Abstract
Bosonic encoding is an approach for quantum information processing, promising lower hardware overhead by encoding in the many levels of a harmonic-oscillator mode. Scaling to multiple modes requires weak interaction for independent control, yet strong interaction for fast control. Applying fast and efficient universal control on multiple modes remains an open problem. Surprisingly, we find that displacements conditioned on the state of a single-qubit ancilla coupled to multiple harmonic oscillators are sufficient for universal control. We present the conditional-no operation concept, which can be used to reduce the duration of entangling gates. Within this guiding concept, we develop the conditional-not displacement control method which enables fast generation and control of bosonic states in multimode systems weakly coupled to a single-ancilla qubit. Our method is fast despite the weak ancilla coupling. The weak coupling in turn allows for excellent separability and thus independent control. We demonstrate our control on a superconducting transmon qubit weakly coupled to a multimode superconducting cavity. We create both entangled and separable cat states in different modes of the multimode cavity, showing entangling operations at low crosstalk while maintaining independent control of the different modes. We show that the operation time is not limited by the inverse of the coupling rate, which is the typical timescale, and we exceed it by almost 2 orders of magnitude. We verify our results with an efficient method for measurement of the multimode characteristic function which employs our conditional-not displacement. Our results inspire a new approach toward general entangling operations and allow for fast and efficient multimode bosonic encoding and measurement.
- Received 31 January 2023
- Revised 30 December 2023
- Accepted 15 February 2024
DOI:https://doi.org/10.1103/PhysRevX.14.011055
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Achieving efficient and fast control over quantum states is paramount for quantum information processing. For many systems, this control has relied on slow entangling operations and multiple control elements, limiting the speed and efficiency, and increasing the hardware overhead. In this work, we develop a method for fast entangling operations that requires only a single control element, even entangling many states. We showcase this by generating entangled bosonic cat-state qubits—superpositions of coherent states—in superconducting microwave cavities almost 100 times faster than any previous method.
Bosonic qubits encode quantum information in harmonic oscillators, such as the electromagnetic modes of a cavity. This offers robust operation and low hardware overhead. To generate entanglement, we apply to such qubits pulses whose resulting operations depend on the state of a control element (e.g., another qubit). However, to make them dependent, the operations rely on narrow-band pulses, and therefore their speed is limited. We introduce “conditional-not” operations and show that they break this speed limit. The essence of this method is to design broadband pulses with nodes at selected frequencies, which allows the control to be fast. We also show that the method can be extended to multiple modes coupled to the single qubit.
Our work introduces a straightforward approach to controlling multimode systems. Its universal and simple design allows for easy adaptation to additional platforms, including trapped ions with vibrational modes and free electrons interacting with electromagnetic modes.
