This pandemic-blighted year isn’t going to top anyone’s list of favourites, but looking on the bright side for a moment, 2020 has seen some remarkable advances in quantum science and technology. Here are a few of the highlights from subfields ranging from quantum fundamentals to quantum computing.
How precise can a thermometer be? In January, Jukka Pekkola, Bayan Karimi and colleagues at the University of Aalto, Finland, and Lund University in Sweden found the answer by building a nanoscale device that can detect fundamental fluctuations in the electron temperature of a sample. The noise level in their thermometer is so low that they could detect the energy change due to the emission of a single microwave photon – all without disturbing the system. Being able to spot such tiny temperature changes could enable advances in fundamental physics, and this “quantum calorimeter” might also be used to make non-invasive measurements of quantum systems such as qubits in superconducting quantum computers.
“Everybody knows” that quantum entanglement is a delicate phenomenon that only survives in ultracold, ultra-low-noise environments. And usually, “everybody” is correct. But in June, physicists at the ICFO in Barcelona, Spain used a technique called a quantum non-demolition measurement to show that at least 1.52 × 1013 out of the 5.32 × 1013 rubidium atoms in their 450 K sample were, in fact, entangled. The team, led by Morgan Mitchell and Jia Kong, also showed that this entanglement was non-local, meaning that it involved atoms that were not close to each other. As well as challenging assumptions about what entanglement looks like, the finding could be important for sensing technologies such as vapour-phase spin-exchange-relaxation-free (SERF) magnetometers that are based on hot, dense clouds of atoms.
As quantum circuits become more complex, so, too, do the elements within them. In June, physicists at the NEST-CNR Nanoscience Institute in Pisa and the University of Salerno, Italy demonstrated the first quantum phase battery: a device that provides a persistent phase bias to the wavefunction of a quantum circuit, similar to the way that a conventional battery provides a persistent voltage bias to an electrical circuit. The device that Francesco Giazotto, Elia Strambini, Andrea Iorio and colleagues built out of InAs nanowires and superconducting Al leads was based on a theoretical concept developed only five years ago by physicists in Spain – a speedy turnaround that illustrates just how fast this field is progressing.
How long does a particle take to tunnel through an energy barrier? To the physicists in the first “golden age” of quantum mechanics, who stumbled across tunnelling while playing around with the Schrödinger equation in the mid-1920s, the question would have seemed outlandish. Such is the progress in quantum fundamentals, however, that we now have an answer. In July, physicists led by Aephraim Steinberg of the University of Toronto, Canada, found that ultracold rubidium-87 atoms spent 0.62 ms tunnelling through a barrier 10 000 times wider than their diameter. While Steinberg acknowledges that his team’s definition of tunnelling time is not the only one available, their experiment sheds much-needed light on a phenomenon that remains poorly understood despite lying at the heart of practical technologies such as scanning tunnelling microscopes and flash memories.
In September 2019, quantum computing experts at Google announced that they had used their Sycamore processor to solve a problem more than a billion times faster than a classical supercomputer. Within weeks, competing experts at IBM were pouring cold water over the claim, suggesting that the upgrade was more like a factor of 1000 (still impressive). Late in 2020, the quest for “quantum advantage” hit the headlines again as researchers led by Jian-Wei Pan and Chao-Yang Lu at the University of Science and Technology of China in Hefei announced that they had performed a quantum computation called Gaussian boson sampling 100 trillion times faster than a supercomputer could. Notably, Pan and Lu constructed their quantum circuit using optical elements rather than superconducting ones. The result is a work of art as well as science, with 100 inputs and 100 outputs generated by some 300 beam splitters and 75 mirrors arranged in a random manner.
Whether such a system can be scaled up is an open question, but it’s also a question that isn’t unique to optical technologies. In a year that many of us would love to forget (and certainly don’t want to relive), developments like this – like the others on our list – are worth cheering.