About

David Schuster is a Professor of Applied Physics at Stanford University. His research areas include Atomic, Molecular, & Optical Physics, Photonics, and Quantum Sensing, Simulation & Computation. The information provided indicates his affiliation with Stanford University and his role within the Applied Physics department, focusing on advanced topics in physics related to atomic and molecular systems, photonics, and quantum technologies.

Research topics

• Physics
• Quantum mechanics
• Particle physics
• Theoretical physics
• Quantum electrodynamics

Selected publications

• Implementation of a quantum addressable router using superconducting qubits

  ArXiv.org · 2025-03-06

  preprintOpen accessSenior author

  The implementation of a quantum router capable of performing both quantum signal routing and quantum addressing (a Q2-router) represents a key step toward building quantum networks and quantum random access memories. We realize a Q2-router that uses fixed-frequency transmon qubits to implement a routing protocol based on two native controlled-iSWAP gates. These gates leverage a large ZZ interaction to selectively route information according to a quantum address. We find an estimated average routing fidelity of 95.3%, with errors arising primarily from decoherence or state preparation and measurement. We present a comprehensive calibration and characterization of both the c-iSWAP gates and the overall routing protocol through randomized benchmarking techniques and state tomography.

  PublisherOA PDFDOI

• Flux-Tunable Cavity for Dark Matter Detection

  Physical Review Letters · 2025-11-13 · 2 citations

  articleOpen access

  Developing a dark matter detector with wide mass tunability is an immensely desirable property, yet, it is challenging due to maintaining strong sensitivity. Resonant cavities for dark matter detection have traditionally employed mechanical tuning, moving parts around to change electromagnetic boundary conditions. However, these cavities have proven challenging to operate in sub-Kelvin cryogenic environments due to differential thermal contraction, low heat capacities, and low thermal conductivities. Instead, we develop an electronically tunable cavity architecture by coupling a superconducting 3D microwave cavity with a dc flux tunable superconducting quantum interference device. With a flux delivery system engineered to maintain high coherence in the cavity, we perform a hidden-photon dark matter search below the quantum-limited threshold. A microwave photon counting technique is employed through repeated quantum nondemolition measurements using a transmon qubit. With this device, we perform a hidden-photon search and constrain the kinetic mixing angle to ϵ<8.2×10^{-15} in a tunable band from 5.672 to 5.694 GHz. By coupling multimode tunable cavities to the transmon, wider hidden-photon searching ranges are possible.

  PublisherDOI

• Implementation of a Quantum Addressable Router Using Superconducting Qubits

  PRX Quantum · 2025-08-20 · 1 citations

  articleOpen accessSenior author

  The implementation of a quantum router capable of performing both quantum signal routing and quantum addressing (a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msup> <a:mrow> <a:mrow> <a:mi mathvariant="normal">Q</a:mi> </a:mrow> </a:mrow> <a:mn>2</a:mn> </a:msup> </a:math> -router) represents a key step toward building quantum networks and quantum random access memories. We realize a <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:msup> <d:mrow> <d:mrow> <d:mi mathvariant="normal">Q</d:mi> </d:mrow> </d:mrow> <d:mn>2</d:mn> </d:msup> </d:math> -router that uses fixed-frequency transmon qubits to implement a routing protocol based on two native controlled- <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>i</g:mi> </g:math> gates. These gates leverage a large <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>Z</i:mi> <i:mi>Z</i:mi> </i:math> interaction to selectively route information according to a quantum address. We find an estimated average routing fidelity of <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mn>95.3</k:mn> <k:mi mathvariant="normal">%</k:mi> </k:math> , with errors arising primarily from decoherence or state preparation and measurement. We present a comprehensive calibration and characterization of both the c- <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mi>i</n:mi> </n:math> gates and the overall routing protocol through randomized benchmarking techniques and state tomography.

  PublisherDOI

• A Cascaded Random Access Quantum Memory

  Research Square · 2025-07-09

  preprintOpen access1st authorCorresponding
  PublisherOA PDFDOI

• Millimeter-Wave Superconducting Qubit

  PRX Quantum · 2025-05-23 · 19 citations

  articleOpen accessSenior author

  Manipulating the electromagnetic spectrum at the single-photon level is fundamental for quantum experiments. In the visible and infrared ranges, this can be accomplished with atomic quantum emitters, and with superconducting qubits such control is extended to the microwave range (below 10 GHz). Meanwhile, the region between these two energy ranges presents an unexplored opportunity for innovation. We bridge this gap by scaling up a superconducting qubit to the millimeter-wave range (near 100 GHz). Working in this energy range greatly reduces sensitivity to thermal noise compared to microwave devices, enabling operation at significantly higher temperatures, up to 1 K. This has many advantages by removing the dependence on rare <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi/><a:mn>3</a:mn></a:msup><a:mi>He</a:mi></a:math> for refrigeration, simplifying cryogenic systems, and providing orders-of-magnitude higher cooling power, lending the flexibility needed for novel quantum sensing and hybrid experiments. Using low-loss niobium trilayer junctions, we realize a qubit at 72 GHz cooled to 0.87 K using only <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msup><c:mi/><c:mn>4</c:mn></c:msup><c:mi>He</c:mi></c:math>. We perform Rabi oscillations to establish control over the qubit state, and measure relaxation and dephasing times of 15.8 and 17.4 ns, respectively. This demonstration of a millimeter-wave quantum emitter offers exciting prospects for enhanced sensitivity thresholds in high-frequency photon detection, provides new options for quantum transduction and for scaling up and speeding up quantum computing, enables integration of quantum systems where <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msup><e:mi/><e:mn>3</e:mn></e:msup><e:mi>He</e:mi></e:math> refrigeration units are impractical, and, importantly, paves the way for quantum experiments exploring a novel energy range.

  PublisherOA PDFDOI

• Cavity QED in a high NA resonator

  Science Advances · 2025-02-26 · 15 citations

  articleOpen access

  From fundamental studies of light-matter interaction to applications in quantum networking and sensing, cavity quantum electrodynamics (QED) provides a toolbox to control interactions between atoms and photons. The coherence of interactions is determined by the single-pass atomic absorption and number of photon round-trips. Reducing the cavity loss has enabled resonators supporting 1 million roundtrips but with limited material choices and increased alignment sensitivity. Here, we present a high–numerical aperture, lens-based resonator that pushes the single-atom single-photon absorption probability near its fundamental limit, reducing the mode size at the atom to order λ. This resonator provides a single-atom cooperativity of 1.6 in a cavity where the light circulates ∼10 times. We load single 87 Rb atoms into this cavity, observe strong coupling, and demonstrate cavity-enhanced atom detection with fidelity of 99.55(6)% and survival of 99.89(4)% in 130 μs. Introducing intracavity imaging systems will enable cavity arrays compatible with Rydberg atom array computing technologies, expanding the applicability of the cavity QED toolbox.

  PublisherDOI

• Efficient quantum tomography of a polynomial subspace

  ArXiv.org · 2025-03-01

  preprintOpen access

  Quantum tomography is crucial for characterizing the quantum states of multipartite systems, but its practicality is often limited by the exponentially large dimension of the Hilbert space. Most existing approaches, such as compressed sensing and tensor network-based tomography, impose structural constraints on the state to enable more resource-efficient characterization. However, not all physical states can be well-approximated with highly structured states. Here, we develop a partial quantum tomography method based on direct fidelity estimation (DFE) that focuses on a neighborhood subspace -- the subspace spanned by states physically close to a given target state. Using this generalized DFE method, we estimate elements of the density operator within this subspace in a self-verifying manner. We investigate the efficiency of this approach under different sets of available measurements for various states and find that the set of available measurements significantly impacts the cost of DFE. For example, we show that Pauli measurements alone are insufficient for performing efficient DFE on all product states, whereas the full set of product measurements is sufficient. This method can be applied in many situations, including characterizing quantum systems with confined dynamics and verifying preparations of quantum states and processes.

  PublisherOA PDFDOI

• Niobium coaxial cavities with internal quality factors exceeding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>1.4</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>9</mml:mn> </mml:msup> </mml:math> for circuit quantum electrodynamics

  Physical Review Applied · 2025-08-25 · 1 citations

  articleSenior author

  Group-V materials such as niobium and tantalum have become popular choices for extending the performance of circuit quantum electrodynamics (cQED) platforms, allowing for quantum processors and memories with reduced error rates and more modes. The complex surface chemistry of niobium, however, makes identifying the main modes of decoherence difficult at millikelvin temperatures and single-photon powers. We use niobium coaxial quarter-wave cavities to study the impact of etch chemistry, prolonged atmospheric exposure, and the significance of cavity conditions prior to and during cooldown---in particular, niobium hydride evolution---on single-photon coherence. We demonstrate cavities with quality factors ${Q}_{\mathrm{int}}\ensuremath{\gtrsim}1.4\ifmmode\times\else\texttimes\fi{}{10}^{9}$ in the single-photon regime, a 15-fold improvement over aluminum cavities of the same geometry. We rigorously quantify the sensitivity of our fabrication process to various loss mechanisms and demonstrate a two- to fourfold reduction in the two-level system loss tangent and a three- to fivefold improvement in the residual resistance over traditional buffered chemical polishing etching techniques. Finally, we demonstrate transmon integration and coherent cavity control while maintaining a cavity coherence of 11.3 ms. The accessibility of our method, which can be easily replicated in academic laboratory settings, together with the demonstration of its performance, mark an advancement in three-dimensional cQED.

  PublisherDOI

• Fabrication and analysis of through-glass vias for glass-based electronic packaging using an ultrashort pulsed laser

  Optics and Lasers in Engineering · 2025-05-28 · 5 citations

  articleOpen access

  We report on a comprehensive study of through-glass via (TGV) drilling for glass-based core packaging using a multi-path scanning strategy with a rotating start point and an ultrashort pulsed laser emitting in the green spectral range. In order to optimize the laser drilling quality in a 200 μm thick Borofloat (BF) 33 glass substrate, processing parameters such as laser pulse duration, laser pulse energy, number of passes and TGV hole diameter are studied in detail. After laser drilling, through-holes are evaluated for the hole diameter, taper angle and crack formation using optical microscopy and scanning electron microscopy. Although small taper angles down to 6 ± 0.3° are obtained using a femtosecond pulse duration, microcracks and backside ablation of the glass substrate are observed. Using a laser pulse duration of 5 ps, a laser pulse energy of 25 μJ and 600 passes for TGV fabrication with a hole diameter of 125 μm, a minimum taper angle of down to 7.3 ± 0.1° is obtained without the formation of cracks. Minimum hole pitches of 180 μm are realized for a hole diameter of 125 μm. To meet industrial requirements for temperature resistance in electronics manufacturing, laser-drilled TGV arrays are tested for thermal sensitivity using a furnace for both thermal cycling and shock at temperatures up to 950°. After thermal tests, no crack formation is observed for a minimum hole pitch of 180 μm and maximal laser-induced stresses is reduced by 37.4% down to 8.2 MPa on average, demonstrating the great potential of the applied scanning strategy for glass-based core packaging. • USP laser drilling of TGVs with a multi-path scanning strategy in Borofloat 33 glass. • Influence of laser parameters on the taper angle, TGV diameter and induced stress. • High packaging density and resistance of TGVs to temperature cycling and shocks.

  PublisherDOI

• High-impedance resonators for strong coupling to an electron on helium

  Physical Review Applied · 2025-02-03 · 6 citations

  preprintOpen access

  The in-plane motion of an electron on helium can couple to superconducting microwave resonators via electrical dipole coupling, offering a robust and rapid readout scheme. In previous efforts, microwave resonator designs for electrons on helium have lacked the coupling strength to reach the strong coupling regime, where coherent quantum effects outlast both electron and resonator decoherence rates. High-impedance superconducting microwave resonators offer a path to strong coupling, but integrating such resonators with electrons on helium remains an outstanding challenge. Here, we introduce a high-impedance resonator design compatible with strong coupling to electrons on helium. We fabricate and measure titanium nitride resonators with median internal quality factors of $3.9\ifmmode\times\else\texttimes\fi{}{10}^{5}$ and average impedance of 2.5 $\mathrm{k}\mathrm{\ensuremath{\Omega}}$, promising a sevenfold increase in coupling strength compared with standard 50-$\mathrm{\ensuremath{\Omega}}$ resonators. In addition, we develop a simplified resonator model from the capacitance matrix and sheet inductance that accurately predicts the mode frequencies, significantly simplifying the design process of future resonators for investigating quantum effects with electrons on helium.

  PublisherOA PDFDOI

Recent grants

• CAREER: Developing Hybrid Quantum Systems Using Superconducting Circuits

  NSF · $600k · 2012–2017

Frequent coauthors

• Robert Schoelkopf

  62 shared

• Luigi Frunzio

  Yale University

  58 shared

• Alexandre Blais

  58 shared

• Jonathan Simon

  53 shared

• Srivatsan Chakram

  University of Chicago

  50 shared

• Jens Koch

  48 shared

• A. Wallraff

  ETH Zurich

  42 shared

• Tanay Roy

  University of Chicago

  42 shared

Labs

• Schuster LabPI

  Not provided

Education

• Ph.D., Applied Physics

  Stanford University

  1989

• B.S., Physics

  University of California, Berkeley

  1984