Expanding coherence with 2D materials

Materials World magazine
31 Jan 2019

A new kind of graphene-based qubit may advance practical quantum computing say researchers from MIT, USA. Katherine Williams found out more.

Mixing up material combinations has allowed researchers at MIT to develop a supercomputing quantum bit (qubit) with unique properties that may enable faster, more efficient computation.

Superconducting qubits rely on a structure known as a Josephson junction, where an insulator is sandwiched between two superconducting materials. 

Professor William Oliver, MIT Physics Professor and Lincoln Laboratory Fellow says, ‘We constructed a superconducting transmon qubit in the usual manner using aluminium on silicon, but we replaced the usual Al/AlOx/Al Josephson junction with an Al/graphene/Al Josephson weak link. The graphene was encapsulated in hexagonal boron nitride (hBN). 

‘Superconducting qubits are solid-state artificial atoms comprising capacitors, inductors, and interconnects. The inductor is usually realised using a Josephson tunneling junction. When cooled to low temperatures, this circuit features quantized states of electric charge or magnetic flux, depending on the choice of parameter values. We studied the lowest-two energy levels of this artificial atom – a qubit – corresponding to quantum states |0> and |1>. We designed these qubit levels to be separated by an energy of approximately h x 5GHz, in the microwave regime, where h is Planck’s constant.

‘The qubit is then a two-level system, represented as a point on a Bloch sphere. If we think of the Bloch sphere as the planet Earth, then the north pole is state |0>, the south pole is state |1>, and everywhere else on the surface of the Earth is a superposition of states |0> and |1>.’

Despite their near-flawless surface quality, only a few research groups have ever applied van der Waals materials to quantum circuits, and none have previously been shown to exhibit temporal coherence.

Time and space

The MIT team recorded the ‘temporal coherence’ of their graphene qubit for the first time. This involved using a pi-pulse to put the qubit into its excited state of |1> and measuring, on average, how long it takes to relax back to the ground state. A Ramsey interferometry experiment was then used to measure the lifetime of a superposition state. 

The amount of time these qubits stay in this superposition state is referred to as their coherence time. The longer the coherence time the more chance of computing complex problems – in this case the coherence was timed at 55 nanoseconds before a return to ground state.

‘Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits,’ says paper first author Joel I-Jan Wang, a postdoc in the Research Laboratory of Electronics (RLE) at MIT. ‘Ours is the first device to show a measurable coherence time — a primary metric of a qubit — that’s long enough for humans to control.’

Changing the voltage

Currently, only about 1,000 qubits can fit on a single chip. Having qubits controlled by voltage will be especially important as millions of qubits start being crammed on a single chip. 

The MIT team can tune the frequency of the qubit after it has been fabricated. Oliver says, ‘With conventional superconducting qubits with Al/AlOx/Al junctions, we can form superconducting loops called SQUIDs, and use a magnetic flux threading the loop to tune the qubit frequency. The magnetic field is realised by flowing a current through a nearby antenna. However, current always is flowing and can dissipate energy continually.

‘Qubits with graphene junctions, however, can be tuned using a voltage applied to a backgate near the graphene junction. This has the advantage that it does not continually dissipate energy, and it should have lower levels of crosstalk. ‘

The MIT graphene qubit has a brief lifetime, conventional superconducting qubits that hold promise for practical application have documented coherence times of a few tens of microseconds, a few hundred times greater than the researchers’ qubit.

The next step is to improve the coherence time of the graphene-based transmon, says Oliver. ‘The coherence times were limited by energy relaxation time through the back gate. We can vastly improve this by designing the impedance seen by the qubit through this back gate. We have done this and are now fabricating new devices that should exhibit longer lifetimes.’

He continued, ‘In the future, we want to explore the limits of coherence when using hybrid superconducting qubits built from 2D materials.’ 

The researchers are keen not to add to the over-hype of quantum computing devices. However, they see potential roles in the simulation and development of new quantum materials and quantum chemistry. Another possible area is optimisation, where quantum computers may lead to a faster time to solution and/or higher quality solutions to optimisation problems. 

The paper Coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures is published in Nature Nanotechnology: https://go.nature.com/2CZ8Okx