Quantum simulation gives a sneak peek into the possibilities of time reversal.
We all mark days with clocks and calendars, but perhaps no timepiece is more immediate than a mirror. The changes we notice over the years vividly illustrate science's "arrow of time" -- the likely progression from order to disorder. We cannot reverse this arrow any more than we can erase all our wrinkles or restore a shattered teacup to its original form.
Or can we?
An international team of researchers from the Moscow Institute of Physics and Technology teamed up with colleagues from the U.S. Department of Energy's (DOE) Argonne National Laboratory and Switzerland, explored this question in a first-of-its-kind experiment, managing to return the state of a quantum computer a fraction of a second into the past. They also calculated the probability that an electron in empty interstellar space will spontaneously travel back into its recent past. The results, published March 13th in the journal Scientific Reports, suggest new paths for exploring the backward flow of time in quantum systems. They also open new possibilities for quantum computer program testing and error correction.
To achieve the time reversal, the research team developed an algorithm for IBM's public quantum computer that simulates the scattering of a particle. In classical physics, this might appear as a billiard ball struck by a cue, travelling in a line. But in the quantum world, one scattered particle takes on a fractured quality, spreading in multiple directions. To reverse its quantum evolution is like reversing the rings created when a stone is thrown into a pond.
IBM Q is an industry first initiative to build universal quantum computers for business and science.
In nature, restoring this particle back to its original state -- in essence, putting the broken teacup back together -- is impossible.
The main problem is that you would need a "supersystem," or external force, to manipulate the particle's quantum waves at every point. But, the researchers note, the timeline required for this supersystem to spontaneously appear and properly manipulate the quantum waves would extend longer than that of the universe itself.
Undeterred, the team set out to determine how this complexity might be overcome, at least in principle. Their algorithm simulated an electron scattering by a two-level quantum system, "impersonated" by a quantum computer qubit -- the basic unit of quantum information -- and its related evolution in time. The electron goes from a localised, or "seen," state, to a scattered one. Then the algorithm throws the process in reverse, and the particle returns to its initial state -- in other words, it moves back in time, if only by a tiny fraction of a second.
Given that quantum mechanics is governed by probability rather than certainty, the odds for achieving this time-travel feat were pretty good: The algorithm delivered the same result 85 percent of the time in a two-qubit quantum computer.
"We did what was considered impossible before," said Argonne senior scientist Valerii Vinokur, who led the research.
The result deepens our understanding of how the second law of thermodynamics -- that a system will always move from order to entropy and not the other way around -- acts in the quantum world. The researchers demonstrated in previous work that, by teleporting information, a local violation of the second law was possible in a quantum system separated into remote parts that could balance each other out.
"The results also give a nod to the idea that irreversibility results from measurement, highlighting the role that the concept of 'measurement' plays in the very foundation of quantum physics," said article coauthor Gordey Lesovik of the Moscow Institute of Physics and Technology.
This is the same notion Austrian physicist Erwin Schrödinger captured with his famous thought experiment, in which a cat sealed in a box might remain both dead and alive until its status is monitored somehow. The researchers suspended their particle in this superposition, or form of quantum limbo, by limiting their measurements.
"This was the essential part of our algorithm," Vinokur said. "We measured the state of the system in the very beginning and at the very end, but did not interfere in the middle."
The four stages of the actual experiment on a quantum computer mirror the stages of the thought experiment involving an electron in space and the imaginary analogy with billiard balls. Each of the three systems initially evolves from order toward chaos, but then a perfectly timed external disturbance reverses this process. Credit: @tsarcyanide/MIPT Press Office
Stage 1: Order. Each qubit is initialized in the ground state, denoted as zero. This highly ordered configuration corresponds to an electron localized in a small region, or a rack of billiard balls before the break.
Stage 2: Degradation. The order is lost. Just like the electron is smeared out over an increasingly large region of space, or the rack is broken on the pool table, the state of the qubits becomes an ever more complex changing pattern of zeros and ones. This is achieved by briefly launching the evolution program on the quantum computer. Actually, a similar degradation would occur by itself due to interactions with the environment. However, the controlled program of autonomous evolution will enable the last stage of the experiment.
Stage 3: Time reversal. A special program modifies the state of the quantum computer in such a way that it would then evolve “backwards,” from chaos toward order. This operation is akin to the random microwave background fluctuation in the case of the electron, but this time it is deliberately induced. An obviously far-fetched analogy for the billiards example would be someone giving the table a perfectly calculated kick.
Stage 4: Regeneration. The evolution program from the second stage is launched again. Provided that the “kick” has been delivered successfully, the program does not result in more chaos but rather rewinds the state of the qubits back into the past, the way a smeared electron would be localized or the billiard balls would retrace their trajectories in reverse playback, eventually forming a triangle.
The finding may eventually enable better methods of error correction on quantum computers, where accumulated glitches generate heat and beget new ones. A quantum computer able to effectively jump back and clean up errors as it works could operate far more efficiently.
"At this moment, it's very hard to imagine all the implications this can have," Vinokur said. "I am optimistic, and I believe that it will be many."
The study also raises the question: can the researchers now figure out a way to make older folks young again? "Maybe," Vinokur jokes, "with the proper funding."
The work was done by international team including researchers from the Moscow Institute of Physics and Technology (Gordey Lesovik, Andrey Lebedev, Mikhail Suslov), ETH Zurich (Andrey Lebedev) and Argonne National Laboratory, U.S. (Valerii Vinokur, Ivan Sadovskyy).
Funding for this research was provided by the DOE Office of Science and Strategic Partnership Projects (Swiss National Foundation and the Foundation for the Advancement of Theoretical Physics "BASIS").
Source materials provided by DOE/Argonne National Laboratory. Originally written by Christina Nunez. Note: Content may be edited for style and length.
Further reading: Physicists reverse time using quantum computer
