In recent years, artificial neural networks have become the flagship algorithm of artificial intelligence1. In these systems, neuron activation functions are static, and computing is achieved through standard arithmetic operations. By contrast, a prominent branch of neuroinspired computing embraces the dynamical nature of the brain and proposes to endow each component of a neural network with dynamical functionality, such as oscillations, and to rely on emergent physical phenomena, such as synchronization2,3,4,5,6, for solving complex problems with small networks7,8,9,10,11. This approach is especially interesting for hardware implementations, because emerging nanoelectronic devices can provide compact and energy-efficient nonlinear auto-oscillators that mimic the periodic spiking activity of biological neurons12,13,14,15,16. The dynamical couplings between oscillators can then be used to mediate the synaptic communication between the artificial neurons. One challenge for using nanodevices in this way is to achieve learning, which requires fine control and tuning of their coupled oscillations17; the dynamical features of nanodevices can be difficult to control and prone to noise and variability18. Here we show that the outstanding tunability of spintronic nano-oscillators—that is, the possibility of accurately controlling their frequency across a wide range, through electrical current and magnetic field—can be used to address this challenge. We successfully train a hardware network of four spin-torque nano-oscillators to recognize spoken vowels by tuning their frequencies according to an automatic real-time learning rule. We show that the high experimental recognition rates stem from the ability of these oscillators to synchronize. Our results demonstrate that non-trivial pattern classification tasks can be achieved with small hardware neural networks by endowing them with nonlinear dynamical features such as oscillations and synchronization.
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The datasets generated and analysed during this study are available from the corresponding authors on reasonable request.
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This work was supported by the European Research Council ERC under grant bioSPINspired 682955, the French National Research Agency (ANR) under grant MEMOS ANR-14-CE26-0021, and a public grant overseen by the ANR as part of the ‘Investissements d’Avenir’ programme (Labex NanoSaclay, reference ANR-10-LABX-0035).
Nature thanks F. Hoppensteadt, A. Kent and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
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Extended data figures and tables
The four coupled vortex nano-oscillators are shown. IRFA and IRFB are the microwave currents injected in the strip line by the two microwave sources. HRF is the resulting microwave field. IDC1–4 are the applied direct currents.
Extended Data Fig. 2 Recognition rates obtained by different neural networks on the formant database.
a, Recognition rates of different neural networks on the formant database as a function of the number of trained parameters. b–e, Schematics of the simulated neural networks: b, multi-layer perceptron; c, perceptron; d, RNN; and e, LSTM.
a, Rectified direct voltage measured between oscillator electrodes when the external microwave signal is injected in the strip line above the oscillator and its frequency is swept. Here, the direct current through the oscillator is 5 mA, the magnetic field is 585 mT and the injected microwave power is +1 dBm. b, Oscillator spectrum emission measured during the same frequency sweep as a. c, Proposed differential measurement configuration for CMOS-based detection of synchronization-induced rectified voltages. d, Two-stage CMOS circuit. e, The first stage, composed of two differential amplifiers (green), is followed by a gain stage (blue). VDD, supply voltage; GND, ground. f, Energy consumption of the CMOS circuit for one synchronization detection event, as a function of the amplitude of the generated rectified direct voltages.
The document describes the numerical simulations that were performed to investigate the important features that oscillators should possess to classify accurately.
This file contains the vowel formant database. The spoken vowels used in this study are characterized by a set of frequencies called formants, that we obtain from a subset of the Hillenbrand database (https://homepages.wmich.edu/~hillenbr/voweldata.html). We use the first three formants (F1, F2 and F3) sampled at four different times of the duration of the spoken vowel: at the steady state and at 20%, 50%, and 80% of the vowel duration, respectively (12 parameters in total). When one of these 12 parameters could not be measured or irresolvable formants mergers occurred, Hillenbrand et al. put a zero in this parameter in the database. For our study, we have removed the vowel utterances whose corresponding set of formants is not complete. Moreover, we use the same number of speakers for each vowel. The resulting formant database comprising 37 female speakers that we used is given. The file also includes the input frequencies fA and fB corresponding to each vowel and the coefficients of the two linear combinations that were used to obtain these input frequencies.
Training spin-torque nano-oscillators to classify. This short video shows the learning process for the spin-torque nano-oscillator based hardware. Here four coupled oscillators are trained to recognize seven American vowels. Each vowel is characterized by two frequencies, fA and fB, and is represented by a dot. The different vowels have different colors. The spread within each vowel is due to the different pronunciations of the 37 speakers. Each vowel is applied to the oscillator system as the sum of two microwave magnetic fields which can phase-lock the oscillators. The different colors in the background map represent the different experimental oscillator synchronization configurations obtained for different input frequencies fA and fB. Recognition is achieved when each vowel cloud corresponds to a single synchronization configuration. At the beginning of the video, the oscillator network is randomly set, and the recognition rate is closed to zero. At each training step, the dc current though each oscillator is modified through a learning rule which brings the oscillator frequency closer to the desired synchronization configuration associated to the vowels that have been applied. At the end of the video, after the networks has learned, the dots corresponding to each vowel are in majority encompassed in different regions of the background map corresponding to different synchronization configurations, which gives a recognition rate of 89% on the training data.
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Romera, M., Talatchian, P., Tsunegi, S. et al. Vowel recognition with four coupled spin-torque nano-oscillators. Nature 563, 230–234 (2018) doi:10.1038/s41586-018-0632-y
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