Research Bits: April 5

Qubits in bulk; storing qubits; storing spin.


Creating qubits in bulk
Researchers from Intel and QuTech, an institute of the Delft University of Technology and the Netherlands Organisation for Applied Scientific Research (TNO), built a qubit using standard semiconductor manufacturing facilities.

The qubit is based on the spin of single electrons that are captured in a silicon nanoscale device, which resembles conventional transistors. The researchers believe that the resemblance could enable design rule techniques used to increase yield and uniformity and decrease defects in the semiconductor industry to be applied to the creation of qubits.

“At present, researchers are mainly working with qubits in laboratories. To make the quantum computer possible, you need to produce these qubits in an accessible way. By working with Intel we wanted to show that we can use current chip production methods to print qubits too. This brings the advent of the quantum computer an important step closer, as you can work with existing facilities,” said Anne-Marije Zwerver, PhD researcher at QuTech, about the collaboration.

“We make the qubits in two steps,” explained Zwerver. “We first create what is called a quantum dot: a kind of box in which we capture an electron. We define this box using electrical voltages that we apply to electrodes. The second step is to make and manipulate qubits using this quantum dot. By making them this way, the qubits resemble the computer chips that are produced on a large scale today.”

“Industrial manufacturing techniques are different from the techniques that are typically used to fabricate such quantum dot samples,” said Zwerver. “It was as if we were first writing with calligraphy and now, we changed to a stencil machine. The former gives more flexibility, the latter gives a significant improvement in yield and uniformity. Moreover, instead of making 20 devices at a time, a fabrication round now gives us tens of thousands of devices, allowing us to collect statistics on the device properties.”

“So many articles state: semiconductor spin qubits in silicon are compatible with CMOS semiconductor manufacturing. But only now, we have proven that to be actually true,” added Lieven Vandersypen, lead scientist at QuTech. “Moreover, the device yield achieved by the Intel team was an unprecedented 98%, compared to 50% on a good day in our university cleanroom.”

Parallel research efforts are geared towards controlling multiple spin qubits and improving the quality of the qubit control. The researchers believe the combined work could build a foundation to realize full-scale quantum computing integrating millions of qubits.

Storing qubits
Researchers from the University of Geneva set a new record for the length of time a qubit could be stored, reaching 20 milliseconds.

“This is a world record for a quantum memory based on a solid-state system, in this case a crystal. We have even managed to reach the 100 millisecond mark with a small loss of fidelity,” said Mikael Afzelius, a senior lecturer in the Department of Applied Physics at the Faculty of Science of the University of Geneva.

The scientists used crystals doped with rare earth metal europium, which are capable of absorbing light and then re-emitting it. These crystals were kept at -273.15°C (absolute zero), because beyond 10°C above this temperature, the thermal agitation of the crystal destroys the entanglement of the atoms.

Crystal used for storing photonic qubits and illuminated by a laser in a cryostat, an instrument for obtaining cryogenic temperatures. (Credit: (c) Antonio Ortu/CC-BY / University of Geneva)

“We applied a small magnetic field of one thousandth of a Tesla to the crystal and used dynamic decoupling methods, which consist in sending intense radio frequencies to the crystal. The effect of these techniques is to decouple the rare-earth ions from perturbations of the environment and increase the storage performance we have known until now by almost a factor of 40,” said Antonio Ortu, a post-doctoral fellow in the Department of Applied Physics at the University of Geneva.

The researchers said that their work could boost the development of long-distance quantum telecommunications networks.

“The challenge now is to extend the storage time further. In theory, it would be enough to increase the duration of exposure of the crystal to radio frequencies, but for the time being, technical obstacles to their implementation over a longer period of time prevent us from going beyond 100 milliseconds. However, it is certain that these technical difficulties can be resolved,” said Afzelius.

They are also working towards memories capable of storing more than a single photon at a time, which would enable ‘entangled’ photons that could guarantee confidentiality. “The aim is to develop a system that performs well on all these points and that can be marketed within ten years,” said Afzelius.

Storing spin
Researchers from the University of Cambridge, University of Technology Sydney, and the Australian National University found that the two-dimensional material hexagonal boron nitride can emit single photons from atomic-scale defects in its structure at room temperature.

The researchers discovered that the light emitted from these isolated defects gives information about a quantum property that can be used to store quantum information, or spin. The quantum spin can be accessed via light and at room temperature, potentially making it useful for future communications networks.

“We can send information from one place to another using photons, but if we’re going to build real quantum networks, we need to send information, store it and send it somewhere else,” said Hannah Stern of Cambridge’s Cavendish Laboratory and a junior research fellow at Trinity College. “We need materials that can hold onto quantum information for a certain amount of time at room temperature, but most current material platforms we’ve got are challenging to make and only work well at low temperatures.”

Hexagonal boron nitride is a two-dimensional material that is grown by chemical vapor deposition, making it cheap and scalable. Recent efforts have revealed the presence of single photon emitters and the presence of a dense ensemble of optically accessible spins, but not individually isolated spin-photon interfaces operating under ambient conditions.

“Usually, it’s a pretty boring material that’s normally used as an insulator,” said Stern. “But we found that there are defects in this material that can emit single photons, which means it could be used in quantum systems. If we can get it to store quantum information in spin, then it’s a scalable platform.”

The team set up a hexagon boron nitride sample near a tiny gold antenna and a magnet of set strength. By firing a laser at the sample at room temperature, they were able to observe lots of different magnetic field-dependent responses on the light being emitted from the material.

The researchers found that when they shone the laser on the material, they were able to manipulate the spin, or inherent angular momentum, of the defects, and use the defects as a way of storing quantum information.

“Typically, the signal is always the same in these systems, but in this case, the signal changes depending on the particular defect we’re studying, and not all defects show a signal, so there is a lot to still discover,” said Qiushi Gu, a PhD candidate at University of Cambridge. “There’s a lot of variation across the material, like a blanket draped over a moving surface – you see lots of ripples, and they’re all different.”

“Now that we have identified optically accessible isolated spins at room temperature in this material, the next steps will be to understand their photophysics in detail and explore the operation regimes for possible applications including information storage and quantum sensing,” said Mete Atature, a professor of physics at University of Cambridge and fellow at St. John’s College. “There will be a stream of fun physics following this work.”

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