Quantum technologies promise a radically new way to solve classically intractable computing problems. Superconducting circuits as a platform are at the forefront of this field. The cryogenic operation temperatures of superconducting circuits however impose challenges for the further scaling to many connected quantum information processing units into a local area or global network. In this work, we present a hardware solution for connecting quantum devices operating at microwave frequencies into local area networks, which enable the exchange of quantum information between spatially separated parties. Specifically, we demonstrate a modular system spanning distances of 5, 10 and 30 meters operated at cryogenic temperatures and connecting two superconducting circuit systems, located in individual dilution refrigerators, through a quantum communication channel. We develop a thermal model to evaluate the heat transfer processes in the setup, optimize the design and select appropriate materials for its construction. The assembled 30-meter-long system achieves operating temperatures of below 50 mK after a cooldown time of about six and a half days. This link enables the execution of distributed quantum computing and communication algorithms. It also adds the resource of non-locality, certified by a loophole-free Bell test, to the field of quantum science and technology with superconducting circuits.
Quantum technologies promise a radically new way to solve classically intractable computing problems. Superconducting circuits as a platform are at the forefront of this field. The cryogenic operation temperatures of superconducting circuits however impose challenges for the further scaling to many connected quantum information processing units into a local area or global network. In this work, we present a hardware solution for connecting quantum devices operating at microwave frequencies into local area networks, which enable the exchange of quantum information between spatially separated parties. Specifically, we demonstrate a modular system spanning distances of 5, 10 and 30 meters operated at cryogenic temperatures and connecting two superconducting circuit systems, located in individual dilution refrigerators, through a quantum communication channel. We develop a thermal model to evaluate the heat transfer processes in the setup, optimize the design and select appropriate materials for its construction. The assembled 30-meter-long system achieves operating temperatures of below 50 mK after a cooldown time of about six and a half days. This link enables the execution of distributed quantum computing and communication algorithms. It also adds the resource of non-locality, certified by a loophole-free Bell test, to the field of quantum science and technology with superconducting circuits.
Scalable quantum information processing with superconducting circuits is expected to advance from individual processors located in single dilution refrigerators to more powerful distributed quantum computing systems. The realization of hardware platforms for quantum local area networks (QLANs) compatible with superconducting technology is of high importance in order to achieve a practical quantum advantage. Here, we present a fundamental prototype platform for a microwave QLAN based on a cryogenic link connecting two separate dilution cryostats over a distance of 6.6 m with a base temperature of 52 mK in the center. Superconducting microwave coaxial cables are employed to form a quantum communication channel between the distributed network nodes. We demonstrate the continuous-variable entanglement distribution between the remote dilution refrigerators in the form of two-mode squeezed microwave states, reaching squeezing of 2.10 ± 0.02 dB and negativity of 0.501 ± 0.011. Furthermore, we show that quantum entanglement is preserved at channel center temperatures up to 1 K, paving the way towards microwave quantum communication at elevated temperatures. Consequently, such a QLAN system can form the backbone for future distributed quantum computing with superconducting circuits.
Superconducting circuits are a strong contender for realizing quantum computing systems and are also successfully used to study quantum optics and hybrid quantum systems. However, their cryogenic operation temperatures and the current lack of coherence-preserving microwave-to-optical conversion solutions have hindered the realization of superconducting quantum networks spanning different cryogenic systems or larger distances. Here, we report the successful operation of a cryogenic waveguide coherently linking transmon qubits located in two dilution refrigerators separated by a physical distance of five meters. We transfer qubit states and generate entanglement on demand with average transfer and target state fidelities of 85.8% and 79.5%, respectively, between the two nodes of this elementary network. Cryogenic microwave links provide an opportunity to scale up systems for quantum computing and create local area superconducting quantum communication networks over length scales of at least tens of meters.
In this study, we investigate the unconditional microwave quantum teleportation of Gaussian states and the feasibility of its realization with real microwave elements between two dilution refrigerators. Our approach involves employing the Braunstein–Kimble protocol for continuous variables, represented in the symplectic domain, with microwave components integrated into a teleportation circuit. This circuit is established between two dilution refrigerators, which are interconnected by a cryogenic link and incorporate a homodyne detector. The proposed framework offers controllability, suitable for implementation within a single refrigerator or between two separate refrigerators. It operates effectively across a temperature range spanning from milliKelvin to 4 K. This platform is useful for advancing superconducting quantum communication within and between refrigeration systems. Its potential extends to facilitating quantum local area networks and enabling distributed quantum computing protocols. Furthermore, we briefly discuss the complex realm of long-range open-air quantum microwave communication under realistic conditions. We also introduce a protocol designed to enhance entanglement distillation of two-mode squeezed states between two refrigerators. This enhancement targets the squeezing factor of the resource and improves the protocol efficiency.
Quantum communication in the microwave regime is set to play an important role in distributed quantum computing and hybrid quantum networks. However, typical superconducting quantum circuits require millikelvin temperatures for operation, which poses a significant challenge for large-scale microwave quantum networks. Here, we present a solution to this challenge by demonstrating the successful quantum teleportation of microwave coherent states between two spatially separated dilution refrigerators over a thermal microwave channel in the temperature range up to 4 K. We distribute two-mode squeezed states over this noisy channel and employ the resulting quantum entanglement for quantum teleportation of coherent states with fidelities of 72.3±0.5% at 1 K and 59.9±2.5% at 4 K, exceeding the no-cloning and classical communication thresholds, respectively. We successfully model the teleportation protocol using a Gaussian operator formalism that includes losses and noise. Our analysis shows that the teleportation infidelity mainly stems from a parasitic heating of the cold quantum nodes due to the hot network connection. These results demonstrate the experimental feasibility of distributed superconducting architectures and motivate further investigations of noisy quantum networks in various frequency regimes.
Our modern information society relies on high-performance local information processing and global information distribution.Complementing today's information technology with quantum networks promises exponentially faster computation, and more efficient and secure communication.First quantum networks have been investigated during the last decades showing the feasibility to utilize quantum technologies locally and globally.In this thesis, we present the experimental implementation of a local quantum network in a circuit quantum electrodynamic (circuit QED) architecture, which has become one of the leading platforms for quantum information processing.Extending the circuit QED framework by reliable, high-bandwidth quantum communication techniques thus paves the way towards distributed quantum computation.Besides quantum computation, combining quantum communication and our fast quantumstate readout techniques provides a basis for studying non-local physics based on Bell tests in circuit QED.Circuit QED systems with typical transition frequencies in the microwave regime need to be operated at temperatures in the tens of millikelvin range to reduce their thermal population.To build circuit QED-based quantum networks we develop a novel modular design of a cryogenic network.We experimentally investigate two cryogenic prototype systems with a length of 4 meter and 10 meter.Based on these characterizations we perform simulations for systems up to 40 meter.The experimental and simulation results indicate that large-scale cryogenic networks can be realized using our modular design.In addition to cryogenic networks, reliable quantum communication at high transfer rates requires low-loss information transmission between distant quantum systems.For that purpose, we characterize coaxial cables and rectangular microwave waveguides down to millikelvin temperatures using a resonant-cavity technique.With this technique we measure attenuation constants down to 1 decibel/kilometer at the single photon level.These low attenuation constants show the potential to perform quantum communication with microwave photons over several tens of meters.In the next part, we realize a direct quantum channel between two remote circuit QED nodes on separate chips.We utilize a fully microwave controlled interaction between a quantum system and a resonator to emit time-symmetric single itinerant photons from the first node and absorb them at the second node.To characterize the performance of the direct quantum channel we transfer quantum states directly and generate entangled states between the remote nodes.We experimentally investigate two different mappings of local quantum states to the itinerant photons.First, we map local quantum states to the Fock-state-basis of the itinerant photons and realize the first deterministic quantum communication between two remote quantum systems.Second, we show that quantum communication errors can be detected by mapping quantum states to a time-bin superposition of a single photon.To herald quantum communication errors by an ancilla quantum systems in future experiments, we test a novel all-microwave interaction between two quantum systems.Quantum gates based on this all-microwave interaction can also be used in more advanced quantum communication algorithms like quantum teleportation or error-correction schemes like entanglement distillation.For all remote quantum communication experiments, we validate the high level of experimental control by observing an excellent agreement with numerical master equation simulations.In conclusion, we present a framework for modular cryogenic quantum networks based on low-loss direct quantum channels for future quantum information processing applications and fundamental studies of non-local physics.ivZusammenfassung Unsere moderne Informationsgesellschaft basiert auf lokalen Hochleistungscomputern und weltweiter Informationsverteilung. Quantennetzwerke, welche die heutige Informationstechnologie ergänzen können, versprechen sowohl exponentiell schnellere Berechnungen als auch eine effizientere und sicherere Kommunikation.Während der letzten Jahrzehnte sind erste Quantennetzwerke erforscht worden, die eine zukünftige Nutzung von Quantentechnologien lokal und weltweit ermöglichen könnten.Die vorliegende Arbeit untersucht die experimentelle Realisierung von lokalen Quantennetzwerken in supraleitenden Schaltkreisen, welche sich zu einer der führenden Plattformen für Quanteninformationsverarbeitung entwickelt haben.Wir erweitern supraleitende Schaltkreise um zuverlässige Techniken zur Quantenkommunikation mit hoher Bandbreite und schaffen somit die Grundlage für verteilte Quantencomputersysteme. Neben Anwendungen im Rahmen der Quanteninformationsverarbeitung schafft die Kombination von Quantenkommunikation und den von uns entwickelten schnellen Messtechniken von Quantenzuständen die Grundlage, um nicht-lokale physikalische Phänomene mit Hilfe bellscher Ungleichungen in supraleitenden Schaltkreisen zu untersuchen.Quantensysteme in supraleitenden Schaltkreisen müssen aufgrund ihrer Übergangsfrequenzen im Mikrowellenbereich auf Temperaturen von einigen zehn Millikelvin abgekühlt werden, um ihre thermische Anregung zu reduzieren.Um zukünftig supraleitenden Quantennetzwerke aufzubauen, entwickeln wir ein neues modulares Design für kryogene Netzwerke.Wir untersuchen experimentell zwei kryogene Prototypsysteme mit einer Länge von 4 Meter und 10 Meter und nutzen diese Charakterisierungsmessungen, um Systeme bis zu 40 Meter Länge zu simulieren.Die experimentellen Ergebnisse und Simulationen deuten darauf hin, dass gross angelegte kryogene Netzwerke mit unserem modularen Design realisiert werden können.Zusätzlich zu kryogenen Netzwerken benötigt eine zuverlässige Quantenkommunikation mit hoher Transferrate eine verlustarme Informationsübertragung zwischen entfernt liegenden Quantensystemen.Wir charakterisieren hierzu Mikrowellenkoaxialkabel und Rechteckhohlleiter bis zu Temperaturen im Millikelvin-Bereich mit Hilfe einer Resonanz-Kavität-Messmethode.Mit dieser Technik messen wir Absorptionskoeffizienten kleiner als 1 Dezibel/Kilometer bei einer Signalstärke auf dem Niveau von einzelnen Photonen.Diese niedrigen Absorptionskoeffizienten zeigen das Potential, dass breitbandige Quantenkommunikation mit Mikrowellenphotonen über mehrere zehn Meter durchgeführt werden kann.Im nächsten Teil dieser Arbeit realisieren wir einen direkten Quantenkanal zwi-schen zwei entfernt liegend, chip-basierten supraleitenden Schaltkreisen.Wir verwenden eine vollständig durch Mikrowellensignale kontrollierte Wechselwirkung zwischen einem Quantensystem und einem Resonator, um einzelne, zeitsymmetrische, sich ausbreitenden Mikrowellenphotonen von einem Netzwerkknoten zu emittieren und an einem zweiten zu absorbieren.Um diesen direkten Quantenkanal zu charakterisieren, übermitteln wir Quantenzustände direkt zwischen zwei Netzwerkknoten und erzeugen verschränkte Zustände zwischen diesen.Wir untersuchen experimentell zwei verschiedene Übertragungsverfahren der lokalen Quantenzustände auf die sich ausbreitenden Mikrowellenphotonen. Zum einen übertragen wir lokale Quantenzustände auf die Fock-Zustands-Basis der sich ausbreitenden Mikrowellenphotonen und realisieren damit die erste deterministische Quantenkommunikation zwischen zwei entfernt liegenden Quantensystemen.Zum anderen zeigen wir, dass Quantenkommunikationsfehler detektiert werden können, indem die lokalen Quantenzustände auf die Superposition von Zeitintervallen eines einzelnen Photons übertragen werden.Um Quantenkommunikationsfehler durch ein Hilfsquantensystem in zukünftigen Experimenten anzuzeigen, testen wir eine neue, mikrowellen-kontrollierte Wechselwirkung zwischen zwei Quantensystemen.Quantengatter, die auf dieser Wechselwirkung basieren, können auch in erweiterten Quantenkommunikationsalgorithmen wie der Quantenteleportation oder zur Fehlerkorrektur mit Hilfe der Verschränkungs-Destillation genutzt werden.In allen Quantenkommunikationsexperimenten belegt eine ausgezeichnete Übereinstimmung mit numerischen Simulationen der Mastergleichung ein hohes Mass an experimenteller Kontrolle unserer Quantensysteme.Zusammenfassend präsentieren wir ein Design für ein modulares kryogenes Quantennetzwerk, das auf verlustarmen Quantenkanälen basiert und das zukünftige Anwendungen im Bereich der Quanteninformationsverarbeitung und der Grundlagenforschung zu nicht-lokalen physikalischen Phänomenen ermöglicht.
Quantum computing promises an exponential speed-up of computation compared to what is nowadays achievable with classical computers. In this way, it enables the evaluation of more complex models and the breaching of current security algorithms. For the operation of a quantum system, many questions remain to be answered. Currently, there are several quantum technologies that promise to be both reliable and scalable, two features required for large scale quantum operations. Common to all technologies is the operating temperature that needs to be close to absolute zero, i.e. below 100 mK, to suppress environment noise and allow the quantum properties to become 'visible'. In order for any quantum processor to be operated, a so-called quantum-classical interface is required for the quantum bit (qubit) read-out and control. This interface consists of various electronic blocks, such as analog-to-digital converters, digital-to-analog converters, mixers, amplifiers and a digital controller. Especially the analog blocks require effort to meet the noise and stability constraints as not to disturb the very sensitive qubits and allow reading of the tiny signals. As the qubits live in extremely deep-cryogenic temperatures, long wires interface the cryogenic with the room temperature environment, where most of the electronics is situated. However, for a scalable system, heat injection becomes a serious problem, with many wires between 300 K and sub-Kelvin. Furthermore, such amount of interconnects is challenging to mechanically place in a dilution refrigerator. Therefore, in this work, we propose to implement the electronics not at room temperature, but at a temperature much closer to the qubits, for example at 4 K. This not only reduces significantly the wiring between room temperature and the qubits, but we can also benefit from lower electronic noise at such a temperature. The operation of conventional electronics almost 200°C below its normal temperature range is not trivial as device properties alter significantly and most circuits no longer operate as intended. In CMOS processes, the main technology in the integrated electronics world, the behaviour of the transistors, required for the implementation of any circuit, deviates considerably at low temperatures. The transistor's threshold voltage goes up, the mobility increases and the subthreshold slope becomes steeper, to name just a few of these deviations. Although there are improvements in performance, there are also some counter effects, and characterization of the transistors is needed to observe the changes. Once devices are characterized at such low temperatures, new models can be built and circuits can be simulated and adapted to operate properly at cryogenic temperatures. Luckily, there are various commercially available devices that can already withstand the chills of cold. We demonstrated various commercially available devices, such as a field-programmable gate array (FPGA) implemented in a 28 nm CMOS process, to be operating without major concerns at 4~K. Its properties alter only slightly, within 5 to 10%, and all tested circuit implementations, working at 300~K, also worked at 4 K. We combined both commercially available devices, that operate 200 K below their specified temperature range, with custom designed CMOS circuits to implement a cryogenic read-out platform for spin qubits. This system comprises amplifiers, an ADC, an FPGA, voltage regulators and a clock generator. It allows to amplify the tiny signal from the qubits and digitize it directly in the FPGA in order to process the data locally at 4~K. This system is one of the first systematic attempts at operating a part of the quantum-classical interface at cryogenic temperatures and forms the basis for future systems comprising the complete electronic interface for qubits to operate at such low temperatures. One of the main problems to tackle for cryogenic electronic systems is their power consumption. Power budgets are simply limited to roughly 1 or 2 Watts at 4 K and exponentially lower at deeper cryogenic temperatures, thus limiting the size of large-scale electronic systems. Our approach of combined commercial and custom circuits will have to be steadily replaced by a single (custom) technology that meets both power and scalability constraints. One of the best candidates is CMOS, a technology that the industry has relied upon for several decades and benefits from many optimizations thanks to Moore's law.