Author Archives: Christopher Monroe

OMG*! Mid-circuit quantum measurement in software!

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Optical control of atomic qubits looks like the only way to scale quantum computers. However, mid-circuit quantum measurement/reset can be challenging in atomic systems, owing to the use of resonance fluorescence. Even a single resonant photon can destroy quantum coherence in data qubits. Conventional approaches to isolate data qubits from those to be measured involve using multiple species (spectral isolation) or atomic shuttling (spatial isolation). Here we perform mid-circuit quantum measurement/reset with a single species atomic qubit system and without any shuttling — all done in software. Our demonstrations, in two different atomic systems, follow previous OMG* ideas on using auxiliary energy levels in a single atom for different types of qubits that are spectrally resolved. Notably, we demonstrate the components of the schemes by leaving the data qubits alone and performing all shelving and measurement operations on jut the qubits to be measured/reset.

  • 171Yb+: “In-situ mid-circuit qubit measurement and reset in a single-species trapped-ion quantum computing system,” Y. Yu, K. Yan, D. Biswas, N. Zhang, B. Harraz, C. Noel, C. Monroe, and A. Kozhanov, arXiv:2504.12544 (2025).
  • 138Ba+: “Non-invasive mid-circuit measurement and reset on atomic qubits,” Z.-Y. Chen, I. Goetting, G. Toh, Y. Yu, M. Shalaev, S. Saha, A. Kalakuntla, H. Shi, C. Monroe, A. Kozhanov, and C. Noel, arXiv 2504.12538 (2025).

*OMG is a goofy acronym for “optical/metastable/ground” state qubits [see Appl. Phys. Lett. 119, 214002 (2021)].

Fast and Hi-Fi Photonic Interconnections between Pristine Quantum Memories

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Trapped ion qubits have the best coherence times and the highest fidelity entangling gate operations of all platforms. In addition to the gates between nearby trapped ion qubits through their Coulomb  interaction, they can also be entangled at a distance through photonic interconnects. Over the past few decades, we have improved the photonic entanglement rate by almost 6 orders of magnitudes, recently hitting 250 Hz. While this seems pretty slow, it is still the fastest photonic interconnect between quantum memories ever demonstrated. We use sympathetic cooling of a different species ion to absorb the heating from the repeated atom excitation, and collect photons with NA/0.8 obectives. By encoding photonic qubits with time-bin wavepackets, we eliminate many errors from conventional polarization encoding, and demonstrate an entanglement fidelity >97%, with much higher fidelities expected in the future. In the course of this research, we discovered a new error source stemming from residual recoil between the two atomic qubits, and show how to eliminate it by synchronizing the atomic motion with the time bins, or adding auxliary operations that “rewind” the recoil.

Many-qubit entangling gate operations

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Typical qubit gate operations are binary, involving operations with up to just two qubits. While this can be universal, it is more efficient for almost all quantum circuits to directly entangle qubits with N-body interactions. With trapped ions, gates are typically realized by applying optical state-dependent displacements to the ions.  By instead using state-dependent squeezing forces, the workhorse quantum gate between pairs of trapped ions is extended to an N-qubit gate. This is an important shortcut for most quantum circuits such as quantum error-correction encoding and quantum optimization circuits, while also providing direct N-body interactions for quantum simulations of many-body quantum systems.

Continuous Quantum Symmetry-Breaking

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Measured correlation between N trapped ion spins after subjecting them to a manybody Ising Hamiltonian. Below shows correlation over distance and vs. N for disordered and symmetry-broken phases.

1D systems exhibiting a continuous symmetry can host exotic quantum phases of matter with true long-range order, but they require long-range interactions. We individually control all the spins in a chain of trapped ions, and their inherent long-range interactions allow the study of such CSB phases with up to 23 spin/qubits.

Certification of a quantum computer without needing another quantum computer

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Flowchart for an interactive prover-verifier process for the certification of a quantum computation.Schematic of an ion trap with multiple zones, and an image of ion chains being separated in space.

We have demonstrated two examples of quantum circuits that allow a verifier to certify a quantum computer, without the verifier having a quantum computer himself. The protocols rely on interactions between prover and verifier and are an extension of “interactive proofs” used in computer science. We demonstrate quantum circuits based on a particular cryptographic function as well as a “learning with errors” model, both passing the threshold for quantum behavior. Importantly, this requires measurement of a subset of qubits and subsequent coherent feedback on the remaining qubits, which we demonstrate in the trapped ion system of up to 15 qubits.

  • “Experimental Implementation of an Efficient Test of Quantumness,” L. Lewis, D. Zhu, A. Gheorghiu, C. Noel, O. Katz, B. Harraz, Q. Wang, A. Risinger, L. Feng, D. Biswas, L. Egan, T. Vidick, M. Cetina, and C. Monroe, arXiv:2209.14316 (2022).
  • “Interactive Protocols for Classically-Verifiable Quantum Advantage,” D. Zhu, G. D. Kahanamoku-Meyer, L. Lewis, C. Noel, O. Katz, B. Harraz, Q. Wang, A. Risinger, L. Feng, D. Biswas, L. Egan, A. Gheorghiu, Y. Nam, T. Vidick, U. Vazirani, N. Y. Yao, M. Cetina, and C. Monroe arXiv:2112.05156 (2022).

Fault-Tolerant Operation of a Quantum Error Correction Code

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Encoding and stabilizer readout schematic for the Bacon-Shor [[9,3,1]] error correction process.

Quantum Circuit Training for Machine Learning Tasks and Simulating Wormholes

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Flowchart for the use of classical feedback to optimize a quantum circuit to generate target quantum states.

We train a small quantum computer to perform “generative modeling” of a particular class of quantum states in one of the first demonstrations of machine learning techniques applied to a quantum computer.  Multiple layers of a standard quantum circuit are generated, with the many classical parameters defining the circuit optimized to minimize a cost function.  We use different types of classical optimization subroutines, showing that in some cases, the classical optimizer is the performance bottleneck.

In a related experiment, we use a variational technique (quantum approximate optimization algorithm or QAOA) to generate “Thermofield Double States.”  These states are pairwise entangled across a ladder network when considered as a whole, but become identical thermal mixed states when considered individually, and their evolution scrambles qubits (see below). TFD states are relevant to theories of quantum gravity and wormholes, where the role of traversing a wormhole takes the form of quantum teleportation across the circuit.

Quantum Scrambling Litmus Test

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Abstract depiction of the information content in a black hole.

Quantum scrambling is the complete diffusion of information throughout a quantum system.  The system is not just entangled, but it is entangled at all depth levels throughout the whole system.  Scrambling is also thought to be the fate of information introduced into a black hole, and is a perfect example of the connection between quantum information and cosmology.  But scrambling it is very difficult to measure, because the space of quantum states is exponentially large.

We didn’t create a black hole in the lab, but instead implemented a 7-qubit circuit on our universal quantum computer that for the first time shows the unambiguous detection of scrambling. The signature is a successful teleportation of information across the circuit.

National Quantum Initiative Hearings

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Profs. Christopher Monroe and Michael Raymer spearheaded the U.S. National Quantum Initiative (NQI), on behalf of the National Photoninics Initiative (NPI).

On May 18, 2018, Monroe testified at the U.S. House of Representatives Committee on Energy and Commerce, “Disruptor Series” hearing on Quantum Computing, then on Oct 24, 2017 at the U.S. House of Representatives Committee on Science, Space and Technology, hearing on American Leadership in Quantum Technology

Optics & Photonics News Coverage
U.S. National Quantum Initiative

Quantum simulation with individual control of 53 qubits

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Tiling of the evolution of the state of 53 trapped ions in a single chain (horizontal) over time (vertical).In one of the largest quantum simulations ever performed, up to 53 trapped ion qubits have been used to simulate properties of many body magnetic interactions.  The qubits are each prepared with individual control, and measured in a single shot with nearly 100% efficiency.  This allows the observation of arbitrary correlation functions that cannot be calculated.  This restricted quantum computer becomes fully programmable and reconfigurable with straightforward modifications in the control lasers.