̽��ֱ�� of Cambridge - Quantum mechanics

Researchers chart the ‘secret’ movement of quantum particles

sc604 — Fri, 22 Dec 2017 11:30:05 +0000

One of the fundamental ideas of quantum theory is that quantum objects can exist both as a wave and as a particle, and that they don’t exist as one or the other until they are measured. This is the premise that Erwin Schrödinger was illustrating with his famous thought experiment involving a dead-or-maybe-not-dead cat in a box.

“This premise, commonly referred to as the wave function, has been used more as a mathematical tool than a representation of actual quantum particles,” said David Arvidsson-Shukur, a PhD student at Cambridge’s Cavendish Laboratory, and the paper’s first author. “That’s why we took on the challenge of creating a way to track the secret movements of quantum particles.”

Any particle will always interact with its environment, ‘tagging’ it along the way. Arvidsson-Shukur, working with his co-authors Professor Crispin Barnes from the Cavendish Laboratory and Axel Gottfries, a PhD student from the Faculty of Economics, outlined a way for scientists to map these ‘tagging’ interactions without looking at them. ̽��ֱ��technique would be useful to scientists who make measurements at the end of an experiment but want to follow the movements of particles during the full experiment.

Some quantum scientists have suggested that information can be transmitted between two people – usually referred to as Alice and Bob – without any particles travelling between them. In a sense, Alice gets the message telepathically. This has been termed counterfactual communication because it goes against the accepted ‘fact’ that for information to be carried between sources, particles must move between them.

“To measure this phenomenon of counterfactual communication, we need a way to pin down where the particles between Alice and Bob are when we’re not looking,” said Arvidsson-Shukur. “Our ‘tagging’ method can do just that. Additionally, we can verify old predictions of quantum mechanics, for example that particles can exist in different locations at the same time.”

̽��ֱ��founders of modern physics devised formulas to calculate the probabilities of different results from quantum experiments. However, they did not provide any explanations of what a quantum particle is doing when it’s not being observed. Earlier experiments have suggested that the particles might do non-classical things when not observed, like existing in two places at the same time. In their paper, the Cambridge researchers considered the fact that any particle travelling through space will interact with its surroundings. These interactions are what they call the ‘tagging’ of the particle. ̽��ֱ��interactions encode information in the particles that can then be decoded at the end of an experiment, when the particles are measured.

̽��ֱ��researchers found that this information encoded in the particles is directly related to the wave function that Schrödinger postulated a century ago. Previously the wave function was thought of as an abstract computational tool to predict the outcomes of quantum experiments. “Our result suggests that the wave function is closely related to the actual state of particles,” said Arvidsson-Shukur. “So, we have been able to explore the ‘forbidden domain’ of quantum mechanics: pinning down the path of quantum particles when no one is observing them.”

Reference
D. R. M. Arvidsson-Shukur, C. H. W. Barnes, and A. N. O. Gottfries. ‘Evaluation of counterfactuality in counterfactual communication protocols’. Physical Review A (2017). DOI: 10.1103/PhysRevA.96.062316

Researchers from the ̽��ֱ�� of Cambridge have taken a peek into the secretive domain of quantum mechanics. In a theoretical paper published in the journal Physical Review A, they have shown that the way that particles interact with their environment can be used to track quantum particles when they’re not being observed, which had been thought to be impossible.

We can verify old predictions of quantum mechanics, for example that particles can exist in different locations at the same time.

David Arvidsson-Shukur

Robert Couse-Baker

2015-12-22 chemistry

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Yes

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Attribution

A tight squeeze for electrons – quantum effects observed in ‘one-dimensional’ wires

sc604 — Thu, 15 Sep 2016 07:00:00 +0000

Scientists have controlled electrons by packing them so tightly that they start to display quantum effects, using an extension of the technology currently used to make computer processors. ̽��ֱ��technique, reported in the journal Nature Communications, has uncovered properties of quantum matter that could pave a way to new quantum technologies.

̽��ֱ��ability to control electrons in this way may lay the groundwork for many technological advances, including quantum computers that can solve problems fundamentally intractable by modern electronics. Before such technologies become practical however, researchers need to better understand quantum, or wave-like, particles, and more importantly, the interactions between them.

Squeezing electrons into a one-dimensional ‘quantum wire’ amplifies their quantum nature to the point that it can be seen, by measuring at what energy and wavelength (or momentum) electrons can be injected into the wire.

“Think of a crowded train carriage, with people standing tightly packed all the way down the centre of the carriage,” said Professor Christopher Ford of the ̽��ֱ�� of Cambridge’s Cavendish Laboratory, one of the paper’s co-authors. “If someone tries to get in a door, they have to push the people closest to them along a bit to make room. In turn, those people push slightly on their neighbours, and so on. A wave of compression passes down the carriage, at some speed related to how people interact with their neighbours, and that speed probably depends on how hard they were shoved by the person getting on the train. By measuring this speed, one could learn about the interactions.”

“ ̽��ֱ��same is true for electrons in a quantum wire – they repel each other and cannot get past, so if one electron enters or leaves, it excites a compressive wave like the people in the train,” said the paper’s first author Dr Maria Moreno, also from the Cavendish Laboratory.

However, electrons have another characteristic, their angular momentum or ‘spin’, which also interacts with their neighbours. Spin can also set off a wave carrying energy along the wire, and this spin wave travels at a different speed to the charge wave. Measuring the wavelength of these waves as the energy is varied is called tunnelling spectroscopy. ̽��ֱ��separate spin and charge waves were detected experimentally by researchers from Harvard and Cambridge Universities.

Now, in the paper published in Nature Communications, the Cambridge researchers have gone one stage further, to test the latest predictions of what should happen at high energies, where the original theory breaks down.

A flurry of theoretical activity in the past decade has led to new predictions of other ways of exciting waves among the electrons — it’s as if the person entering the train pushes so hard some people fall over and knock into others much further down the carriage. These new ‘modes’ are weaker than the spin and charge waves and so are harder to detect.

̽��ֱ��collaborators of the Cambridge researchers from the ̽��ֱ�� of Birmingham predicted that there would be a hierarchy of modes corresponding to the variety of ways in which the interactions can affect the quantum-mechanical particles, and the weaker modes should be strongest in very short wires.

To make a set of such short wires, the Cambridge group set about devising a way of making contact to a set of 6000 narrow strips of metal that are used to create the quantum wires from the semiconducting material gallium arsenide (GaAs). This required an extra layer of metal in the shape of bridges between the strips.

By varying the magnetic field and voltage, the tunnelling from the wires to an adjacent sheet of electrons could be mapped out, and this revealed evidence for the extra curves predicted, where it can be seen as an upside-down replica of the spin curve.

These results will now be applied to better understand and control the behaviour of electrons in the building blocks of a quantum computer.

Reference:
Moreno et al. ‘Nonlinear spectra of spinons and holons in short GaAs quantum wires.’ Nature Communications (2016).DOI: 10.1038/ncomms12784

Researchers have observed quantum effects in electrons by squeezing them into one-dimensional ‘quantum wires’ and observing the interactions between them. ̽��ֱ��results could be used to aid in the development of quantum technologies, including quantum computing.

Dr Maria Moreno

Regime of a single 1D wire subband filled

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Yes

Laser technique promises super-fast and super-secure quantum cryptography

sc604 — Mon, 04 Apr 2016 15:06:38 +0000

Researchers have developed a new method to overcome one of the main issues in implementing a quantum cryptography system, raising the prospect of a useable ‘unbreakable’ method for sending sensitive information hidden inside particles of light.

By ‘seeding’ one laser beam inside another, the researchers, from the ̽��ֱ�� of Cambridge and Toshiba Research Europe, have demonstrated that it is possible to distribute encryption keys at rates between two and six orders of magnitude higher than earlier attempts at a real-world quantum cryptography system. ̽��ֱ��results are reported in the journal Nature Photonics.

Encryption is a vital part of modern life, enabling sensitive information to be shared securely. In conventional cryptography, the sender and receiver of a particular piece of information decide the encryption code, or key, up front, so that only those with the key can decrypt the information. But as computers get faster and more powerful, encryption codes get easier to break.

Quantum cryptography promises ‘unbreakable’ security by hiding information in particles of light, or photons, emitted from lasers. In this form of cryptography, quantum mechanics are used to randomly generate a key. ̽��ֱ��sender, who is normally designated as Alice, sends the key via polarised photons, which are sent in different directions. ̽��ֱ��receiver, normally designated as Bob, uses photon detectors to measure which direction the photons are polarised, and the detectors translate the photons into bits, which, assuming Bob has used the correct photon detectors in the correct order, will give him the key.

̽��ֱ��strength of quantum cryptography is that if an attacker tries to intercept Alice and Bob’s message, the key itself changes, due to the properties of quantum mechanics. Since it was first proposed in the 1980s, quantum cryptography has promised the possibility of unbreakable security. “In theory, the attacker could have all of the power possible under the laws of physics, but they still wouldn’t be able to crack the code,” said the paper’s first author Lucian Comandar, a PhD student at Cambridge’s Department of Engineering and Toshiba’s Cambridge Research Laboratory.

However, issues with quantum cryptography arise when trying to construct a useable system. In reality, it is a back and forth game: inventive attacks targeting different components of the system are constantly being developed, and countermeasures to foil attacks are constantly being developed in response.

̽��ֱ��components that are most frequently attacked by hackers are the photon detectors, due to their high sensitivity and complex design – it is usually the most complex components that are the most vulnerable. As a response to attacks on the detectors, researchers developed a new quantum cryptography protocol known as measurement-device-independent quantum key distribution (MDI-QKD).

In this method, instead of each having a detector, Alice and Bob send their photons to a central node, referred to as Charlie. Charlie lets the photons pass through a beam splitter and measures them. ̽��ֱ��results can disclose the correlation between the bits, but not disclose their values, which remain secret. In this set-up, even if Charlie tries to cheat, the information will remain secure.

MDI-QKD has been experimentally demonstrated, but the rates at which information can be sent are too slow for real-world application, mostly due to the difficulty in creating indistinguishable particles from different lasers. To make it work, the laser pulses sent through Charlie’s beam splitter need to be (relatively) long, restricting rates to a few hundred bits per second (bps) or less.

̽��ֱ��method developed by the Cambridge researchers overcomes the problem by using a technique known as pulsed laser seeding, in which one laser beam injects photons into another. This makes the laser pulses more visible to Charlie by reducing the amount of ‘time jitter’ in the pulses, so that much shorter pulses can be used. Pulsed laser seeding is also able to randomly change the phase of the laser beam at very high rates. ̽��ֱ��result of using this technique in a MDI-QKD setup would enable rates as high as 1 megabit per second, representing an improvement of two to six orders of magnitude over previous efforts.

“This protocol gives us the highest possible degree of security at very high clock rates,” said Comandar. “It could point the way to a practical implementation of quantum cryptography.”

Reference:
L.C. Comandar et al. ‘Quantum key distribution without detector vulnerabilities using optically seeded lasers.’ Nature Photonics (2016). DOI: 10.1038/nphoton.2016.50

A new method of implementing an ‘unbreakable’ quantum cryptographic system is able to transmit information at rates more than ten times faster than previous attempts.

This protocol gives us the highest possible degree of security at very high clock rates

Lucian Comandar

Depiction of indistinguishable photons leaving through the same output port of a beam splitter

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Yes

Graphene means business – two-dimensional material moves from the lab to the UK factory floor

sc604 — Fri, 06 Nov 2015 15:50:52 +0000

More than 40 companies, mostly from the UK, are in Cambridge this week to demonstrate some of the new products being developed from graphene and other two-dimensional materials.

Graphene is a two-dimensional material made up of sheets of carbon atoms. With its combination of exceptional electrical, mechanical and thermal properties, graphene has the potential to revolutionise industries ranging from healthcare to electronics.

On Thursday, the Cambridge Graphene Technology Day – an exhibition of graphene-based technologies organised by the Cambridge Graphene Centre, together with its partner companies – took place, showcasing new products based on graphene and related two-dimensional materials.

Some of the examples of the products and prototypes on display included flexible displays, printed electronics, and graphene-based heaters, all of which have potential for consumer applications. Other examples included concrete and road surfacing incorporating graphene, which would mean lighter and stronger infrastructure, and roads that have to be resurfaced far less often, greatly lowering the costs to local governments.

“At the Cambridge Graphene Technology Day we saw several real examples of graphene making its way from the lab to the factory floor – creating jobs and growth for Cambridge and the UK,” said Professor Andrea Ferrari, Director of the Cambridge Graphene Centre and of the EPSRC Centre for Doctoral Training in Graphene Technology. “Cambridge is very well-placed in the network of UK, European and global initiatives targeting the development of new products and devices based on graphene and related materials.”

Cambridge has a long history of research and application into carbon-based materials, since the identification of the graphite structure in 1924, moving through to diamond, diamond-like carbon, conducting polymers, and carbon nanotubes, with a proven track-record in taking carbon research from the lab to the factory floor.

Cambridge is also one of the leading centres in graphene technology. Dr Krzysztof Koziol from the Department of Materials Science & Metallurgy sits on the management board of the EPSRC Centre for Doctoral Training in Graphene Technology. He is developing hybrid electrical wires made from copper and graphene in order to improve the amount of electric current they can carry, functional graphene heaters, anti-corrosion coatings, and graphene inks which can be used to draw printed circuit boards directly onto paper and other surfaces.

Koziol has established a spin-out company, Cambridge Nanosystems, which produces high volume amounts of graphene for industrial applications. ̽��ֱ��company, co-founded by recent Cambridge graduate Catharina Paulkner, has recently established a partnership with a major auto manufacturer to start developing graphene-based applications for cars.

Other researchers affiliated with the Cambridge Graphene Centre include Professor Clare Grey of the Department of Chemistry, who is part of the Cambridge Graphene Centre Management Board. She is incorporating graphene and related materials into next-generation batteries and has recently demonstrated a breakthrough in Lithium air batteries by exploiting graphene. Professor Mete Atature from the Department of Physics, is one of the supervisors of the Centre for Doctoral Training in Graphene Technology. He uses two-dimensional materials for research in quantum optics, including the possibility of a computer network based on quantum mechanics, which would be far more secure and more powerful than classical computers.

“ ̽��ֱ��Cambridge Graphene Centre is a great addition to the Cambridge technology and academic cluster,” said Chuck Milligan, CEO of FlexEnable, which is developing technology for flexible displays and other electronic components. "We are proud to be a partner of the Centre and support its activities. Graphene and other two dimensional materials are very relevant to flexible electronics for displays and sensors, and we are passionate about taking technology from labs to the factory floor. Our unique manufacturing processes for flexible electronics, together with the exponential growth expected in the flexible display and Internet of Things sensor markets, provide enormous opportunity for this exciting class of materials. It is for this reason that today we placed in the Cambridge Graphene Centre Laboratories a semi-automatic, large area EVG Spray coater. This valuable tool, donated to the ̽��ֱ��, will be a good match between the area of research of solution processable graphene and Flexenable long term technological vision."

FlexEnable is supporting efforts to scale the graphene technology for use in tomorrow's factories. ̽��ֱ��company has donated a large area deposition machine to the ̽��ֱ��, which is used for depositing large amounts of graphene onto various substrates.

“ ̽��ֱ�� ̽��ֱ�� is at the heart of the largest, most vibrant technology cluster in Europe,” said Professor Sir Leszek Borysiewicz, the ̽��ֱ��’s Vice-Chancellor. “Our many partnerships with industry support the continued economic success of the region and the UK more broadly, and the Cambridge Graphene Centre is an important part of that – working with industry to bring these promising materials to market.”

Professor David Cardwell, Head of the Cambridge Engineering Department, indicated the planned development in Cambridge of a scale-up centre, where research will be nurtured towards higher technology readiness levels in collaboration with UK industry. “ ̽��ֱ��Cambridge Graphene Centre is a direct and obvious link to this scale-up initiative, which will offer even more exciting opportunities for industry university collaborations,” he said.

Among the many local companies with an interest in graphene technologies are FlexEnable, the R&D arm of global telecommunications firm Nokia, printed electronics pioneer Novalia, Cambridge Nanosystems, Cambridge Graphene, and Aixtron, which specialises in the large-scale production of graphene powders, inks and films for a variety of applications.

Underpinning this commercial R&D effort in Cambridge and the East of England is public and private investment in the Cambridge Graphene Centre via the Graphene Flagship, part funded by the European Union. ̽��ֱ��flagship is a pan-European consortium, with a fast-growing number of industrial partners and associate members.

A major showcase of companies developing new technologies from graphene and other two-dimensional materials took place this week at the Cambridge Graphene Centre.

Cambridge is very well-placed in the network of UK, European and global initiatives targeting the development of new products and devices based on graphene and related materials

Andrea Ferrari

Graphene: A 2D materials revolution

̽��ֱ�� of Cambridge

Some of the products and prototypes on display at Cambridge Graphene Technology Day.

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Yes

Entanglement at heart of 'two-for-one' fission in next-generation solar cells

sc604 — Mon, 26 Oct 2015 16:15:13 +0000

An international team of scientists have observed how a mysterious quantum phenomenon in organic molecules takes place in real time, which could aid in the development of highly efficient solar cells.

̽��ֱ��researchers, led by the ̽��ֱ�� of Cambridge, used ultrafast laser pulses to observe how a single particle of light, or photon, can be converted into two energetically excited particles, known as spin-triplet excitons, through a process called singlet fission. If singlet fission can be controlled, it could enable solar cells to double the amount of electrical current that can be extracted.

In conventional semiconductors such as silicon, when one photon is absorbed it leads to the formation of one free electron that can be harvested as electrical current. However certain materials undergo singlet fission instead, where the absorption of a photon leads to the formation of two spin-triplet excitons.

Working with researchers from the Netherlands, Germany and Sweden, the Cambridge team confirmed that this ‘two-for-one’ transformation involves an elusive intermediate state in which the two triplet excitons are ‘entangled’, a feature of quantum theory that causes the properties of each exciton to be intrinsically linked to that of its partner.

By shining ultrafast laser pulses – just a few quadrillionths of a second – on a sample of pentacene, an organic material which undergoes singlet fission, the researchers were able to directly observe this entangled state for the first time, and showed how molecular vibrations make it both detectable and drive its creation through quantum dynamics. ̽��ֱ��results are reported today (26 October) in the journal Nature Chemistry.

“Harnessing the process of singlet fission into new solar cell technologies could allow tremendous increases in energy conversion efficiencies in solar cells,” said Dr Alex Chin from the ̽��ֱ��’s Cavendish Laboratory, one of the study’s co-authors. “But before we can do that, we need to understand how exciton fission happens at the microscopic level. This is the basic requirement for controlling this fascinating process.”

̽��ֱ��key challenge for observing real-time singlet fission is that the entangled spin-triplet excitons are essentially ‘dark’ to almost all optical probes, meaning they cannot be directly created or destroyed by light. In materials like pentacene, the first stage – which can be seen – is the absorption of light that creates a single, high-energy exciton, known as a spin singlet exciton. ̽��ֱ��subsequent fission of the singlet exciton into two less energetic triplet excitons gives the process its name, but the ability to see what is going on vanishes as the process take place.

To get around this, the team employed a powerful technique known as two-dimensional spectroscopy, which involves hitting the material with a co-ordinated sequence of ultrashort laser pulses and then measuring the light emitted by the excited sample. By varying the time between the pulses in the sequence, it is possible to follow in real time how energy absorbed by previous pulses is transferred and transformed into different states.

Using this approach, the team found that when the pentacene molecules were vibrated by the laser pulses, certain changes in the molecular shapes cause the triplet pair to become briefly light-absorbing, and therefore detectable by later pulses. By carefully filtering out all but these frequencies, a weak but unmistakable signal from the triplet pair state became apparent.

̽��ֱ��authors then developed a model which showed that when the molecules are vibrating, they possess new quantum states that simultaneously have the properties of both the light-absorbing singlet exciton and the dark triplet pairs. These quantum ‘super positions’, which are the basis of Schrödinger’s famous thought experiment in which a cat is – according to quantum theory – in a state of being both alive and dead at the same time, not only make the triplet pairs visible, they also allow fission to occur directly from the moment light is absorbed.

“This work shows that optimised fission in real materials requires us to consider more than just the static arrangements and energies of molecules; their motion and quantum dynamics are just as important,” said Dr Akshay Rao, from the ̽��ֱ��’s Cavendish Laboratory. “It is a crucial step towards opening up new routes to highly efficiency solar cells.”

̽��ֱ��research was supported by the European LaserLab Consortium, Royal Society, and the Netherlands Organization for Scientific Research. ̽��ֱ��work at Cambridge forms part of a broader initiative to harness high tech knowledge in the physical sciences to tackle global challenges such as climate change and renewable energy. This initiative is backed by the UK Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability.

Reference:
Bakulin, Artem et. al. ‘Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy.’ Nature Chemistry (2015). DOI: 10.1038/nchem.2371

̽��ֱ��mechanism behind a process known as singlet fission, which could drive the development of highly efficient solar cells, has been directly observed by researchers for the first time.

Harnessing the process of singlet fission into new solar cell technologies could allow tremendous increases in energy conversion efficiencies in solar cells

Alex Chin

Lawrence W Chin, David Turban and Alex W Chin

Pentacene molecules convert a single photon into two molecular excitations via the quantum mechanics of singlet fission

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Yes

Scientists "squeeze" light one particle at a time

tdk25 — Tue, 01 Sep 2015 04:04:46 +0000

A team of scientists has successfully measured particles of light being “squeezed”, in an experiment that had been written off in physics textbooks as impossible to observe.

Squeezing is a strange phenomenon of quantum physics. It creates a very specific form of light which is “low-noise” and is potentially useful in technology designed to pick up faint signals, such as the detection of gravitational waves.

̽��ֱ��standard approach to squeezing light involves firing an intense laser beam at a material, usually a non-linear crystal, which produces the desired effect.

For more than 30 years, however, a theory has existed about another possible technique. This involves exciting a single atom with just a tiny amount of light. ̽��ֱ��theory states that the light scattered by this atom should, similarly, be squeezed.

Unfortunately, although the mathematical basis for this method – known as squeezing of resonance fluorescence – was drawn up in 1981, the experiment to observe it was so difficult that one established quantum physics textbook despairingly concludes: “It seems hopeless to measure it”.

So it has proven – until now. In the journal Nature, a team of physicists report that they have successfully demonstrated the squeezing of individual light particles, or photons, using an artificially constructed atom, known as a semiconductor quantum dot. Thanks to the enhanced optical properties of this system and the technique used to make the measurements, they were able to observe the light as it was scattered, and proved that it had indeed been squeezed.

Professor Mete Atature, from the Cavendish Laboratory, Department of Physics, and a Fellow of St John’s College at the ̽��ֱ�� of Cambridge, led the research. He said: “It’s one of those cases of a fundamental question that theorists came up with, but which, after years of trying, people basically concluded it is impossible to see for real – if it’s there at all.”

“We managed to do it because we now have artificial atoms with optical properties that are superior to natural atoms. That meant we were able to reach the necessary conditions to observe this fundamental property of photons and prove that this odd phenomenon of squeezing really exists at the level of a single photon. It’s a very bizarre effect that goes completely against our senses and expectations about what photons should do.”

Like a lot of quantum physics, the principles behind squeezing light involve some mind-boggling concepts.

It begins with the fact that wherever there are light particles, there are also associated electromagnetic fluctuations. This is a sort of static which scientists refer to as “noise”. Typically, the more intense light gets, the higher the noise. Dim the light, and the noise goes down.

But strangely, at a very fine quantum level, the picture changes. Even in a situation where there is no light, electromagnetic noise still exists. These are called vacuum fluctuations. While classical physics tells us that in the absence of a light source we will be in perfect darkness, quantum mechanics tells us that there is always some of this ambient fluctuation.

"If you look at a flat surface, it seems smooth and flat, but we know that if you really zoom in to a super-fine level, it probably isn't perfectly smooth at all," Atature said. " ̽��ֱ��same thing is happening with vacuum fluctuations. Once you get into the quantum world, you start to get this fine print. It looks like there are zero photons present, but actually there is just a tiny bit more than nothing."

Importantly, these vacuum fluctuations are always present and provide a base limit to the noise of a light field. Even lasers, the most perfect light source known, carry this level of fluctuating noise.

This is when things get stranger still, however, because, in the right quantum conditions, that base limit of noise can be lowered even further. This lower-than-nothing, or lower-than-vacuum, state is what physicists call squeezing.

In the Cambridge experiment, the researchers achieved this by shining a faint laser beam on to their artificial atom, the quantum dot. This excited the quantum dot and led to the emission of a stream of individual photons. Although normally, the noise associated with this photonic activity is greater than a vacuum state, when the dot was only excited weakly the noise associated with the light field actually dropped, becoming less than the supposed baseline of vacuum fluctuations.

Explaining why this happens involves some highly complex quantum physics. At its core, however, is a rule known as Heisenberg’s uncertainty principle. This states that in any situation in which a particle has two linked properties, only one can be measured and the other must be uncertain.

In the normal world of classical physics, this rule does not apply. If an object is moving, we can measure both its position and momentum, for example, to understand where it is going and how long it is likely to take getting there. ̽��ֱ��pair of properties – position and momentum – are linked.

In the strange world of quantum physics, however, the situation changes. Heisenberg states that only one part of a pair can ever be measured, and the other must remain uncertain.

In the Cambridge experiment, the researchers used that rule to their advantage, creating a tradeoff between what could be measured, and what could not. By scattering faint laser light from the quantum dot, the noise of part of the electromagnetic field was reduced to an extremely precise and low level, below the standard baseline of vacuum fluctuations. This was done at the expense of making other parts of the electromagnetic field less measurable, meaning that it became possible to create a level of noise that was lower-than-nothing, in keeping with Heisenberg’s uncertainty principle, and hence the laws of quantum physics.

Plotting the uncertainty with which fluctuations in the electromagnetic field could be measured on a graph creates a shape where the uncertainty of one part has been reduced, while the other has been extended. This creates a squashed-looking, or “squeezed” shape, hence the term, “squeezing” light.

Atature added that the main point of the study was simply to attempt to see this property of single photons, because it had never been seen before. “It’s just the same as wanting to look at Pluto in more detail or establishing that pentaquarks are out there,” he said. “Neither of those things has an obvious application right now, but the point is knowing more than we did before. We do this because we are curious and want to discover new things. That’s the essence of what science is all about.”

Additional image: ̽��ֱ��left diagram represents electromagnetic activity associated with light at what is technically its lowest possible level. On the right, part of the same field has been reduced to lower than is technically possible, at the expense of making another part of the field less measurable. This effect is called “squeezing” because of the shape it produces.

Reference:
Schulte, CHH, et al. Quadrature squeezed photons from a two-level system. Nature (2015). DOI: 10.1038/nature14868.

A team of scientists have measured a bizarre effect in quantum physics, in which individual particles of light are said to have been “squeezed” – an achievement which at least one textbook had written off as hopeless.

It’s just the same as wanting to look at Pluto in more detail or establishing that pentaquarks are out there. Neither of those things has an obvious application right now, but the point is knowing more than we did before. We do this because we are curious and want to discover new things. That’s the essence of what science is all about.

Mete Atature

An image from an experiment in the quantum optics laboratory in Cambridge. Laser light was used to excite individual tiny, artificially constructed atoms known as quantum dots, to create “squeezed” single photons

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Yes

New understanding of electromagnetism could enable ‘antennas on a chip’

sc604 — Wed, 08 Apr 2015 23:01:03 +0000

A team of researchers from the ̽��ֱ�� of Cambridge have unravelled one of the mysteries of electromagnetism, which could enable the design of antennas small enough to be integrated into an electronic chip. These ultra-small antennas – the so-called ‘last frontier’ of semiconductor design – would be a massive leap forward for wireless communications.

In new results published in the journal Physical Review Letters, the researchers have proposed that electromagnetic waves are generated not only from the acceleration of electrons, but also from a phenomenon known as symmetry breaking. In addition to the implications for wireless communications, the discovery could help identify the points where theories of classical electromagnetism and quantum mechanics overlap.

̽��ֱ��phenomenon of radiation due to electron acceleration, first identified more than a century ago, has no counterpart in quantum mechanics, where electrons are assumed to jump from higher to lower energy states. These new observations of radiation resulting from broken symmetry of the electric field may provide some link between the two fields.

̽��ֱ��purpose of any antenna, whether in a communications tower or a mobile phone, is to launch energy into free space in the form of electromagnetic or radio waves, and to collect energy from free space to feed into the device. One of the biggest problems in modern electronics, however, is that antennas are still quite big and incompatible with electronic circuits – which are ultra-small and getting smaller all the time.

“Antennas, or aerials, are one of the limiting factors when trying to make smaller and smaller systems, since below a certain size, the losses become too great,” said Professor Gehan Amaratunga of Cambridge’s Department of Engineering, who led the research. “An aerial’s size is determined by the wavelength associated with the transmission frequency of the application, and in most cases it’s a matter of finding a compromise between aerial size and the characteristics required for that application.”

Another challenge with aerials is that certain physical variables associated with radiation of energy are not well understood. For example, there is still no well-defined mathematical model related to the operation of a practical aerial. Most of what we know about electromagnetic radiation comes from theories first proposed by James Clerk Maxwell in the 19th century, which state that electromagnetic radiation is generated by accelerating electrons.

However, this theory becomes problematic when dealing with radio wave emission from a dielectric solid, a material which normally acts as an insulator, meaning that electrons are not free to move around. Despite this, dielectric resonators are already used as antennas in mobile phones, for example.

“In dielectric aerials, the medium has high permittivity, meaning that the velocity of the radio wave decreases as it enters the medium,” said Dr Dhiraj Sinha, the paper’s lead author. “What hasn’t been known is how the dielectric medium results in emission of electromagnetic waves. This mystery has puzzled scientists and engineers for more than 60 years.”

Working with researchers from the National Physical Laboratory and Cambridge-based dielectric antenna company Antenova Ltd, the Cambridge team used thin films of piezoelectric materials, a type of insulator which is deformed or vibrated when voltage is applied. They found that at a certain frequency, these materials become not only efficient resonators, but efficient radiators as well, meaning that they can be used as aerials.

̽��ֱ��researchers determined that the reason for this phenomenon is due to symmetry breaking of the electric field associated with the electron acceleration. In physics, symmetry is an indication of a constant feature of a particular aspect in a given system. When electronic charges are not in motion, there is symmetry of the electric field.

Symmetry breaking can also apply in cases such as a pair of parallel wires in which electrons can be accelerated by applying an oscillating electric field. “In aerials, the symmetry of the electric field is broken ‘explicitly’ which leads to a pattern of electric field lines radiating out from a transmitter, such as a two wire system in which the parallel geometry is ‘broken’,” said Sinha.

̽��ֱ��researchers found that by subjecting the piezoelectric thin films to an asymmetric excitation, the symmetry of the system is similarly broken, resulting in a corresponding symmetry breaking of the electric field, and the generation of electromagnetic radiation.

̽��ֱ��electromagnetic radiation emitted from dielectric materials is due to accelerating electrons on the metallic electrodes attached to them, as Maxwell predicted, coupled with explicit symmetry breaking of the electric field.

“If you want to use these materials to transmit energy, you have to break the symmetry as well as have accelerating electrons – this is the missing piece of the puzzle of electromagnetic theory,” said Amaratunga. “I’m not suggesting we’ve come up with some grand unified theory, but these results will aid understanding of how electromagnetism and quantum mechanics cross over and join up. It opens up a whole set of possibilities to explore.”

̽��ֱ��future applications for this discovery are important, not just for the mobile technology we use every day, but will also aid in the development and implementation of the Internet of Things: ubiquitous computing where almost everything in our homes and offices, from toasters to thermostats, is connected to the internet. For these applications, billions of devices are required, and the ability to fit an ultra-small aerial on an electronic chip would be a massive leap forward.

Piezoelectric materials can be made in thin film forms using materials such as lithium niobate, gallium nitride and gallium arsenide. Gallium arsenide-based amplifiers and filters are already available on the market and this new discovery opens up new ways of integrating antennas on a chip along with other components.

“It’s actually a very simple thing, when you boil it down,” said Sinha. “We’ve achieved a real application breakthrough, having gained an understanding of how these devices work.”

̽��ֱ��research has been supported in part by the Nokia Research Centre, the Cambridge Commonwealth Trust and the Wingate Foundation. Additional support was provided through the East of England Development Agency, Cambridge ̽��ֱ�� Entrepreneurs, and investment from Cambridge Angels.

Reference: Dhiraj Sinha & Gehan Amaratunga, Electromagnetic Radiation Under Explicit symmetry Breaking, Physical Review Letters, 114, 147701 (2015)

New understanding of the nature of electromagnetism could lead to antennas small enough to fit on computer chips – the ‘last frontier’ of semiconductor design – and could help identify the points where theories of classical electromagnetism and quantum mechanics overlap.

This is the missing piece of the puzzle of electromagnetic theory

Gehan Amaratunga

Generated using Mathematica from Wolfram Inc

̽��ֱ��radiation pattern from a dipole antenna showing symmetry breaking of the electric field

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Spin with a new twist

tdk25 — Fri, 10 Oct 2014 06:56:28 +0000

A new method of controlling the “spin” of an electron, one of the fastest-developing research topics in quantum-based technologies and widely seen as the potential foundation of numerous future advances, has been demonstrated by scientists.

In quantum physics, the “spin” of a particle refers to its intrinsic angular momentum. This can be controlled so that it is aligned with one of two directions, typically referred to as “up” or “down”.

Usually, researchers define the direction by applying a magnetic field to orientate the electron, called the “quantisation axis”. ̽��ֱ��process can, however, be distorted by the natural magnetic environment around the electron itself, which is usually seen as one of the biggest obstacles to controlling spin.

Uniquely, the new study, by a team at the ̽��ֱ�� of Cambridge and the Joint Quantum Institute (JQI) in the USA, instead turned this magnetic field into a natural advantage which allowed the electron to be held in place. ̽��ֱ��researchers did this by firing two precisely-tuned lasers at the particle, creating what they call a “dark state” from which it could be manipulated and measured.

̽��ֱ��implications for controlling quantum systems are significant because, since the 1990s, researchers have theorised that a particle’s spin could be used to store and manipulate information. Using the “up” or “down” as an alternative to the binary coding of 0s and 1s that characterises computers today, spin-based quantum computers would be able to compute difficult problems and vast amounts of data much more efficiently.

Any such development, however, depends on finding ways to bring electron spin under control in the first place. To date, researchers have had to find ways to do this in spite of the randomising effect that the magnetic field around an electron has on the orientation of its spin.

̽��ֱ��spin of an electron cannot be observed continuously without altering it, so it has to be measured before and after an attempt to manipulate its quantum state. This measurement reveals whether the spin is up or down, but the surrounding magnetic environment can also take effect at any time. If it does so, the quantisation axis of the electron is altered, and the whole picture is distorted. ̽��ֱ��effect is similar to trying to measure longitude and latitude in a world where the positions of the north and south poles are changing randomly all the time.

Dr Mete Atatüre, a researcher at St John’s College, Cambridge who led part of the new study, said: “In order to perform reliable measurements, we constantly have to fight against this fluctuating magnetic environment. In fact, most research is about trying to keep electrons detached or isolated from it. What is unique about this experiment is that we did the opposite and used this environment as a resource. We created a quantum state that wouldn’t be accessible if the magnetic field wasn’t there.”

̽��ֱ��electron was trapped inside a self-assembled “quantum dot”, a tiny structure made from a 10 nanometre-thick indium arsenide droplet, surrounded by gallium arsenide. While both materials are semiconductors, an electron can have a lower energy inside the “quantum dot” than in gallium arsenide. “This forms a natural and stable trap for single electrons within a semiconductor device, providing the desired conditions for defining a spin quantum bit” explained Carsten Schulte, a Cambridge graduate student who worked on the project.

An electron isolated in this fashion can then be targeted with lasers to manipulate its spin. If a laser strikes the quantum dot at certain wavelengths, the electron is optically excited and emits light, or fluoresces, which changes its spin. If, however, the magnetic environment interferes with this the change becomes uncontrollable.

To resolve this, the researchers fired two separate lasers at the quantum dot – one tuned to excite the “up” spin state, the other to excite the “down” state. These interfered with each other destructively, preventing any fluorescence at all and creating a so-called “dark state”.

“You would expect two lasers to raise the level of optical excitation even more, but in fact when this is done no light comes out of the quantum dot,” Jack Hansom, another graduate student in the research team, said. “Optical excitation ceases and the electron finds a unique quantum superposition state, which is neither up nor down.”

By changing the relative phase between the two lasers, they were able to redefine the specific dark state and force the electron into it. This showed that the electron could be manipulated in its own up/down coordinate system without the researchers ever knowing the orientation of up and down during the whole process.

“What is profound is that the electron is always in the same quantum superposition state, but the basis in which it is represented evolves with the nuclear field that remains unknown to us,” Atatüre added. “This research shows that the magnetic environment around the quantum dot does not need to remain a problem, but can be utilized for the definition and control of a quantum bit.”

̽��ֱ��full report appears in the October issue of Nature Physics.

Scientists have successfully demonstrated a new way to control the “spin” of an electron – the natural intrinsic angular momentum of electrons which could underpin faster computing in the future. ̽��ֱ��technique counterintuitively makes use of the ever-changing magnetic field of the electron’s environment - one of the main obstacles to traditional methods of spin control.

Most research is about trying to keep electrons isolated from the magnetic environment. What is unique about this experiment is that we did the opposite and used it as a resource

Mete Atature

Carsten Schulte

Spin manipulation in a noisy environment. In the quest for ever more precise manipulation of quantum systems, any uncontrolled interaction with the environment is usually considered detrimental.

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