Seeking to prevent the loss of information in quantum computers, physicists have found a possible route using laser pulses that create a symmetry in time rather than in space, and do so in two time dimensions.
To physicists, time is just another dimension, albeit one that is curiously resistant to reversing. However, a paper in Nature reports that trapped ions can be made to behave as if experiencing time as two-dimensional, with symmetry in both. The utter strangeness and incomprehensibility of the whole thing aside, the property could be key to avoiding one of quantum computing's greatest problems.
Dr Philipp Dumitrescu of the Flatiron Institute and colleagues created a line of 10 ytterbium ions, which are each held by electric fields in an ion trap and can serve as an individual “qubit”. As in all quantum computers, the qubits can, like ordinary computer bits, be in a 1 or a 0 state, but can also exist in a superposition of both, with the state manipulated by the laser pulses.
The unparalleled potential power of quantum computing has inspired dozens of research teams worldwide to spend decades chasing it, but it has a key flaw: exposure to the outside world, which can lead to decoherence. Even a little warmth getting into systems that need to be kept at temperatures close to absolute zero can be enough to destroy the precious coherence. As the paper notes, “to perform a quantum computation, one faces a trade-off between the desire to isolate qubits to preserve their coherence, and the need to strongly interact qubits to perform computations. “
“Even if you keep all the atoms under tight control, they can lose their quantumness by talking to their environment, heating up or interacting with things in ways you didn’t plan,” first author Dumitrescu said in a statement. “In practice, experimental devices have many sources of error that can degrade coherence after just a few laser pulses.”
One approach to holding onto qubits' information is to induce “time symmetry”. In space, symmetry can operate in a single dimension, such as an object that looks the same in a mirror, or in more than one. Rotational symmetry means that turning something 60 or 120 degrees leaves them exactly as they were. Time symmetry, exemplified by “time crystals”, creates a similar robustness to translations in time.
The authors reasoned that qubits would be more robust if symmetrical on two time axes, rather than one, just as an object with multiple symmetries will be more likely to appear unchanged. This might make the qubits more resilient to disturbance, since anything that caused their state to flip would leave them back where they started.
Alternating different sorts of pulses to manipulate the ions has already been shown to produce time symmetry on a single axis.
To advance this to two dimensions, Dumitrescu and co-authors got inspiration from 13th Century mathematics and Penrose tiling. Instead of adding the last two numbers, as in the original Fibonacci sequence 1, 1, 2, 3, 5, 8..., the team used a series of pulses A, AB, ABA, ABAAB, ABAABABA... This creates a pattern that has order but no repetition, like Penrose's covering of planes with non-overlapping polygons with shapes that have rotational but not translational symmetry.
When the ytterbium ions were exposed to this Fibonacci-like sequence, the qubits at either end of the line maintained their superpositions for 5.5 seconds. To outsiders, this may appear a measly return on such an elaborate investment, but in a world where coherence times are often measured in microseconds, it represents significant progress. Crucially, it is an advance on the 1.5 seconds achieved with an identical set up but a periodic series of laser pulses, which produced only one-dimensional time symmetry.
“With this quasi-periodic sequence, there’s a complicated evolution that cancels out all the errors that live on the edge,” Dumitrescu said. “Because of that, the edge stays quantum-mechanically coherent much, much longer than you’d expect.”
The fact the extended coherence only works for the qubits at either end of the line poses a problem for practical computing, which requires vast numbers of qubits. Nevertheless, Dumitrescu describes the result as “tantalizing” in demonstrating what may be possible.