Physicist Dirk Bouwmeester discovers a promising route for combined optical and solid state-based quantum information processing
Tiny units of matter and chemistry that they are, atoms constitute the entire universe. Some rare atoms can store quantum information, an important phenomenon for scientists in their ongoing quest for a quantum Internet.
New research from UC Santa Barbara scientists and their Dutch colleagues exploits a system that has the potential to transfer optical quantum information to a locally stored solid-state quantum format, a requirement of quantum communication. The team’s findings appear in the journal Nature Photonics.
“Our research aims at creating a quantum analog of current fiber optic technology in which light is used to transfer classical information — bits with values zero or one — between computers,” said author Dirk Bouwmeester, a professor in UCSB’s Department of Physics. “The rare earth atoms we’re studying can store the superpositions of zero and one used in quantum computation. In addition, the light by which we communicate with these atoms can also store quantum information.”
Atoms are each composed of a nucleus typically surrounded by inner shells full of electrons and often have a partially filled outer electron shell. The optical and chemical properties of the atoms are mainly determined by the electrons in the outer shell.
Rare earth atoms such as erbium and ytterbium have the opposite composition: a partially filled inner shell surrounded by filled outer shells. This special configuration is what enables these atoms to store quantum information.
However, the unique composition of rare earth atoms leads to electronic transitions so well shielded from the surrounding atoms that optical interactions are extremely weak. Even when implanted in a host material, these atoms maintain those shielded transitions, which in principle can be addressed optically in order to store and retrieve quantum information.
Bouwmeester collaborated with John Bowers, a professor in UCSB’s Department of Electrical and Computer Engineering, and investigators at Leiden University in the Netherlands to strengthen these weak interactions by implanting ytterbium into ultra-high-quality optical storage rings on a silicon chip.
“The presence of the high-quality optical ring resonator — even if no light is injected — changes the fundamental optical properties of the embedded atoms, which leads to an order of magnitude increase in optical interaction strength with the ytterbium,” Bouwmeester said. “This increase, known as the Purcell effect, has an intricate dependence on the geometry of the optical light confinement.”
The team’s findings indicate that new samples currently under development at UCSB can enable optical communication to a single ytterbium atom inside optical circuits on a silicon chip, a phenomenon of significant interest for quantum information storage. The experiments also explore the way in which the Purcell effect enhances optical interaction with an ensemble of a few hundred rare earth atoms. The grouping itself has interesting collective properties that can also be explored for the storage of quantum information.
Key is an effect called a photon echo, the result of two distinct light pulses, the first of which causes atoms in ytterbium to become partially excited.
“The first light pulse creates a set of atoms we ‘talk’ to in a specific state and we call that state ‘in phase’ because all the atoms are created at the same time by this optical pulse,” Bouwmeester explained. “However, the individual atoms have slightly different frequencies because of residual coupling to neighboring atoms, which affects their time evolution and causes decoherence in the system.” Decoherence is the inability to keep track of how the system evolves in all its details.
“The trick is that the second light pulse changes the state of the system so that it evolves backwards, causing the atoms to return to the initial phase,” he continued. “This makes everything coherent and causes the atoms to collectively emit the light they absorbed from the first pulse.”
The strength of the photon echo contains important information about the fundamental properties of the ytterbium in the host material. “By analyzing the strength of these photon echoes, we are learning about the fundamental interactions of ytterbium with its surroundings,” Bouwmeester said. “Now we’re working on strengthening the Purcell effect by making the storage rings we use smaller and smaller.”
According to Bouwmeester, quantum computation needs to be compatible with optical communication for information to be shared and transmitted. “Our ultimate goal is to be able to communicate to a single ytterbium atom; then we can start transferring the quantum state of a single photon to a single ytterbium atom,” he added. “Coupling the quantum state of a photon to a quantum solid state is essential for the existence of a quantum Internet.”
Learn more: Rare Earth Atoms See the Light
Why send a message back in time, but lock it so that no one can ever read the contents? Because it may be the key to solving currently intractable problems. That’s the claim of an international collaboration who have just published a paper in npj Quantum Information.
It turns out that an unopened message can be exceedingly useful. This is true if the experimenter entangles the message with some other system in the laboratory before sending it. Entanglement, a strange effect only possible in the realm of quantum physics, creates correlations between the time-travelling message and the laboratory system. These correlations can fuel a quantum computation.
Around 10 years ago, researcher Dave Bacon, now at Google, showed that a time-travelling quantum computer could quickly solve a group of problems, known as NP-complete, which mathematicians have lumped together as being hard.
The problem was, Bacon’s quantum computer was travelling around ‘closed timelike curves.’ These are paths through the fabric of spacetime that loop back on themselves. General relativity allows such paths to exist through contortions in spacetime known as wormholes.
Physicists argue something must stop such opportunities arising, because it would threaten ‘causality’ — in the classic example, someone could travel back in time and kill their grandfather, negating their own existence.
And it’s not only family ties that are threatened. Breaking the causal flow of time has consequences for quantum physics, too. Over the past two decades, researchers have shown that foundational principles of quantum physics break in the presence of closed timelike curves: you can beat the uncertainty principle, an inherent fuzziness of quantum properties, and the no-cloning theorem, which says quantum states can’t be copied.
However, the new work shows that a quantum computer can solve insoluble problems, even if it is travelling along ‘open timelike curves,’ which don’t create causality problems. That’s because they don’t allow direct interaction with anything in the object’s own past: the time travelling particles (or data they contain) never interact with themselves. Nevertheless, the strange quantum properties that permit ‘impossible’ computations are left intact.
“We avoid ‘classical’ paradoxes, like the grandfathers paradox, but you still get all these weird results,” says Mile Gu, who led the work.
Gu is at the Centre for Quantum Technologies (CQT) at the National University of Singapore and Tsinghua University in Beijing. His eight other coauthors come from these institutions, the University of Oxford, UK, Australian National University in Canberra, the University of Queensland in St Lucia, Australia, and QKD Corp in Toronto, Canada.
“Whenever we present the idea, people say no way can this have an effect,” says Jayne Thompson, a co-author at CQT. But it does: quantum particles sent on a timeloop could gain super computational power, even though the particles never interact with anything in the past. “The reason there is an effect is because some information is stored in the entangling correlations: this is what we’re harnessing,” Thompson says.
There is a caveat — not all physicists think that these open timeline curves are any more likely to be realizable in the physical universe than the closed ones. One argument against closed timelike curves is that no-one from the future has ever visited us. That argument, at least, doesn’t apply to the open kind, because any messages from the future would be locked.
Read more: Computing with Time Travel