Quantum-controlled synthetic materials

Analog quantum simulators and digital quantum computers are two distinct paradigms driving near-term applications in modern quantum science, from probing many-body phenomena to identifying computational advantage over classical systems. A transformative opportunity on the horizon is merging the high-fidelity many-body evolution in analog simulators with the robust control and measurement of digital machines. Such a hybrid platform would unlock new capabilities in state preparation, characterization and dynamical control. Here, we embed digital quantum control in the analog evolution of a synthetic quantum material by entangling the lattice potential landscape of a Bose-Hubbard circuit with an ancilla qubit. This Hamiltonian-level control induces dynamics under a superposition of different lattice configurations and guides the many-body system to novel strongly-correlated states where different phases of matter coexist – ordering photons into superpositions of solid and fluid eigenstates. Leveraging hybrid control modalities, we adiabatically introduce disorder to localize the photons into an entangled cat state and enhance its coherence using a many-body echo technique. This work illustrates the potential for entangling quantum computers with quantum matter – synthetic and solid-state – for advantage in sensing and materials characterization.

💡 Research Summary

In this work the authors demonstrate a hybrid quantum platform that merges the many‑body dynamics of an analog quantum simulator with the precise control of a digital quantum processor. The experimental system is a one‑dimensional Bose‑Hubbard circuit realized with a chain of capacitively coupled superconducting transmon qubits. Photons (microwave excitations) hop between neighboring sites with tunneling rate J ≈ ‑9 MHz, experience strong on‑site interaction U ≈ ‑240 MHz, and have a lifetime T₁ ≈ 45 µs, ensuring that coherent many‑body effects dominate over dissipation.

A central innovation is the entanglement of the lattice potential with an ancillary transmon (the “ancilla”). By preparing the ancilla in either the ground state |0⟩, the excited state |1⟩, or a coherent superposition (|0⟩ + |1⟩)/√2, the energy of a selected middle lattice site is shifted by U only when the ancilla is occupied. Consequently, the transport of photons across the lattice becomes conditional on the ancilla’s quantum state. If the ancilla is |0⟩ the photons remain localized on the left half, forming a Mott‑insulating domain; if the ancilla is |1⟩ the middle site resonantly couples to a doubly‑occupied (“doublon”) state, allowing the photons to delocalize into a superfluid‑like configuration. When the ancilla is in a superposition, the system evolves under a coherent superposition of these two distinct lattice Hamiltonians, creating a “solid + fluid” many‑body entangled state.

The protocol proceeds in three stages. First, a strong static disorder (δ_i ≫ J) is applied to localize all eigenstates, and three photons are injected into the leftmost three sites (Q₀‑Q₂) using site‑resolved π‑pulses, while the ancilla (Q₃) is prepared in the desired quantum state. Second, the disorder is adiabatically removed, exposing a transport configuration where the middle site is detuned by U. Depending on the ancilla, the photons either stay localized (solid) or spread (fluid). Third, the ancilla is placed in the superposition (|0⟩ + |1⟩)/√2, and disorder is re‑introduced in an inverted fashion so that the highest‑energy three‑photon state now resides on the right half (Q₄‑Q₆). This maps the solid + fluid superposition onto a highly entangled N00N (cat) state of the form (|L⟩ + |R⟩)/√2, where |L⟩ denotes photons on the left with ancilla |0⟩ and |R⟩ denotes photons on the right with ancilla |1⟩.

To verify the coherence of the N00N state, the authors employ many‑body Ramsey interferometry. After preparation, the system is allowed to evolve for a variable hold time Δt, during which the two components acquire a relative phase Δφ = (ω_R − ω_L)Δt set by the energy difference between the left and right clusters. A reverse transport sequence then disentangles the lattice, mapping the accumulated phase onto the ancilla. A final π/2 pulse followed by a projective measurement of the ancilla yields sinusoidal Ramsey fringes. The fringe frequency matches the expected energy splitting, confirming that a coherent superposition of many‑qubit eigenstates has been generated. The contrast is limited primarily by qubit dephasing and a small systematic offset in the ancilla’s precession frequency.

To mitigate low‑frequency noise, a many‑body echo protocol is introduced. By inverting the ancilla‑conditioned interaction midway through the evolution, slow phase fluctuations are effectively cancelled, extending the coherence time of the cat state by a factor of roughly two to three compared with the plain Ramsey sequence.

Beyond state verification, the authors benchmark the N00N state as a quantum sensor. By applying opposite energy offsets ±δ to the left and right clusters during the Ramsey interval, they observe a shift in the Ramsey frequency that scales as (N − 1)δ, where N is the number of photons in the cat state (N = 5 or 7 in their experiments). This demonstrates the expected Heisenberg‑like enhancement in sensitivity provided by entangled resources.

The paper also discusses scalability. The ancilla‑conditioned lattice potential is a Hamiltonian‑level control mechanism that can be extended to larger transmon arrays, to multiple ancilla qubits, or to two‑dimensional lattices. Similar conditional potentials could be realized in Rydberg atom arrays or optical lattices, opening pathways toward real‑time control of topological phases, quantum error‑correcting codes, and complex material simulations.

In summary, the authors achieve three major technical milestones: (1) converting the lattice potential into a quantum‑controlled variable via ancilla entanglement, (2) generating and characterizing a many‑body N00N state that simultaneously embodies solid‑type (Mott) and fluid‑type (superfluid) order, and (3) demonstrating many‑body Ramsey and echo techniques that preserve coherence and enable quantum‑enhanced sensing. This work establishes a concrete route to hybrid analog‑digital quantum devices, offering new tools for quantum materials engineering, high‑precision metrology, and the broader quest to harness many‑body quantum phenomena for computational advantage.