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Research news: Using a single molecule to create a new magnetic field sensor

By Oli Usher, on 26 January 2015

Researchers at UCL and the University of Liverpool have shown a new way to use a single molecule as a magnetic field sensor. In the study, published today in Nature Nanotechnology, the scientists show how magnetism can manipulate the way electricity flows through a single molecule, a key step that could enable the development of magnetic field sensors for hard drives that are a tiny fraction of their present size.

Topographic image generated by a scanning tunnelling microscope (STM) of an iron phthalocyanine molecule separated from an underlying metallic copper surface by a thin barrier of copper nitride. Because the coupling between the molecule and the copper surface is much stronger than the coupling between the molecule and the STM tip (located above the molecule), the electrical response of the molecule to a magnetic field can be enhanced to create new magnetoresistive effects. Credit: Ben Warner, Fadi El Hallak and Cyrus F Hirjibehedin

Topographic image generated by a scanning tunnelling microscope (STM) of an iron phthalocyanine molecule separated from an underlying metallic copper surface by a thin barrier of copper nitride. Because the coupling between the molecule and the copper surface is much stronger than the coupling between the molecule and the STM tip (located above the molecule), the electrical response of the molecule to a magnetic field can be enhanced to create new magnetoresistive effects.
Credit: Ben Warner, Fadi El Hallak and Cyrus F Hirjibehedin

Magnetoresistive effects, in which the electrical conductivity of a substance is manipulated by a magnetic field, have many applications today, including the magnetic field sensors in computer hard drives and unconventional types of computer memory.

In hard drives, magnetised areas on spinning disks are used to store information. As the magnetised areas pass a magnetic sensor, they trigger fluctuations in electric current flowing through the sensor, allowing the data to be read. Making these areas smaller increases a hard drive’s storage capacity without making it bigger, but also requires a smaller sensor.

“However, making smaller sensors isn’t trivial” says Fadi El Hallak, the researcher who conceived of the study and who is now a Magnetics Specialist at hard drive manufacturer Seagate Technology. “It is difficult to use magnetism to control the current flowing through objects the size of single molecules because the response to changes in the magnetic field is often very weak. The magnetic field enclosed by a single molecule is very small and these fields can only induce very small shifts in the molecule’s energy levels, making it very hard to detect.”

To get around this problem, the researchers, based at the London Centre for Nanotechnology, developed a method of magnifying the effect of the magnetism on the flow of current in the detector.

First, the researchers created a junction in which a single magnetic molecule was weakly coupled to two metallic leads. The barriers between the molecule and the nearby metals were high enough that electrical charge in the metals could not flow over the barriers. However a small fraction of the charge can effectively go through the barriers by undergoing quantum mechanical tunnelling. This well-studied process, which is analogous to a person repeatedly walking into a closed door and occasionally magically appearing on the other side, enables a very small current to flow through the molecule when a voltage is applied across it.

To control the molecule’s energy levels, the scientists configured the experiment so that the molecule was much more strongly connected to one metal lead than the other. “This creates a large lever arm in which small shifts in the molecule’s orbitals caused by the magnetic field are magnified” says Cyrus Hirjibehedin, the project’s lead scientist.

Ben Warner, the first author of the paper, adds: “The effect we observed is like that of a very lopsided see-saw, where a small change at one end causes a dramatic change at the other end.”

The team’s research demonstrates a new kind of sensor for magnetic fields that can be created in systems as small as a single molecule. The technology is promising for creating new computer technologies, and further refinement is being explored for commercial applications.


The research is presented in a paper entitled ‘Tunable magnetoresistance in an asymmetrically coupled single 2 molecule junction’, published online today by the journal Nature Nanotechnology.

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Cyrus Hirjibehedin
London Centre for Nanotechnology at UCL
020 7679 2617

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UCL Faculty of Mathematical and Physical Sciences
020 7679 7964


EU News: The race towards quantum computation

By uccasg0, on 27 November 2014

It’s said that the 21st century will be the quantum era. But how close are we to having quantum computers? And are EU researchers leading the charge?

‘Perhaps the quantum computer will change our everyday lives in this century in the same radical way as the classical computer did in the last century’: these were the words of the Nobel committee in 2012 upon awarding Serge Haroche and David Wineland the Nobel Prize for Physics for their work on quantum systems.

It’s said that the 21st century will indeed be the quantum era. But how close are we to having quantum computers? What will their capabilities be? And what challenges does the EU research community face along the way? Researchers, industry representatives and policy makers gathered at last week’s Innovation Summit to explore these questions.

Read the full CORDIS article.

Research news: Spins in silicon are feeling electric

By Oli Usher, on 29 October 2014

Scientists at UCL have developed a new way of changing information stored in quantum bits – a vital technology for ensuring computers continue to increase in power over the next century.

Classical computer architecture is coming close to its limits. The ever increasing power of computer chips rests in part on making the circuits inside them ever smaller – but these are now so small, just a few atoms across, that there is not much further to go.

Quantum computing, in which the 1s and 0s of binary code that computers process are replaced by values that can be both 0 and 1 at the same time, is a promising technology for further improving computer performance. In theory, quantum computers should be able to carry out multiple operations in parallel.

Artist's depiction of donor spins controlled by electric fields. This is a key requirement for scalable quantum computers.

Artist’s depiction of donor spins controlled by electric fields. This is a key requirement for scalable quantum computers.

Classical computers represent 0s and 1s with circuits which are either open or closed – essentially, many billions of microscopic switches. These 0s and 1s are known as ‘bits’. If they are to become a viable replacement for classical computers, quantum computers need to have a chip technology which can physically encode bits which are both 1 and 0 at the same time, using the ability of quantum systems to exist in several states at once (known as quantum superposition). These quantum bits are known as ‘qubits’.

“One way of creating a qubit is to encode the information using the spin in a particle,” explains Gary Wolfowicz, the study’s lead author and a PhD researcher in the London Centre for Nanotechnology at UCL. “The direction of a particle’s spin, and hence its magnetic orientation, follows quantum principles, and can exist in both states at once. The challenge is building a system in which the spin is quite stable, and so doesn’t change on its own, but still easy to modify when you want to manipulate it.”

The team experimented with a material that is already used in classical computer design: the silicon wafers which integrated circuits are etched onto. As in classical computer chips, the silicon had a small amount of a different element – in this case, antimony – dispersed through it. Since antimony atoms have one extra electron in their outer shells, this normally creates a sea of unbound electrons that can move throughout the silicon, the key property that makes them behave as semiconductors and allows transistors to be made out of them. The team’s technique departs from classical computing in what happens next: they immobilise these electrons by cooling down the silicon, then use the antimony atoms as their qubits, encoding information in their spin.

Using a technique that is already widely used in the construction of computer chips is a great advantage. Techniques for creating silicon wafers are now very advanced, meaning that they are extremely pure and have very few defects. This gives silicon an advantage over more novel materials – for example, this purity means the spins in silicon can keep their quantum state for up to a few hours, so the information encoded in them is long-lasting.

“Spin is a magnetic phenomenon, so the easiest way to change its orientation is to apply an oscillating magnetic field whose frequency resonates with the particle’s spin,” Wolfowicz says. “Unfortunately, it is very difficult to apply such magnetic fields locally to individual quantum bits within a processor, but this is necessary in order to be able to control the states of different qubits.” Electric fields are much easier to apply locally – you merely need to apply a voltage to a tiny wire close to your qubit, without the need for large and heat-wasting coils. The trouble is that electric fields do not directly affect spins.

The team’s solution was to use the electric field to pull at the electrons, moving them slightly further away from the antimony nuclei, changing their resonance frequency. This makes it possible to subject the entire silicon crystal to an oscillating magnetic field, but antimony atoms that are being tugged at by the electric field don’t respond. “It’s like a sergeant barking the same orders at a whole platoon of spins” said Prof John Morton, who leads the Quantum Spin Dynamics group at UCL, “but we give some of them earplugs.”

This is the first practical demonstration for spin-qubits in silicon of how to use electric fields to ”switch” on and off the qubit response to the magnetic field. The team has not yet managed to narrow down the effect to a single atom, but combined with recent demonstrations of control of single spins by collaborators in UNSW, this represent a major step towards selective control of qubits, and more importantly a scalable silicon-based quantum computer.


  • The research appears in a paper entitled “Conditionalk control of donor nuclear spins in silicon using Stark shifts”, published in the journal Physical Research Letters

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UCLQ Fellowships 2nd Call

By uccasg0, on 2 October 2014

The UCL Quantum Science and Technology Institute (UCLQ) welcomes proposals from early career researchers to support research projects in quantum science and technology at UCL (there is no nationality restriction, and applications from candidate currently based outside UCL are encouraged) . The aim of this call is help recent graduates develop an independent research career within the remit of the Institute.


UCLQ Secondments 2nd Call

By uccasg0, on 2 October 2014

The UCL Quantum Science and Technology Institute (UCLQ) welcomes proposals to support UCL staff working in this field and wishing to promote their engagement in knowledge transfer, through secondments with companies, social enterprises, charities and other non-academic organisations.


UCLQ Visitors 2nd Call

By uccasg0, on 2 October 2014

The UCL Quantum Science and Technology Institute (UCLQ) welcomes proposals from the UCL community for financial support to invite high profile visitors (from academia, industry and other organisations) to UCL to develop collaborative research ideas in the area of quantum science and technology.


Research news: Pairing old technologies with new for next generation electronic devices

By news editor, on 11 August 2014

UCL scientists have discovered a new method to efficiently generate and control currents based on the magnetic nature of electrons in semi-conducting materials, offering a radical way to develop a new generation of electronic devices.

One promising approach to developing new technologies is to exploit the electron’s tiny magnetic moment, or ‘spin’. Electrons have two properties – charge and spin – and although current technologies use charge, it is thought that spin-based technologies have the potential to outperform the ‘charge’-based technology of semiconductors for the storage and process of information.


In order to utilise electron spins for electronics, or ‘spintronics’, the method of electrically generating and detecting spins needs to be efficient so the devices can process the spin information with low-power consumption. One way to achieve this is by the spin-Hall effect, which is being researched by scientists who are keen to understand the mechanisms of the effect, but also which materials optimise its efficiency. If research into this effect is successful, it will open the door to new technologies.

The spin-Hall effect helps generate ‘spin currents’ which enable spin information transfer without the flow of electric charge currents. Unlike other concepts that harness electrons, spin current can transfer information without causing heat from the electric charge, which is a serious problem for current semiconductor devices. Effective use of spins generated by the spin-Hall effect can also revolutionise spin-based memory applications.

The study published in Nature Materials shows how applying an electric field in a common semiconductor material can dramatically increase the efficiency of the spin-Hall effect which is key for generating and detecting spin from an electrical input.

The scientists reported a 40-times-larger effect than previously achieved in semiconductor materials, with the largest value measured comparable to a record high value of the spin-Hall effect observed in heavy metals such as Platinum. This demonstrates that future spintronics might not need to rely on expensive, rare, heavy metals for efficiency, but relatively cheap materials can be used to process spin information with low-power consumption.

As there are limited amounts of natural resources in the earth and prices of materials are progressively going up, scientists are looking for more accessible materials with which to develop future sustainable technologies, potentially based on electron spin rather than charge. Added to this, the miniaturisation approach of current semiconductor technology will see a point when the trend, predicted by Moore’s law, will come to an end because transistors are as small as atoms and cannot be shrunk any further. To address this, fundamentally new concepts for electronics will be needed to produce commercially viable alternatives which meet demands for ever-growing computing power.

Lead author of the study, Dr Hidekazu Kurebayashi (UCL London Centre for Nanotechnology), said, “We borrowed 50 year old semiconductor phenomena for our modern spintronic research. Our results are the start of the story but are a proof of principle with a promising future for spins; as we know that there is existing matured semiconductor growth technology, we can stand on the shoulders of the giants.”

An international research team of scientists from UCL and University of Cambridge in the UK, Mainz University in Germany, the Institute of Physic of the Academy of Sciences in Czech Republic, and Tohoku University in Japan worked on the study.


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Bex Caygill
UCL Media Relations
020 3108 3846

News update: UCL Quantum Science & Technology Institute launches

By Oli Usher, on 12 May 2014

UCL is today inaugurating a new Quantum Science & Technology Institute (UCLQ). The new institute will coordinate and support research into quantum science and technology across UCL, helping to develop this fast-advancing field of research.

The institute will link researchers across a range of departments and disciplines, including the London Centre for Nanotechnology, and the Departments of Physics & Astronomy, Electronic & Electrical Engineering, Computer Science, Science & Technology Studies, and Chemistry, as well as UCL’s new EPSRC Centre for Doctoral Training in Delivering Quantum Technologies.

Research in UCL Quantum labs. Photo credit: O. Usher (UCL MAPS)

Research in UCL Quantum labs. Photo credit: O. Usher (UCL MAPS)

Prof Michael Arthur, UCL President & Provost said: “The UCL Quantum Science & Technology Institute will push the boundaries of our understanding of quantum science and use these insights to deliver disruptive future technologies. Meeting these challenges requires a major interdisciplinary effort, such as that brought together in UCLQ.”

Quantum science and technology harnesses the special properties of matter on tiny scales, to go beyond the limits of classical physics and conventional technologies. It has potentially revolutionary applications in electronics, computer science, communications and many other important fields.

Key areas of expertise at UCL include:

  • Quantum communication:
    Taking advantage of quantum properties of systems, research in quantum communications aims to develop means of communicating which are completely impervious to eavesdropping – something of crucial importance both to industry and private users.
  • Quantum computation:
    Exploiting the complexity of quantum systems, it is possible to achieve computing power far superior to what is achievable with existing technologies.
  • Quantum metrology and sensing:
    The sensitivity of quantum systems to tiny variations in their environment makes them excellent tools for measurement, with precision reaching single-molecule or single-electron levels. This has potential applications ranging from healthcare to space and defence technologies.

Prof David Price, UCL Vice-Provost (Research) said: “Leading UCL researchers have made pioneering contributions to quantum technologies over several decades, and helped to develop a thriving community of quantum researchers across the UK. UCLQ is our flagship commitment to the next phase of our quantum technologies, which will see these breakthroughs translated to applications in partnership with industry. This will require both large-scale investment in and expert coordination of cross-disciplinary research and development.”

The institute will promote engagement with quantum science and technology, both for the public and for end users, including a range of public events, a network of government and industry stakeholders, and research into the responsible innovation policies.

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Oli Usher
UCL Faculty of Mathematical and Physical Sciences
020 7679 7964

Research news: “Like melting an entire iceberg with a hot poker” – spotting phase transitions triggered by impurities

By Oli Usher, on 7 May 2014

“What a curious feeling,” says Alice in Lewis Carroll’s tale, as she shrinks to a fraction of her size, and everything around her suddenly looks totally unfamiliar. Scientists too have to get used to these curious feelings when they examine matter on tiny scales and at low temperatures: all the behaviour we are used to seeing around us is turned on its head.

In research published today in the journal Nature Communications, UCL scientists have made a startling discovery about a familiar physical effect in this unfamiliar setting.

Phase transitions are a category of physical phenomena in which the properties of a sample and the relationships between the particles that make it up suddenly change. Phase transitions include familiar events such as water turning to ice when temperature drops, or a magnet losing its magnetisation when temperature rises.

On human scales, and at ambient temperatures, phase transitions are well understood. They are linked to the temperature (and hence the vibration, orientation and movement of particles) of an object. In quantum physics, however, phase transitions behave slightly differently, and can even occur close to absolute zero, when virtually no heat is present in a sample. These occur when a factor which affects the whole sample, such as a magnetic field, is changed. But in certain cases, quantum phase transitions can also happen when varying a highly localised factor, such as changing the coupling between a single pair of particles on the edge of such a system. These are called boundary phase transitions and would be like melting an entire iceberg by touching a corner of it with a hot poker.

Dr Abolfazl Bayat and Prof Sougato Bose (both UCL Physics & Astronomy), along with colleagues at other institutions, have probed the nature of boundary phase transitions in quantum systems. For the first time they have identified a measurable quantity that can label the distinct phases of such a system.

“When phase transitions happen, scientists often talk of an ‘order parameter’,” says Bayat. “This is the value which suddenly changes, such as the orientation of magnetisation in a metal, or the spacing of atoms in a sample which determines whether it is liquid or solid. Getting a handle on order parameters in quantum boundary phase transitions is much harder, but we have identified one in this research.”Understanding quantum phase transitions

Bayat and Bose studied a system in which a tiny change applied to a single pair of impurities causes a change in the entire system. The two impurities can either be entangled with each other, or can separately be entangled with the left and right hand parts of the system. Changing the pairing of the impurities acts like a switch on the whole system, triggering a phase transition in the form of a sudden change in the entire sample.

“We found that that a quantity called the Schmidt gap has all the features of an order parameter for this phase transition,” says Bose. “That means that mathematically it characterizes all the features of the phase transition in an analogous way to how magnetisation does for a magnet”.

However, unlike magnetisation, which can be easily measured, the Schmidt gap is a non-localised quantity which requires every particle in a system to be individually measured. Although, this a challenging task, it is becoming viable in recent experiments in ultra-cold atoms.

This research opens up new possibilities for exploring phase transitions in quantum physics. In particular, where conventional methods which use localised order parameters are not applicable, this gives a new means of studying phase transitions. It may help to determine new phases of matter and shed light into the structure of complex systems.


  • The research appears in a paper published in the journal Nature Communications, entitled “An order parameter for impurity systems at quantum criticality”

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Abolfazl Bayat
020 7679 3647

Sougato Bose
020 7679 3485

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Oli Usher
UCL Faculty of Mathematical and Physical Sciences
020 7679 7964

In depth: Extending the life of the spin of single atoms

By news editor, on 27 January 2014

by Alexander Hellemans

A single quantum spin, found in single atoms and molecules, is the smallest magnet available.  The possibility to control the spin of single atoms is now intensively researched because they could serve as carriers of quantum information in future quantum computers.  Also within reach is their use for storing ordinary binary information, and this would allow the increase of the density of magnetic data storage by orders of magnitude. However, unlike the magnetization of materials currently used for data storage, the spin of single atoms is often too short-lived for data storage or quantum computing because of their interaction with other nearby physical systems.

Now a team lead by Cyrus Hirjibehedin in the London Centre for Nanotechnology at UCL has set out to investigate whether it would be possible to control the interactions between single magnetic atoms and their surrounding structure so that their spins can survive longer. Their research was published in the journal Nature Nanotechnology.

Cyrus Hirjibehedin in his lab at the London Centre for Nanotechnology

Cyrus Hirjibehedin in his lab at the London Centre for Nanotechnology. Photo: O. Usher (UCL MAPS)

In their work, the researchers focused primarily on the electrical interactions between single atom spins and nearby metallic electrodes, which can significantly decrease the lifetime of the spin in higher energy (or “excited”) states. One way to reduce these interactions is to separate the atoms from the conducting substrate by an insulating film.  To do this, the team grew large islands of an insulating copper nitride layer on a copper substrate, and on these islands they deposited cobalt atoms.

“As a test we put down a material we understood pretty well, cobalt, to see if there was a difference,” says Hirjibehedin.

First the team located a number of cobalt atoms on a copper nitride island with a scanning tunneling microscope (STM). Then they selected various atoms, starting at the edge of the island and then at increasing distances from the edge. They hovered the STM tip over each of the selected atoms, applied a bias voltage that ranged from – 15 mV to 15 mV, and measured the differential conductance (the ratio of the change in current passing through the atom to the change in bias voltage for very small changes in bias voltage)–a technique called scanning tunneling spectroscopy (STS).

The STM used by Hirjibehedin's team in the research

The STM used in the research

They found that the STS spectra for the atoms depended on their distance to the edge of the island. The researchers had expected that the surrounding atomic structure might play a small role, but the differences were much greater than expected. In particular, atoms at the edge of the island showed a prominent peak in the conductance centered on the bias voltage equalling zero, but this peak diminished the further the atom was from the edge of the island. At the same time, the energy needed to excite the atomic spin to a higher energy state, which appears as steps in the differential conductance at both positive and negative bias voltages, almost doubled as the atom’s distance from the edge increased. In the STS spectra, they found a clear correlation between the height of the conductance peak and the spin excitation energy.

“In principle,” says Hirjibehedin, “you can have various strain effects near the edges, so we thought there might be something small, but we were quite surprised that what we saw was very dramatic.”

“What we found is that the interaction between the cobalt atom and the copper is much stronger near the edges than it is at the center,” he says.

An obvious explanation is that the insulating copper nitride layer played an important role in reducing the effect of the copper substrate on the spin of the single atoms. This interaction is known as Kondo exchange. If the atom with spin is close enough to the metal, its spin couples to the spins of the conduction electrons in the metal, causing these electrons to hop on and off the atom. An electron with opposite spin to the spin on the atom will hop onto the atom, and the other electron with opposite spin will hop off the atom.  This process switches the spin of the atom up and down continuously, which makes data storage impossible.

Remarkably, the team also found that the same Kondo exchange that is responsible for this spin switching or “screening” also dramatically reduces the energy of the spin’s excitations, which also makes it less stable. This is significant because the excitation energy determines the spin’s stability, and normally this property is thought to be controlled by the surrounding physical structure rather than by electrical coupling.

It was clear to the team that near the edge of the island the Kondo exchange was strongly affecting the magnetic properties of the atom. What the team observed was that at the edges of the island, the Kondo effect was predominant, while more at the center the STS spectrum did not display this effect.  This allowed them to correlate the change in the Kondo effect with the changes in the spin excitation energies.  Two different calculations, performed by team members in Portugal and Germany, further support this explanation. “We use the Kondo effect as a sort of measurement for how strong this coupling is,” says Hirjibehedin, “and therefore how much it should affect the spin’s excitation energy.” He adds: “It still is an open question why the coupling is so much stronger at the edges of the island than in the middle.”

These new results suggest that tuning the electron density of a conducting material, as is often done in a field effect transistor, would enable the continuous tuning of the lifetime of the spins of nearby magnetic atoms. This could be done, for example, on a semiconductor substrate, which can be electrically gated to enhance or decrease the electron density. This would in turn affect the energies between the different spin states of nearby magnetic atoms, which would then modify the stability of the spins.  “The stability of the spin is very important, in particular if you want to store data with a single atom,” concludes Hirjibehedin.

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Scanning tunneling microscope

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Cyrus Hirjibehedin