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arXiv:2206.07607 · Applied Physics Letters

Preserving entanglement in a solid-spin system using quantum autoencoders

Feifei Zhou, Yu Peng Tian, Yumeng Song, Chu-Dan Qiu, Xiangyu Wang, Mingti Zhou, Bing Chen, Nanyang Xu, Dawei Lu

Published Sep 26, 2022• 7 citations
arXivHTMLPDF

Abstract

Entanglement, as a key resource for modern quantum technologies, is extremely fragile due to the decoherence. Here, we show that a quantum autoencoder, which is trained to compress a particular set of quantum entangled states into a subspace that is robust to decoherence, can be employed to preserve entanglement. The training process is based on a hybrid quantum-classical approach to improve the efficiency in building the autoencoder and reduce the experimental errors during the optimization. Using nitrogen-vacancy centers in diamond, we demonstrate that the entangled states between the electron and nuclear spins can be encoded into the nucleus subspace, which has much longer coherence time. As a result, lifetime of the Bell states in this solid-spin system is extended from 2.22 ± 0.43 μs to 3.03 ± 0.56 ms, yielding a three orders of magnitude improvement. The quantum autoencoder approach is universal, paving the way of utilizing long lifetime nuclear spins as immediate-access quantum memories in quantum information tasks.

DOI
10.1063/5.0120060

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Entanglement, as a key resource for modern quantum technologies, is extremely fragile due to the decoherence. Here, we show that a quantum autoencoder, which is trained to compress a particular set of quantum entangled states into a subspace that is robust to decoherence, can be employed to preserve entanglement. The training process is based on a hybrid quantum-classical approach to improve the efficiency in building the autoencoder and reduce the experimental errors during the optimization. Using nitrogen-vacancy centers in diamond, we demonstrate that the entangled states between the electron and nuclear spins can be encoded into the nucleus subspace, which has much longer coherence time. As a result, lifetime of the Bell states in this solid-spin system is extended from 2.22 ± 0.43 μs to 3.03 ± 0.56 ms, yielding a three orders of magnitude improvement. The quantum autoencoder approach is universal, paving the way of utilizing long lifetime nuclear spins as immediate-access quantum memories in quantum information tasks.
We experimentally demonstrate room-temperature storage of quantum entanglement using two nuclear spins weakly coupled to the electronic spin carried by a single nitrogen-vacancy center in diamond. We realize universal quantum gate control over the three-qubit spin system and produce entangled states in the decoherence-free subspace of the two nuclear spins. By injecting arbitrary collective noise, we demonstrate that the decoherence-free entangled state has coherence time longer than that of other entangled states by an order of magnitude in our experiment.
Abstract Solid-state quantum registers are exceptional for storing quantum information at room temperature with long coherence time. Nevertheless, practical applications toward quantum supremacy require even longer coherence time to allow for more complex algorithms. In this work we propose a quantum register that lies in a decoherence-protected subspace to be implemented with nuclear spins nearby a nitrogen-vacancy center in diamond. The quantum information is encoded in two logical states composed of two carbon-13 nuclear spins, while an electron spin is used as ancilla for initialization and control. Moreover, by tuning an off-axis magnetic field we enable non-nuclear-spin-preserving transitions that we use for preparing and manipulating the register through stimulating Raman adiabatic passage. Furthermore, we consider more elaborated sequences to improve simultaneous control over the system yielding decreased gate time.
The realization of a network of quantum registers is an outstanding challenge in quantum science and technology.We experimentally investigate a network node that consists of a single nitrogen-vacancy center electronic spin hyperfine coupled to nearby nuclear spins.We demonstrate individual control and readout of five nuclear spin qubits within one node.We then characterize the storage of quantum superpositions in individual nuclear spins under repeated application of a probabilistic optical internode entangling protocol.We find that the storage fidelity is limited by dephasing during the electronic spin reset after failed attempts.By encoding quantum states into a decoherence-protected subspace of two nuclear spins, we show that quantum coherence can be maintained for over 1000 repetitions of the remote entangling protocol.These results and insights pave the way towards remote entanglement purification and the realization of a quantum repeater using nitrogen-vacancy center quantum-network nodes.
Understanding and protecting the coherence of individual quantum systems is a central challenge in quantum science and technology.Over the past decades, a rich variety of methods to extend coherence have been developed.A complementary approach is to look for naturally occurring systems that are inherently protected against decoherence.Here, we show that pairs of identical nuclear spins in solids form intrinsically long-lived qubits.We study three carbon-13 pairs in diamond and realize high-fidelity measurements of their quantum states using a single nitrogen-vacancy center in their vicinity.We then reveal that the spin pairs are robust to external perturbations due to a combination of three phenomena: a decoherence-free subspace, a clock transition, and a variant on motional narrowing.The resulting inhomogeneous dephasing time is T Ã 2 ¼ 1.9ð3Þ min, the longest reported for individually controlled qubits.Finally, we develop complete control and realize an entangled state between two spin pairs through projective parity measurements.These long-lived qubits are abundantly present in diamond and other solids and provide new opportunities for ancilla-enhanced quantum sensing and for robust memory qubits for quantum networks.
Spins associated with single defects in solids provide promising qubits for quantum-information processing and quantum networks.Recent experiments have demonstrated long coherence times, highfidelity operations, and long-range entanglement.However, control has so far been limited to a few qubits, with entangled states of three spins demonstrated.Realizing larger multiqubit registers is challenging due to the need for quantum gates that avoid cross talk and protect the coherence of the complete register.In this paper, we present novel decoherence-protected gates that combine dynamical decoupling of an electron spin with selective phase-controlled driving of nuclear spins.We use these gates to realize a ten-qubit quantum register consisting of the electron spin of a nitrogen-vacancy center and nine nuclear spins in diamond.We show that the register is fully connected by generating entanglement between all 45 possible qubit pairs and realize genuine multipartite entangled states with up to seven qubits.Finally, we investigate the register as a multiqubit memory.We demonstrate the protection of an arbitrary single-qubit state for over 75 s-the longest reported for a single solid-state qubit-and show that two-qubit entanglement can be preserved for over 10 s.Our results enable the control of large quantum registers with long coherence times and therefore open the door to advanced quantum algorithms and quantum networks with solid-state spin qubits.
Abstract Quantum networks can enable quantum communication and modular quantum computation. A powerful approach is to use multi-qubit nodes that provide quantum memory and computational power. Nuclear spins associated with defects in diamond are promising qubits for this role. However, dephasing during optical entanglement distribution hinders scaling to larger systems. Here, we show that a 13C-spin quantum memory in isotopically engineered diamond is robust to the optical link operation of a nitrogen-vacancy centre. The memory lifetime is improved by two orders-of-magnitude upon the state-of-the-art, surpassing reported times for entanglement distribution. Additionally, we demonstrate that the nuclear-spin state can survive ionisation and recapture of the nitrogen-vacancy electron. Finally, we use simulations to show that combining this memory with previously demonstrated entanglement links and gates can enable key network primitives, such as deterministic non-local two-qubit gates, paving the way for test-bed quantum networks capable of investigating complex algorithms and error correction.