Research

Preprint articles

2025

1. 

   Bias-preserving and error-detectable entangling operations in a superconducting dual-rail system

   Nitish Mehta, James D. Teoh, Taewan Noh, and 26 more authors

   Mar 2025

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   For useful quantum computation, error-corrected machines are required that can dramatically reduce the inevitable errors experienced by physical qubits. While significant progress has been made in approaching and exceeding the surface-code threshold in superconducting platforms, large gains in the logical error rate with increasing system size remain out of reach. This is due both to the large number of required physical qubits and the need to operate far below threshold. Importantly, by exploiting the biases and structure of the physical errors, this threshold can be raised. Erasure qubits achieve this by detecting certain errors at the hardware level. Dual-rail qubits encoded in superconducting cavities are a promising erasure qubit wherein the dominant error, photon loss, can be detected and converted to an erasure. In these approaches, the complete set of operations, including two qubit gates, must be high performance and preserve as much of the desirable hierarchy or bias in the errors as possible. Here, we design and realize a novel two-qubit gate for dual-rail erasure qubits based on superconducting microwave cavities. The gate is high-speed (∼500 ns duration), and yields a residual gate infidelity after error detection below 0.1%. Moreover, we experimentally demonstrate that this gate largely preserves the favorable error structure of idling dual-rail qubits, making it ideal for error correction. We measure low erasure rates of ∼0.5% per gate, as well as low and asymmetric dephasing errors that occur at least three times more frequently on control qubits compared to target qubits. Bit-flip errors are practically nonexistent, bounded at the few parts per million level. This error asymmetry has not been well explored but is extremely useful in quantum error correction and flag-qubit contexts, where it can create a faster path to effective error-corrected systems.

2024

1. 

   Niobium coaxial cavities with internal quality factors exceeding 1.5 billion for circuit quantum electrodynamics

   Andrew E. Oriani, Fang Zhao, Tanay Roy, and 6 more authors

   Mar 2024

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   Group-V materials such as niobium and tantalum have become popular choices for extending theperformance of circuit quantum electrodynamics (cQED) platforms allowing for quantum proces-sors and memories with reduced error rates and more modes. The complex surface chemistry ofniobium however makes identifying the main modes of decoherence difficult at millikelvin tempera-tures and single-photon powers. We use niobium coaxial quarter-wave cavities to study the impactof etch chemistry, prolonged atmospheric exposure, and the significance of cavity conditions priorto and during cooldown, in particular niobium hydride evolution, on single-photon coherence. Wedemonstrate cavities with quality factors of Qint ≳ 1.4 × 109 in the single-photon regime, a 15 foldimprovement over aluminum cavities of the same geometry. We rigorously quantify the sensitivityof our fabrication process to various loss mechanisms and demonstrate a 2 − 4× reduction in thetwo-level system (TLS) loss tangent and a 3 − 5× improvement in the residual resistivity over tradi-tional BCP etching techniques. Finally, we demonstrate transmon integration and coherent cavitycontrol while maintaining a cavity coherence of 11.3 ms. The accessibility of our method, which caneasily be replicated in academic-lab settings, and the demonstration of its performance mark anadvancement in 3D cQED.

Journal articles

2024

1. 

   Stimulated Emission of Signal Photons from Dark Matter Waves

   Ankur Agrawal, Akash V. Dixit, Tanay Roy, and 5 more authors

   Phys. Rev. Lett. Apr 2024

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   The manipulation of quantum states of light has resulted in significant advancements in both dark matter searches and gravitational wave detectors. Current dark matter searches operating in the microwave frequency range use nearly quantum-limited amplifiers. Future high frequency searches will use photon counting techniques to evade the standard quantum limit. We present a signal enhancement technique that utilizes a superconducting qubit to prepare a superconducting microwave cavity in a nonclassical Fock state and stimulate the emission of a photon from a dark matter wave. By initializing the cavity in an |n=4⟩ Fock state, we demonstrate a quantum enhancement technique that increases the signal photon rate and hence also the dark matter scan rate each by a factor of 2.78. Using this technique, we conduct a dark photon search in a band around 5.965 GHz (24.67  μeV), where the kinetic mixing angle ε≥4.35×10−13 is excluded at the 90% confidence level.

2022

1. 

   The QICK (Quantum Instrumentation Control Kit): Readout and control for qubits and detectors

   Leandro Stefanazzi, Kenneth Treptow, Neal Wilcer, and 14 more authors

   Review of Scientific Instruments Apr 2022

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   We introduce a Xilinx RF System-on-Chip (RFSoC)-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short), which supports the direct synthesis of control pulses with carrier frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC field-programmable gate array, custom firmware, and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average gate fidelity of Favg=99.93\%. All of the schematics, firmware, and software are open-source.

2021

1. 

   Searching for Dark Matter with a Superconducting Qubit

   Akash V. Dixit, Srivatsan Chakram, Kevin He, and 4 more authors

   Phys. Rev. Lett. Apr 2021

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   Detection mechanisms for low mass bosonic dark matter candidates, such as the axion or hidden photon, leverage potential interactions with electromagnetic fields, whereby the dark matter (of unknown mass) on rare occasion converts into a single photon. Current dark matter searches operating at microwave frequencies use a resonant cavity to coherently accumulate the field sourced by the dark matter and a near standard quantum limited (SQL) linear amplifier to read out the cavity signal. To further increase sensitivity to the dark matter signal, sub-SQL detection techniques are required. Here we report the development of a novel microwave photon counting technique and a new exclusion limit on hidden photon dark matter. We operate a superconducting qubit to make repeated quantum nondemolition measurements of cavity photons and apply a hidden Markov model analysis to reduce the noise to 15.7 dB below the quantum limit, with overall detector performance limited by a residual background of real photons. With the present device, we perform a hidden photon search and constrain the kinetic mixing angle to ε≤1.68×10−15 in a band around 6.011 GHz (24.86  μeV) with an integration time of 8.33 s. This demonstrated noise reduction technique enables future dark matter searches to be sped up by a factor of 1,300. By coupling a qubit to an arbitrary quantum sensor, more general sub-SQL metrology is possible with the techniques presented in this Letter.

2. 

   Seamless High-Q Microwave Cavities for Multimode Circuit Quantum Electrodynamics

   Srivatsan Chakram, Andrew E. Oriani, Ravi K. Naik, and 5 more authors

   Phys. Rev. Lett. Aug 2021

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   Multimode cavity quantum electrodynamics—where a two-level system interacts simultaneously with many cavity modes—provides a versatile framework for quantum information processing and quantum optics. Because of the combination of long coherence times and large interaction strengths, one of the leading experimental platforms for cavity QED involves coupling a superconducting circuit to a 3D microwave cavity. In this work, we realize a 3D multimode circuit QED system with single photon lifetimes of 2 ms across 9 modes of a novel seamless cavity. We demonstrate a variety of protocols for universal single-mode quantum control applicable across all cavity modes, using only a single drive line. We achieve this by developing a straightforward flute method for creating monolithic superconducting microwave cavities that reduces loss while simultaneously allowing control of the mode spectrum and mode-qubit interaction. We highlight the flexibility and ease of implementation of this technique by using it to fabricate a variety of 3D cavity geometries, providing a template for engineering multimode quantum systems with exceptionally low dissipation. This work is an important step towards realizing hardware efficient random access quantum memories and processors, and for exploring quantum many-body physics with photons.

3. PRD

   Axion dark matter experiment: Run 1B analysis details

   C. Bartram, T. Braine, R. Cervantes, and 44 more authors

   Phys. Rev. D Feb 2021

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4. PRL

   Search for Invisible Axion Dark Matter in the 3.3–4.2  μeV Mass Range

   C. Bartram, T. Braine, E. Burns, and 48 more authors

   Phys. Rev. Lett. Dec 2021

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2020

1. PRL

   Extended Search for the Invisible Axion with the Axion Dark Matter Experiment

   T. Braine, R. Cervantes, N. Crisosto, and 40 more authors

   Phys. Rev. Lett. Mar 2020

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2018

1. PRL

   Piezoelectrically Tuned Multimode Cavity Search for Axion Dark Matter

   C. Boutan, M. Jones, B. H. LaRoque, and 31 more authors

   Phys. Rev. Lett. Dec 2018

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Thesis

2022

1. 

   Agrawal, A. (2022). Superconducting Qubit Advantage for Dark Matter (SQuAD). The University of Chicago.

   Bib HTML PDF

   @thesis{Thesis2022,
     author = {Agrawal, Ankur},
     title = {Superconducting Qubit Advantage for Dark Matter (SQuAD)},
     school = {The University of Chicago},
     address = {Chicago, IL, USA},
     year = {2022},
     month = dec,
   }