--- title: "Comprehensive Quantum Feedback and Feed-Forward" date: "2023-10-29T15:15:25+00:00" modified: "2025-07-31T11:16:17+00:00" url: "https://www.quantum-machines.co/technology/comprehensive-quantum-feedback-and-feed-forward/" description: "A powerful classical processor that orchestrates classical computations and control flow with low-level quantum pulses in real-time" --- # Comprehensive Quantum Feedback and Feed-Forward ![](https://www.quantum-machines.co/wp-content/uploads/2023/10/Banner-feedback1.png) ![](https://www.quantum-machines.co/wp-content/uploads/2023/10/Banner-feedback1.png) [Get Started](https://www.quantum-machines.co/contact-us/) **Adaptive Quantum Circuites (AQCs)** – means having pulses, readout, thresholds, other circuit parameters and control flow dynamically modified, in real-time, based on computation and outcomes of mid-circuit measurements, allowing for unlimited branching and fully adaptive quantum-classical circuits. ![](https://www.quantum-machines.co/wp-content/uploads/2023/10/rect2797-4444444.png) ![](https://www.quantum-machines.co/wp-content/uploads/2023/11/rect2797-11111222222211.png) **Feedback and Feed-Forward** are generally composed of three steps: (1) qubit readout or other input from the quantum system; (2) real-time ultra-fast computation and decision making; (3) dynamical and parametric generation of response drive. The Pulse Processing Unit (PPU) of the OPX does all three, offering arbitrary feedback capabilities and the most advanced quantum controller in the industry. ## Quantum Feedback in Numbers Quantum feedback, in its most arbitrary form, allows utilizing and reacting in real-time to information processed from measurements. This requires a high level of integration between classical and quantum resources. Here we focus on quantum real-time (QRT) latency benchmarks, which refer to the delay required for operations that must occur faster than qubit decoherence. We present the numbers for our [Quantum Machines’ OPX controllers](https://www.quantum-machines.co/press-release/opx1000-to-power-the-quantum-computing-lab-at-skku/). To learn more about the control benchmarking system we use click below. [Read More on Control Benchmarks ](https://www.quantum-machines.co/technology/benchmarking-quantum-controllers/) ![](https://www.quantum-machines.co/wp-content/uploads/2023/10/inNout2.png) [Read More on Control Benchmarks ](https://www.quantum-machines.co/technology/benchmarking-quantum-controllers/) ## OPX+ Latency Benchmarks OPX+ controller’s architecture and its QUA programming language, allow for intuitive programming of comprehensive feedback. The PPU responds to measurements, performs calculations and orchestrates the experiment’s control flow and qubit drives based on measurements. As a result, OPX can do much more than the traditional “play/no play” feedback or two-paths-only branching. It easily supports the implementation of advanced dynamic circuits right out of the box. Here we show OPX+ latency for a few QRT feedback benchmarks, such as *Active Reset* using standard feedback in under 250 ns and *π-pulse calibration* under 300 ns (numbers for analog-to-analog, while analog-to-digital requires half the latency). Read below for an explanation of the protocols. [Get the Full OPX+ Spec Sheet](https://www.quantum-machines.co/see-full-opx-spec-sheet/) ![](https://www.quantum-machines.co/wp-content/uploads/2023/10/rect170111111111111111111851-1.png) [Get the Full OPX+ Spec Sheet](https://www.quantum-machines.co/see-full-opx-spec-sheet/) ## Latency numbers explained ## Conditional Pulse #### Ultra-fast active reset example A pulse is either generated or not, according to the Boolean result of classical processing of measurement results. From the moment the last point of a measurement input pulse comes to the controller, OPX processing unit performs state discrimination and decision making, and then produces a reset pulse conditionally on the qubit being in the excited state. This takes 224 ns for one qubit, the fastest arbitrary analog-to-analog feedback in the industry. Easily extendable to any number of qubits simultaneously, and can be performed aggregating or distributing the processing. ![](https://www.quantum-machines.co/wp-content/uploads/2023/10/conditional-1.png) \#Ultra-fast Active Reset ``` play('pi', 'qubit', condition = I > threshold) `````` ``` ## Real-time Control Flow #### Repeat-until-success active reset example OPX can change the flow of the program in real-time based on the classical processing of measurement results. The ability to respond to events with real-time decision making based on measurements allows for repeat-until-success (RUS) sequences, where the controller responds dynamically to events as they unfold, in a non-deterministic process. For example, RUS active reset allows to continuously reset until the qubit is in the required state with a given probability, to perform experimental shots always in the required conditions without wasting time. ![](https://www.quantum-machines.co/wp-content/uploads/2023/10/control.png) \#Repeat-until-success Active Reset ``` with while_(I > threshold): play('pi', 'qubit') measure('readout', 'resonator', ..., I, Q) ``` ## Parametric Updates #### π-pulse calibration example The OPX can dynamically change parameters (e.g. amplitude, frequency, phase, chirp, threshold, etc.) of a pulse-level operation based on processed measurement results. For example, in a π-pulse calibration, we instruct the pulse processor to change the amplitude of a the pulse dynamically, responding to a measurement, comparing the result to a previous value. Parametric changes are done with ultra-low latency, improving repetition rates of experiments, calibration rates and reducing overhead. ![](https://www.quantum-machines.co/wp-content/uploads/2023/10/parametric.png) \#Dynamic π-pulse Calibration ``` measure('readout', 'resonator', ..., I, Q) `````` assign(A, 1+slope*(I - I0)) play('pi'*amp(A), 'qubit') ``` ## See OPX Comprehensive Feedback in Action ![](https://www.quantum-machines.co/wp-content/uploads/2023/07/naturecommunicationsweb-637598726191877261-1024x375.jpg) #### Real-time two-axis control of a spin qubit Feb 2024 [ Read More ](https://www.nature.com/articles/s41467-024-45857-0) ![](https://www.quantum-machines.co/wp-content/uploads/2023/07/naturecommunicationsweb-637598726191877261-1024x375.jpg) #### Inductively shunted transmons exhibit noise insensitive plasmon states and a fluxon decay exceeding 3 hours Jul 2022 [ Read More ](https://www.nature.com/articles/s41467-023-39656-2) ![](https://www.quantum-machines.co/wp-content/uploads/2023/05/800_492-nature-physics.png) #### Two-level system hyperpolarization using a quantum Szilard engine Jun 2023 [ Read More ](https://www.nature.com/articles/s41567-023-02082-8) ![](https://www.quantum-machines.co/wp-content/uploads/2023/05/800_492-nature-physics.png) #### A quantum electromechanical interface for long-lived phonons June 2023 [ Read More ](https://www.nature.com/articles/s41567-023-02080-w) ![](https://www.quantum-machines.co/wp-content/uploads/2023/01/image-41-1024x251.png) #### 2022 Year in Review: Quantum Research Highlights from Our Customers 2022, what a year it has been! As the research in quantum computing continues to increase ... [Learn more](https://www.quantum-machines.co/blog/2022-year-in-review-quantum-research-highlights-from-our-customers/) ![](https://www.quantum-machines.co/wp-content/uploads/2023/03/Asset-1.svg) #### Extreme Speed-Up Speed-up protocols with loops and dynamic circuits running on the lowest level of the hardware, without uploading or memory issues. ![](https://www.quantum-machines.co/wp-content/uploads/2023/03/best-fidelity.svg) #### Highest Fidelities Calibrate coefficients and parameters in real-time to achieve the highest operation fidelities keeping the up-time high. ![](https://www.quantum-machines.co/wp-content/uploads/2023/04/parametic-pulse-programming.svg) #### Mitigating Eerrors Track important parameters dynamically and with the lowest latencies, to mitigate errors and drifts. For example, update the qubit drive frequency every 500 ns. ![](https://www.quantum-machines.co/wp-content/uploads/2023/04/Quantum-Computing-at-Scale.svg) #### At Any Scale Scale up your system seamlessly, without changes of codes and marginal latency buildup. Your flexible quantum control solution at any scale. ## Additional Resources Blog Finally, a Practical Way to Benchmark Quantum Controllers [ Read More ](https://www.quantum-machines.co/blog/finally-a-practical-way-to-benchmark-quantum-controllers/) Tutorials **How to Dramatically Increase the Initialization Fidelity of Your Qubits with QUA** [ Read More ](https://www.quantum-machines.co/resources/tutorials/how-to-dramatically-increase-the-initialization-fidelity-of-your-qubits-with-qua/) Blog **One Little Push at a Time:** Quantum Trajectories and Weak Measurements [ Read More ](https://www.quantum-machines.co/blog/one-little-push-at-a-time-quantum-trajectories-and-weak-measurements/) Scientific Publications Mid-circuit measurements on a neutral atom quantum processor [ Read More ](https://www.quantum-machines.co/resources/scientific-publications/mid-circuit-measurements-on-a-neutral-atom-quantum-processor/) Scientific Publications Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit [ Read More ](https://www.quantum-machines.co/resources/scientific-publications/direct-manipulation-of-a-superconducting-spin-qubit-strongly-coupled-to-a-transmon-qubit-2/) Blog Finally, a Practical Way to Benchmark Quantum Controllers [Read More](https://www.quantum-machines.co/blog/finally-a-practical-way-to-benchmark-quantum-controllers/) Tutorials **How to Dramatically Increase the Initialization Fidelity of Your Qubits with QUA** [Read More](https://www.quantum-machines.co/resources/tutorials/how-to-dramatically-increase-the-initialization-fidelity-of-your-qubits-with-qua/) Blog **One Little Push at a Time:** Quantum Trajectories and Weak Measurements [Read More](https://www.quantum-machines.co/blog/one-little-push-at-a-time-quantum-trajectories-and-weak-measurements/) Scientific Publications Mid-circuit measurements on a neutral atom quantum processor [Read More](https://www.quantum-machines.co/resources/scientific-publications/mid-circuit-measurements-on-a-neutral-atom-quantum-processor/) Scientific Publications Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit [Read More](https://www.quantum-machines.co/resources/scientific-publications/direct-manipulation-of-a-superconducting-spin-qubit-strongly-coupled-to-a-transmon-qubit-2/) ![](https://www.quantum-machines.co/wp-content/uploads/2023/03/OPX2.png) Request the Full OPX+ Spec Sheet [ Read More ](https://www.quantum-machines.co/see-full-opx-spec-sheet/) ![OPX1000](https://www.quantum-machines.co/wp-content/uploads/2023/08/OPX1000.png) Request the Full OPX1000 Spec Sheet [ Read More ](https://www.quantum-machines.co/request-the-full-opx1000-spec-sheet/) ## Accelerate the Realization of Practical Quantum Computing [Contact us](/contact-us/) ## Take the Next Step ![](https://www.quantum-machines.co/wp-content/uploads/2023/05/Asset-3.svg) Have a specific experiment in mind and wondering about the best quantum control and electronics setup? [Talk to an Expert](https://www.quantum-machines.co/contact-us/) ![](https://www.quantum-machines.co/wp-content/uploads/2023/02/Group-175897-1.svg) Want to see what our quantum control and cryogenic electronics solutions can do for your qubits? [Request a Demo](https://www.quantum-machines.co/request-demo/)