A quantum CCD: new and useful

Say ‘CCD’ and people think of megapixels and electronic cameras, which is indeed the most common application of charge-coupled-devices. But when the CCD was invented in the AT&T Bell Labs way back in 1969, it was primarily meant as a shift register. Imagine you have a sequence of charges representing 1’s and 0’s stored in a memory, and you want to read the sequence out. A shift register can help you because at every electronic pulse it transports the charges along the surface of a semiconductor towards a charge reading device. The inventors of the CCD, Willard Boyle and George Smith, in their 1970 article envisioned uses of their device as a memory, a delay line and as an imaging device.

Indeed 40 years on, CCD is the standard for any imaging device: light falls on a photoactive part of silicon and liberates an amount of electric charge that is proportional to the amount of light falling on the pixel. Now imagine thousands of rows, each containing thousands of charges being read out one by one within a fraction of a second. That’s what happens inside your smartphone when you snap a picture.

The introduction to CCD development serves to show what enormous amount of progress a technology can undergo once it has revealed its promise. A similar development could be about to take off with the single-spin CCD that PhD candidate Tim Baart MSc. has developed at Qutech, part of the Kavli Institute of Nanoscience at the Faculty of Applied Sciences of the TU Delft.

In contrast to the normal CCD, the quantum CCD works with single electrons while preserving their electron spins, even after 500 jumps from one electron well to another. The electron spin (in states ‘up’ or ‘down’) can be used to store information in a quantum bit (qubit). In an article in Nature Nanotechnology Baart has shown a qubit stability of about 10 milliseconds before the spin state is destroyed. The article was co-authored by his former colleague Dr Mohammed Shafiei.

Just like Boyle and Smith 45 years ago, Baart and colleagues envision multiple useful applications for their device: storage of quantum information, transferring electron spins from one processor to the next, shuttling information between two quantum wells or transferring quantum information between electron spins and polarised photons. “A lot of technology is developed,” Baart said, “but this device might turn out to be very useful indeed.” As a component of quantum computers, one should add.

That said, engineering at the nanometre scale remains challenging. First of all, the temperature needs to be extremely low (below 0,3 Kelvin) to be able to distinguish between spin up and down. Also, the semiconductor material is not completely homogenous at the submicron scale. Local variations cause differences in the voltages needed to transport the electrons between the wells. Plus: the tiniest imperfection (one other atom) in the crystal structure will interfere with the device’s operation. That’s why the presented prototype contained no more than three quantum wells between which the electron was shuttled hundreds of times.

Further research aims to speed up CCD operation by a factor of ten thousand or more. That would make the single-spin CCD fast enough not only to preserve single quantum states, but also a superposition of these.

Tim Baart, M. Shafiei, T. Fujita, C. Reichl, W. Wegscheider and L.M.K. Vandersypen, ‘Single-Spin CCD’, Nature Nanotechnology, January 4, 2016.