Low-Latency Digital Signal Processing for Feedback and Feedforward in Quantum Computing and Communication

Salathé, Yves; Kurpiers, Philipp; Karg, Thomas M.; Lang, C. B.; Andersen, Christian Kraglund; Akın, Abdulkadir; Krinner, Sebastian; Eichler, Christopher; Wallraff, Andreas

doi:10.1103/physrevapplied.9.034011

Cited by 72 publications

(39 citation statements)

References 111 publications

(166 reference statements)

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“…Our comparison with Pauli frame updating provides evidence for potential advantages of real-time feedback control for quantum error correction by avoiding errors due to relaxation of the ancilla qubits, a topic to be further investigated. We find the measured steady state fidelity to be mainly limited by decoherence during the feedback delay time of 1 µs, which we expect to further decrease in the future by reducing the latency of feedback electronics [36] and the readout duration [51][52][53][54]. Our results constitute an important step towards the realtime stabilization of entangled multi-qubit states beyond Bell states using higher-weight parity detection [32,33] as required, for example, in quantum error correction codes such as the Bacon-Shor code [55] and the surface code [8,9].…”

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confidence: 88%

“…Repeated parity detection was also achieved for the cat code [34].In most previous implementations of ancilla-based parity detection, changes in the measured parity were accounted for in post-processing rather than actively compensated for using feedback. Conditional feedback, however, was previously used in superconducting circuits to arXiv:1902.06946v2 [quant-ph] 20 Feb 2019 2 initialize and reset qubit states [35,36], to demonstrate a deterministic quantum teleportation protocol [37], and to extend the lifetime of a qubit state encoded as a cat state in a superconducting cavity [20].Here, we report on the experimental realization of repeated XX and ZZ parity detection of two superconducting qubits. In contrast to previous experiments in superconducting circuits, we perform real-time conditional feedback to stabilize the data qubits in a Bell state and to actively reset the ancillary qubit to the ground state, see Fig.…”

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confidence: 99%

“…In most previous implementations of ancilla-based parity detection, changes in the measured parity were accounted for in post-processing rather than actively compensated for using feedback. Conditional feedback, however, was previously used in superconducting circuits to arXiv:1902.06946v2 [quant-ph] 20 Feb 2019 2 initialize and reset qubit states [35,36], to demonstrate a deterministic quantum teleportation protocol [37], and to extend the lifetime of a qubit state encoded as a cat state in a superconducting cavity [20].…”

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Entanglement stabilization using ancilla-based parity detection and real-time feedback in superconducting circuits

et al. 2019

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Fault tolerant quantum computing relies on the ability to detect and correct errors, which in quantum error correction codes is typically achieved by projectively measuring multi-qubit parity operators and by conditioning operations on the observed error syndromes. Here, we experimentally demonstrate the use of an ancillary qubit to repeatedly measure the ZZ and XX parity operators of two data qubits and to thereby project their joint state into the respective parity subspaces. By applying feedback operations conditioned on the outcomes of individual parity measurements, we demonstrate the real-time stabilization of a Bell state with a fidelity of F ≈ 74% in up to 12 cycles of the feedback loop. We also perform the protocol using Pauli frame updating and, in contrast to the case of real-time stabilization, observe a steady decrease in fidelity from cycle to cycle. The ability to stabilize parity over multiple feedback rounds with no reduction in fidelity provides strong evidence for the feasibility of executing stabilizer codes on timescales much longer than the intrinsic coherence times of the constituent qubits.The inevitable interaction of quantum mechanical systems with their environment renders quantum information vulnerable to decoherence [1][2][3]. Quantum error correction aims to overcome this challenge by redundantly encoding logical quantum states into a larger-dimensional Hilbert space and performing repeated measurements to detect and correct for errors [4][5][6][7]. For sufficiently small error probabilities of individual operations, logical errors are expected to become increasingly unlikely when scaling up the number of physical qubits per logical qubit [8,9]. As the concept of quantum error correction provides a clear path toward fault tolerant quantum computing [10], it has been explored in a variety of physical systems ranging from nuclear magnetic resonance [11], to trapped ions [12-15] and superconducting circuits, both for conventional [16][17][18][19] and for continuous variable based encoding schemes [20].Quantum error correction typically relies on the measurement of a set of commuting multi-qubit parity operators, which ideally project the state of the data qubits onto a subspace of their Hilbert space -known as the code space -without extracting information about the logical qubit state [7]. A change in the outcome of repeated parity measurements signals the occurrence of an error, which brings the state of the qubits out of the code space. Such errors can either be corrected for in real-time by applying conditional feedback, or by keeping track of the measurement outcomes in a classical register to reconstruct the quantum state evolution in post-processing. The latter approach -also known as Pauli frame updating [21] -has the advantage of avoiding errors introduced by imperfect feedback and additional decoherence due to feedback latency. Real-time feedback, on the other hand, could be beneficial in the presence of * These authors contributed equally to this work. asymmetric relaxation e...

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Entanglement stabilization using ancilla-based parity detection and real-time feedback in superconducting circuits

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“…In general, this includes signal demodulation, filtering, decimation, signal integration and state discrimination. Special techniques can be used to bring down the latency, such as using frequency-selective kernels to combine the demodulation, filtering and integration in a single processing stage [76], or using digital mixing and multiplier-less filters [77].…”

Section: Tailor-made Room-temperature Controllersmentioning

confidence: 99%

The electronic interface for quantum processors

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2019

Microprocessors and Microsystems

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Quantum computers can potentially provide an unprecedented speed-up with respect to traditional computers. However, a significant increase in the number of quantum bits (qubits) and their performance is required to demonstrate such quantum supremacy. While scaling up the underlying quantum processor is extremely challenging, building the electronics required to interface such large-scale processor is just as relevant and arduous. This paper discusses the challenges in designing a scalable electronic interface for quantum processors. To that end, we discuss the requirements dictated by different qubit technologies and present existing implementations of the electronic interface. The limitations in scaling up such state-of-the-art implementations are analyzed, and possible solutions to overcome those hurdles are reviewed. The benefits offered by operating the electronic interface at cryogenic temperatures in close proximity to the low-temperature qubits are discussed. Although several significant challenges must still be faced by researchers in the field of cryogenic control for quantum processors, a cryogenic electronic interface appears the viable solution to enable large-scale quantum computers able to address world-changing computational problems.

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“…Examples include enabling deterministic one-way optical computing [20][21][22][23], improving the single-photon probability of heralded single-photon sources without increasing the multiphoton pulse probability [24][25][26][27][28][29][30], reducing light intensity in quantum sensing [31], and enabling deterministic quantum teleportation [32][33][34]. For applications in quantum computing and quantum communications, the latency of the measurement and feedforward mechanism are important considerations, especially if multiple iterations of the mechanism are required [35].…”

Section: Introductionmentioning

confidence: 99%

Quantum state correction using a measurement-based feedforward mechanism

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One of the weaknesses of quantum optical state postselection schemes is the low success probability. Typically there is a trade-off between amplifier properties such as success probability and output state fidelity. However, here we present a state comparison amplifier for optical coherent states, which features an active measurement and feedforward mechanism to correct for errors made during the initial amplification. The simple and relatively low latency mechanism allows us to correct for a binary phase alphabet. We demonstrate a significant simultaneous improvement in the amplifier characteristic parameters: output state fidelity, correct state fraction, and success probability. This demonstrates that nondeterministic quantum amplification can be enhanced significantly by measurement and feedforward.

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Low-Latency Digital Signal Processing for Feedback and Feedforward in Quantum Computing and Communication

Cited by 72 publications

References 111 publications

Entanglement stabilization using ancilla-based parity detection and real-time feedback in superconducting circuits

Entanglement stabilization using ancilla-based parity detection and real-time feedback in superconducting circuits

The electronic interface for quantum processors

Quantum state correction using a measurement-based feedforward mechanism

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