Speaker
Description
A system of transmon qubits is proposed as a potential platform to detect dark matter (DM) [1]. The sensitivity of the detector was shown to be enhanced by entangling a large number of qubits under the assumption that the hidden photon associated with the DM acts equally on all qubits as the same unitary operator $U_\mathrm{DM}$. We call this signal collective noise.
Qubits can be protected from collective noise by the so-called noiseless subsystem [2]. Consider a system of 3 qubits. Using the permutation symmetry of collective noise, it can be shown that there is a unitary transformation $U_E \in U(2^3)$, such that [2]
$$
U_E^\dagger (U_\mathrm{DM}^{\otimes 3})U_E|0\rangle |\phi\rangle| \psi\rangle =
|0\rangle |\phi\rangle U_\mathrm{DM}|\psi\rangle.
$$
$|\phi\rangle$ is immune to noise under this error-avoiding encoding. We take $|\phi\rangle=|0\rangle$ and $|\psi \rangle \in {|0\rangle, |+\rangle, |y+\rangle}$ in the following.
[1] showed that the sensitivity of the proposed detector is $\propto n_q^2 \delta^2$, where $n_q$ is the number of qubits that participate in the entanglement and $\delta=\eta \tau$ is a small parameter where $\eta$ is a coupling parameter between hidden photon DM and transmons and $\tau$ is the integration time in detector.
In this new proposal, deviation of the measurement outcome from the no-noise case is proportional to $\sin \delta\sim \delta$, although the sensitivity enhancement factor may not be literally $\sim 1/\delta$ if the standard quantum limit is taken into account, for example. The output state $|00\rangle$ of the first two qubits signals that the noise is collective.
In our talk, we will introduce the quantum circuit implementing $U_E$ and propose a QPU design that can be fabricated within near-future technology.
$[1]$ Chen, Shion and Fukuda, Hajime and Inada, Toshiaki and Moroi, Takeo and Nitta, Tatsumi and Sichanugrist, Thanaporn, Phys. Rev. D **110**, 115021 (2024)
$[2]$ G\"ung\"ord\"u, Utkan and Li, Chi-Kwong and Nakahara, Mikio and Poon, Yiu-Tung and Sze, Nung-Sing, Phys. Rev. A 89, 042301 (2014)