Physicists bang the drum for quantum memory

Physicists in the US say they are the first to store – and then retrieve – quantum information in a mechanical oscillator. Their device consists of an extremely thin disc of aluminium that is connected to a microwave circuit. Quantum information encoded in a microwave signal is transferred to the disc, which vibrates much like a drumhead. This information can then be retrieved by converting the mechanical oscillations back into microwaves. Built by Konrad Lehnert and colleagues at the National Institute of Standards and Technology in Boulder and the neighbouring University of Colorado, the team says the device can store quantum information for long enough to make it a potential candidate for a future quantum computer. Such machines could, in principle, outperform conventional computers at certain tasks, but require quantum bits (or qubits) that can store and transfer quantum information without it being destroyed by interacting with the outside world. Cool stuff Like many attempts to make "quantum memory", the qubits built by Lehnert and colleagues are "mesoscopic" objects – small enough to behave as quantum systems, yet large enough to fabricate on a chip and connect to other qubits. Weighing just 48 pg, their drumhead is a circular sheet of aluminium 15 μm in diameter and 100 nm thick. The entire device was chilled to 25 mK and the drum itself is put into its lowest-energy vibrational state (its ground state) using a microwave cooling technique. Quantum information is first encoded in the amplitude and phase of a microwave pulse. This pulse is then sent along a waveguide that sits next to a spiral-like resonant circuit that includes the drum (see figure; click to enlarge). The circuit is designed such that the pulse is completely absorbed by the circuit and the microwave energy is converted to vibrational energy stored in the drum. "At the end of the process, the mechanical oscillator quivers with a particular amplitude and phase, which is the amplitude and phase that the microwave signal had before it was absorbed," explains Lehnert. Towards improved efficiency This absorption process is controlled using a second microwave signal, with the presence or absence of this "transfer field" determining whether the pulse can move from the waveguide to the resonant circuit and vice versa. To read out the quantum information, the transfer field is adjusted so that the vibration of the drum is converted back to a microwave pulse, which can then jump into the waveguide and be measured by Lehnert and colleagues. In a typical experiment the team was able to store quantum information for about 25 μs without it degrading significantly. The team was also able to successfully store and retrieve the pulses in about 65% of attempts. According to Lehnert, this inefficiency arises because the microwave circuit is not perfect and some of the microwave signal is lost. But he is confident that smaller circuits will improve the performance. "We believe the prospects for improving this number are quite good, as other researchers have recently demonstrated microwave circuits with loss about 100 times smaller than we achieved," says Lehnert. Lehnert told physicsworld.com that his team's drums could be used in conjunction with superconducting qubits, which also operate at low temperatures and microwave frequencies. While mechanical oscillators cannot easily be connected with each other to create logic devices, they could be used to store quantum information that is then processed in superconducting devices. In particular, Lehnert points out that the linear nature of the device means that it could be used to store more than one qubit at the same time. Optical boost Another possible application in quantum information is using mechanical oscillators as quantum transducers, which convert quantum information from one form to another. "We are working on using mechanical oscillators to convert quantum information encoded in microwave electrical signals into information encoded as an optical field [light]," Lehnert says. Converting qubits from microwave to optical signals could play an important role in large-scale quantum information systems of the future. Although microwaves are very well suited for transferring quantum information in small ultracold devices, they cannot carry information over large distances. Optical signals, on the other hand, can travel tens or even hundreds of kilometres without losing their quantum nature.

Quantum simulation targets particle physics

A major challenge for physics in the last century was connecting two of its leading theories – quantum physics and special relativity. Researchers are still grappling with the relativistic quantum theories that resulted. Writing in Physical Review Letters, CQT researchers propose a solution to one outstanding problem: how to make predictions from the Thirring Model. The Thirring Model, which dates back to the 1950s, describes the interactions of relativistic fermions. Such particles are moving at speeds close enough to the speed of light to significantly experience time slowing and length contracting, as predicted by special relativity. Weird effects occur when the particles interact, such as the 'dressing' or changing of the particle's mass. Fermions are particles having the quantum property of half-integer spin. But researchers only know exactly what the Thirring Model predicts for fermions that have no mass. In this case, an exact solution to the equation of motion exists. However, when the model needs a mass term – for example, to describe the interactions of electrons or quarks – the mathematical solutions are cumbersome. Predictions for the behaviour of matter at these regimes are then hard or impossible to make. In their new paper, CQT's Dimitris Angelakis and his collaborators suggest a way to make predictions by quantum simulation. Over the past few years, Dimitris and his team have pioneered the idea of simulating quantum systems using photons. This approach was initially surprising because photons do not easily interact, making them apparently poor candidates to simulate interacting particles. The proposed simulations use techniques from nonlinear optics to create the effect of interactions between the photons. The team has previously shown that such photon-based quantum simulators could recreate phenomena such as the 'Luttinger liquid', an elusive state predicted in condensed matter physics, and the Cooper pairing of fermions in a superconductor. A collaboration with mathematical physicist Vladimir Korepin of the State University of New York at Stony Brook, US, who visited CQT in 2012, has now helped Dimitris and his collaborators show that the same kind of simulator system can mimic the Thirring Model, offering a way to tackled the unsolved cases. "He spoke maths and we spoke physics, but we managed to get something that makes sense from our collaboration," jokes Dimitris. The team proposes that polarised photons are confined in a one-dimensional system such as an optical fibre or waveguide, together with atoms that act as a nonlinear medium. Dimitris' student MingXia Huo, a co-author on the paper, performed calculations and simulations to show how the effective interactions could be controlled by outside lasers to make the photons behave like fermions. A photon with vertical polarisation simulates a fermion with spin 'up' and one with horizontal polarisation a fermion with spin 'down'. The proposal could point the way to simulations for other exotic fields theories, such as those describing the interaction of quarks in quantum chromodynamics. The photon-based simulator would be implementable in a table-top set-up, and Dimitris says some groups are close to having the kind of system required to test the simulation proposals. Handily, the fact that the Thirring model has already been solved for massless particles provides a way to test the simulator's output. The unknown solutions for massive particles can then be probed by tuning optical parameters such as the photon-atom interactions which give mass to the particles.. Dimitris is a Principal Investigator at CQT and faculty at the Science Department, Technical University of Crete in Greece. He, MingXia and Vladimir carried out the work with CQT PI Leong Chuan Kwek and Darrick Chang from the Institut de Ciencies Fotoniques (ICFO) near Barcelona, Spain. MingXia is a final-year PhD student co-supervised by Dimitris and Kwek. For more details, see the research article "Mimicking interacting relativistic theories with stationary pulses of light", Phys. Rev. Lett. 110, 100502 (2013); arXiv: 1207.727