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We have demonstrated that a long conduction electron spin lifetime in metallic-like material made up of carbon nanospheres can be achieved at room temperature.
This material was produced simply by burning naphthalene, the active ingredient in mothballs.
The material is produced as a solid powder and handled in air. It can then be dispersed in ethanol and water solvents, or deposited directly onto a surface like glass. As the material was remarkably homogeneous, the measurements could be made on the bulk solid powder.
This allowed us to achieve a new record electron spin lifetime of 175 nanoseconds at room temperature. This might not sound like a long time, but it exceeds the prerequisite for applications in quantum computing and is about 100 times longer than that found in graphene.
In principal, this may provide an initial avenue to high-density qubit arrays of nanospheres that are integrated onto existing silicon technologies or thin-film-based electronics.
While traditional bits represent data as 0s or 1s, qubits are distinguished by what's known as superposition, or the ability to be both 0 and 1 at once.
Superposition is the heart of quantum computing's exciting potential, but it's also proved a thorny challenge. While calculations require that qubits not only maintain their state but also interact with one another, the quantum objects that have been used to create qubits -- ions or electrons, for example -- have so far only been able to maintain a certain quantum state for a short time. In a system with dozens or hundreds of qubits, the problem gets even trickier.
That's where physicists from the Moscow Institute of Physics and Technology and the Russian Quantum Center are proposing a different approach. Rather than trying to maintain the stability of a large qubit system, they sought instead to increase the capacity of the units doing the calculations. For that, they turned to the "qudit," a qubit alternative.
A qudit with four or five levels is able to function as a system of two 'ordinary' qubits, and eight levels is enough to imitate a three-qubit system," explained Aleksey Fedorov, a researcher at the Russian Quantum Center.
The new module builds on decades of research into trapping and controlling ions. It uses standard techniques but also introduces novel methods for control and measurement. This includes manipulating many ions at once using an array of tightly-focused laser beams, as well as dedicated detection channels that watch for the glow of each ion.
The module performs these tasks using different colors of laser light. One color prepares the ions using a technique called optical pumping, in which each qubit is illuminated until it sits in the proper quantum energy state. The same laser helps read out the quantum state of each atomic ion at the end of the process. In between, a separate laser strikes the ions to drive quantum logic gates.
“By reducing an algorithm into a series of laser pulses that push on the appropriate ions, we can reconfigure the wiring between these qubits from the outside," he says. "It becomes a software problem, and no other quantum computing architecture has this flexibility."
The team believes that eventually more qubits—perhaps as many as 100—could be added to their quantum computer module. It is also possible to link separate modules together, either by physically moving the ions or by using photons to carry information between them.
NREL's [National Renewable Energy Lab] scientists were able to observe the [optical Stark] effect quite readily at room temperature in materials grown using solution processing.
The NREL researchers used the optical Stark effect to remove the degeneracy of the excitonic spin states within the perovskite sample. An electron can have either "up" or "down" spins, and electrons with opposite spins can occupy the same electronic state. Circularly polarized light can be used to only interact with one of the spin states, shifting its transition energy.
The optical Stark effect possesses promising applications, including the potential to be used as an ultrafast optical switch. In addition, the optical Stark effect can be used to control or address individual spin states, which is needed for spin-based quantum computing.
The Stanford team has built what's called an Ising machine, named for a mathematical model of magnetism. The machine acts like a reprogrammable network of artificial magnets where each magnet only points up or down and, like a real magnetic system, it is expected to tend toward operating at low energy.
The theory is that, if the connections among a network of magnets can be programmed to represent the problem at hand, once they settle on the optimal, low-energy directions they should face, the solution can be derived from their final state…
Rather than using magnets on a grid, the Stanford team used a special kind of laser system, known as a degenerate optical parametric oscillator, that, when turned on, will represent an upward- or downward-pointing "spin."
Nearly all of the materials used to make this machine are off-the-shelf elements that are already used for telecommunications. That, in combination with the simplicity of the programming, makes it easy to scale up. Stanford's machine is currently able to solve 100-variable problems with any arbitrary set of connections between variables, and it has been tested on thousands of scenarios.
A group at NTT in Japan that consulted with Stanford's team has also created an independent version of the machine.
“Quantum coherence underlies all quantum information technologies, such as quantum communication and quantum sensing. However, the coherence time in materials is eventually limited by the magnetic noise produced by the fluctuating nuclear spins in a crystal,” said Hosung Seo…
However, spin qubits in silicon carbide have been expected to have inherently short coherence times because of the high concentration of magnetic nuclei in the crystals. Counterintuitively, the electron coherence time in silicon carbide reaches 1.3 milliseconds—the longest time measured in a naturally isotopic crystal.
“Our work has important implications beyond silicon carbide. The essential physics and the dynamics responsible for the coherence found in silicon carbide, a binary crystal, may allow qubits in ternary and quaternary crystals to have even longer spin coherence times,” said Abram Falk.
"Typically, for surface electrode traps, the laser beam is coming from an optical table and entering this system, so there's always this concern about the beam vibrating or moving," said engineer Rajeev Ram. "With photonic integration, you're not concerned about beam-pointing stability, because it's all on the same chip that the electrodes are on. So now everything is registered against each other, and it's stable."
The team’s successful demonstration of their prototype suggests that large-scale trapped-ion quantum systems could employ similar techniques. However, one remaining barrier is that the integrated photonic system has no mechanism for varying the amount of light delivered to the ions. The researchers are investigating the addition of light modulators to the gratings in order to address this issue.