arXiv subjects

Z. M. McIntyre

Publications and source records attributed to Z. M. McIntyre.

12 recordsLinked to original sources

arXiv2026

One-clean-qubit spectroscopy of simulated Kitaev chains

Spin qubits in gate-defined quantum dots provide a highly programmable platform for simulating condensed-matter phenomena. In this work, we introduce a digital-analog quantum simulation protocol for extracting the single-particle spectrum of a Kitaev chain. The Kitaev chain is mapped onto qubits via the standard Jordan-Wigner transformation and implemented as a drive-engineered, $N$-site transverse-field Ising model (TFIM) in a linear array of quantum dots. We show that periodically toggling the analog-simulation parameters conditioned on the state of a control qubit causes the dynamics of this control qubit to stroboscopically match the output of the one-clean-qubit (DQC1) model of computation, thereby yielding the full spectrum of the TFIM from measurements of a single spin. Classical postprocessing can then be used to isolate the $N$ single-particle energies of the Kitaev chain from the $2^N$ eigenenergies of the TFIM. By varying the strength of the Rabi drive used to engineer the synthetic transverse field, the spectral signature of the crossover from the trivial to the topological regime of the Kitaev chain could then be mapped out with measurements of just one spin.

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arXiv2026

Dimensionality reduction for closed-loop quantum gate calibration

Numerical gate design typically makes use of high-dimensional parameterizations enabling sophisticated, highly expressive control pulses. Developing efficient experimental calibration methods for such gates is a long-standing challenge in quantum control, as on-device calibration requires the optimization of noisy experimental data over high-dimensional parameter spaces. To improve the efficiency of calibrations, we present a systematic method based on SVD stacking for reducing the dimensionality of the parameter space traversed in gate calibration, starting from an arbitrary high-dimensional pulse representation. We use this approach to design and calibrate an $X_{π/2}$ gate robust against amplitude and detuning errors, as well as an $X_{π/2}$ gate robust against coherent errors due to a spectator qubit.

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arXiv2026

Spin Kerr-cat qubits

The use of noise-robust qubit encodings provides a way of extending the lifetime of quantum information at the hardware level. In this work, we introduce the spin Kerr-cat encoding, which leverages a clock transition in the spectrum of quadrupolar nuclei (having spin length $I\geq 1$) to achieve a first-order suppression of noise leading to qubit dephasing. The basis states of the spin Kerr-cat qubit are given by the two lowest levels of a $\mathbb{Z}_2$-symmetric nuclear-spin Hamiltonian and are well approximated by spin cat states. We compute the dephasing time of the spin Kerr-cat qubit under a model of $1/f$ noise, as well as relaxation of the qubit due to breaking of the $\mathbb{Z}_2$ symmetry by charge-noise-induced fluctuations of the quadrupolar tensor. Using measured parameters for antimony (${}^{123}\mathrm{Sb}$) donors in silicon, we estimate that a coherence time of $T_2^*=100$ s could be achieved with this encoding. We propose a two-qubit gate mediated by hopping electrons and estimate that with an enhancement of measured quadrupolar splittings by a factor of $\approx 4$, a gate fidelity of $99\%$ could be achieved for spin Kerr-cat qubits encoded in ${}^{123}\mathrm{Sb}$ nuclear spins, neglecting errors that impact the electron while it is being shuttled and read out.

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arXiv2026

Theory of spin qubits and the path to scalability

Spin qubits have emerged as a leading platform for quantum information processing due to their long coherence times, small footprint, and compatibility with the existing semiconductor industry. We first provide an introduction to the different qubit implementations currently being investigated, including single electron-spin qubits, hole-spin qubits, donor qubits, and multispin encodings. We discuss how the confinement and strain present in semiconductor heterostructures produce addressable levels whose spin degree of freedom can be used to encode a qubit. A large emphasis is placed on reviewing the theoretical foundations and recent experimental demonstrations of proposed mechanisms for long-range coupling, including hybrid approaches based on circuit QED and Andreev qubits, as well as spin shuttling. Finally, we review a recent proposal for linking spin qubits using topological spin textures.

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arXiv2025

Heisenberg-limited metrology from the quantum-quench dynamics of an anisotropic ferromagnet

The emerging field of quantum magnonics seeks to understand and harness the quantum properties of magnons -- quantized collective spin excitations in magnets. Squeezed magnon states arise naturally as the equilibrium ground states of anisotropic ferromagnets and antiferromagnets, representing an important class of nonclassical magnon states. In this work, we show how a qubit-conditioned quantum quench of an anisotropic ferromagnet can be used for Heisenberg-limited parameter estimation based on measurements of the qubit only. In the presence of ground-state squeezing, the protocol yields information about the eigenmode frequency of the coupled magnon-qubit system, whereas no information is gained in the absence of such squeezing. The protocol therefore leverages genuine quantum correlations in the form of magnonic squeezing while simultaneously relying on the equilibrium character of this squeezing -- a feature distinctive to magnetic systems.

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arXiv2025

Loss-tolerant parallelized Bell-state generation with a hybrid cat qudit

Having multiple Bell pairs shared by distant quantum registers provides a key resource for both quantum networks and distributed quantum computing. In this paper, we present a protocol for parallelized Bell-pair generation that uses the phase of a coherent light pulse to encode a qudit, enabling the simultaneous generation of multiple Bell pairs. By encoding a qudit in a basis of light-matter Schrödinger's cat states, the loss of a photon in transit can be detected through an $XX$ parity syndrome, allowing the backaction due to the lost photon to be deterministically corrected through single-qubit rotations. The protocol presented here is compatible with existing technologies in both optical and microwave (circuit QED) architectures, supporting near-term implementation across diverse quantum platforms.

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arXiv2025

Extended Coherent States

Using the formalism of Maya diagrams and ladder operators, we describe the algebra of annihilating operators for the class of rational extensions of the harmonic oscillator. This allows us to construct the corresponding coherent state in the sense of Barut and Girardello. The resulting time-dependent function is an exact solution of the time-dependent Schrödinger equation and a joint eigenfunction of the algebra of annihilators. Using an argument based on Schur functions, we also show that the newly exhibited coherent states asymptotically minimize position-momentum uncertainty.

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arXiv2025

Protocols for inter-module two-qubit gates mediated by time-bin encoded photons

As quantum devices scale to larger numbers of qubits, entangling gates between distant stationary qubits will help provide flexible, long-range connectivity in modular architectures. In this work, we present protocols for implementing long-range two-qubit gates mediated by either Fock-state or time-bin qubits -- photonic encodings that are both compatible with the coplanar waveguide resonators commonly used in circuit quantum electrodynamics (QED). These protocols become deterministic in the limit of vanishing photon loss. Additionally, photon loss can be heralded, signaling a failed two-qubit gate attempt. We model the loss of a time-bin qubit to a dielectric environment consisting of an ensemble of two-level systems (TLSs), which are believed to be the dominant mechanism for dielectric loss in circuit QED architectures. The backaction (on the stationary qubits) associated with the loss of the time-bin qubit is strongly suppressed in a non-Markovian regime where the temporal separation of the time bins is short compared to the dielectric environment's correlation time. This result suggests strategies based on a combination of materials-fabrication and time-bin-qubit optimization for ensuring that the loss of a time-bin qubit is not only heralded, but also approximately backaction-free.

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arXiv2025

Quantum information processing in modular cavity QED architectures

This thesis contains a collection of articles exploring various aspects of quantum information processing with cavity quantum electrodynamics (QED), starting with qubit noise spectroscopy and building towards the longer-term goal of modular quantum-computing architectures equipped with protocols for controlling and correcting the states of distantly separated qubits. The first chapter presents a self-contained introduction to the field of cavity QED. Following this introductory material, we show in Chapter 2 how measurements of the field emitted by a cavity can be leveraged for in-situ qubit noise spectroscopy in the presence of significant inhomogeneous broadening. We also identify a signature of genuinely quantum noise in the cavity output field originating from the non-commutation of bath operators acting on the qubit. In Chapter 3, we present a novel quantum-optical effect whereby a suitable modulation of a longitudinal cavity-qubit coupling can be used to entangle the state of a qubit with the path taken by a multiphoton wavepacket. Entanglement between a qubit and a which-path degree-of-freedom can in turn be used to generate entanglement between distant stationary qubits. As shown in Chapter 4, qubit-which-path entanglement can also be exploited for maximally sensitive estimation of a phase in a Mach-Zehnder interferometry setup, i.e., sensing at the quantum Cramér-Rao bound. In Chapter 5, we discuss strategies for realizing stabilizer measurements using qubit-conditioned phase shifts applied to propagating pulses of radiation. We find that in the context of a subsystem surface code, photon loss during such stabilizer measurements would not introduce any horizontal hook errors on the code qubits. In the sixth and final chapter, we give protocols for performing entangling gates between distant stationary qubits using Fock- or time-bin encoded photons.

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arXiv2024

Flying-cat parity checks for quantum error correction

Long range, multi-qubit parity checks have applications in both quantum error correction and measurement-based entanglement generation. Such parity checks could be performed using qubit-state-dependent phase shifts on propagating pulses of light described by coherent states $\vertα\rangle$ of the electromagnetic field. We consider "flying-cat" parity checks based on an entangling operation that is quantum non-demolition (QND) for Schrödinger's cat states $\vertα\rangle\pm \vert-α\rangle$. This operation encodes parity information in the phase of maximally distinguishable coherent states $\vert\pm α\rangle$, which can be read out using a phase-sensitive measurement of the electromagnetic field. In contrast to many implementations, where single-qubit errors and measurement errors can be treated as independent, photon loss during flying-cat parity checks introduces errors on physical qubits at a rate that is anti-correlated with the probability for measurement errors. We analyze this trade-off for three-qubit parity checks, which are a requirement for universal fault-tolerant quantum computing with the subsystem surface code. We further show how a six-qubit entangled "tetrahedron" state can be prepared using these three-qubit parity checks. The tetrahedron state can be used as a resource for controlled quantum teleportation of a two-qubit state, or as a source of shared randomness with potential applications in three-party quantum key distribution. Finally, we provide conditions for performing high-quality flying-cat parity checks in a state-of-the-art circuit QED architecture, accounting for qubit decoherence, internal cavity losses, and finite-duration pulses, in addition to transmission losses.

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arXiv2024

Homodyne detection is optimal for quantum interferometry with path-entangled coherent states

We present measurement schemes that do not rely on photon-number resolving detectors, but that are nevertheless optimal for estimating a differential phase shift in interferometry with either an entangled coherent state or a qubit-which-path state (where the path taken by a coherent-state wavepacket is entangled with the state of a qubit). The homodyning schemes analyzed here achieve optimality (saturate the quantum Cramér-Rao bound) by maximizing the sensitivity of measurement outcomes to phase-dependent interference fringes in a reduced Wigner distribution. In the presence of photon loss, the schemes become suboptimal, but we find that their performance is independent of the phase to be measured. They can therefore be implemented without any prior information about the phase and without adapting the strategy during measurement, unlike strategies based on photon-number parity measurements or direct photon counting.

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arXiv2024

A photonic which-path entangler based on longitudinal cavity-qubit coupling

We show that a modulated longitudinal cavity-qubit coupling can be used to control the path taken by a multiphoton coherent-state wavepacket conditioned on the state of a qubit, resulting in a qubit-which-path (QWP) entangled state. QWP states can generate long-range multipartite entanglement using strategies for interfacing discrete- and continuous-variable degrees-of-freedom. Using the approach presented here, entanglement can be distributed in a quantum network without the need for single-photon sources or detectors.

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