Imperfect-Information Games on Quantum Computers: A Case Study in Skat
Pith reviewed 2026-05-23 08:26 UTC · model grok-4.3
The pith
Skat rules and hidden cards can be encoded in quantum registers so a score operator and quantum counting identify the move with highest expected payoff.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
By representing the current Skat position with quantum registers that hold the visible cards and the superposition over possible hidden assignments, constructing unitary gates that advance the game according to the rules, and applying a score operator whose eigenvalues mark winning terminal states, the winning probability for each legal action is obtained via quantum counting. The resulting payoff function is maximized to recommend the action that yields the highest long-run success rate, an evaluation the authors state cannot be performed classically for sufficiently large instances because of the size of the underlying search tree.
What carries the argument
Quantum registers holding the game state together with a score operator that projects onto the winning subspace, used inside a quantum counting routine to estimate the number of paths leading to victory.
If this is right
- Selecting the legal move whose measured winning fraction is largest gives the action that maximizes the payoff function.
- Quantum advantage is expected once the number of possible hidden-card assignments exceeds the scale at which classical exhaustive search remains practical.
- The method supplies a statistical recommendation rather than a guaranteed winning line, consistent with the imperfect-information setting.
- Game rules are enforced exactly through the choice of unitary operators that implement legal card play and scoring.
Where Pith is reading between the lines
- The same register-and-score-operator construction could be applied to other imperfect-information card games such as bridge once their rule sets are expressed as quantum gates.
- Running the circuit on near-term hardware with a reduced deck would expose whether decoherence or measurement overhead prevents useful tree sizes from being reached.
- Hybrid schemes that pre-filter the superposition with classical sampling might lower the qubit and gate resources required for the initial state.
Load-bearing premise
The rules of Skat together with the distribution of hidden cards can be faithfully realized as a quantum circuit and score operator whose measurement statistics match the true winning probabilities without being overwhelmed by decoherence or state-preparation cost.
What would settle it
Exact classical enumeration of winning probabilities for a concrete Skat position with a small but non-trivial number of hidden cards, followed by direct comparison against the output of the corresponding quantum counting circuit executed on a simulator.
Figures
read the original abstract
For decades it is known that Quantum Computers might serve as a tool to solve a very specific kind of problems that have long thought to be incalculable. Some of those problems are of a combinatorial nature, with the quantum advantage arising from the exploding size of a huge decision tree. Although this is of high interest as well, there are more opportunities to make use of the quantum advantage among non-perfect information games with a limited amount of steps within the game. Even though it is not possible to answer the question for the winning move in a specific situation, people are rather interested in what choice gives the best outcome in the long run. This leads us to the search for the highest number of paths within the game's decision tree despite the lack of information and, thus, to a maximum of the payoff-function. We want to illustrate on how Quantum Computers can play a significant role in solving these kind of games, using an example of the most popular German card game Skat. Therefore we use quantum registers to encode the game's information properly and construct the corresponding quantum gates in order to model the game progress and obey the rules. Finally, we use a score operator to project the quantum state onto the winning subspace and therefore evaluate the winning probability for each alternative decision by the player to be made by using quantum algorithms, such as quantum counting of the winning paths to gain a possible advantage in computation speed over classical approaches. Thus, we get a reasonable recommendation of how to act at the table due to the payoff-function maximization. This approach is clearly not doable on a classical computer due to the huge tree-search problem and we discuss peculiarities of the problem that may lead to a quantum advantage when exceeding a certain problem size.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes encoding Skat (an imperfect-information card game) on a quantum computer by mapping game states to quantum registers, implementing rules via quantum gates, and applying a score operator whose measurement via quantum counting yields winning probabilities for payoff-maximizing decisions. It asserts that the exponential size of the decision tree renders the problem intractable classically but potentially amenable to quantum advantage at large scale.
Significance. If the encoding, gate constructions, and resource scaling can be shown to work, the approach would extend quantum algorithms from perfect-information games to a new class of imperfect-information combinatorial problems, with possible practical implications for decision-making under uncertainty. The emphasis on payoff maximization rather than single-instance winning moves is a constructive framing.
major comments (3)
- [Abstract] Abstract: the claim that quantum registers 'encode the game's information properly' and that a score operator 'project[s] the quantum state onto the winning subspace' is not supported by any explicit mapping from card configurations (including hidden hands) to basis states, nor by any derivation showing that measurement statistics recover the correct payoff function.
- [Abstract] Abstract and main text: no circuit construction, gate count, depth estimate, or scaling analysis is supplied to justify the assertion of quantum advantage 'when exceeding a certain problem size,' despite this being the central contrast with classical tree search.
- [Abstract] The manuscript provides neither a toy-model verification on a reduced game instance nor any resource estimate (qubits, gates, shots) that would allow assessment of whether decoherence or the continuous distribution of hidden-card assignments remains tractable.
minor comments (2)
- [Abstract] Abstract contains minor grammatical issues (e.g., 'For decades it is known' should read 'For decades it has been known').
- [Abstract] Several long sentences in the abstract could be split for clarity.
Simulated Author's Rebuttal
We thank the referee for their careful reading and constructive comments on our manuscript. We address each major comment below, clarifying the conceptual scope of the proposal while indicating where revisions will strengthen the presentation.
read point-by-point responses
-
Referee: [Abstract] Abstract: the claim that quantum registers 'encode the game's information properly' and that a score operator 'project[s] the quantum state onto the winning subspace' is not supported by any explicit mapping from card configurations (including hidden hands) to basis states, nor by any derivation showing that measurement statistics recover the correct payoff function.
Authors: The manuscript is a conceptual outline of a quantum-register encoding for Skat states (including hidden hands) and a score operator whose measurement yields payoff estimates via quantum counting. We agree that the abstract and main text do not supply explicit basis-state mappings or a derivation of the payoff function. We will revise the abstract to qualify the claims as high-level and add a dedicated subsection with a concrete example mapping for a reduced card configuration together with the corresponding measurement statistics. revision: yes
-
Referee: [Abstract] Abstract and main text: no circuit construction, gate count, depth estimate, or scaling analysis is supplied to justify the assertion of quantum advantage 'when exceeding a certain problem size,' despite this being the central contrast with classical tree search.
Authors: The central contrast drawn in the manuscript is the exponential growth of the decision tree under imperfect information, which quantum counting can in principle address by estimating the measure of winning paths. We acknowledge that no explicit circuit constructions, gate counts, or quantitative scaling analysis are provided. We will add a new section that supplies qualitative scaling arguments based on the qubit requirements for encoding card distributions and the known complexity of quantum counting, while making clear that a full resource analysis lies beyond the scope of the present case study. revision: partial
-
Referee: [Abstract] The manuscript provides neither a toy-model verification on a reduced game instance nor any resource estimate (qubits, gates, shots) that would allow assessment of whether decoherence or the continuous distribution of hidden-card assignments remains tractable.
Authors: We agree that the absence of a toy-model verification and concrete resource estimates limits the ability to assess practical tractability, including decoherence and the handling of hidden-card distributions. The work is framed as a proposal for the overall approach rather than an implementation study. We will revise the manuscript to include a qualitative discussion of qubit counts needed to encode a Skat hand and to note that detailed toy-model simulations and decoherence analysis are reserved for follow-up work. revision: partial
Circularity Check
No circularity; high-level proposal lacks explicit derivations or self-referential reductions.
full rationale
The manuscript outlines a conceptual mapping of Skat rules to quantum registers, gates, and a score operator followed by quantum counting, but supplies no equations, circuit constructions, or parameter fits. No step reduces a claimed prediction or advantage to an input by definition, self-citation chain, or renaming. The discussion of potential quantum advantage at large problem size remains an unelaborated assertion rather than a derivation that collapses onto its premises. The paper is therefore self-contained at the level of a proposal and exhibits no load-bearing circularity.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Quantum superposition and entanglement can represent distributions over hidden information without loss of correctness for payoff estimation.
- domain assumption Quantum gates exist that simulate every legal sequence of moves while obeying Skat rules.
Reference graph
Works this paper leans on
-
[1]
Jean R. S. Blair, David Mutchler, and Cheng Liu. Games with imperfect information. AAAI Technical Report, FS- 93-02:59–67, 1993
work page 1993
-
[2]
H. J. Bremermann. Quantum noise and information. Berkeley Symp. Math. Statistics and Probability, 5:15–20, 1967
work page 1967
-
[3]
Peter W. Shor. Polynomial-time algorithms for prime factorization and discrete logarithms on a quantum com- puter. SIAM Journal on Computing , 26(5):1484–1509, October 1997
work page 1997
-
[4]
Alexander M. Dalzell, Aram W. Harrow, Dax Enshan Koh, and Rolando L. La Placa. How many qubits are needed for quantum computational supremacy? Quan- tum, 4:264, May 2020
work page 2020
-
[5]
Burch, Akihiro Kishimoto, Martin M¨ uller, Robert Lake, Paul Lu, and Steve Sutphen
Jonathan Schaeffer, Yngvi Bj¨ ornsson, N. Burch, Akihiro Kishimoto, Martin M¨ uller, Robert Lake, Paul Lu, and Steve Sutphen. Solving checkers. In IJCAI, pages 292– 297, 2005
work page 2005
-
[6]
R. Gasser. Harnessing Computational Resources for Effi- cient Exhaustive Search. PhD thesis, ETH Z¨ urich, 1995
work page 1995
-
[7]
Mastering Chess and Shogi by self-play with a gen- eral reinforcement learning algorithm
David Silver, Thomas Hubert, Julian Schrittwieser, Ioan- nis Antonoglou, Matthew Lai, Arthur Guez, Marc Lanc- tot, Laurent Sifre, Dharshan Kumaran, Thore Graepel, Timothy Lillicrap, Karen Simonyan, and Demis Hass- abis. Mastering Chess and Shogi by self-play with a gen- eral reinforcement learning algorithm. Technical Report 1712.018, arxiv, 2017
work page 2017
-
[8]
Theαµ search algorithm for the game of bridge
Tristan Cazenave and V´ eronique Ventos. Theαµ search algorithm for the game of bridge. CoRR, abs/1911.07960, 2019
-
[9]
Challenging human supremacy in Skat
Stefan Edelkamp. Challenging human supremacy in Skat. In Symposium on Combinatorial Search (SOCS) , pages 52–60, 2019
work page 2019
-
[10]
F. Schettler and G. Kirschbach. Das große Skatvergn¨ ugen. Urania Verlag, Leipzig, Jena, Berlin, 1988
work page 1988
-
[11]
David A. Meyer. Quantum strategies. Physical Review Letters, 82(5):1052–1055, February 1999
work page 1999
-
[12]
Quantum games and quantum strategies
Jens Eisert, Martin Wilkens, and Maciej Lewenstein. Quantum games and quantum strategies. Physical Re- view Letters, 83(15):3077–3080, October 1999
work page 1999
-
[13]
Quantum formulation of classical two person zero-sum games
Andreas Boukas. Quantum formulation of classical two person zero-sum games. Open Systems & Information Dynamics, 7:19–32, 03 2000
work page 2000
-
[14]
C. E. Lemke and J. T. Howson, Jr. Equilibrium points of bimatrix games. Journal of the Society for Industrial and Applied Mathematics , 12(2):413–423, 1964
work page 1964
- [15]
-
[16]
Regret minimization in games with incomplete information
Martin Zinkevich, Michael Johanson, Michael Bowling, and Carmelo Piccione. Regret minimization in games with incomplete information. In J. Platt, D. Koller, Y. Singer, and S. Roweis, editors, Advances in Neural Information Processing Systems, volume 20. Curran As- sociates, Inc., 2007. 19
work page 2007
-
[17]
Emanuel Lasker. Das verst¨ andige Kartenspiel. August Scherl Verlag, Berlin, 1929
work page 1929
-
[18]
Emanuel Lasker. Strategie der Spiele – Skat . August Scherl Verlag, Berlin, 1938
work page 1938
-
[19]
Joseph Petrus Wergin. Wergin on Skat and Sheepshead . Wergin Distributing, Mc. Farland, USA, 1975
work page 1975
-
[20]
S. Grandmontagne. Meisterhaft Skat spielen . Selfpub- lisher, Kr¨ uger Druck+Verlag, 2005
work page 2005
-
[21]
Skat-R¨ atsel – 50 lehrreiche Skatauf- gaben mit L¨ osungen und Analysen
Thomas Kinback. Skat-R¨ atsel – 50 lehrreiche Skatauf- gaben mit L¨ osungen und Analysen. Books on Demand, Norderstedt, 2007
work page 2007
-
[22]
Gl¨ aserne Karten – Gewinnen beim Skat
Manfred Quambusch. Gl¨ aserne Karten – Gewinnen beim Skat. Stomi Verlag, Schwerte Rau Verlag, D¨ usseldorf, 1990
work page 1990
-
[23]
Siegfried Harmel. Skat–Zahlen. Klabautermann-Verlag, P¨ underich (Mosel), 2016
work page 2016
-
[24]
Der Skatfuchs – Gewinnen im Skatspiel mit Mathematische Methoden
Rainer G¨ oßl. Der Skatfuchs – Gewinnen im Skatspiel mit Mathematische Methoden . Selfpublisher. D¨ ammig, Chemnitz, Available from the Author or via DSKV Al- tenburg, 2019
work page 2019
-
[25]
Search, Inference and Opponent Modelling in an Expert-Caliber Skat Player
Jeffrey Richard Long. Search, Inference and Opponent Modelling in an Expert-Caliber Skat Player . PhD thesis, University of Alberta, 2011
work page 2011
-
[26]
Policy Based Inference in Trick-Taking Card Games
Douglas Rebstock, Christopher Solinas, Michael Buro, and Nathan R. Sturtevant. Policy based inference in trick-taking card games. CoRR, abs/1905.10911, 2019
work page internal anchor Pith review Pith/arXiv arXiv 1905
-
[27]
Improving computer play in skat with hope cards
Stefan Edelkamp. Improving computer play in skat with hope cards. In Computers and Games - International Conference (CG), Revised Selected Papers, volume 13865 of LNCS, pages 133–145. Springer, 2022
work page 2022
-
[28]
Learning Policies from Human Data for Skat
Douglas Rebstock, Christopher Solinas, and Michael Buro. Learning policies from human data for Skat. CoRR, abs/1905.10907, 2019
work page internal anchor Pith review Pith/arXiv arXiv 1905
-
[29]
On the power of refined skat selection
Stefan Edelkamp. On the power of refined skat selection. CoRR, abs/2104.02997, 2021
-
[30]
ELO system for skat and other games of chance
Stefan Edelkamp. ELO system for skat and other games of chance. CoRR, abs/2104.05422, 2021
-
[31]
Entwicklung eines Double- Dummy Skat Solvers mit einer Anwendung f¨ ur verdeckte Skatspiele
Sebastian Kupferschmid. Entwicklung eines Double- Dummy Skat Solvers mit einer Anwendung f¨ ur verdeckte Skatspiele. Master’s thesis, University of Freiburg, 2006
work page 2006
-
[32]
Automatic bidding for the game of Skat
Thomas Keller and Sebastian Kupferschmid. Automatic bidding for the game of Skat. In KI, pages 95–102, 2008
work page 2008
-
[33]
Michael Buro, Jeffrey Richard Long, Timothy Furtak, and Nathan R. Sturtevant. Improving state evaluation, inference, and search in trick-based card games. In Sim- ulation, pages 135–147, 2006
work page 2006
-
[34]
Improving search with supervised learning in trick- based card games
Christopher Solinas, Douglas Rebstock, and Michael Buro. Improving search with supervised learning in trick- based card games. CoRR, abs/1903.09604, 2019
-
[35]
Symmetries and Search in Trick-Taking Card Games
Timothy Michael Furtak. Symmetries and Search in Trick-Taking Card Games. PhD thesis, University of Al- berta, 2013
work page 2013
-
[36]
Knowledge-based paranoia search
Stefan Edelkamp. Knowledge-based paranoia search. In 2021 IEEE Conference on Games (CoG), Copenhagen, Denmark, August 17-20, 2021 , pages 1–8. IEEE, 2021
work page 2021
-
[37]
Gal Cohensius, Reshef Meir, Nadav Oved, and Roni Stern. Bidding in spades. In Giuseppe De Giacomo, Alejandro Catal´ a, Bistra Dilkina, Michela Milano, Sen´ en Barro, Alberto Bugar´ ın, and J´ erˆ ome Lang, editors, ECAI, volume 325 of Frontiers in Artificial Intelligence and Applications, pages 387–394. IOS Press, 2020
work page 2020
-
[38]
Generalisation of alpha-beta search for AND-OR graphs with partially ordered values
Junkang Li, Bruno Zanuttini, Tristan Cazenave, and V´ eronique Ventos. Generalisation of alpha-beta search for AND-OR graphs with partially ordered values. In IJCAI, pages 4769–4775, 2022
work page 2022
-
[39]
Trick costs for αµ and new relatives
Samuel Bounan and Stefan Edelkamp. Trick costs for αµ and new relatives. In International Symposium on Artificial Intelligence and Mathematics (ISAIM) , 2024
work page 2024
-
[40]
Connor, Neil Burch, Thomas Anthony, Stephen McAleer, Romuald Elie, Sarah H
Julien Perolat, Bart De Vylder, Daniel Hennes, Eu- gene Tarassov, Florian Strub, Vincent de Boer, Paul Muller, Jerome T. Connor, Neil Burch, Thomas Anthony, Stephen McAleer, Romuald Elie, Sarah H. Cen, Zhe Wang, Audrunas Gruslys, Aleksandra Malysheva, Mina Khan, Sherjil Ozair, Finbarr Timbers, Toby Pohlen, Tom Eccles, Mark Rowland, Marc Lanctot, Jean-Bapt...
work page 2022
-
[41]
Efficient boolean methods for preparing uniform quantum states
Fereshte Mozafari, Heinz Riener, Mathias Soeken, and Giovanni De Micheli. Efficient boolean methods for preparing uniform quantum states. IEEE Transactions on Quantum Engineering , 2:1–12, 2021
work page 2021
-
[42]
Michael A. Nielsen and Isaac L. Chuang. Quantum Com- putation and Quantum Information: 10th Anniversary Edition. Cambridge University Press, 2010
work page 2010
-
[43]
Could the declarer have discarded it? refined anticipation of cards in skat
Stefan Edelkamp. Could the declarer have discarded it? refined anticipation of cards in skat. In Andreas Hotho and Sebastian Rudolph, editors, KI, Lecture Notes in Computer Science, pages 60–72. Springer, 2024
work page 2024
-
[44]
Dynamic play via suit factorization search in skat
Stefan Edelkamp. Dynamic play via suit factorization search in skat. InKI, Lecture Notes in Computer Science, pages 18–32. Springer, 2020
work page 2020
-
[45]
Technology and performance benchmarks of iqm’s 20-qubit quantum computer, 2024
Leonid Abdurakhimov, Janos Adam, Hasnain Ahmad, Olli Ahonen, Manuel Algaba, Guillermo Alonso, Ville Bergholm, Rohit Beriwal, Matthias Beuerle, Clinton Bockstiegel, Alessio Calzona, Chun Fai Chan, Daniele Cucurachi, Saga Dahl, Rakhim Davletkaliyev, Olexiy Fedorets, Alejandro Gomez Frieiro, Zheming Gao, Jo- han Guldmyr, Andrew Guthrie, Juha Hassel, Hermanni...
work page 2024
-
[46]
Eric Hyypp¨ a, Antti Veps¨ al¨ ainen, Miha Papiˇ c, Chun Fai Chan, Sinan Inel, Alessandro Landra, Wei Liu, J¨ urgen Luus, Fabian Marxer, Caspar Ockeloen-Korppi, Sebas- tian Orbell, Brian Tarasinski, and Johannes Heinsoo. Reducing leakage of single-qubit gates for superconduct- ing quantum processors using analytical control pulse en- velopes. PRX Quantum,...
work page 2024
-
[47]
J. Heinsoo. Gate performance statistics from quantum processing units with tens of qubits. Bulletin of the American, 2024
work page 2024
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.