A Time Machine in a Superconducting Circuit ✛ ✎
In a dark laboratory where superconducting chips are cooled to thousandths of a kelvin, a qubit spins under the influence of a magnetic field. A regular qubit rotates in one direction. An antiqubit, its mirror twin, rotates in the opposite direction. Together they form a pair that senses the field twice as sharply as any ordinary pair.
Physicists in China have built such a time mirror on a superconducting platform. They took three transmons, superconducting qubits based on Josephson junctions, and turned one of them into an antiqubit. To do this, they used two techniques: Z-gates before and after the field is applied to invert the transverse components, and an off-resonant microwave drive at the "magic" frequency of 4.177 GHz, which shifts the longitudinal component of the field to the opposite value. Thus the qubit began to behave like a positron: the same field, but rotation in the opposite direction.
Previously, reversing an unknown evolution required complex algorithms and additional qubits. Here, the inversion is built into the physics of the device: you only need to flip the sign of the effective gyromagnetic ratio, the coupling between magnetic moment and angular momentum. An entangled qubit-antiqubit pair measures the strength of a field of unknown direction with sensitivity close to the Heisenberg limit. This opens the way to more efficient quantum magnetic field sensors, quantum cryptography, and algorithms that need to apply an operation and its inverse simultaneously.
The time mirror is no longer a metaphor. In a superconducting circuit, it has become a working tool that brings us closer to the limits of precision allowed by quantum mechanics.
A positron, the antiparticle of the electron, has the same magnitude of gyromagnetic moment but with the opposite sign. It is precisely this property that allowed Feynman to interpret the positron as an electron moving backward in time. In the experiment, the "antiqubit" rotates in the opposite direction under the same field, as if time were flowing backward for it.
The idea of particles moving backward in time is explored in Kurt Vonnegut's novel Slaughterhouse-Five, where the protagonist becomes "unstuck in time" and experiences events in reverse order. Here, time reversal is not a literary device but a working tool of quantum sensing.