At Scientific American, we were told last month: “Physicists have devised a mind-bending error-correction technique that could dramatically boost the performance of quantum computers”:
“It is very exciting to see this unusual phase of matter realized in an actual experiment, especially because the mathematical description is based on a theoretical ‘extra’ time dimension,” says team member Philipp Dumitrescu, who was at the Flatiron Institute in New York City when the experiments were carried out. A paper describing the work was published in Nature on July 20.
Opening a portal to an extra time dimension—even just a theoretical one—sounds thrilling, but it was not the physicists’ original plan. “We were very much motivated to see what new types of phases could be created,” says study co-author Andrew Potter, a quantum physicist at the University of British Columbia. Only after envisioning their proposed new phase did the team members realize it could help protect data being processed in quantum computers from errors.Zeeya Merali, “New Phase of Matter Opens Portal to Extra Time Dimension” at Scientific American (July 26, 2022)
So, time travel? Not really. Physicist Philipp Dumitrescu and colleagues (the paper requires a fee or subscription) were studying phases of matter and realized that one of them could be used as an error correction technique for quantum computers. They used a pulse frequency that was neither periodic nor random but rather followed the Fibonacci sequence of numbers.
Experimental physicist Rob Sheldon offers an explanation:
They are constructing “time crystals”, where moving atoms return to the same position after some time.
A simple example is to connect two pendulums with a spring and set them in motion. After a while, one pendulum comes to rest and the other oscillates wildly. But then the stationary one starts to move and oscillates wildly while the first one stops. This goes on for some time.
If we make a graph with time on the x-axis, and positions of two pendulums on y-axis, the pattern repeats with time. This is an example of a “time crystal.” The researchers wanted to do it for 11 atoms in a quantum computer, that were acting as “qubits”, or quantum states. So you can think of this as 11 pendulums connected by springs. But the “springs” are actually two lasers beams that push them around.
The reason for this arrangement is that we need to “entangle” the 11 atoms in a coherent wavefunction to make a quantum computation. But the slightest disturbances “perturb” the entangled state and destroy it or “decohere” it into random, uncoordinated motions.
However to make a quantum computer useful, the entangled state must last long enough to do a calculation and be read out. The perturbations were too strong, so the entangled state “decayed” too quickly to be useful.
In the past decade, people realized that one can “digitize” these entangled states by making them wrap around a crystal or some physical symmetry. Then, as in Bohr’s electron model of the atom, only a very few waves have the right “size” ( or energy) to wrap around an object and match the ends. It’s like a jump rope. You can have waves of 1/2 wavelength with one jumper (that’s the normal one) or, with talented rope handlers, twice that for two jumpers. But you can’t have .75 wavelengths and 1 1/2 jumpers. It has to come out even.
This effect is what turns squishy waves into digitized units of 1/2 wavelengths. It’s a “topological” effect of wrapping waves into a package that turns them into integers. That’s how the “quantum” in quantum mechanics (QM) comes about.
What physicists realized in the past three decades is that this applies to large groups of atoms as well as to Bohr’s single atom. There are waves that wrap around a million atoms or even a trillion atoms, but have to match at the ends. This allows one to construct (with silicon etching) macroscopic (visible to the eye) shapes with distinct, quantized wavefunctions called “topological” states.
With such a wavefunction, little perturbations don’t have enough oomph to push the entangled state to another wrapping number (higher energy). So the topological state is very stable and robust. This gives the quantum computer the stability it needs to carry out computations on qubits.
This is how the experiment started out: They took 11 atoms, connected the springs, and made a time crystal with topological (in time) symmetry. If it helps, think of the two dimensions of a donut as polar and azimuthal angles that wrap back to the beginning. Now for a time crystal, the pulses of the two lasers have time lags, called phases, that also wrap back to the beginning. So we are making a “donut” time crystal.
Their time crystal didn’t work. The results were a mishmash. Too many perturbations “resonated” with the time or phase of the crystal, and spoiled the effect. So they decided to lengthen the size. If it were space they would expand from microns to meters in size, but since they are using time crystals, they “size” is a really long repetition time. In nuclear fusion tokomaks, this is the “wrapping angle” around the donut torus. If chosen correctly, it keeps the hydrogen ions from repeating an orbit, as they densely fill all the possible area of the donut like winding thread on a spool.
This means that a bump or imperfection in the walls of the tokamak donut — a tokamak is a donut-shaped vacuum chamber — only affects a hydrogen atom once, and doesn’t resonate or add perturbations with each orbit. So with two lasers, they made a time crystal where the phases or timing of two lasers pulses adjusted the “wrapping angle” in time. When they found these “long repeat” wrapping angles, they discovered that their entangled states lasted a great deal longer, which made a quantum computer using atoms for qubits possible.
They found a useful error correction technique that may help with the development of quantum computers but it’s not really a portal to an extra time dimension. For that, we need science fiction.
You may also wish to read: Even if a time machine didn’t kill you, it wouldn’t change much. Here are some interesting reflections by science buffs on time machines, as seen in the movies. Are they even possible? If time machines that would take us any distance into the past or future are possible, they would suggest that we live in a fatalistic universe.