A variety of new colour centres (luminescing crystal defects that can emit individual photons) have been found in light-emitting diodes made from silicon carbide (SiC). The result confirms once again that it is a promising single-photon source and a good material out of which to make quantum bits (qubits).

Single-photon emitters operating at room temperature could be used in on-chip quantum communication applications, and as a source of “flying” qubits for quantum computers. Such computers exploit the ability of quantum particles to be in a “superposition” of two or more states at the same time unlike classical computers that store and process information as “bits” that can have one of two logic states – “0” or “1”

Quantum computers could, in principle, outperform classical computers on certain tasks, like code decryption for example, because their processing speed should increase exponentially with the number of qubits of information involved. In reality, it is difficult to create even the simplest quantum computer, however, because the fragile nature of these quantum states means that they are easily destroyed and are difficult to control.

Colour centres in SiC

In recent years, there have many studies on point defects (or colour centres) in SiC, a material that is already widely used in high-power electronics thanks to its high thermal conductivity and high maximum current density to name but two good properties. These defects possess electron spin states that can be coherently controlled and manipulated as qubits using light.

Researchers led by Jorg Wrachtrup of the University of Stuttgart in Germany have now confirmed these previous findings.  They have discovered a variety of new colour centres in lateral p-i-n diodes made from a polytype (a crystal structure) of silicon carbide called 4H-SiC that contains naturally occurring defects (or “divacancies”). These defects, which correspond to a missing silicon atom next to a missing carbon atom in the crystal, are very much like the defects in diamond known as “nitrogen-vacancy centres” – that form when a nitrogen impurity finds itself next to a missing carbon atom in the diamond lattice.

Both types of defect form a muti-electron system that has a net angular momentum (or spin) that can be aligned either parallel (“1”) or antiparallel (“0”) to an applied magnetic field, and can so be exploited as a qubit. SiC has an advantage, however, in that it is CMOS-compatible and so could more easily be scaled up to larger systems than hard diamond can.

Photoluminescence experiments

The newly-discovered centres in 4H-SiC emit non-classical light in the visible and near-infrared range. One type of defect can even be excited using electrical means. This means that it might be integrated into compact electronics devices as there would be no need for an additional bulky laser system to optically excite it.

As in previous experiments on diamond nitrogen vacancy centres and SiC point defects, Wrachtrup and co-workers measured the spin of the divacancies in 4H-SiC using photoluminescence. This involved shining laser light onto the sample and collecting the fluorescence light subsequently emitted by it. And, as for diamond nitrogen vacancies, the fluorescence of the silicon carbide divacancies depends on their spin state, so it is possible to “read out” the state of the qubits in this way.

“In this work, a key concept is the generation and manipulation of individual particles of light – photons,” says Marina Radulaski of Stanford University, who was not involved in this study.  “The researchers have not only discovered new colour centres in silicon carbide that can generate single photons at high rates, they have also succeeded in integrating these with electronic elements that can turn the light emission on and off. In a way, they have developed an early prototype of a ‘quantum telegraph’.”

Sophia Economou of Virginia Tech, who was not involved in the study either, agrees: “Electrically operated single photon sources are of interest for miniaturized quantum devices and silicon carbide is an especially promising material for such devices thanks to its industrial maturity, compatibility with CMOS fabrication techniques and low cost. Despite its maturity, however, new colour centres are still being discovered in SiC and these will provide a range of properties in terms of emission frequency, spin structure and photon polarization.

“Wrachtrup and colleagues’ work opens new directions both for device engineering and for basic physics studies,” she adds. “It will be good to further understand the nature of the newly discovered colour centres — their composition, their spin structure, their dynamics under optical versus electrical excitation, and whether they can provide useful transitions for spin-photon entanglement interfaces.”

Full details of the research are reported in Applied Physics Letters 10.1063/1.5032291.

“Just like the transistor is the basis of computers, the ion channel is the basic element in many processes in biology,” explains Peter Burke, who heads the nanotechnology group at the University of California at Irvine. He lists some of the aspects of the body they affect, which include neurons (and hence human thought) but also a variety of other processes, such as synthesis of the energy-storage molecule ATP in mitochondria. In fact, ion channels are crucial to so many physiological functions that they are the target of 25% of drugs produced, yet the technology for studying them using external current amplifiers has remained largely unchanged for 30 years. “Since we can manipulate and sense individual electrons in circuits, I thought it would be interesting to apply some of this knowledge in biology,” adds Burke.

Unsurprisingly, Burke and his colleagues are not the first to try to measure the currents of single ion channels. However, these measurements require a wide range of expertise covering both nanowires and physiology. Fortunately, Burke who holds multiple professorships at UCI was able to draw on his expertise in both these fields, as well as the extensive knowledge and experience of his collaborator Mark Reed from Yale University alongside co-authors Weiwei Zhou, LuyeMu, and Jinfeng Li.

“Previously people were looking at the action potential on a membrane, and treating the membrane like a capacitor;” says Burke.  “This looks at a single channel so it’s a more difficult measurement.” Their nanowire device is the first to integrate the current measurement into the ion channel, which should ultimately allow much greater measurement sensitivity.

Sensing success

The researchers first demonstrated the ability to measure single ion channel currents using carbon-nanotube and graphene devices. However, Burke and his colleagues were keen to demonstrate the capability in a top-down fabricated silicon device that could leverage on the decades of development in that industry. This meant finding the right surface chemistry to attach the lipid bilayers, which act as artificial cell walls, to the silicon nanowire. They could then observe the behaviour of ion channel molecules on these bilayers.

They tested the device with two text book ion channels – alamethicin and gramicidin A. Both have antibiotic functions and work by making the bacterial membrane permeable. Ion channels responsible for vision and other neural functions have additional voltage and light dependencies and multiple states making observations of their behaviour more complicated. In their study the researchers observed the opening and closing of the gramicidin A ion channel and three of the four states in alamethicin.

The channels can open and close due to conformational changes in response to thermal fluctuations or interactions between peptides and the cell wall or by admitting some ions and blocking others. “One of the hopes of this work is to be able to apply this technology to answer in more detail: How do ion channels function?” says Burke.

While the behaviour of these ion channels was not affected by being integrated into the sensor, he points out that for larger ion channels that extrude into the space between the nanowire and membrane, this could be an issue. To tackle this the team have been able to demonstrate how to vary the space between nanowire and membrane with a tether.

Dynamic modelling

Different ion channels also have different opening times, leading to different “spike widths” in the observed current. “To be able to understand the dynamic behaviour of the ion channel itself we need to understand the behaviour of the ion channel within the circuit it makes with the sensing device,” Burke tells Physics World Materials. “We have a comprehensive circuit model that explains the connected system, which I call the ‘modified Hodgkin-Huxley model’.”

Alan Lloyd Hodgkin and Andrew Fielding Huxley won the Nobel Prize in Physiology and Medicine in 1963 for their mathematical description of how ionic transport gives rise to action potentials and allows them to propagate and carry signals through the neural system, enabling movement and other physical responses to sensory input. In the Hodgkin-Huxley model circuit elements represent different parts of the cell – for example a capacitor represents the lipid bilayer; a voltage- and time-dependent conductance represents the ion channel. Since the capacitance of the nanowire device is comparable to the lipid bilayer, Burke and co-authors could add this as another capacitance in the circuit model of the cell.

Next the team will look into scaling up the system to run millions of devices on a chip in parallel. They are also interested in measuring single ion channels from specific systems such as the heart, brain and other organs. Full details are reported in Nano Futures