Quantum records fall: most entangled photons plus 'twistiest' light


Physicists split and twirled light to generate the most entangled photons to date. Cathal O'Connell reports.


One team entangled multiple photons – not two, or three, or even eight, but 10 – while another used mirrors to send photons spinning off with different corkscrew patterns, then entangled them too.
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Quantum computers hold huge promise, but the current state of the art is dishearteningly basic – the equivalent of an abacus with just two or three beads.

To build a practical, large-scale quantum computer, scientists must figure out how to manipulate vastly more information – like thousands of beads on an abacus – than is currently possible.

Now, two record-breaking experiments have pushed the boundaries of how much quantum information can be shuttled around using photons, particles of light. In China, researchers broke the record for the most entangled photons at one time. Meanwhile, an international team created the twistiest light ever, advancing a new way to cram more information into a single stream of light.

Entanglement is when two particles (such as photons) are intimately connected so that measurement on one instantly affects the other, even if it’s light-years away. Because entanglement appeared to break the laws of relativity, Albert Einstein labelled it “spooky action at a distance”.

In a quantum computer, it’s this connection which links multiple quantum bits, called qubits, allowing them to process in concert and perform some tasks in seconds which would take a regular computer billions of years to compute.

One way to create entangled photons is to use a special crystal that can split a high energy photon into two of lower energy. Like twins in a horror movie, the daughters possess a mystical connection between them.

The problem is only a fraction of incoming photons get entangled, and that makes entangling more than two fiendishly difficult. You can try to place a few crystals in a row, but the more photons you try to entangle, the less efficient the whole process becomes.

In 2012, Jian-Wei Pan and his team at the University of Science and Technology of China steamrolled the issue by developing a laser system that produced entangled photon pairs at a much higher rate. That’s how they set a record of the time of eight entangled photons.

Now, the same team have raised the bar even higher by improving their photon-splitter design.

They placed two β-barium borate crystals in a new sandwich structure that spat the two entangled photons in the same direction, with the same polarisation.

This way, they were able to almost double the collection efficiency from 40% to 70%.

The team then placed five of these crystal sandwiches in a row. When they fired a laser beam through the lot, the five crystals created 10 photons. All 10 were directed into a maze of mirrors and beam-splitters to entangle them all together.

They describe the work in Physical Review Letters.

Besides being a record-breaker, the advance is a milestone because it brings what’s known as Shor’s quantum error correction code within reach.

The team also entangled the highly twisted photons, setting a record for the largest entangled quantum number ever demonstrated.

That’s a way to protect against copying errors inside a quantum computer by storing the information of one qubit onto an entangled state of nine other qubits.

Meanwhile, an international team of researchers from Austria, Canada and Australia has created the twistiest light ever.

The usual picture of a photon is a self-generating electric field and magnetic field, travelling at right angles to one another. But the photon can also be twisted to corkscrew through the air.

Increasing the twistiness of light can dramatically increase the amount of information a single light source can transmit.

Scientists can split one beam of light into multiple channels, each of a different twistiness, and send many different signals simultaneously along the same path.

In 2012, scientists transmitted 2.5 terabits (about 66 DVDs’ worth) per second through a single optical fibre using light with eight different kinds of twistiness.

The cool thing is there is no theoretical upper limit to how twisty the light can get. That means there is potentially no limit to the number of different channels sent down a single fibre at the same time.

The challenge, though, is to reliably create highly twisted light.

Using special mirrors with a helical spiral imprinted onto its surface, physicists have done just that. The work is reported in the Proceedings of the National Academy of Sciences.

Physicists at the Australian National University used a diamond lathe to create patterned mirrors with more precision than ever before. Any photon that bounces off the mirrors gets twisted according to the helical pattern.

Collaborators in Austria and Canada took those mirrors to create photons with an optical angular momentum (physics speak for “twistiness”) of 500, 1,000 and 10,010 quanta. (As a quantum property, the twistiness of light increases in tiny steps – called quanta – in proportion to Planck’s constant.)

The team also entangled the highly twisted photons, setting a record for the largest entangled quantum number ever demonstrated.

According to the authors: “The results show how complex the structure of entangled photons can be and hint at the large information content a single quantum system is able to carry.”

Besides quantum computing, the work could be useful for beefing up how much data can be transmitted through the uncrackable quantum hotline of quantum communication.

Quantum computing is still a long way off. But with these two advances, physicists are reaching for one more bead to thread on their quantum abacus.

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Cathal O'Connell is a science writer based in Melbourne.