Super benzene oligomers pave the way for new types of quantum computers

December 19, 2012
High-resolution microscopy reveals that a benzene-like molecule known as HBC has a quantized electron density around its ring framework (left). Theoretical calculations show that the observed quantum states change with different tip positions (right, upper/lower images, respectively). Credit: 2012 American Chemical Society

Scanning tunneling microscopy (STM) is routinely employed by physicists and chemists to capture atomic-scale images of molecules on surfaces. Now, an international team led by Christian Joachim and co-workers from the A*STAR Institute of Materials Research and Engineering has taken STM a step further: using it to identify the quantum states within 'super benzene' compounds using STM conductance measurements. Their results provide a roadmap for developing new types of quantum computers based on information localized inside molecular bonds.

To gain access to the quantum states of hexabenzocoronene (HBC)—a flat aromatic molecule made of interlocked benzene rings—the researchers deposited it onto a gold substrate. According to team member We-Hyo Soe, the weak electronic interaction between HBC and gold is crucial to measuring the system's 'differential conductance'—an instantaneous rate of current charge with voltage that can be directly linked to electron densities within certain quantum states.

After cooling to near-absolute zero temperatures, the team maneuvered its STM tip to a fixed location above the HBC target. Then, they scanned for differential conductance resonance signals at particular voltages. After detecting these voltages, they mapped out the around the entire HBC framework using STM. This technique provided real-space pictures of the compound's molecular orbitals—quantized states that control chemical bonding.

When Joachim and co-workers tried mapping a molecule containing two HBC units, a , they noticed something puzzling. They detected two quantum states from STM measurements taken near the dimer's middle, but only one state when they moved the STM tip to the dimer's edge (see image). To understand why, the researchers collaborated with theoreticians who used high-level calculations to identify which molecular orbitals best reproduced the experimental maps.

Traditional theory suggests that STM differential conductance signals can be assigned to single, unique molecular orbitals. The researchers' calculations, however, show that this view is flawed. Instead, they found that observed quantum states contained mixtures of several molecular orbitals, with the exact ratio dependent upon the position of the ultra-sharp STM tip.

Soe notes that these findings could have a big impact in the field of . "Each measured resonance corresponds to a of the system, and can be used to transfer information through a simple energy shift. This operation could also fulfill some logic functions." However, he adds that advanced, many-body theories will be necessary to identify the exact composition and nature of molecular orbitals due to the location-dependent tip effect.

Explore further: Next up: 'The Nano Lisa'

More information: Soe, W.-H., Wong, H. S., Manzano, C., Grisolia, M., Hliwa, M., Feng, X., Müllen, K. & Joachim, C. Mapping the excited states of single hexa-peri-benzocoronene oligomers. ACS Nano 6, 3230–3235 (2012).

Related Stories

Next up: 'The Nano Lisa'

August 14, 2007

Two atomic-sized images resulting from scientific work at IBM's labs are part of an art exhibit opening today at the United States Patent and Trademark Museum in Alexandria, Virginia.

A molecule that switches on and off

June 10, 2011

A single molecule whose charge state and shape can be changed at will: the latest breakthrough at the CEMES should prove a key advantage in the race for miniaturization. In addition to controlling its charge in a completely ...

Recommended for you

Mathematicians identify limits to heat flow at the nanoscale

November 24, 2015

How much heat can two bodies exchange without touching? For over a century, scientists have been able to answer this question for virtually any pair of objects in the macroscopic world, from the rate at which a campfire can ...

New sensor sends electronic signal when estrogen is detected

November 24, 2015

Estrogen is a tiny molecule, but it can have big effects on humans and other animals. Estrogen is one of the main hormones that regulates the female reproductive system - it can be monitored to track human fertility and is ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (1) Dec 19, 2012
Schrodedingers principle at work? The tips observation Interference influencing the quantum state?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.