While reversing time remains science fiction, scientists have demonstrated a concept called “quantum reverse” in laboratories. This involves precisely manipulating tiny quantum systems to make them evolve backward in a controlled setting, effectively returning them to a previous state. This concept offers insights into how quantum mechanics behaves over time.
The Quantum Arrow of Time
In our everyday experience, time seems to flow in one direction, from past to future. A dropped teacup shatters, and it never spontaneously reassembles itself; a cup of hot coffee cools down, but it does not spontaneously heat back up. This irreversible progression is governed by the Second Law of Thermodynamics, which states that the total disorder, or entropy, of an isolated system tends to increase over time. This principle gives rise to what physicists call the “arrow of time” in the macroscopic world.
The fundamental equations describing the quantum world, such as the Schrödinger equation, present an interesting contrast. These equations are time-symmetric, meaning they work equally well whether time is moving forward or backward. If you could perfectly reverse all the motions of every particle in a quantum system, it would, in theory, retrace its steps back to its original configuration. This inherent reversibility at the quantum level poses a puzzle when contrasted with the macroscopic world’s clear one-way flow of time.
Simulating a Quantum Rewind
In 2019, scientists from the Moscow Institute of Physics and Technology, collaborating with researchers in Switzerland and the United States, used an IBM quantum computer to simulate quantum reversal. They demonstrated the ability to rewind quantum states.
This process can be visualized like a game of pool where a perfectly arranged triangle of balls is broken apart by a cue ball, scattering in all directions. Imagine a special “kick” could then be applied, causing them to precisely reverse their trajectories and reform the original perfect triangle. In the experiment, the “balls” were qubits, the basic units of quantum information. Researchers initiated an “evolution program” that caused these qubits to become increasingly complex and disordered.
A precisely engineered microwave pulse then acted as the “kick,” reversing their evolution back to their initial, simpler state. In trials, a two-qubit system returned to its initial state approximately 85% of the time. A three-qubit system achieved about a 50% success rate due to inherent quantum computer imperfections.
Distinguishing Fact from Fiction
Despite its intriguing nature, “quantum reverse” is not time travel as depicted in movies. This demonstration works only for tiny, isolated quantum systems, not macroscopic objects. It cannot send information to the past or alter the overall flow of time.
This experiment does not violate the Second Law of Thermodynamics, which states that the total entropy of a closed system must increase or remain constant. While the qubit system temporarily moved from higher to lower disorder, the quantum computer and its environment generated a much larger amount of disorder. The energy consumed and heat produced by the computer contributed to an overall increase in the universe’s total entropy, upholding this fundamental law.
Practical Implications for Quantum Computing
The ability to reverse quantum state evolution is promising for quantum computing. A major obstacle for quantum computers is quantum decoherence, where fragile quantum states lose coherence and become corrupted by environmental interactions, leading to errors and unreliable calculations.
The principles of quantum reverse could provide a tool for error correction in quantum computers. By understanding how to “rewind” a quantum state, scientists may develop methods to identify and undo decoherence errors. This could involve applying inverse operations to restore qubits to a correct state without disturbing ongoing computation. Such techniques are a pathway toward building robust, fault-tolerant quantum computers, necessary for complex problems beyond conventional supercomputers.