Ultra-fast computers of the future might consist of tiny pieces of superconducting material linked electrically to equally small mechanical resonators, the former providing the processing power and the latter the memory. That is the prospect raised by new work carried out by an international group of physicists, showing that quantum information can be passed between the two kinds of component in such a way that this delicate information might be protected from environmental interference.
Quantum computers exploit the counterintuitive idea that tiny objects can exist in more than one state at the same time. Rather than processing bits – which are either 0 or 1 – such devices instead manipulate qubits – which can be 0 and 1 simultaneously – potentially allowing vast numbers of operations to be carried out in parallel and rendering these devices far quicker than classical computers.
Physicists are working on a number of different kinds of quantum computer but all have their downsides. Some exploit the spin of individual particles, such as atoms, molecules or photons. The quantum states in these devices can be made quite robust against interference from outside – one of the biggest challenges in building a workable quantum computer – but they require bulky apparatus that is not well suited to building computers with large numbers of qubits. Suitable scaling up should not be a problem for solid-state designs, however, such as devices that exploit the quantum-mechanical properties of superconductors. But these devices are extremely susceptible to electromagnetic interference.
“Hybrid quantum systems” attempt to overcome these problems by combining the best aspects of different approaches. In the latest research, Mika Sillanpää and colleagues at Aalto University in Finland have combined a superconducting qubit with two kinds of resonators – one mechanical and the other electrical. They have shown that vibrational quanta – known as phonons – from the mechanical resonator can be sent to and from the superconducting qubit, which acts like an artificial atom, and then be detected in the form of electromagnetic quanta (photons) using the electrical resonator.
All three components are made from aluminium laid down onto a single sapphire substrate measuring a little over 1 mm2. The moving part of the mechanical resonator consists of a flat piece of aluminium measuring 5 μm by 4 μm suspended some 50 nm above one end of the superconducting circuit, while its main components are two Josephson junctions – pairs of superconductors separated by a thin insulator. This circuit, in turn, is connected to one end of the electrical resonator – a waveguide into which microwaves are fed.
The researchers’ first step in testing their device was to expose the superconducting circuit to a magnetic field so as to set up two “charge states” within the circuit as if they had created an atom with two energy levels. They then fed an alternating current into the circuit with a frequency equal to the energy-level difference of the created atom. This stimulated the atom to “Rabi oscillate” between these two states. Having determined that the qubit worked as planned, the researchers then coupled the qubit to the mechanical resonator. This was done by reducing the frequency of the alternating current feeding the qubit, to the point where the resulting shortfall in the energy quantum required for Rabi oscillations was exactly equal to the energy quantum of the vibrating arm.
To prove that they really had coupled the qubit to the resonator, the researchers monitored the phase of the microwaves in the waveguide. As predicted, they found that they got almost exactly the same phase change as they did when applying all the energy directly to the qubit. Put another way, the phonons had combined with the states of the artificial atom, and these combined states modulated the photons in the waveguide just as the qubit alone had done.
Collaboration member Pertti Hakonen says that the result opens up interesting possibilities for exploring various non-classical states, such as those with a well-defined quantum number of phonons. This is particularly the case, he adds, if the amount by which a single phonon changes the energy spacing in the qubit can be made large as compared with the decay rates of mechanical or electrical oscillations.
Down memory lane
According to Hakonen, the latest research might also form the basis for a quantum memory analogous to the read-only memory of conventional computers in which quantum information is stored as superpositions of different vibrational amplitudes. Many hurdles would need to be overcome to realize this kind of memory, he cautions, such as increasing the frequency of the mechanical resonators in order to raise their energy spacings above the level of thermal noise. This would require making the resonators shorter, which would reduce their coupling with the qubit, thereby making the experiment even harder to perform.
Andrew Briggs of Oxford University in the UK believes that the latest work is an “important step” on the road to long-lived quantum memory. “This shows that an adequate strength of coupling can be achieved to move into the quantum regime,” he says. “It also constitutes progress towards demonstrating quantum phenomena in increasingly macroscopic structures.” He adds that it will be important to extend this research to demonstrate mechanical resonators in the ground state. The lowest state of the Finnish device, which was operated at a temperature of about 25 millikelvin, corresponded to an energy of about 20 quanta.
The research is published in Nature.