Publications
2024

(2024) arXiv.org. Abstract
Decoherence in qubits, caused by their interaction with a noisy environment, poses a significant challenge to developing reliable quantum processors. Monitoring the qubit's environment enables not only to identify decoherence events but also to reverse these errors, thereby restoring the qubit coherence. This approach is particularly beneficial for superconducting cavity qubits, whose unavoidable interaction with auxiliary transmons impacts their coherence. In this work, we uncover the intricate dynamics of cavity decoherence by tracking the noisy trajectory of a transmon acting as the cavity's environment. Using realtime feedback, we successfully recover the lost coherence of the cavity qubit, achieving a fivefold increase in its dephasing time. Alternatively, by detecting transmon errors and converting them into erasures, we improve the cavity phase coherence by more than an order of magnitude. These advances are essential for implementing longlived cavity qubits with highfidelity gates and can enable more efficient bosonic quantum error correction codes.
2023

(2023) PRX Quantum. 4, 3, 030336. Abstract[All authors]
Storing quantum information for an extended period of time is essential for running quantum algorithms with low errors. Currently, superconducting quantum memories have coherence times of a few milliseconds, and surpassing this performance has remained an outstanding challenge. In this work, we report a singlephoton qubit encoded in a novel superconducting cavity with a coherence time of 34 ms, representing an order of magnitude improvement compared to previous demonstrations. We use this longlived quantum memory to store a Schrödinger cat state with a record size of 1024 photons, indicating the cavity's potential for bosonic quantum error correction.
2022

(2022) Quantum. 6, 821. Abstract
Bosonic qubits encoded in continuousvariable systems provide a promising alternative to twolevel qubits for quantum computation and communication. So far, photon loss has been the dominant source of errors in bosonic qubits, but the significant reduction of photon loss in recent bosonic qubit experiments suggests that dephasing errors should also be considered. However, a detailed understanding of the combined photon loss and dephasing channel is lacking. Here, we show that, unlike its constituent parts, the combined lossdephasing channel is nondegradable, pointing towards a richer structure of this channel. We provide bounds for the capacity of the lossdephasing channel and use numerical optimization to find optimal singlemode codes for a wide range of error rates.
2020

(2020) Physical Review Letters. 125, 11, 110503. Abstract
Ancilla systems are often indispensable to universal control of a nearly isolated quantum system. However, ancilla systems are typically more vulnerable to environmental noise, which limits the performance of such ancillaassisted quantum control. To address this challenge of ancillainduced decoherence, we propose a general framework that integrates quantum control and quantum error correction, so that we can achieve robust quantum gates resilient to ancilla noise. We introduce the path independence criterion for faulttolerant quantum gates against ancilla errors. As an example, a pathindependent gate is provided for superconducting circuits with a hardwareefficient design.

(2020) Nature Physics. 16, 8, p. 822826 Abstract
To reach their full potential, quantum computers need to be resilient to noise and decoherence. In such a faulttolerant quantum computer, errors must be corrected in real time to prevent them from propagating between components(1,2). This requirement is especially pertinent while applying quantum gates, where the interaction between components can cause errors to spread quickly throughout the system. However, the large overhead involved in most faulttolerant architectures(2,3) makes implementing these systems a daunting task, motivating the search for hardwareefficient alternatives(4,5). Here, we present a gate enacted by an ancilla transmon on a cavityencoded logical qubit that is faulttolerant to ancilla decoherence and compatible with logical error correction. We maintain the purity of the encoded qubit by correcting ancillainduced errors in real time, yielding a reduction of the logical gate error by a factor of two in the presence of naturally occurring decoherence. We also demonstrate a sixfold suppression of the gate error with increased ancilla relaxation errors and a fourfold suppression with increased ancilla dephasing errors. The results demonstrate that bosonic logical qubits can be controlled by errorprone ancilla qubits without inheriting the ancilla's inferior performance. As such, errorcorrected ancillaenabled gates are an important step towards faulttolerant processing of bosonic qubits.Errorcorrected quantum gates that can tolerate dominant errors during the execution of quantum operations have been demonstrated. Substantial improvement of the gate fidelity sheds light on faulttolerant universal quantum computation.

(2020) Physical Review X. 10, 1, 011001. Abstract[All authors]
Qubit measurements are central to quantum information processing. In the field of superconducting qubits, standard readout techniques are limited not only by the signaltonoise ratio, but also by state relaxation during the measurement. In this work, we demonstrate that the limitation due to relaxation can be suppressed by using the manylevel Hilbert space of superconducting circuits: In a multilevel encoding, the measurement is corrupted only when multiple errors occur. Employing this technique, we show that we can directly resolve transmon gate errors at the level of one part in 10(3). Extending this idea, we apply the same principles to the measurement of a logical qubit encoded in a bosonic mode and detected with a transmon ancilla, implementing a proposal by Hann et al. [Phys. Rev. A 98, 022305 (2018)]. Qubit state assignments are made based on a sequence of repeated readouts, further reducing the overall infidelity. This approach is quite general, and several encodings are studied; the codewords are more distinguishable when the distance between them is increased with respect to photon loss. The tradeoff between multiple readouts and state relaxation is explored and shown to be consistent with the photonloss model. We report a logical assignment infidelity of 5.8 x 10(5) for a Fockbased encoding and 4.2 x 10 (3) for a quantum error correction code (the S = 2, N = 1 binomial code). Our results not only improve the fidelity of quantum information applications, but also enable more precise characterization of process or gate errors.
2018

(2018) Nature Physics. 14, 10, p. 9961000 Abstract
Deterministic quantum interactions between single photons and single quantum emitters are a vital building block towards the distribution of quantum information between remote systems(14). Deterministic photonatom state transfer has previously been demonstrated with protocols that include active feedback or synchronized control pulses(510). Here we demonstrate a passive swap operation between the states of a single photon and a single atom. The underlying mechanism is singlephoton Raman interaction(1115)an interferencebased scheme that leads to deterministic interaction between two photonic modes and the two ground states of a Lambdasystem. Using a nanofibrecoupled microsphere resonator coupled to single Rb atoms, we swap a photonic qubit into the atom and back, demonstrating fidelities exceeding the classical threshold of 2/3 in both directions. In this simultaneous write and read process, the returning photon, which carries the readout of the atomic qubit, also heralds the successful arrival of the write photon. Requiring no control fields, this singlestep gate takes place automatically at the timescale of the atom's cavityenhanced spontaneous emission. Applicable to any waveguidecoupled Lambdasystem, this mechanism, which can also be harnessed to construct universal gates(16,17), provides a versatile building block for the modular scaling up of quantum information systems.
[All authors] 
(2018) Science. 361, 6399, p. 266269 Abstract
A critical component of any quantum errorcorrecting scheme is detection of errors by using an ancilla system. However, errors occurring in the ancilla can propagate onto the logical qubit, irreversibly corrupting the encoded information. We demonstrate a faulttolerant errordetection scheme that suppresses spreading of ancilla errors by a factor of 5, while maintaining the assignment fidelity. The same method is used to prevent propagation of ancilla excitations, increasing the logical qubit dephasing time by an order of magnitude. Our approach is hardwareefficient, as it uses a single multilevel transmon ancilla and a cavityencoded logical qubit, whose interaction is engineered in situ by using an offresonant sideband drive. The results demonstrate that hardwareefficient approaches that exploit systemspecific error models can yield advances toward faulttolerant quantum computation.

(2018) Physical Review X. 8, 2, 021073. Abstract
Interference experiments provide a simple yet powerful tool to unravel fundamental features of quantum physics. Here we engineer a driven, timedependent bilinear coupling that can be tuned to implement a robust 50:50 beam splitter between stationary states stored in two superconducting cavities in a threedimensional architecture. With this, we realize highcontrast HongOuMandel interference between two spectrally detuned stationary modes. We demonstrate that this coupling provides an efficient method for measuring the quantum state overlap between arbitrary states of the two cavities. Finally, we showcase concatenated beam splitters and differential phase shifters to implement cascaded MachZehnder interferometers, which can control the signature of the twophoton interference on demand. Our results pave the way toward implementation of scalable boson sampling, the application of linear optical quantum computing protocols in the microwave domain, and quantum algorithms between longlived bosonic memories.

(2018) Nature Communications. 9, 1, 652. Abstract[All authors]
Entangling gates between qubits are a crucial component for performing algorithms in quantum computers. However, any quantum algorithm must ultimately operate on errorprotected logical qubits encoded in highdimensional systems. Typically, logical qubits are encoded in multiple twolevel systems, but entangling gates operating on such qubits are highly complex and have not yet been demonstrated. Here we realize a controlled NOT (CNOT) gate between two multiphoton qubits in two microwave cavities. In this approach, we encode a qubit in the highdimensional space of a single cavity mode, rather than in multiple twolevel systems. We couple two such encoded qubits together through a transmon, which is driven by an RF pump to apply the gate within 190 ns. This is two orders of magnitude shorter than the decoherence time of the transmon, enabling a highfidelity gate operation. These results are an important step towards universal algorithms on errorcorrected logical qubits.
2017

(2017) Physical Review A. 95, 3, 033814. Abstract
The longstanding goal of deterministic quantum interactions between single photons and single atoms was recently realized in various experiments. Among these, an appealing demonstration relied on singlephoton Raman interaction (SPRINT) in a threelevel atom coupled to a singlemode waveguide. In essence, the interferencebased process of SPRINT deterministically swaps the qubits encoded in a single photon and a single atom, without the need for additional control pulses. It can also be harnessed to construct passive entangling quantum gates, and can therefore form the basis for scalable quantum networks in which communication between the nodes is carried out only by singlephoton pulses. Here we present an analytical and numerical study of SPRINT, characterizing its limitations and defining parameters for its optimal operation. Specifically, we study the effect of losses, imperfect polarization, and the presence of multiple excited states. In all cases we discuss strategies for restoring the operation of SPRINT.

(2017) Quantum Information and Measurement (QIM) 2017. Abstract[All authors]
Using singlephoton Raman interaction, we demonstrate a deterministic and passive SWAP gate and quantum memory between the states of a single photon and a single ^{87}Rb atom coupled to highQ microresonator.
2016

(2016) Nature Photonics. 10, 1, p. 1922 Abstract
Removing a single photon from a pulse is one of the most elementary operations that can be performed on light, having both fundamental significance and practical applications in quantum communication and computation. So far, photon subtraction, in which the removed photon is detected and therefore irreversibly lost, has been implemented in a probabilistic manner with inherently low success rates using lowreflectivity beam splitters. Here we demonstrate a scheme for the deterministic extraction of a single photon from an incoming pulse. The removed photon is diverted to a different mode, enabling its use for other purposes, such as a photon numbersplitting attack on quantum key distribution protocols. Our implementation makes use of singlephoton Raman interaction (SPRINT) with a single atom near a nanofibrecoupled microresonator. The singlephoton extraction probability in our current realization is limited mostly by linear loss, yet probabilities close to unity should be attainable with realistic experimental parameters.

(2016) Proceedings of SPIE  The International Society for Optical Engineering. 9727, 972706. Abstract
We demonstrate a passive scheme for deterministic interactions between a single photon and a single atom. Relying on singlephoton Raman interaction (SPRINT), this controlfields free scheme swaps a flying qubit, encoded in the two possible input modes of a photon, with a stationary qubit, encoded in the two ground states of the atom, and can be also harnessed to perform universal quantum gates. Using SPRINT we experimentally demonstrated alloptical switching of single photons by single photons, and deterministic extraction of a single photon from an optical pulse. Applicable to any atomlike Lambda system, SPRINT provides a versatile building block for scalable quantum networks based on completely passive nodes interconnected and activated solely by single photons.
2015

(2015) Nature Communications. 6, 6788. Abstract
Spectroscopy of whisperinggallery mode microresonators has become a powerful scientific tool, enabling the detection of single viruses, nanoparticles and even single molecules. Yet the demonstrated timescale of these schemes has been limited so far to milliseconds or more. Here we introduce a scheme that is orders of magnitude faster, capable of capturing complete spectral snapshots at nanosecond timescales  cavity ringup spectroscopy. Based on sharply rising detuned probe pulses, cavity ringup spectroscopy combines the sensitivity of heterodyne measurements with the highestpossible, transformlimited acquisition rate. As a demonstration, we capture spectra of microtoroid resonators at time intervals as short as 16 ns, directly monitoring submicrosecond dynamics of their optomechanical vibrations, thermorefractive response and Kerr nonlinearity. Cavity ringup spectroscopy holds promise for the study of fast biological processes such as enzyme kinetics, protein folding and light harvesting, with applications in other fields such as cavity quantum electrodynamics and pulsed optomechanics.

(2015) Nonlinear Optics. Abstract
We demonstrate deterministic photonatom and photonphoton interactions with a single atom coupled to a highQ fibercoupled microresonator. Based on Deterministic One Photon Raman Interaction (DOPRI), this scheme can form the basis for alloptical quantum information processing.

(2015) CLEO:QELS  Fundamental Science 2015. Abstract
We demonstrate alloptical deterministic photonatom and photonphoton interactions with a single Rb atom coupled to highQ fibercoupled microresonator. This scheme enables alloptical photon routing, passive quantum memory and quantum gates activated solely by single photons.
2014

(2014) Science. 345, 6199, p. 903906 Abstract
The prospect of quantum networks, in which quantum information is carried by single photons in photonic circuits, has long been the driving force behind the effort to achieve alloptical routing of single photons.We realized a singlephotonactivated switch capable of routing a photon from any of its two inputs to any of its two outputs. Our device is based on a single atom coupled to a fibercoupled, chipbased microresonator. A single reflected control photon toggles the switch from high reflection (R ∼ 65%) to high transmission (T ∼ 90%), with an average of ∼1.5 control photons per switching event (∼3, including linear losses). No additional control fields are required.The control and target photons are both infiber and practically identical, making this scheme compatible with scalable architectures for quantum information processing.
2013

(2013) Physical review letters. 112, 12, 120403. Abstract
We experimentally demonstrate firstorder (fold) and secondorder (cusp) catastrophes in the density of an atomic cloud reflected from an optical barrier in the presence of gravity and show their corresponding universal asymptotic behavior. These catastrophes, arising from classical dynamics, enable robust, fieldfree refocusing of an expanding atomic cloud with a wide velocity distribution. Specifically, the density attained at the cusp point in our experiment reached 65% of the peak density of the atoms in the trap prior to their release. We thereby add caustics to the various phenomena with parallels in optics that can be harnessed for manipulation of cold atoms. The structural stability of catastrophes provides inherent robustness against variations in the system's dynamics and initial conditions, making them suitable for manipulation of atoms under imperfect conditions and limited controllability.

(2013) Physical review letters. 111, 2, 023604. Abstract
We demonstrate a new type of weak measurement based on the dynamics of spontaneous emission. The pointer in our scheme is given by the Lorentzian distribution characterizing atomic exponential decay via emission of a single photon. We thus introduce weak measurement, so far demonstrated nearly exclusively with laser beams and Gaussian statistics, into the quantum regime of single emitters and single quanta, enabling the exploitation of a wide class of sources that are abundant in nature. We describe a complete analogy between our scheme and weak measurement with conventional Gaussian pointers. Instead of a shift in the mean of a Gaussian distribution, an imaginary weak value is exhibited in our scheme by a significantly slowerthannatural exponential distribution of emitted photons at the postselected polarization, leading to a large shift in their mean arrival time. The dynamics of spontaneous emission offer a broader view of the measurement process than is usually considered within the weak measurement formalism. Our scheme opens the path for the use of atoms and atomlike systems as sensitive probes in weak measurements, one example being optical magnetometry.
2011

(2011) Physical Review A. 84, 3, 033854. Abstract
The most simple and seemingly straightforward application of the photon blockade effect, in which the transport of one photon prevents the transport of others, would be to separate two incoming indistinguishable photons to different output ports. We show that timeenergy uncertainty relations inherently prevent this ideal situation when the blockade is implemented by a twolevel system. The fundamental nature of this limit is revealed in the fact that photon blockade in the strong coupling regime of cavity QED, resulting from the nonlinearity of the JaynesCummings energy level structure, exhibits efficiency and temporal behavior identical to those of photon blockade in the bad cavity regime, where the underlying nonlinearity is that of the atom itself. We demonstrate that this limit can be exceeded, yet not avoided, by exploiting timeenergy entanglement between the incident photons. Finally, we show how this limit can be circumvented completely by using a threelevel atom coupled to a singlesided cavity, enabling an ideal and robust photon routing mechanism.