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Keeping a quantum bit alive by feed-forward decoupling

Georg Braunbeck, Maximilian Kaindl, A. M. Waeber, Friedemann Reinhard

Published Apr 18, 2020• 0 citations
OpenAlex

Abstract

We present feed-forward decoupling, a scheme to protect a qubit against pure dephasing induced by noise of a classical environment. The scheme builds upon the key idea that this kind of noise can be recorded by a classical device during the qubit evolution, and that its effect can be undone by a suitable control sequence that is conditioned on the measurement result. We specifically demonstrate this scheme on a nitrogen-vacancy (NV) center that strongly couples to current noise in a nearby conductor. By conditioning the readout observable on a measurement of the current, we recover the full qubit coherence and the qubit's intrinsic coherence time $T_2$. We demonstrate that this scheme provides a simple way to implement single-qubit gates with an infidelity of $10^{-2}$ even if they are driven by noisy sources, and we estimate that an infidelity of $10^{-5}$ could be reached with additional improvements. We anticipate this method to find widespread adoption in experiments using fast control pulses driven from strong currents, in particular in nanoscale magnetic resonance imaging, where control of peak currents of 100 mA with a bandwidth of 100 MHz is required.

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We present feed-forward decoupling, a scheme to protect a qubit against pure dephasing induced by noise of a classical environment. The scheme builds upon the key idea that this kind of noise can be recorded by a classical device during the qubit evolution, and that its effect can be undone by a suitable control sequence that is conditioned on the measurement result. We specifically demonstrate this scheme on a nitrogen-vacancy (NV) center that strongly couples to current noise in a nearby conductor. By conditioning the readout observable on a measurement of the current, we recover the full qubit coherence and the qubit's intrinsic coherence time T2T_2T2​. We demonstrate that this scheme provides a simple way to implement single-qubit gates with an infidelity of 10−210^{-2}10−2 even if they are driven by noisy sources, and we estimate that an infidelity of 10−510^{-5}10−5 could be reached with additional improvements. We anticipate this method to find widespread adoption in experiments using fast control pulses driven from strong currents, in particular in nanoscale magnetic resonance imaging, where control of peak currents of 100 mA with a bandwidth of 100 MHz is required.
We present a scheme to neutralize the dephasing effect induced by classical noise on a qubit. The scheme builds upon the key idea that this kind of noise can be recorded by a classical device during the qubit evolution, and that its effect can be undone by a suitable control sequence that is conditioned on the measurement result. We specifically demonstrate this scheme on a nitrogen-vacancy center that strongly couples to current noise in a nearby conductor. By conditioning the readout observable on a measurement of the current, we recover the full qubit coherence and the qubit's intrinsic coherence time T2. We demonstrate that this scheme provides a simple way to implement single-qubit gates with an infidelity of 10−2 even if they are driven by noisy sources, and we estimate that an infidelity of 10−5 could be reached with additional improvements. We anticipate this method to find widespread adoption in experiments using fast control pulses driven from strong currents, in particular, in nanoscale magnetic resonance imaging, where control of peak currents of 100 mA with a bandwidth of 100 MHz is required.
Implementing precise operations on quantum systems is one of the biggest challenges for building quantum devices in a noisy environment. Dynamical decoupling attenuates the destructive effect of the environmental noise, but so far, it has been used primarily in the context of quantum memories. Here, we experimentally demonstrate a general scheme for combining dynamical decoupling with quantum logical gate operations using the example of an electron-spin qubit of a single nitrogen-vacancy center in diamond. We achieve process fidelities >98% for gate times that are 2 orders of magnitude longer than the unprotected dephasing time T2.
Dephasing--phase randomization of a quantum superposition state--is a major obstacle for the realization of high fidelity quantum logic operations. Here, we implement a two-qubit controlled-NOT gate using dynamical decoupling (DD), despite the gate time being more than 1 order of magnitude longer than the intrinsic coherence time of the system. For realizing this universal conditional quantum gate, we have devised a concatenated DD sequence that ensures robustness against imperfections of DD pulses that otherwise may destroy quantum information or interfere with gate dynamics. We compare its performance with three other types of DD sequences. These experiments are carried out using a well-controlled prototype quantum system--trapped atomic ions coupled by an effective spin-spin interaction. The scheme for protecting conditional quantum gates demonstrated here is applicable to other physical systems, such as nitrogen vacancy centers, solid state nuclear magnetic resonance, and circuit quantum electrodynamics.
Realistic quantum computing is subject to noise. Therefore, an important frontier in quantum computing is to implement noise-resilient quantum control over qubits. At the same time, dynamical decoupling can protect the coherence of qubits. Here we demonstrate non-trivial quantum evolution steered by dynamical decoupling control, which simultaneously suppresses noise effects. We design and implement a self-protected controlled-NOT gate on the electron spin of a nitrogen-vacancy centre and a nearby carbon-13 nuclear spin in diamond at room temperature, by employing an engineered dynamical decoupling control on the electron spin. Final state fidelity of 0.91(1) is observed in preparation of a Bell state using the gate. At the same time, the qubit coherence time is elongated at least 30 fold. The design scheme does not require the dynamical decoupling control to commute with the qubit interaction and therefore works for general qubit systems. This work marks a step towards implementing realistic quantum computing systems. Realistic quantum computers require a high degree of qubit control and must also be resilient to noise. Using dynamical decoupling control techniques, Liuet al.implement a self-protected controlled-NOT gate for electron and nuclear spins that retains a high final state fidelity.
Realizing individual control on single qubits in a spin-based quantum register is an ever-increasing challenge due to the close proximity of the qubits' resonance frequencies. Current schemes typically suffer from an inherent trade-off between fidelity and qubits selectivity. Here, we report on a scheme which combines noise protection by dynamical decoupling and magnetic gradient based selectivity, to enhance both the fidelity and the selectivity. With a single nitrogen-vacancy center in diamond, we experimentally demonstrate quantum gates with fidelity $=0.9\phantom{\rule{4pt}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{4pt}{0ex}}0.02$ and a $50\ensuremath{-}\mathrm{kHz}$ spectral bandwidth, which is almost an order of magnitude narrower than the unprotected bandwidth. Our scheme will enable selective control of an individual nitrogen-vacancy qubit in an interacting qubits array using relatively moderate gradients of about 1mG/nm1\phantom{\rule{4pt}{0ex}}\mathrm{mG}/\mathrm{nm}1mG/nm.
Methods that preserve coherence broadly impact all quantum information processing and metrology applications. Dynamical decoupling methods accomplish this by protecting qubits in noisy environments but are typically constrained to the limit where the qubits themselves are non-interacting. Here we consider the alternate regime wherein the inter-qubit couplings are of the same order as dephasing interactions with the environment. We propose and demonstrate a multi-pulse protocol that protects transverse spin states by suitably Hamiltonian engineering the inter-spin coupling while simultaneously suppressing dephasing noise on the qubits. We benchmark the method on 13C nuclear spin qubits in diamond, dipolar coupled to each other and embedded in a noisy electronic spin bath, and hyperpolarized via optically pumped NV centers. We observe effective state lifetimes of 13C nuclei $T_2^{\prime}\approx$2.5s at room temperature, an extension of over 4700-fold over the conventional T2∗T_2^{\ast}T2∗​ free induction decay. The spins are continuously interrogated during the applied quantum control, resulting in 13C NMR line narrowing and an $>$500-fold boost in SNR due to the lifetime extension. Together with hyperpolarization spin interrogation is accelerated by >1011>10^{11}>1011 over conventional 7T NMR. This work suggests strategies for the dynamical decoupling of coupled qubit systems with applications in a variety of experimental platforms.