- Physicists have successfully created a quantum state that moves in the opposite direction of the thermodynamic arrow of time.
- They have demonstrated this state in 2- and 3-qubit quantum computer.
- It could be used to make quantum computers more precise while reducing errors and noise.

How does the irreversibility emerge from the seemingly time-symmetric, laws of physics? Scientists have been trying to find the answer for years.

Within the framework of classical statistical mechanics, the problem is linked with the 2nd law of thermodynamics, which states that entropy in a closed system increases when energy changes from one form to the other, or matter moves freely.

In 2018, Russian researchers at Moscow Institute of Physics and Technology reported the violation of the 2nd law of thermodynamics through a device known as Maxwell’s demon. Now, they have tackled the problem from a different angle: they have developed a quantum state that moves in the opposite direction of the thermodynamic arrow of time.

The team first evaluated the probability that an electron in a vacuum will instinctively travel back into its recent past. They found that observing 10 billion newly localized electrons every second across the entire lifetime of the universe would reveal reverse evolution of the electron’s state only once.

Even in this case, the particle would not travel more than a nanosecond into the past. This is why we don’t observe things going in reverse time, because that would require an astounding number of particles unfolding on much greater timescales.

*Reference: arXiv:1712.10057 | MIPT*

### The Four-Stage Experiment

The researchers analyzed the state of a quantum computer made of 2- and 3-qubits (quantum bits).

**Order:**Initially, each qubit is in the ground state, representing zero.**Degradation:**Launch an ‘evolution program’ to change the states of qubits to ones and zeros, or both at the same time.**Time reversal:**A unique algorithm developed in this study changes the quantum computer in such a way that it would then move in the reverse direction, from chaos towards order.**Regeneration:**Launch the evolution program again to rewind the qubits’ states back into the past.

Courtesy of researchers

The 2-qubit quantum computer returned back into its initial state in 85% of the cases, while 3-qubit showed more errors, resulting in a 50% success rate. The errors occurred due to the defects in the existing quantum computer. As more advanced quantum computing techniques are developed, the error rate is expected to decrease significantly.

### What’s Next?

This time reversal algorithm could be used to develop more precise quantum computers. In the near future, it could be modified to test software written for quantum computers and reduce errors and noise.

Read: 18 Most Interesting Facts About Quantum Computers

The schemes developed in this study were scrolling one by one through the state components but didn’t exploit the quantum parallelism in its full power. The next question is whether it’s possible at all to develop a quantum algorithm that would carry out time-reversal more efficiently than using O(N) elementary gates.