Quantum-controlled synthetic materials

TL;DR

The work addresses interfacing digital quantum control with an analog quantum material to steer complex many-body dynamics. It implements a 1D Bose-Hubbard circuit of superconducting transmons with an ancilla qubit that conditions transport on lattice occupancy, effectively acting as a quantum-controlled photonic transistor. The authors demonstrate solid+fluid joint states and a long-range entangled N00N cat state via adiabatic disorder control and ancilla-conditioned transport, verified by many-body Ramsey interferometry and protected by a many-body echo. This hybrid approach points to practical quantum advantage in sensing and materials characterization by bridging quantum computers with quantum matter.

Abstract

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.

Figures (3)

• Figure 1: Quantum control of transport in a Bose-Hubbard circuit. a. The physical platform used to investigate transport dynamics maps to a 1D Bose-Hubbard model where bosonic particles coherently propagate in a tight-binding lattice and experience strong local interactions. This system is implemented as a chain of capacitively coupled superconducting transmon qubits (blue), serving as the lattice sites that host photonic particles (microwave excitations of the qubits). Inter-site capacitive coupling mediates particle tunneling ($J$), and the transmon anharmonicity provides local interactions ($U$). Site-resolved control of the lattice potential landscape is achieved by tuning the transmon resonance energies using inductively coupled bias lines to thread magnetic flux through their SQUID loops. Leveraging the quantum nature of the lattice sites, specifically their occupancy-dependent energy transition, offers a unique capability for steering the entanglement dynamics in our many-body system, by marrying the state preparation protocol of correlated fluids AdbPaper with the quantum control of the lattice energy landscape. b. Schematic representation of a quantum-controlled photonic transistor, where the many-body dynamics is regulated by the state of a control site. A superposition of zero and one particles in the control site drives interference between different lattice configurations to generate entangled states with long-range correlations, which we characterize by extracting the phase information of the control qubit.
• Figure 2: Many-body entangling operation via ancilla-conditioned transport. a. Starting in a highly disordered configuration (computational basis), we initialize the highest energy state by exciting the qubits on the left-hand side, and adiabatically remove disorder to a quantum-controlled transport configuration, where the middle site is detuned by $U$ and mediates dynamics between the two halves of the system through its doublon state. If this ancilla qubit is in its ground (excited) state, the particles localize (delocalize) into a solid (fluid), as shown by the measured density profile in blue (red). Preparing the ancilla in a particle + hole superposition ($\frac{\pi}{2}$ pulse) produces a solid + fluid superposition, which maps to a many-body entangled (N00N) state in the computational basis when we adiabatically relocalize the photons to an inverted disordered configuration. b. The coherence of such entangled states is probed through many-body Ramsey interferometry: we coherently evolve the entangled state for a time $\Delta t$ to accumulate a relative phase proportional to the energy difference of the left and right qubit clusters, and apply the corresponding disentangling operation to relocalize the phase information into the ancilla qubit accessible from its population measurement following a second $\frac{\pi}{2}$ pulse. c. The frequency difference in the ancilla interference fringe when it is uncoupled (blue) and entangled (purple) with the lattice matches the energy difference between the left and right qubit clusters $\omega_L - \omega_R$, consistent with the preparation of the coherent cat-like superposition of these two collective states. d. To leverage this N00N state, we benchmark our system's capability as a sensor: we offset the energy of the right/left qubit clusters by $\pm\delta$ during the phase evolution and observe an increased sensitivity, where the shift in the many-body Ramsey frequency scales with system size $(N-1)\delta$ for both $N=5$ (left) & $N=7$ (right) qubit N00N states. e. Adiabaticity is characterized by varying the ramp time $t_\mathrm{ramp}$, where discrete changes in the Ramsey Fourier frequencies reveal the transition from a product to a fully entangled state.
• Figure 3: Phonon-assisted SWAP operation. a. Entanglement generation using conditional phonon dynamics is demonstrated for $N=5$ qubits. Phonons are excited in the adiabatically prepared fluid through the potential modulation of a single lattice site (flux driving the transmon), creating a particle-hole excitation. Driving the lattice site at $\omega_d \approx 3.6 J$ induces a two-phonon process that inverts the population of the free fermion eigenstates (diagram). b. Adiabatically reintroducing disorder turns the phonon creation process into a many-body SWAP operation in the computational basis, where the highest frequency qubits ($Q_2,\, Q_3$) simultaneously exchange photons with the lowest frequency qubits ($Q_5,\, Q_6$). In contrast, this lattice perturbation does not affect the solid (Mott insulator) eigenstate, since it is incompressible ($\omega_d \ll U$). Therefore, applying this phonon drive to the $|\text{solid}\rangle + |\text{fluid}\rangle$ phase leads to a similar swap operation conditioned on the state of the ancilla ($Q_4$). c. The preparation of the target entangled state is clearly captured in a many-body Rabi experiment: the ancilla Ramsey frequency is probed as a function of the phonon drive duration, and we observe a cyclic transition from the product state to the entangled cat state.