitations and advance the frontiers of quantum technologies. Harnessing the
untapped potential of the so-called qudits necessitates the development of quantum protocols beyond the established qubit methodologies. Here, we present a robust, hardware-efficient, and extensible approach for operating multidimensional solid-state systems using Raman-assisted two-photon interactions. To demonstrate its efficacy, we construct a set of multi-qubit operations, realize highly entangled multidimensional states including atomic squeezed states and Schrödinger cat states, and implement programmable entanglement distribution along a qudit array. Our work illuminates the quantum electrodynamics of strongly driven multi-qudit systems and provides the experimental foundation for the future development of high-dimensional quantum applications.

Dynamically Reconfigurable Photon Exchange in a Superconducting Quantum Processor

  1. Brian Marinelli,
  2. Jie Luo,
  3. Hengjiang Ren,
  4. Bethany M. Niedzielski,
  5. David K. Kim,
  6. Rabindra Das,
  7. Mollie Schwartz,
  8. David I. Santiago,
  9. and Irfan Siddiqi
Realizing the advantages of quantum computation requires access to the full Hilbert space of states of many quantum bits (qubits). Thus, large-scale quantum computation faces the challenge
of efficiently generating entanglement between many qubits. In systems with a limited number of direct connections between qubits, entanglement between non-nearest neighbor qubits is generated by a series of nearest neighbor gates, which exponentially suppresses the resulting fidelity. Here we propose and demonstrate a novel, on-chip photon exchange network. This photonic network is embedded in a superconducting quantum processor (QPU) to implement an arbitrarily reconfigurable qubit connectivity graph. We show long-range qubit-qubit interactions between qubits with a maximum spatial separation of 9.2 cm along a meandered bus resonator and achieve photon exchange rates up to gqq=2π×0.9 MHz. These experimental demonstrations provide a foundation to realize highly connected, reconfigurable quantum photonic networks and opens a new path towards modular quantum computing.

Quantum Computation of Frequency-Domain Molecular Response Properties Using a Three-Qubit iToffoli Gate

  1. Shi-Ning Sun,
  2. Brian Marinelli,
  3. Jin Ming Koh,
  4. Yosep Kim,
  5. Long B. Nguyen,
  6. Larry Chen,
  7. John Mark Kreikebaum,
  8. David I. Santiago,
  9. Irfan Siddiqi,
  10. and Austin J. Minnich
The quantum computation of molecular response properties on near-term quantum hardware is a topic of significant interest. While computing time-domain response properties is in principle
straightforward due to the natural ability of quantum computers to simulate unitary time evolution, circuit depth limitations restrict the maximum time that can be simulated and hence the extraction of frequency-domain properties. Computing properties directly in the frequency domain is therefore desirable, but the circuits require large depth when the typical hardware gate set consisting of single- and two-qubit gates is used. Here, we report the experimental quantum computation of the response properties of diatomic molecules directly in the frequency domain using a three-qubit iToffoli gate, enabling a reduction in circuit depth by a factor of two. We show that the molecular properties obtained with the iToffoli gate exhibit comparable or better agreement with theory than those obtained with the native CZ gates. Our work is among the first demonstrations of the practical usage of a native multi-qubit gate in quantum simulation, with diverse potential applications to the simulation of quantum many-body systems on near-term digital quantum computers.

Programmable Heisenberg interactions between Floquet qubits

  1. Long B. Nguyen,
  2. Yosep Kim,
  3. Akel Hashim,
  4. Noah Goss,
  5. Brian Marinelli,
  6. Bibek Bhandari,
  7. Debmalya Das,
  8. Ravi K. Naik,
  9. John Mark Kreikebaum,
  10. Andrew N. Jordan,
  11. David I. Santiago,
  12. and Irfan Siddiqi
The fundamental trade-off between robustness and tunability is a central challenge in the pursuit of quantum simulation and fault-tolerant quantum computation. In particular, many emerging
quantum architectures are designed to achieve high coherence at the expense of having fixed spectra and consequently limited types of controllable interactions. Here, by adiabatically transforming fixed-frequency superconducting circuits into modifiable Floquet qubits, we demonstrate an XXZ Heisenberg interaction with fully adjustable anisotropy. This interaction model is on one hand the basis for many-body quantum simulation of spin systems, and on the other hand the primitive for an expressive quantum gate set. To illustrate the robustness and versatility of our Floquet protocol, we tailor the Heisenberg Hamiltonian and implement two-qubit iSWAP, CZ, and SWAP gates with estimated fidelities of 99.32(3)%, 99.72(2)%, and 98.93(5)%, respectively. In addition, we implement a Heisenberg interaction between higher energy levels and employ it to construct a three-qubit CCZ gate with a fidelity of 96.18(5)%. Importantly, the protocol is applicable to various fixed-frequency high-coherence platforms, thereby unlocking a suite of essential interactions for high-performance quantum information processing. From a broader perspective, our work provides compelling avenues for future exploration of quantum electrodynamics and optimal control using the Floquet framework.

High-Fidelity Qutrit Entangling Gates for Superconducting Circuits

  1. Noah Goss,
  2. Alexis Morvan,
  3. Brian Marinelli,
  4. Bradley K. Mitchell,
  5. Long B. Nguyen,
  6. Ravi K. Naik,
  7. Larry Chen,
  8. Christian Jünger,
  9. John Mark Kreikebaum,
  10. David I. Santiago,
  11. Joel J. Wallman,
  12. and Irfan Siddiqi
Ternary quantum information processing in superconducting devices poses a promising alternative to its more popular binary counterpart through larger, more connected computational spaces
and proposed advantages in quantum simulation and error correction. Although generally operated as qubits, transmons have readily addressable higher levels, making them natural candidates for operation as quantum three-level systems (qutrits). Recent works in transmon devices have realized high fidelity single qutrit operation. Nonetheless, effectively engineering a high-fidelity two-qutrit entanglement remains a central challenge for realizing qutrit processing in a transmon device. In this work, we apply the differential AC Stark shift to implement a flexible, microwave-activated, and dynamic cross-Kerr entanglement between two fixed-frequency transmon qutrits, expanding on work performed for the ZZ interaction with transmon qubits. We then use this interaction to engineer efficient, high-fidelity qutrit CZ† and CZ gates, with estimated process fidelities of 97.3(1)% and 95.2(3)% respectively, a significant step forward for operating qutrits on a multi-transmon device.

Effects of Laser-Annealing on Fixed-Frequency Superconducting Qubits

  1. Hyunseong Kim,
  2. Christian Jünger,
  3. Alexis Morvan,
  4. Edward S. Barnard,
  5. William P. Livingston,
  6. M. Virginia P. Altoé,
  7. Yosep Kim,
  8. Chengyu Song,
  9. Larry Chen,
  10. John Mark Kreikebaum,
  11. D. Frank Ogletree,
  12. David I. Santiago,
  13. and Irfan Siddiqi
As superconducting quantum processors increase in complexity, techniques to overcome constraints on frequency crowding are needed. The recently developed method of laser-annealing provides
an effective post-fabrication method to adjust the frequency of superconducting qubits. Here, we present an automated laser-annealing apparatus based on conventional microscopy components and demonstrate preservation of highly coherent transmons. In one case, we observe a two-fold increase in coherence after laser-annealing and perform noise spectroscopy on this qubit to investigate the change in defect features, in particular two-level system defects. Finally, we present a local heating model as well as demonstrate aging stability for laser-annealing on the wafer scale. Our work constitutes an important first step towards both understanding the underlying physical mechanism and scaling up laser-annealing of superconducting qubits.

Scalable High-Performance Fluxonium Quantum Processor

  1. Long B. Nguyen,
  2. Gerwin Koolstra,
  3. Yosep Kim,
  4. Alexis Morvan,
  5. Trevor Chistolini,
  6. Shraddha Singh,
  7. Konstantin N. Nesterov,
  8. Christian Jünger,
  9. Larry Chen,
  10. Zahra Pedramrazi,
  11. Bradley K. Mitchell,
  12. John Mark Kreikebaum,
  13. Shruti Puri,
  14. David I. Santiago,
  15. and Irfan Siddiqi Singh
The technological development of hardware heading toward universal fault-tolerant quantum computation requires a large-scale processing unit with high performance. While fluxonium qubits
are promising with high coherence and large anharmonicity, their scalability has not been systematically explored. In this work, we propose a superconducting quantum information processor based on compact high-coherence fluxoniums with suppressed crosstalk, reduced design complexity, improved operational efficiency, high-fidelity gates, and resistance to parameter fluctuations. In this architecture, the qubits are readout dispersively using individual resonators connected to a common bus and manipulated via combined on-chip RF and DC control lines, both of which can be designed to have low crosstalk. A multi-path coupling approach enables exchange interactions between the high-coherence computational states and at the same time suppresses the spurious static ZZ rate, leading to fast and high-fidelity entangling gates. We numerically investigate the cross resonance controlled-NOT and the differential AC-Stark controlled-Z operations, revealing low gate error for qubit-qubit detuning bandwidth of up to 1 GHz. Our study on frequency crowding indicates high fabrication yield for quantum processors consisting of over thousands of qubits. In addition, we estimate low resource overhead to suppress logical error rate using the XZZX surface code. These results promise a scalable quantum architecture with high performance for the pursuit of universal quantum computation.

Optimizing frequency allocation for fixed-frequency superconducting quantum processors

  1. Alexis Morvan,
  2. Larry Chen,
  3. Jeffrey M. Larson,
  4. David I. Santiago,
  5. and Irfan Siddiqi
Fixed-frequency superconducting quantum processors are one of the most mature quantum computing architectures with high-coherence qubits and low-complexity controls. However, high-fidelity
multi-qubit gates pose tight requirements on individual qubit frequencies in these processors and their fabrication suffers from the large dispersion in the fabrication of Josephson junctions. It is inefficient to make a large number of processors because degeneracy in frequencies can degrade the processors‘ quality. In this article, we propose an optimization scheme based on mixed-integer programming to maximize the fabrication yield of quantum processors. We study traditional qubit and qutrit (three-level) architectures with cross-resonance interaction processors. We compare these architectures to a differential AC-Stark shift based on entanglement gates and show that our approach greatly improves the fabrication yield and also increases the scalability of these devices. Our approach is general and can be adapted to problems where one must avoid specific frequency collisions.

Microscopic Theory of Magnetic Disorder-Induced Decoherence in Superconducting Nb Films

  1. Evan Sheridan,
  2. Thomas F. Harrelson,
  3. Eric Sivonxay,
  4. Kristin A. Persson,
  5. M. Virginia P. Altoé,
  6. Irfan Siddiqi,
  7. D. Frank Ogletree,
  8. David I. Santiago,
  9. and Sinéad M. Griffin
The performance of superconducting qubits is orders of magnitude below what is expected from theoretical estimates based on the loss tangents of the constituent bulk materials. This
has been attributed to the presence of uncontrolled surface oxides formed during fabrication which can introduce defects and impurities that create decoherence channels. Here, we develop an ab initio Shiba theory to investigate the microscopic origin of magnetic-induced decoherence in niobium thin film superconductors and the formation of native oxides. Our ab initio calculations encompass the roles of structural disorder, stoichiometry, and strain on the formation of decoherence-inducing local spin moments. With parameters derived from these first-principles calculations we develop an effective quasi-classical model of magnetic-induced losses in the superconductor. We identify d-channel losses (associated with oxygen vacancies) as especially parasitic, resulting in a residual zero temperature surface impedance. This work provides a route to connecting atomic scale properties of superconducting materials and macroscopic decoherence channels affecting quantum systems.

High-fidelity iToffoli gate for fixed-frequency superconducting qubits

  1. Yosep Kim,
  2. Alexis Morvan,
  3. Long B. Nguyen,
  4. Ravi K. Naik,
  5. Christian Jünger,
  6. Larry Chen,
  7. John Mark Kreikebaum,
  8. David I. Santiago,
  9. and Irfan Siddiqi
The development of noisy intermediate-scale quantum (NISQ) devices has extended the scope of executable quantum circuits with high-fidelity single- and two-qubit gates. Equipping NISQ
devices with three-qubit gates will enable the realization of more complex quantum algorithms and efficient quantum error correction protocols with reduced circuit depth. Several three-qubit gates have been implemented for superconducting qubits, but their use in gate synthesis has been limited due to their low fidelity. Here, using fixed-frequency superconducting qubits, we demonstrate a high-fidelity iToffoli gate based on two-qubit interactions, the so-called cross-resonance effect. As with the Toffoli gate, this three-qubit gate can be used to perform universal quantum computation. The iToffoli gate is implemented by simultaneously applying microwave pulses to a linear chain of three qubits, revealing a process fidelity as high as 98.26(2)%. Moreover, we numerically show that our gate scheme can produce additional three-qubit gates which provide more efficient gate synthesis than the Toffoli and Toffoli gates. Our work not only brings a high-fidelity iToffoli gate to current superconducting quantum processors but also opens a pathway for developing multi-qubit gates based on two-qubit interactions.