Simulating quantum ‘time travel’ disproves butterfly effect in quantum realm
Evolving quantum processes backwards on a quantum computer to damage information in the simulated past causes little change when returned to the ‘present’
July 28, 2020
In research by a team at Los Alamos National Laboratory, Alice prepares her qubit and applies the information scrambling unitary U to this and many other qubits altogether. Bob measures her qubit in any basis, flipping the qubit to the state not known to Alice. Alice still can reconstruct her information via a single decoding unitary U†.
On a quantum computer, there is no problem simulating opposite-in-time evolution, or simulating running a process backwards into the past.- Nikolai Sinitsyn
LOS ALAMOS, N.M., July 28, 2020—Using a quantum computer to simulate time travel, researchers have demonstrated that, in the quantum realm, there is no “butterfly effect.” In the research, information—qubits, or quantum bits—“time travel” into the simulated past. One of them is then strongly damaged, like stepping on a butterfly, metaphorically speaking. Surprisingly, when all qubits return to the “present,” they appear largely unaltered, as if reality is self-healing.
“On a quantum computer, there is no problem simulating opposite-in-time evolution, or simulating running a process backwards into the past,” said Nikolai Sinitsyn, a theoretical physicist at Los Alamos National Laboratory and coauthor of the paper with Bin Yan, a post doc in the Center for Nonlinear Studies, also at Los Alamos. “So we can actually see what happens with a complex quantum world if we travel back in time, add small damage, and return. We found that our world survives, which means there’s no butterfly effect in quantum mechanics.”
In Ray Bradbury’s 1952 science fiction story, “A Sound of Thunder,” a character used a time machine to travel to the deep past, where he stepped on a butterfly. Upon returning to the present time, he found a different world. This story is often credited with coining the term “butterfly effect,” which refers to the extremely high sensitivity of a complex, dynamic system to its initial conditions. In such a system, early, small factors go on to strongly influence the evolution of the entire system.
Instead, Yan and Sinitsyn found that simulating a return to the past to cause small local damage in a quantum system leads to only small, insignificant local damage in the present.
This effect has potential applications in information-hiding hardware and testing quantum information devices. Information can be hidden by a computer by converting the initial state into a strongly entangled one.
“We found that even if an intruder performs state-damaging measurements on the strongly entangled state, we still can easily recover the useful information because this damage is not magnified by a decoding process,” Yan said. “This justifies talks about creating quantum hardware that will be used to hide information.”
This new finding could also be used to test whether a quantum processor is, in fact, working by quantum principles. Since the newfound no-butterfly effect is purely quantum, if a processor runs Yan and Sinitsyn’s system and shows this effect, then it must be a quantum processor.
To test the butterfly effect in quantum systems, Yan and Sinitsyn used theory and simulations with the IBM-Q quantum processor to show how a circuit could evolve a complex system by applying quantum gates, with forwards and backwards cause and effect.
Presto, a quantum time-machine simulator.
In the team’s experiment, Alice, a favorite stand-in agent used for quantum thought experiments, prepares one of her qubits in the present time and runs it backwards through the quantum computer. In the deep past, an intruder – Bob, another favorite stand-in – measures Alice’s qubit. This action disturbs the qubit and destroys all its quantum correlations with the rest of the world. Next, the system is run forward to the present time.
According to Ray Bradbury, Bob’s small damage to the state and all those correlations in the past should be quickly magnified during the complex forward-in-time evolution. Hence, Alice should be unable to recover her information at the end.
But that’s not what happened. Yan and Sinitsyn found that most of the presently local information was hidden in the deep past in the form of essentially quantum correlations that could not be damaged by minor tampering. They showed that the information returns to Alice’s qubit without much damage despite Bob’s interference. Counterintuitively, for deeper travels to the past and for bigger “worlds,” Alice’s final information returns to her even less damaged.
“We found that the notion of chaos in classical physics and in quantum mechanics must be understood differently,” Sinitsyn said.
Paper: Bin Yan and Nikolai A. Sinitsyn. Recovery of Damaged Information and the Out-of-Time-Ordered Correlators, Phys. Rev. Lett. 125, 040605 (2020); available online at https://doi.org/10.1103/PhysRevLett.125.040605. Authors: Bin Yan and Nikolai Sinitsyn.
Funding: This work was supported by the U.S. Department of Energy Office of Science.
Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is managed by Triad, a public service oriented, national security science organization equally owned by its three founding members: Battelle Memorial Institute (Battelle), the Texas A&M University System (TAMUS), and the Regents of the University of California (UC) for the Department of Energy’s National Nuclear Security Administration.
Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.
Time travel movies have different rules about what happens when you start messing around with the timeline. If you’ve ever wondered which ones make the most sense, we may now have an answer. According to experiments using a quantum time travel simulator, reality is more or less “self-healing,” so changes made to the past won’t drastically alter the future you came from – at least, in the quantum realm.
The classic Back to the Future rules of time travel say that whatever you change in the past can have huge effects on the future. That’s why Marty McFly can almost erase his own existence by accidentally stopping his parents from meeting, and why Biff Tannen can get rich by giving his younger self a book of sports scores to bet on.
Other movies handle things differently. In Avengers: Endgame, the superheroes travel back in time to steal versions of the Infinity Stones out of different time periods to revive their fallen friends (look, it doesn’t make much sense unless you’ve seen all 20-something movies). Anyway, they can dabble in the past without ruining the future because the universe has a knack for correcting those paradoxes so that both versions of events did happen.
But which set of rules has more basis in science? According to a new study, quantum mechanics supports the Avengers model.
In a way, it all comes down to the butterfly effect, where a tiny action in one system can escalate into huge consequences. The name comes from the principle of chaos theory that a butterfly flapping its wings could set off a cascade of events that causes a hurricane in another part of the world.
In terms of time travel though, the term might have originated in reference to Ray Bradbury’s 1952 sci-fi story, A Sound of Thunder. In this story, time tourism is readily available, but carefully controlled so as not to affect the present. While on one of these trips into the distant past, a character steps off the beaten track and accidentally kills a butterfly, and when he returns to his own time, things have dramatically changed.
So how can we test these ideas? Since we can’t exactly jump into a TARDIS and start experimenting ourselves, researchers at Los Alamos National Laboratory used a quantum computer to develop a simulation of time travel. And their results were somewhat surprising.
“On a quantum computer, there is no problem simulating opposite-in-time evolution, or simulating running a process backwards into the past,” says Nikolai Sinitsyn, co-author of the study. “So we can actually see what happens with a complex quantum world if we travel back in time, add small damage, and return. We found that our world survives, which means there’s no butterfly effect in quantum mechanics.”
Using an IBM-Q quantum processor, the team created a complex system using quantum gates and demonstrating cause and effect, running both forwards and backwards in time. The simulation involved two hypothetical people, Alice and Bob, who each have a qubit – a quantum bit of information.
In the scenario, Alice prepares her qubit in the present, then sends it backwards in time. At some point in the past, Bob interferes with the qubit by measuring it. Then, the system is run forwards again to the present time, and Alice checks her qubit.
What you think “should” happen next depends on which time travel rules you subscribe to. The butterfly effect says that because the qubit is tied to so many variables, Bob’s small interference should completely change the system by the time we get back to the future (or present, to Alice).
In this diagram, Bob's interference with the qubit in the deep past doesn't affect Alice's ability to read it in the future
Los Alamos National Laboratory
But the team found that that wasn’t the case. Alice’s qubit comes back relatively unscathed, and she can recover the information on it. Interestingly, the fact that it’s tied to so many variables seems to be what actually saves it from damage – the information in the present qubit was hidden in the quantum correlations in the deep past. This web of connections isn’t so easily disturbed by Bob’s amateur efforts at timeline vandalism.
Stranger still, the further back in time the qubit travels, and the more complicated the whole system is, the less damage the qubit takes from interference. That might seem counterintuitive – since there are more things for the butterfly’s wings to knock over, so logically the effects should be more dramatic. But by the team’s reasoning, this just creates a stronger web of quantum correlations to protect the qubit from damage.
“We found that the notion of chaos in classical physics and in quantum mechanics must be understood differently,” says Sinitsyn.
If we want to take these findings to their logical conclusion, the argument could be made that Back to the Future represents time travel through classical physics, while Avengers: Endgame is a model of quantum time travel. Intriguingly, the latter movie justifies time travel by having the characters manipulate the “quantum realm.”
While the study may mostly seem like just a fun thought experiment, the team says that the simulation could have some real-world benefits. For one, since there’s no way a classical processor can handle this kind of simulation, it could be used as a way of testing whether a quantum computer is actually working on quantum principles.
The team also says that the concept could be used to create new security protocols for information in quantum systems. If a more sinister real-world Bob tries to mess with the qubits, the system can convert the information into a strongly entangled state to protect it.
“We found that even if an intruder performs state-damaging measurements on the strongly entangled state, we still can easily recover the useful information because this damage is not magnified by a decoding process,” says Bin Yan, co-author of the study. “This justifies talks about creating quantum hardware that will be used to hide information.”
Imagina que viajas al pasado y haces algo aparentemente inofensivo; impides un accidente, por ejemplo. Al volver al presente, descubres que todo es completamente distinto; te ves en una silla de ruedas, con algunas extremidades amputadas. Este es el tipo de consecuencias involuntarias que se producen gracias lo que se conoce como ‘efecto mariposa’.
Ahora, un equipo de investigadores acaba de usar una computadora cuántica para simular este tipo de consecuencias en los viajes en el tiempo. El resultado es sorprendente: al menos en el ámbito cuántico, no existe un ‘efecto mariposa’.
“Descubrimos que nuestro mundo sobrevive, lo que significa que no hay efecto mariposa en la mecánica cuántica», describió Nikolai Sinitsyn, físico teórico en el Laboratorio Nacional de Los Alamos
En 1952, Ray Bradbury publicó una historia de ciencia ficción llamada “A Sound of Thunder”. En ella nos cuenta cómo un personaje usó una máquina del tiempo para viajar al pasado, donde pisó una mariposa. Al regresar al presente, encontró un mundo completamente diferente.
Esta es la historia a la que comúnmente se le atribuye el término ‘efecto mariposa’, aunque también existen otras frases populares en internet. “El batir de las alas de una mariposa puede provocar un huracán en otra parte del mundo”, es una de ellas.
El ‘efecto mariposa’ se refiere a la alta sensibilidad que tienen los sistemas complejos y dinámicos a sus condiciones iniciales. Es decir, un pequeño factor podría tener una fuerte influencia en la evolución de todo el sistema.
No existe un efecto mariposa a nivel cuántico
En la investigación, la información (qubits o bits cuánticos) viajaron en el tiempo hacia un pasado simulado. Aquí, uno de ellos fue dañado severamente. Sorprendentemente, cuando todos los qubits regresaron al presente, parecían inalterados, como si la realidad se hubiera autocorregido.
«En una computadora cuántica, no hay problema simulando la evolución opuesta en el tiempo, o simulando ejecutar un proceso hacia atrás en el pasado. Entonces podemos ver qué sucede con un mundo cuántico complejo si viajamos en el tiempo, agregamos daños pequeños y regresamos”, sostuvo Sinitsyn, coautor del estudio.
El resultado es sorprendente porque descubrieron que un daño local pequeño en un sistema cuántico conduce a un daño local pequeño e insignificante en el presente.
«Descubrimos que la noción de caos en la física clásica y en la mecánica cuántica debe entenderse de manera diferente», dijo Sinitsyn.
Posibles aplicaciones
Este efecto tiene aplicaciones potenciales en hardware para ocultar información y poner a prueba dispositivos de información cuántica. La información puede ser ocultada por una computadora al convertir el estado inicial en uno fuertemente entrelazado.
«Descubrimos que incluso si un intruso realiza mediciones que dañan el estado en el estado fuertemente enredado, aún podemos recuperar fácilmente la información útil porque este daño no se magnifica por un proceso de decodificación», dijo Yan, autor principal del estudio. «Esto justifica las conferencias sobre la creación de hardware cuántico que se utilizará para ocultar información».
Además, el nuevo hallazgo podría usarse para probar si un procesador cuántico está funcionando según los principios cuánticos. Por ejemplo, si un procesador muestra este efecto, entonces debe ser un procesador cuántico.
Hallan, durante un experimento, una nueva clase de tiempo cuántico
Un equipo de investigadores mezcla los conceptos de tiempo clásico y tiempo cuántico para alterar el orden en el que se producen dos o más acontecimientos
Un equipo de investigadores de la Universidad de Queensland, en Australia, acaba de hacer un descubrimiento excepcional durante uno de sus experimentos. La forma más sencilla de describirlo sería que han encontrado un «nuevo tipo de orden en el tiempo cuántico». El hallazgo, en el que se mezclan la física clásica y la física cuántica, permite alterar el orden temporal lógico de dos o más acontecimientos.
La física Magdalena Zych, que ha dirigido la investigación, afirma que el descubrimiento surgió de un experimento diseñado por su equipo para unir elementos de las dos mayores, aunque contradictorias, teorías de la Física. Los resultados de este singular trabajo se acaban de publicar en Nature Communications.
«Nuestro propósito –asegura la investigadora– era descubrir qué sucede cuando un objeto lo suficientemente masivo como para influir en el flujo del tiempo se coloca en un estado cuántico».
El tiempo en la cuántica y en la relatividad
Conviene aclarar, en este punto, que el concepto del tiempo y su flujo cambia mucho de la física clásica a la mecánica cuántica. Según explican los autores en su artículo, «El tiempo tiene un carácter fundamentalmente diferente en la mecánica cuántica y en la relatividad general. En la teoría cuántica, los eventos se desarrollan en un orden fijo, mientras que en la relatividad general el orden temporal está influenciado por la distribución de la materia. Cuando la materia requiere una descripción cuántica, se espera que el orden temporal se vuelva no clásico, un escenario más allá del alcance de las teorías actuales. Aquí proporcionamos una descripción directa de tal escenario».
La teoría de Einstein, por ejemplo, predice que la presencia de un objeto muy masivo puede ralentizar el tiempo. Y de ahí parte precisamente el experimento de los investigadores.
«Imaginemos dos naves espaciales –explica Zych– a las que se les ordena dispararse mutuamente en un momento específico, al mismo tiempo que tratan de esquivar el ataque de su oponente».
Evidentemente, el primero que efectúe su disparo será el vencedor y destruirá a la otra nave. Pero las cosas no siempre son como parecen.
Ralentizar el tiempo en combate...
«Según la teoría de Einstein –continúa Zych– un enemigo lo suficientemente poderoso podría usar los principios de la relatividad general y colocar un objeto muy masivo, como un planeta, cerca de la nave enemiga para que en ella se ralentice el paso del tiempo. A causa de este lapso temporal, la nave más alejada del objeto masivo disparará antes, y destruirá a su adversario».
Y justo aquí entra la segunda teoría. La mecánica cuántica, en efecto, dice que un objeto puede estar en un estado de «superposición». «Lo cual significa –prosigue la investigadora– que podemos encontrarlo en diferentes estados al mismo tiempo, como sucede con el célebre gato de Schrödinger». (Ya saben, el gato encerrado en una caja junto a un frasco de veneno y que, según la mecánica cuántica, está en un estado de «vivo/muerto» hasta que abrimos la caja y «materializamos» uno de los dos posibles estados).
O frenar el flujo del tiempo
Pues bien, según Zych, si aplicamos la mecánica cuántica al caso de la batalla espacial, la nave que debería ser destruida a causa de la ralentización del tiempo podría colocar todo el objeto masivo (el planeta entero) en un estado de superposición cuántica, con lo cual interrumpiría de inmediato el flujo del tiempo.
Para Zych, «esta sería una forma totalmente nueva de establecer el orden de los eventos, sin que ninguno de ellos sea primero o segundo. En un estado cuántico genuino, en efecto, ambos serían primero y segundo al mismo tiempo».
Según explica por su parte Fabio Costa, coautor del estudio, «aunque una superposición de planetas, como se describe en el estudio, puede que nunca sea posible, la tecnología sí que nos permitió simular cómo funcionaría el tiempo en el mundo cuántico, sin usar la gravedad. Incluso si el experimento nunca llegara a hacerse, el estudio resulta relevante para las tecnologías futuras».
Como ejemplo de esas tecnologías, Costa afirma que «actualmente, estamos trabajando en computadoras cuánticas que, dicho de forma sencilla, podrían saltar efectivamente en el tiempo para realizar sus operaciones de manera mucho más eficiente que los dispositivos que operan siguiendo una secuencia temporal fija, tal y como la conocemos en nuestro mundo "normal"».
You know how sometimes you find yourself facing off against an enemy spaceship in a Wild West-inspired laser duel where whoever fires first wins?
What if I told you that a group of researchers (a murder of physicists?) came up with a way for you to manipulate the very fabric of time and space so that, no matter who fires first, you both die? That’s probably not the best pitch you’ve heard for developing quantum computers. But it’s an interesting one.
The researchers, led by scientists at the University of Queensland, have discovered “a new kind of quantum time order.” It’s like time travel, but for the universe instead of you. The research focuses on a thought experiment where an object large enough to affect local gravity gets placed in a state of quantum superposition at the moment a series of events unfold.
University of Queensland physicist Magdalena Zych explained it to Science Daily:
Imagine two space ships, asked to fire at each other at a specified time while dodging the other’s attack.
If either fires too early, it will destroy the other.
In Einstein’s theory, a powerful enemy could use the principles of general relativity by placing a massive object — like a planet — closer to one ship to slow the passing of time. Because of the time lag, the ship furthest away from the massive object will fire earlier, destroying the other.
Einstein’s theories on relativity are only half the ingredients to the thought experiment though. The rest comes from another theory: quantum mechanics. According to quantum mechanics, the prevailing theory on how our universe works, any object should be able to be placed into a state of superposition – even an entire planet.
Superposition is a quantum concept where a particle, or system – in this case a planet – is in two distinctly different physical states at the same time. This was best explained through Schodinger’s Cat, another thought experiment. Think of a particle in superposition like a spinning coin: it’s both heads and tails until it lands.
If the planet next to one of the spaceships was placed into a state of superposition, the researchers claim, these quantum affects would logically extend to time as well. Zych’s explanation continues:
There would be a new way for the order of events to unfold, with neither of the events being first or second — but in a genuine quantum state of being both first and second.
In essence, no matter who fired first, the quantum state of the planet would have a greater influence over how the events unfolded – my guess is both spaceships blow up and then the lasers go off a few seconds later like sad space confetti.
This all sounds impossible but as any Douglas Adams fan knows, time travel is merely improbable. And we can work with that. According to Zych, this trippy thought experiment (and the math that backs it up) has the potential to directly inform the development of tomorrow’s quantum computers. She told Science Daily:
We are currently working towards quantum computers that — very simply speaking — could effectively jump through time to perform their operations much more efficiently than devices operating in fixed sequence in time, as we know it in our ‘normal’ world.
Classical computers have to do things in order. If, then, goto; that’s how they work. Quantum computers can, theoretically, just “goto” the answer. It’s a bit more complex than that, but essentially physicists are writing the rules of the universe in real time in 2019, and will continue to do so for the imaginable future.
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