  Phase change memory (PCM) is one of the leading candidates for neuromorphic
hardware and has recently matured as a storage class memory. Yet, energy and
power consumption remain key challenges for this technology because part of the
PCM device must be self-heated to its melting temperature during reset. Here,
we show that this reset energy can be reduced by nearly two orders of magnitude
by minimizing the pulse width. We utilize a high-speed measurement setup to
probe the energy consumption in PCM cells with varying pulse width (0.3 to 40
nanoseconds) and uncover the power dissipation dynamics. A key finding is that
the switching power (P) remains unchanged for pulses wider than a short thermal
time constant of the PCM ({\tau}$_t$$_h$ < 1 ns in 50 nm diameter device),
resulting in a decrease of energy (E=P{\tau}) as the pulse width {\tau} is
reduced in that range. In other words, thermal confinement during short pulses
is achieved by limiting the heat diffusion time. Our improved programming
scheme reduces reset energy density below 0.1 nJ/{\mu}m$^2$, over an order of
magnitude lower than state-of-the-art PCM, potentially changing the roadmap of
future data storage technology and paving the way towards energy-efficient
neuromorphic hardware
