Heat flow usually evens everything out: hot things cool down, cold things warm up, and in the end you are left with a uniform warm swamp. The same holds in the quantum world: the environment, whether a crystal lattice or a gas of photons, tends to destroy the fragile entanglement between particles. Qubits in a quantum computer are isolated, cooled to millikelvins, and shielded from the slightest noise, all to protect them from dissipation. But what if there is a crack in this dam of thermal equilibrium? A crack through which the flow does not even things out but turns a turbine of order.
Theorists at the University of California, Los Angeles, have found such a crack. They considered two noninteracting qubits weakly coupled to a thermal bath. Normally, the system and bath exchange a conserved quantity, such as spin. The conservation law acts like an iron wall: as much spin leaves the system, exactly that much enters the bath. This leads to thermalization: the qubits relax to a Gibbs state, which has no entanglement. But if the coupling Hamiltonian has not one but two channels that change spin by different integer amounts, the conservation law is broken. The bath then behaves as several reservoirs with different chemical potentials. In the weak-coupling limit, the dynamics are described by a Lindblad equation in which local and nonlocal dissipation have different effective temperatures. This temperature difference is what stabilizes entanglement.
For illustration, they chose a concrete realization: two NV centers in diamond coupled to a two-dimensional inverted ferromagnet via anisotropic dipole-dipole interaction. The magnet is pumped into an inverted state with spin accumulation µ < −b, where b is the Zeeman field. Magnons, the quanta of spin waves, carry correlations between the centers over finite distances. In such a system, entanglement persists at distances up to r ≲ 2√(As/∆), where As is the spin stiffness, and the field b must lie in a narrow range ∆ ≤ b ≲ 1.1∆. The maximum concurrence (a measure of entanglement from 0 to 1) is achieved at r → 0, but then the convergence time to the steady state diverges: this is a manifestation of the universal trade-off between the amount of entanglement and the relaxation speed.
The work shifts the focus from controlling coherence to controlling the symmetries of the interaction. Previously, it was thought that a thermal bath destroys entanglement, and stabilizing it required active pumping or complex reservoir engineering. Now it is shown that it suffices to break the conservation law for the quantity exchanged between system and bath, and to maintain a nonzero chemical potential in the bath; then entanglement becomes a steady state. This opens a path to passive stabilization of entangled states by dissipation, which could simplify the creation of entanglement for quantum sensors and quantum communication, especially in solid-state platforms such as NV centers in diamond.
A magnon with negative energy in an inverted magnet is not exotic; it is a direct analog of a "hole" in a semiconductor: an excitation that decreases the system's energy but increases its spin. Such antimagnons carry correlations between NV centers in the proposed scheme.
The idea of using a "wrong" environment to create order echoes Stanislaw Lem's novel "Solaris": an ocean-environment that violates familiar laws gives rise to structures that seem impossible from the standpoint of earthly physics. Here, an environment with a broken spin conservation law stabilizes quantum entanglement, also a kind of "guest from nowhere."