A theoretical control method combines measurements, feedback, and quantum fields to cancel, blur, and even reverse the arrow of time created in a monitored quantum system. The researchers propose to harness the energy that the measurements introduce into the system, but this is not a time machine or a violation of the second law of thermodynamics.
In everyday life, time seems to have a clear and irreversible direction. A glass breaks but does not reconnect on its own, heat flows from a hot body to a cold body, and our memories refer to the past rather than the future. But many of the fundamental equations of microscopic physics make no fundamental distinction between moving forward in time and moving backward.
New research suggests a way to control the observed arrow of time in quantum systems subjected to continuous measurement. The researchers have developed theoretical protocols that can cancel out the measurement effect, amplify it, or compensate for it to an extent that is too large—until the resulting trajectories seem more plausible when interpreted as moving backward in time than forward. [1][2]
The study, by Luis Pedro Garcia-Pintos, Yi-Kai Liu, and Alexey V. Gorshkov, was published in the journal Physical ReviewIt is important to clarify: the researchers did not make time in the universe flow backwards, did not return an object to its previous state, and did not build a time machine. They designed the statistical evolution of a quantum system so that the arrow of time that emerges from its measurement results could change. [2][3]
How does measurement create an arrow of time?
In classical physics, it is sometimes possible to measure a system without significantly changing it. Measuring the position of a car, for example, does not deviate it from its path. In quantum mechanics, the situation is different: the measurement itself affects the system and changes its state probabilistically.
When a series of measurements are made on a quantum system, the results create a trajectory of random changes. Statistically, one can ask whether a particular trajectory is more likely to evolve forward in time or backward. It is the difference in the likelihood of the two descriptions that gives the measured trajectory an arrow of time.
A simple example is a film of a glass breaking. It is easy to determine which direction the film is projected, because the reverse process – shards rising and reconnecting themselves – almost never occurs. In a small quantum system, however, the fluctuations and measurements make it possible to precisely examine the probability of each trajectory and its opposite version.
The researchers wanted to find out if it was possible to intervene in these pathways so that the preferred direction would weaken, disappear, or reverse.
The Hamiltonian mimics measurement
To this end, the researchers constructed a control Hamiltonian – a mathematical description of a sequence of fields and pulses applied to the system. The Hamiltonian was designed to mimic the change that a quantum measurement would cause, but in a controlled manner.
When the control is combined with information about the results of the measurements that have already been received, a feedback loop is created. The researchers have shown theoretically that the feedback can be directed to several situations:
- Compensate for measurement disturbance and reduce the arrow of time;
- Strengthen the impact of measurement and increase irreversibility;
- Overcompensate, so that the system will produce routes that are more suitable for reverse time pressure.
In other words, the system doesn't travel back in time. Instead, the researchers change its dynamics so that the observed sequence of states appears as if the time-reversed version is the more natural one.
The method can also simulate the backward evolution of an open quantum system—a system that interacts with its environment. Such a capability could aid in the study of noise, information loss, and the preparation of desired quantum states. [2]
Maxwell's Demon in a Quantum Version
The study is related to the thought experiment known as “Maxwell’s Demon.” In the 19th century, James Clerk Maxwell proposed an imaginary creature that could sort fast and slow molecules and create a temperature difference without seemingly putting in any work. The result appeared to violate the second law of thermodynamics, which states that entropy in an isolated system should not decrease.
The modern solution to the paradox is that the collection, processing, and disposal of information also incur a thermodynamic cost. When the entire system is taken into account, the second law is not violated.
The system proposed by the researchers also uses information from measurement results to drive an unusual process. The measurement changes the system and injects energy into it, and the feedback redirects some of this energy in a useful direction.
Motor operated by measurement
As one application, the team developed a model of a continuous measurement engine. The engine draws energy from the changes that measurements generate in the quantum system. In principle, the energy could be used to drive another process or stored in a quantum battery.
There is no energy being created out of thin air. The measurement, control, data collection, and feedback are physical resources. The energy extracted from the system comes from the energy that was injected into it during the monitoring process. The researchers have also theoretically examined imperfect conditions, including feedback delay and limited measurement efficiency. [2]
The result is interesting because it transforms the measurement from a factor that disrupts the system into a thermodynamic resource that can be exploited.
Not yet a lab experiment
The current work is a theoretical study that includes mathematical analyses and simulations. The researchers have not reported any experimental implementation of reversing the arrow of time using the new protocol.
The next step they propose is to test the method in superconducting qubits. Such systems allow for fast measurements, precise control, and short-time feedback, and have already been used in previous experiments to realize quantum versions of Maxwell’s demon. The researchers are also exploring using tools they have developed to prepare quantum states. [1]
What the study does not prove
The title “Time Flows Backward” may give the wrong impression. The study did not send a person, particle, or event back into the past, nor did it change the thermodynamic direction of the macroscopic world.
The term “arrow of time” here refers to the statistical asymmetry in the trajectories of a monitored quantum system: whether the sequence of outcomes appears more likely in the direction in which it is measured, or when its order is reversed.
The work shows that these probabilities can be engineered through control and feedback, providing a new tool for investigating the relationship between measurement, information, energy, and irreversibility in quantum mechanics—and perhaps a more practical way to control quantum systems in the future.
Questions and Answers
Did the researchers turn back time?
No. They changed the probabilities of trajectories in a monitored quantum system, so that the time-reversed trajectory became more likely than the normal trajectory.
What is the arrow of time?
The arrow of time is the difference between naturally occurring processes that move forward and their reverse version, which usually does not. The study examines the statistical arrow of time that results from a series of quantum measurements.
Has the second law of thermodynamics been violated?
No. Measurement and feedback require information and energy. When all resources are taken into account, there is no creation of energy out of thin air.
What is a quantum measurement engine?
A system that utilizes the energy that quantum measurements introduce into the system and routes it through feedback to produce work or for storage.
Has the method been tested on real qubits?
Not yet in the scope of this study. The researchers propose to examine it in the future in superconducting qubits.
Featured main image
Caption:
Illustration of quantum trajectories in different time directions. The researchers developed a control method capable of blurring and even reversing the arrow of time observed in a monitored quantum system.
Sources
[1] The Los Alamos National Laboratory announcement makes it clear that the protocols change the The arrow of time observed in a quantum system, suggests future experiments in superconducting qubits and describes a measurement-driven engine. (Los Alamos National Laboratory)
[2] The abstract and open-source version of the paper describe a Hamiltonian that mimics measurement trajectories, feedback that creates trajectories that correspond to inverse time pressure, backward dynamics simulation, and a measurement engine that operates even under non-ideal conditions. (arXiv)
[3] The article was published inPhysical Review, Volume 16, Article 011028, titled “Reshaping the Quantum Arrow of Time”; the official DOI is 10.1103/l18s-9vmhsystem. (journals.aps.org)
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