Quantum dial

Abstract

Accurate control of quantum degrees of freedom is promising for sensing, communication, and computing, but building a useful quantum computer faces a central isolation-and-control challenge: qubits must remain well isolated from their environment to preserve coherence, yet also be coupled strongly enough for control, readout, and reset. Existing approaches address this challenge only partially, using separate reset elements, drive schemes, and Purcell filters, often with added complexity and tradeoffs such as heating and crosstalk. Here we introduce and demonstrate a first-generation quantum dial: a device that on demand mediates the coupling of a qubit to selected auxiliary degrees of freedom. Our implementation uses a band-stop filter between a high-coherence transmon qubit and a broadband transmission line, enabling the coupling strength to be tuned by several orders of magnitude on nanosecond timescales without significant Stark shift. In the reset configuration, we reduce the qubit energy relaxation time T1 from >150 $μ$s to about 200 ns and demonstrate reset independent of the initial state. In the control configuration, we obtain 99.99% idle fidelity and 99.9% gate fidelities for 40 ns pulses at about -110 dBm. The same device also enables thermometry of the qubit environment, reaching a noise-equivalent temperature of 0.6 mK/$\sqrt{Hz}$ at 60 mK and approaching the Cramér-Rao bound at higher temperatures. The quantum dial thus offers fast, on-demand switching between isolation and strong coupling, with potential to reduce noise and errors in future quantum processors.

Figures (28)

• Figure 1: Quantum-dial concept. The quantum dial (centre) chooses the environment coupled to a high-coherence qubit (left). In the first experiments reported here, we have access to three core dial configurations (shaded in gray): Control selects moderate coupling for fast, high-fidelity single-qubit gates, Idle isolates the qubit for state preservation, and Reset implements strong coupling to a cold $50~\Omega$ bath for fast initialization and thermometry. Two additional dial configurations completes the scheme (future work): Readout facilitates selective coupling at the resonator frequency $f_{\mathrm{r}}$ to open the measurement linewidth $\kappa$ only during readout while keeping the qubit protected at $f_\text{q}$, and Qubit–qubit coupling implements a tunable bus that is off at idle (low residual coupling) and turned on for two-qubit gate operations, for example, via parametric exchange (iSWAP) or a static $ZZ$ phase (CZ). This architecture enables duty-cycled protection of all elements, deterministic hardware reset, and two-qubit gates without moving qubits away from their flux sweet spots.
• Figure 2: Device layout and operation principle.a, Schematic diagram of a tunable-frequency drive-line filter implemented as a $50~\Omega$ transmission line that is strongly capacitively coupled to the qubit at the position $x=0$ (port P2). From the coupling point, the line extends a length $L_\mathrm{f}$ and is weakly terminated to ground through a small capacitance, forming a quarter-wave filter. A series array of direct-current (dc) superconducting quantum interference devices (SQUIDs) with an effective inductance $L_\mathrm{J}^{\mathrm{arr}}$ is integrated along the line to tune the filter. b, Voltage magnitude profile $|V|$ of the modes of the transmission line along the filter as a function of the position $x$ for several values of the flux-tunable $L_\mathrm{J}^{\mathrm{arr}}$. For a given $L_\mathrm{J}^{\mathrm{arr}}$, the filter decouples the qubit from the $50~\Omega$ line at the frequency that yields zero voltage at the qubit; here this occurs for a qubit designed near $4.5$ GHz. The SQUID placement and the number of elements are chosen to provide a large tunability while operating in the linear regime where the ac drive current is well below the critical current of the SQUIDs. c, Transmission magnitude $|S_{21}|$ from the source port P1 to the qubit port P2 as a function of drive frequency for different $L_\mathrm{J}^{\mathrm{arr}}$. Biasing the SQUID loops changes $L_\mathrm{J}^{\mathrm{arr}}$ and shifts the location of the minimum in the transmission. d, False-colored optical micrograph of the fabricated device with four double-pad transmon qubits (yellow). Qubit Q1 is flux-tunable, whereas Q2--Q4 are fixed-frequency. Each qubit couples to its own readout resonator (olive), which in turn couples to a common readout Purcell filter (blue). Q1 connects to a fixed-frequency drive filter (cyan), Q2 to a standard weakly-coupled drive line (magenta), and Q3--Q4 to tunable-frequency drive filters (purple). Each tunable-frequency drive line is inductively coupled to a fast-flux line (red) adjacent to the dc SQUID array.
• Figure 3: Simulation and experimental characterization of tunable-frequency drive line filters.a, Simulated Rabi frequency $\Omega/2\pi$ as a function of the flux bias $\Phi$ and qubit frequency using a model (\ref{['supp_note:device_design']}), in which the dc SQUID array is represented by a tunable series inductance. The flux quantum is denoted by $\Phi_0$. The filter produces strong suppression of the Rabi frequency where the qubit and filter frequencies are equal. b, As a, but for the simulated external relaxation time $T_1^{\mathrm{ext}}$, showing an inverse trend to that of a, with large enhancements at the filter frequency. c, Simulated ground-state probability of the qubit as a function of qubit pulse length and flux bias, showing clear Rabi oscillations. d, As c, but for measurement data acquired from Q4, showing good agreement with simulated results in c. e, Measured Rabi frequency as a function of the flux bias for the fixed-frequency qubits Q3 and Q4. The markers denote experimental data obtained from a fit to Rabi oscillations similar to those in d, solid lines are simulations evaluated with the designed coupling and measured qubit frequency. The inset shows the behavior around the half flux quantum. f, Measured relaxation time $T_1$ as a function of flux bias for Q3 and Q4. The markers denote experimental data and the solid and dashed curves correspond to simulations with and without dielectric loss, respectively. The inset shows the behavior around the half flux quantum. From the comparison we estimate the zero-bias filter frequency lies near $4.24$ GHz, corresponding to offsets of about $300$ MHz for Q3 and about $360$ MHz for Q4 from the design target. We observe almost three-orders-of-magnitude suppression of the Rabi frequency accompanied by a comparable enhancement of the relaxation times when the filter is decoupled from the qubit. The error bars represent $1\sigma$ fitting uncertainty.
• Figure 4: Resonant control and idle fidelity of Q4.a, Required qubit-drive power at the sample (red line, left axis) to realize a target Rabi frequency $\Omega/2\pi=12.5$ MHz implementing gates of length $t_\text{g}=40$ ns as a function of the flux bias $\Phi$ of the tunable drive line filter. The qubit relaxation time $T_1$ (right axis) is also shown with (solid line) and without (dashed line) dielectric loss. The edges of the shaded regions indicate the positions of the dial: the idle configuration with the filter frequency parked at $f_\text{q}$ and the control configuration where the filter is detuned to provide a moderate coupling. The orange band denotes the fast-flux sweep used for dialing. b, Ground-state probability of the qubit as a function of the sequence length in randomized benchmarking for static dialing. Extracted average Clifford fidelity and interleaved idle fidelity are $\mathit{F}_\mathrm{avg}^\mathrm{Cliff.}=0.9987\pm0.0002$ and $\mathit{F}_\mathrm{avg}^\mathrm{I}=0.9986\pm0.0004$, respectively. The inset shows the pulse schedule (fixed filter bias, resonant single-qubit gates, and readout). c, As b, but for dynamic dialing, where a short flux pulse detunes the filter away from the idle dial configuration only during the gate pulse. The fits yield $\mathit{F}_\mathrm{avg}^\mathrm{Cliff.}=0.9986\pm0.0002$ and $\mathit{F}_\mathrm{avg}^\mathrm{I}=0.9998\pm0.0002$, reflecting the strongly suppressed idle error when the qubit remains at the idle position of the quantum dial. The insets depict the corresponding flux and single-qubit gate sequences together with the readout pulse. The error bars on the data represent $1\sigma$ standard deviation over a multiple repetitions.
• Figure 5: Single-shot measurements and reset of Q4. a, Measured reference single-shot data in the in-phase--quadrature-phase (IQ) plane for the four lowest-energy states $\ket{\text{g}},\ket{\text{e}},\ket{\text{f}}$, and $\ket{\text{h}}$. This data is used to fit a four-component Gaussian mixture model (GMM). The dashed line is a circle fit to the means (red crosses) of each state returned by the GMM model fit. The inset shows the pulse sequence (weak reference readout, state preparation, final readout). b, Readout assignment matrix obtained by preparing each basis state and classifying the outcome. High fidelities are observed on the diagonal for $\ket{\text{g}}$ and $\ket{\text{e}}$, with slightly lower fidelities for $\ket{\text{f}}$ and $\ket{\text{h}}$ consistent with their shorter $T_1$ during readout. c, As a, but for preparations in $\ket{\text{e}}$, $\ket{\text{f}}$, and $\ket{\text{h}}$ after a fixed flux pulse of duration $t_\text{reset}=0.8~\mu\text{s}$; inferred ground-state populations $P_\mathrm{g}$ are indicated. d, Measured (markers) residual populations as functions of the flux-pulse length for the three state preparations in c. The solid curves represent a fit of a four-state Pauli master equation with only downward transitions allowed to the whole data set. The fit yields the following decay times $T_1^{mk}$ from the state $\ket{k}$ to the state $\ket{m}$: $T_1^\text{ge}=238.2\pm 3.3~\text{ns}$, $T_1^\text{ef}=136.8\pm 2.8~\text{ns}$, and $T_1^\text{fh}=128.8\pm 3.4~\text{ns}$. At $t_\text{reset}\approx 1.2~\mu\text{s}$ we obtain $P_\mathrm{g}\gtrsim 97\%$ from $\ket{\text{e}}$, $95\%$ from $\ket{\text{f}}$, and $91\%$ from $\ket{\text{h}}$, with saturation near $98.5\%$ set by a thermal floor corresponding to $T\approx 45~\text{mK}$.